The femoral ontogeny and long bone histology of the Middle Triassic (?late Anisian) dinosauriform Asilisaurus kongwe and implications for the growth of early dinosaurs

Abstract

The ontogeny of early-diverging dinosauromorphs is poorly understood because few ontogenetic series from the same species-level taxon are known and what is available has not been extensively documented. The large numbers of skeletal elements of the silesaurid Asilisaurus kongwe recently recovered from Tanzania provide an opportunity to examine the ontogenetic trajectory of the earliest known member of Ornithodira and one of the closest relatives to Dinosauria. We examined the ontogeny of the femur and the histology of a series of long bone elements. We observed bone scar variation in a series of femora (n = 27) of different lengths (73.8–177.2 mm). We hypothesize that most femora follow a similar developmental trajectory; however, we observed sequence polymorphism in the order of appearance and shape of bone scars, and we quantified this polymorphism using ontogenetic sequence analysis (OSA). Additionally, five femora, three tibiae, a fibula, and a humerus were thin-sectioned to examine osteological tissues. No lines of arrested growth (LAGs) are present in any specimen, and there is little histological information about the ontogenetic stage of femora, although none have slowed or ceased growth. The woven-fibered bone present in the cortex of elements sectioned is similar to that of the earliest dinosaurs. This sequence polymorphism provides an alternate hypothesis for the robust/gracile dichotomy found in early dinosaurs often interpreted as sexual dimorphism. The shared femoral features found in Asilisaurus and early dinosaurs suggest that this ontogenetic pattern is plesiomorphic for Dinosauria, and that size is a poor predictor of maturity in early dinosauriforms.SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVPCitation for this article: Griffin, C. T., and S. J. Nesbitt. 2016. The femoral ontogeny and long bone histology of the Middle Triassic (?late Anisian) dinosauriform Asilisaurus kongwe and implications for the growth of early dinosaurs. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1111224.

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The femoral ontogeny and long bone histology of
the Middle Triassic (?late Anisian) dinosauriform
Asilisaurus kongwe and implications for the
growth of early dinosaurs
C. T. Griffin & Sterling J. Nesbitt
To cite this article: C. T. Griffin & Sterling J. Nesbitt (2016): The femoral ontogeny and long
bone histology of the Middle Triassic (?late Anisian) dinosauriform Asilisaurus kongwe and
implications for the growth of early dinosaurs, Journal of Vertebrate Paleontology
To link to this article: http://dx.doi.org/10.1080/02724634.2016.1111224
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ARTICLE
THE FEMORAL ONTOGENY AND LONG BONE HISTOLOGY OF THE MIDDLE TRIASSIC
(?LATE ANISIAN) DINOSAURIFORM ASILISAURUS KONGWE AND IMPLICATIONS FOR THE
GROWTH OF EARLY DINOSAURS
C. T. GRIFFIN* and STERLING J. NESBITT
Department of Geosciences, Virginia Polytechnic and State University, 4044 Derring Hall, 1405 Perry Street, Blacksburg, Virginia
24061, U.S.A., ctgriff@vt.edu; sjn2104@vt.edu
ABSTRACT—The ontogeny of early-diverging dinosauromorphs is poorly understood because few ontogenetic series from
the same species-level taxon are known and what is available has not been extensively documented. The large numbers of
skeletal elements of the silesaurid Asilisaurus kongwe recently recovered from Tanzania provide an opportunity to examine
the ontogenetic trajectory of the earliest known member of Ornithodira and one of the closest relatives to Dinosauria. We
examined the ontogeny of the femur and the histology of a series of long bone elements. We observed bone scar variation in a
series of femora (n D27) of different lengths (73.8–177.2 mm). We hypothesize that most femora follow a similar
developmental trajectory; however, we observed sequence polymorphism in the order of appearance and shape of bone scars,
and we quantified this polymorphism using ontogenetic sequence analysis (OSA). Additionally, five femora, three tibiae, a
fibula, and a humerus were thin-sectioned to examine osteological tissues. No lines of arrested growth (LAGs) are present in
any specimen, and there is little histological information about the ontogenetic stage of femora, although none have slowed
or ceased growth. The woven-fibered bone present in the cortex of elements sectioned is similar to that of the earliest
dinosaurs. This sequence polymorphism provides an alternate hypothesis for the robust/gracile dichotomy found in early
dinosaurs often interpreted as sexual dimorphism. The shared femoral features found in Asilisaurus and early dinosaurs
suggest that this ontogenetic pattern is plesiomorphic for Dinosauria, and that size is a poor predictor of maturity in early
dinosauriforms.
SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP
Citation for this article: Griffin, C. T., and S. J. Nesbitt. 2016. The femoral ontogeny and long bone histology of the Middle
Triassic (?late Anisian) dinosauriform Asilisaurus kongwe and implications for the growth of early dinosaurs. Journal of
Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1111224.
INTRODUCTION
Knowledge of ontogeny is often crucial to understanding the
evolution and paleobiology of a clade (e.g., S
anchez-Villagra,
2012; Thewissen et al., 2012). Ontogenetic series of fossil organ-
isms allow for the study of the development of phylogenetic
characters, character polarity, heterochronic evolution, and espe-
cially the homology of characters (e.g., S
anchez-Villagra, 2012;
Thewissen et al., 2012). Although hypotheses can be proposed
about the evolution of development within a clade based on
extant members of that clade, paleontology provides the sole
source of information on phenotypic evolution in geological
time, and as such the study of ontogeny in fossil organisms is vital
to studying development in an evolutionary context.
Studies of dinosaur ontogeny are common (Chinsamy-Turan,
2005; Padian and Lamm, 2013) and have been utilized to under-
stand various aspect of dinosaur paleobiology, including life his-
tory and the evolution of growth rates (e.g., Horner et al., 1999,
2000, 2001; Padian et al., 2001; Erickson et al., 2004; Horner and
Padian, 2004), the synonymy of species (e.g., Carr, 1999; Horner
and Goodwin, 2009; Scannella and Horner, 2010), ecology and
extinction (Codron et al., 2012), sexual dimorphism (Raath,
1990; Klein and Sander, 2008), and even the evolution of feathers
(Xu et al., 2009). Understanding the development of dinosaurs
and their relatives is therefore a key factor in understanding the
evolution and paleobiology of one of history’s most dominant
group of animals.
Studies of Jurassic and Cretaceous dinosaur ontogeny are also
common (e.g., Chinsamy, 1993; Varricchio, 1993; Rimblot-Baly
et al., 1995; Curry, 1999; Horner et al., 1999, 2000; Erickson and
Tumanova, 2000; Erickson et al., 2004; Horner and Padian,
2004). Yet, our understanding of dinosaurian growth decreases
as we approach their origins in the Late Triassic (Langer, 2004;
Langer and Benton, 2006), and although early dinosaur remains
are represented in most Upper Triassic deposits, the absence of
abundant well-preserved skeletal elements of the same species-
level taxon limits our understanding of early dinosaurian paleo-
biology, especially ontogeny. Besides discussions of postcranial
skeletal variation (Colbert, 1989, 1990; Genin, 1992) and allome-
try (Rinehart et al., 2009) of the early neotheropod Coelophysis
bauri, a study on the long bone histology of Plateosaurus engel-
hardti (Sander and Klein, 2005), and a brief discussion of a speci-
men interpreted as a juvenile Thecodontosaurus antiquus
(Benton et al., 2000; this specimen was later renamed T. caducus
[Yates, 2003] before being placed in the genus Pantydraco [Gal-
ton et al., 2007]), Triassic dinosaur ontogeny is still relatively
unexplored.
Most studies of early dinosaurian ontogeny focus on growth fea-
tures through histological analyses (Massospondylus,Chinsamy,
*Corresponding author.
Color versions of one or more of the figures in this article can be found
online at www.tandfonline.com/ujvp.
Journal of Vertebrate Paleontology e1111224 (22 pages)
Óby the Society of Vertebrate Paleontology
DOI: 10.1080/02724634.2016.1111224
Downloaded by [Christopher Griffin] at 09:42 04 March 2016

1993; Euskelosaurus,Ricql

es, 1968; Theocodontosaurus,Sander
et al., 2004; Plateosaurus, Sander et al., 2004; Sander and Klein,
2005; Klein and Sander, 2007; Syntarsus,Chinsamy,1990;Lesotho-
saurus, Knoll et al., 2010; Scutellosaurus, Padian et al., 2004) or
allometry (Rinehart et al., 2009), whereas others have focused on
differences of ‘robustness’ within a sample of the same species of
dinosaur (Raath, 1977, 1990; Colbert, 1990; Rinehart et al., 2009).
For example, when variation has been reported in early dinosaurs,
polymorphism is often interpreted as indicative of sexual dimor-
phism rather than individual variation or ontogenetic patterns. A
study of Thecodontosaurus antiquus interpreted the presence of
more robust individuals as indicative of sexual dimorphism (Benton
et al., 2000). Variation between two individuals of the Late Triassic
theropod Coelophysis bauri collected from Ghost Ranch, New
Mexico (differences in skull, neck, limb length, and sacral fusion),
was interpreted as the result of sexual dimorphism (Colbert, 1990).
However, other workers studying Ghost Ranch C. bauri have found
no evidence for dimorphism in either limb length or sacral fusion,
but did report a “relatively small” degree of sexual dimorphism in
the neck and skull lengths (Rinehart et al., 2009:117). Morphologi-
cal variation in several bones, most importantly in six muscle fea-
tures of the proximal portion of the femur, has been recognized in
the Early Jurassic theropod ‘Syntarsus’(DCoelophysis,Bristowe
and Raath, 2004) rhodesiensis (Raath, 1977, 1990). Raath (1977,
1990) concluded that the apparent bimodal variation of ‘robust’
and ‘gracile’ features indicated sexual dimorphism, rather than tax-
onomic diversity or individual variation, and that robust features
developed at the onset of sexual maturity in only one sex.
Until recently, the features present in the ‘robust and
‘gracile’ morphological suites sensu Raath (1977, 1990), par-
ticularly of the femur, were only thought to be present in
coelophysoid theropod dinosaurs (Tykoski and Rowe, 2004).
However, it is now clear that these features have a wider dis-
tribution among dinosaurs, because they are present in the
noasaurid theropod Masiakasaurus knopfleri (Carrano et al.,
2002; Lee and O’Connor, 2013), the early sauropodomorph
Saturnalia tupiniquim (S.J.N., pers. obs.), other early thero-
pods (Tykoski, 2005), and even outside Dinosauria in close
relatives (Silesaurus opolensis, Piechowski et al., 2014; Asili-
saurus kongwe, see below). The possibility that differences
between the two morphs represent changes in ontogeny has
only been touched on (Raath, 1977, 1990; Lee and O’Connor,
2013), although several studies have suggested that bone
scarsofhindlimbelementsincrease in prominence during
ontogeny in both extant (Alligator mississippiensis, Brochu,
1992, 1996, Tumarkin-Deratzian et al., 2007; Branta canaden-
sis, Tumarkin-Deratzian et al., 2006) and extinct archosaurs
(Dromomeron gregorii, Nesbitt et al., 2009).
Here, we use a close relative of dinosaurs, Asilisaurus kongwe—an
early-diverging silesaurid from the early Middle Triassic of Tanzania
(Nesbitt et al., 2010)—to understand the origin of the gracile-robust
dichotomy present in dinosaurs. To this end, we critically examined
the changes in femoral scars of A. kongwe across a series of different
sizes and examined the long bone histology of A. kongwe.
Asilisaurus kongwe is an ideal taxon to examine the dichotomy
of gracile and robust femoral morphs because (1) the femora of
A. kongwe exhibit a combination of gracile and robust morphol-
ogies over different lengths; (2) the taxon is known from hun-
dreds of long bones from two localities; and (3) it is one of the
oldest known members of Ornithodira and is part of one of the
proximal outgroups of Dinosauria (Nesbitt et al., 2010). In this
study, we describe the ontogenetic trajectory of a series of Asili-
saurus kongwe femora, as well as histological characteristics of
the long bones, and hypothesize that the pattern of development
and ontogenetic variation found in Asilisaurus may be plesio-
morphic for Dinosauria. Therefore, this study has implications
for understanding the growth, development, and skeletal varia-
tion of early dinosaurs.
Institutional Abbreviations—AMNH, American Museum of
Natural History, New York, New York, U.S.A.; GR, Ghost
Ranch Ruth Hall Museum of Paleontology, Abiquiu, New Mex-
ico; NMT, National Museum of Tanzania, Dar es Salaam, Tanza-
nia; QG, Natural History Museum of Zimbabwe, Bulawayo,
Zimbabwe; SAM, Iziko South African Museum, Cape Town,
South Africa; TMM, Vertebrate Paleontology Laboratory, The
University of Texas at Austin, Austin, Texas, U.S.A.
MATERIALS AND METHODS
Provenance and Taxonomic Justification
All of the femora assigned to a silesaurid and specifically Asili-
saurus kongwe originate from a series of localities within the
Anisian (Lucas, 1998; Hancox, 2000; Abdala et al., 2005;
Rubidge, 2005; Hancox et al., 2013) Lifua Member of the Manda
beds in two major areas sampled by our team within the Ruhuhu
Basin: a western area and an eastern area. Within the Lifua
Member in both areas, all of the remains come from a similar
stratigraphic horizon (i.e., about »2/3 the stratigraphic distance
from the contact with the underlying Kingori Sandstone) in
bands of small outcrops (»100–1000 m
2
) that follow along strike.
Admittedly, the localities in which these remains are found are
difficult to correlate to exact stratigraphic level because of
patchy outcrops surrounded by dense vegetation, but the locali-
ties appear to be within tens of meters of each other stratigraphi-
cally. Most of the femora come from the western area localities,
which include the holotype locality of A. kongwe (NMT Z34).
The holotype was found among largely disarticulated elements
of silesaurids (see below) that were collected from the surface
after weathering out of mudstone. Most of the femora (17 of
total) and two tibiae (NMT RB209, 214) in this study were from
this near monotypic bonebed, and a large serrated tooth and a
single maxilla of a cynodont were also found here. In the same
geographic area, and similar estimated stratigraphic position,
another bonebed with silesaurids was also recovered (NMT Z90)
with a similar taphonomic signature (i.e., largely disarticulated,
elements from throughout the skeleton). Only one femur, one
humerus, and one tibia were sampled from this locality (NMT
RB226). One of the complete femora (NMT RB171) was found
at a nearby locality among partial remains of an archosaur refer-
able to a silesaurid (NMT Z29).
In the eastern area, silesaurid remains are much more rare, but
occur as isolated partial skeletons of individuals. The most com-
plete skeleton of Asilisaurus kongwe (NMT RB159) was col-
lected from a locality (NMT Z137) with the remains of our
smallest individual (NMT RB169). The dentary of the nearly
complete skeleton (NMT RB159) shares all the autapomorphies
of the holotype from the western area. From these associations,
a general similarity of stratigraphic level for all of the A. kongwe
specimens, and the identification of each element assignable to
Silesauridae (see below), we hypothesize that all of the femora
and long bone elements originate from a single species-level
taxon, A. kongwe.
Femora—NMT RB19, 102, 109, 112, 159, 169, 171–172, 179,
185, 211–223, 226, 228–229. All of the femora bear character
states of silesaurids, but unfortunately, there are no autapomor-
phies present in the femora of Asilisaurus kongwe. The femora
preserving the proximal portion of the femur bear the following
two character states found in most silesaurids: notch ventral to
proximal end of the femur (character state 304[1] of Nesbitt
et al., 2010; for detailed explanation and illustration of this and
other Nesbitt et al., 2010, character states, see Nesbitt, 2011);
and straight medial articular facet of the proximal portion of the
femur (character state 309[1] of Nesbitt, 2010). Additionally, the
proximal portions of the femora bear a straight groove on the
proximal surface (character state 314[1] of Nesbitt, 2010), three
similarly sized proximal tubera (sensu Nesbitt, 2010), and a
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distinct fourth trochanter—character states consistent with, but
not exclusive to, silesaurids and Asilisaurus kongwe. The mid-
shaft of the femur does not bear any character state exclusive to
silesaurids or even Archosauria. Yet, the midshaft does bear a
fourth trochanter, a character state present in some archosauri-
forms and all archosaurs. The ridge-like fourth trochanter is simi-
lar to that of the smaller specimens of Silesaurus opolensis. All
femora except NMT RB226 were found among the holotype and
referred specimens of Asilisaurus kongwe at a locality (NMT
Z34) consisting of similarly sized, disarticulated skeletons of Asi-
lisaurus (Nesbitt et al., 2010). Specimen NMT RB226 was found
in another bonebed of small silesaurids at the same stratigraphic
horizon (NMT RB Z90). To date, Asilisaurus kongwe is the only
species-level taxon of silesaurid known from the Manda beds,
and all evidence available to us indicates that there is not more
than one species-level taxon of silesaurid.
Humerus—NMT RB225. There are no character states that
clearly place NMT RB225 within Silesauridae or within Archo-
sauria. However, the distal condyles of NMT RB225 are reduced
as in other silesaurids (e.g., Silesaurus opolensis, Dzik, 2003) and
in a partial skeleton of Asilisaurus kongwe (NMT RB159). Speci-
men NMT RB225 is from a locality (NMT RB Z90) consisting of
thousands of disarticulated remains of a small silesaurid. To
date, it is not clear if all the remains belong to one species-level
taxon of silesaurid and/or Asilisaurus kongwe, but here we refer
it to Asilisaurus kongwe based on similarity and proximity to
other diagnostic skeletal elements.
Tibiae—NMT RB209, 214, 224. The proximal and distal ends
of the tibiae do not possess any unique character states of Asili-
saurus kongwe or any other silesaurid. They do possess character
states placing them within Dinosauriformes. For example, the
anterior portion of NMT RB209 bears a straight cnemial crest
(character state 328[1] of Nesbitt, 2010), whereas the posterior
portion of the proximal part bears two nearly equally sided con-
dyles (character state 331[1] of Nesbitt, 2011). The distal ends of
the tibiae bear a slot for a small ascending process of the astraga-
lus (character state 356[1] of Nesbitt, 2010). Each of the tibiae is
either from the type locality (NMT Z34) or from another locality
that has produced thousands of silesaurid remains that likely per-
tain to Asilisaurus kongwe (NMT Z90).
Fibula—NMT RB209, articulated with a tibia (NMT RB209;
see above). Currently, there are no fibula character states exclu-
sive to silesaurids. The proximal end of the fibula is mediolater-
ally compressed, and the anterior end of the proximal edge is
slightly curved and pointed as in other dinosauriforms (Nesbitt,
2010, 2011). This fibula was found in near articulation with a
tibia, and the tibia clearly bears three character states with dino-
sauriforms. Additionally, NMT RB209 is from the holotype
locality of Asilisaurus kongwe (see Femora above).
Preparation
All specimens for which preparation was necessary were
washed with sulfamic acid (10% concentration) and scrubbed
with a toothbrush before being cleaned of matrix with a Micro-
Jack 1 airscribe (www.paleotools.com). All specimens that were
histologically sampled were first molded with PolyTek PlatSil
71-10 RTV silicone rubber prior to thin sectioning. Casts using
Smooth-Cast 300 casting polyurethane resin (www.reynoldsam.
com) were made of each specimen in order to preserve morpho-
logical detail. Additionally, all specimens that underwent
destructive sampling were first photographed using a Canon
Rebel Xsi digital SLR camera.
Femoral Ontogeny
Twenty-six femora ranging from 73.8 to 177.2 mm in length
were included in our series, in addition to a cast of one femoral
element, SAM-PK-10598 (n D27). We measured the maximum
width of the proximal end of the femur and the maximum femo-
ral length (both in mm) of the five complete femora (NMT
RB169, 172, 171, 159 [left and right elements]) in our sample
with a Cen-Tech 6 inch digital caliper. We used R (www.r-proj
ect.org) to construct a simple linear regression (R
2
D0.974) of
these five femora, with the width of the proximal femoral end as
the independent variable and the maximum femoral length as
the dependent variable. A simple, or least squares, linear regres-
sion was preferable because only one variable, maximum femo-
ral length (y), was uncertain. We then used the linear model
returned by the regression (y D4.69x C9.19; in which x is width
of the proximal femoral end and y is the maximum femoral
length) to estimate the maximum femoral length of incomplete
femora by inputting the measured width of the proximal femoral
ends into the linear formula as x, allowing a hypothesized onto-
genetic sequence of femoral lengths (the returned y values) to be
estimated (Fig. 1; Table S1, Supplementary Data); 95% confi-
dence intervals were calculated for each estimated femur length
in R (Table S1).
Bone scars were identified based on morphology and location
when compared with femoral muscle scars of extant crocodilians
and avian dinosaurs (Rowe, 1986; Baumel and Witmer, 1996;
Hutchinson, 2001), hypothesized muscle scars of non-avian dino-
saurs (Sereno, 1991; Novas, 1993; Bonaparte et al., 1999; Hutch-
inson, 2001; Butler, 2010), and other silesaurids (Dzik, 2003;
Ferigolo and Langer, 2006; Langer and Ferigolo, 2013). Eleven
muscle features or states showing variation were used in this
study: the fourth trochanter, which was present in all specimens
in a blade-like, gracile morph or a rounded, robust morph; a pro-
trusion on the distolateral side of the fourth trochanter; the
attachment scar of the M. caudofemoralis brevis; the linea inter-
muscularis cranialis and linea intermuscularis caudalis; two por-
tions of a scar on the lateral surface of the ‘greater trochanter’
that we hypothesize is homologous to the dorsolateral trochanter
observed in several dinosaurs (Bonaparte et al., 1999; Langer,
2003; Butler, 2010) and silesaurids (Ferigolo and Langer, 2006;
Nesbitt et al., 2007; Langer and Ferigolo, 2013; unnamed Otis
FIGURE 1. Simple linear regression (R
2
D0.974) of femur length
against maximum width of femoral head in Asilisaurus kongwe. Com-
plete femora (n D5) were used to generate the regression and estimate
the femur lengths of incomplete specimens. Regression line is solid; 95%
confidence interval bounded by dashed lines.
Griffin and Nesbitt—Ontogeny of Asilisaurus (e1111224-3)
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Chalk silesaurid, TMM 31100-1309); the anterior trochanter and
the trochanteric shelf, which were fused in some specimens but
separate in others (and therefore treated as an independent char-
acter); and a large muscle scar on the anterolateral surface of the
proximal end of the femur that does not seem to be homologous
with any described archosaurian muscle scar, which we here
name the anterolateral scar. The presence, absence, or morpho-
logical variant of each muscle scar was recorded for each speci-
men, and this allowed us to construct a hypothesized
developmental trajectory followed by the majority of the individ-
uals studied.
Histological Sectioning
We sampled bone tissue for study as close to the midshaft
region as possible in all elements (femora, NMT RB211, 210,
212, 226, 213; all other non-femoral elements) with the exception
of NMT RB213, which was serially sectioned along the entire
length of the element. Histological signals can differ between
elements, even between different elements in the same individ-
ual (e.g., Horner et al., 2000), and this has been reported in sile-
saurids as well (Silesaurus opolensis, Fostowicz-Frelik and Sulej,
2010). Because of this, we sampled several elements, not just
femora, in order to understand the long bone histology of Asili-
saurus kongwe in a more comprehensive manner. Sections of the
bone to be thin-sectioned were removed from the specimen by
sawing with an IsoMet 4000 diamond wafering blade at
5000 rpm, with the exception of NMT RB213, for which this was
unnecessary. The removed bone was then embedded in Castolite
AP, a clear polyester resin, and immediately vacuumed for
1.5 minutes to remove bubbles before allowing several days for
the resin to cure. In the case of NMT RB213, the entire specimen
was embedded in resin. Wafers of resin and embedded bone
were cut with the IsoMet 4000 diamond wafering blade at
200 rpm, and then ground with a 1200-grit grinding disc and pol-
ished with a microcloth polishing disc with 0.3-micron slurry on
an Ecomet 2 speed grinder-polisher at 240 rpm. We glued each
polished wafer to a 20.3-mm-thick plastic slide with Aron Alpha
(Type 201) cyanoacrylate, which was recommended by Lamm
(2013) for mounting smaller specimens, before removing excess
material by first cutting then grinding the wafer down with a Hill-
quist thin-section machine. All slides were then ground down to
appropriate thicknesses by hand, first with a 1200-grit grinding
disc, then a 2400-grit disc and microcloth disc with 0.3-micron
slurry to polish. Slides were examined with an Olympus BX60
microscope under both plane- and cross-polarized light (the lat-
ter with a quartz wedge), and images were captured with cellSens
standard software. The exception to this is the histology slides of
the humerus (NMT RB225), which were examined with an
Olympus BX51 research microscope under plane- and cross-
polarized light, and images were captured with Lumenera Infin-
ity capture imaging software. High-resolution images of these
histological slides were uploaded to Morphobank (www.morpho
bank.com, project 2188).
Ontogenetic Sequence Analysis
Ontogenetic sequence analysis (OSA) is a parsimony-based
method of reconstructing ontogenetic sequences based on dis-
crete ontogenetic events independent of size while accounting
for developmental sequence polymorphism (Colbert, 1999; Col-
bert and Rowe, 2008), and is therefore an ideal method for
reconstructing the developmental sequence(s) of bone scars in
our hypothesized ontogenetic series of Asilisaurus kongwe.
Although cladistic ontogeny is a common and powerful method
for reconstructing developmental sequences with a small amount
of sequence variation (e.g., Brochu, 1992, 1996; Tumarkin-Derat-
zian et al., 2006), this method is less useful for samples with a
large amount of variation (e.g., Tumarkin-Deratzian et al., 2007)
because variation is eliminated to form a consensus tree, result-
ing in a low-resolution reconstructed sequence. The OSA
method predicts all equally parsimonious sequences, allowing all
potential ontogenetic sequences (with all semaphoronts sensu
Hennig [1966]) in the studied population to be determined, even
when data are missing (as is often the case in fossil ontogenetic
series, which are inherently incomplete). A relatively large sam-
ple size is necessary for this method to be most effective, because
OSA has been shown to better detect sequence polymorphism
with larger sample sizes (De Jong et al., 2009). We followed the
procedure of Colbert and Rowe (2008; detailed by Z. Morris,
unpubl.). In summary, after constructing a data set of irreversible
developmental event characters in each specimen, a parsimony-
based cladistics program is used to optimize these developmental
events onto trees, which are in turn used to construct reticulating
diagrams showing hypothesized developmental pathways.
Because all pathways must connect the least mature semaphor-
ont to the most mature, the procedure is first undertaken with all
trees rooted to the least mature semaphoront (normal treat-
ment), then again with all trees rooted to the most mature sema-
phoront (reverse treatment).
We constructed a table of developmental events (the 11 bone
scars referenced above, scored as characters; Table 1) similar to
cladistic analysis, with each individual specimen treated as an
operational taxonomic unit (OTU). Because femoral scars
increase in number and prominence during ontogeny in many
archosaurs (e.g., Brochu, 1992, 1996; Tumarkin-Deratzian et al.,
2006, 2007; Nesbitt et al., 2009), we hypothesize that the absence
of a scar is the immature character state, and the presence of a
scar is the mature state. Further, because the smaller, gracile
morph of the fourth trochanter is present in almost all the small-
est individuals, and the robust morph is present in the largest
individuals, we hypothesize that the more prominent morph is
the more mature of the two. Characters were either absent
(scored as [0]) or present (scored as [1]), except in the case of the
fourth trochanter, which was present in all specimens as either a
gracile (scored as [0]) or robust (scored as [1]) morph. Missing
information was scored as [?]. We then removed redundancies,
or specimens that showed the identical suite of character states,
because they represent the same semaphoront. In these cases,
multiple specimens (NMT RB211 and 215; NMT RB223 and
213; NMT RB228 and 217; NMT RB218 and 112) were treated
as the same OTU in the table. For the reverse treatment (with
the mature semaphoront as the outgroup), the identical proce-
dure was followed except that absent/immature characters were
scored as [1] and present/mature characters were scored as [0].
Because specimens representing the least mature (NMT RB169;
all characters scored as [0]) and most mature (NMT RB216; all
characters scored as [1]) character suites possible were present in
the data set, there was no need to include hypothetical least
mature or most mature semaphoronts in the data set. Tables
were constructed in Excel and converted to NEXUS files with
Mesquite (v. 2.75; Maddison and Maddison, 2011; NEXUS files
for the normal and reverse treatments are available in Supple-
mentary Data).
Using PAUP* (v. 4.0b10; Swofford, 2002), two heuristic
searches were run with the tree-bisection-reconnection algo-
rithm, adding specimens randomly and running 20,000 replicates:
one with the least mature semaphoront as the outgroup and one
with the most mature as the outgroup. We then condensed trees
that had any branches with minimum length of zero so that polyt-
omies would be recovered. We recovered 1260 trees rooted to
the juvenile semaphoront and 160 rooted to the mature sema-
phoront, and visualized character changes along all branches for
each tree. Reticulating sequence diagrams were then constructed
following the procedure of Colbert and Rowe (2008), resulting
in a single sequence diagram with semaphoronts arranged by
reconstructed maturity score (i.e., the number of reconstructed
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TABLE 1. Asilisaurus kongwe femora arranged by measured and estimated length, with bone scar information and observed and reconstructed maturity score for each specimen.
Specimen dltp (1) cfb (2) dlp (3) als (4) dlta (5) ts (6) at (7) tsCat (8) lia (9) lip (10) 4th (11)
Observed
maturity score
Reconstructed
maturity score
% of max.
femoral length Semaphoront(s)
NMT RB169 0 0 0 0 0 0 0 0 0 0 0 0 0 41.66 S1
NMT RB221 1 1 ? 1 1 1 1 1 1 0 0 8 9 46.67 S18
NMT RB220 0 ? ? 0 0 0 1 0 1 ? ? 2 2, 3, 4 54.22 S2, S3, S4
NMT RB172 ? 1 0 1 1 1 1 0 1 0 0 6 6, 7 54.39 S8, S11
NMT RB185 0 1 ? 0 0 1 1 1 0 ? ? 4 4, 8 54.8 S5, S15
NMT RB109 0 1 1 1 1 1 1 0 1 1 0 8 8 56.37 S12
NMT RB19 0 0 1 1 1 1 1 0 1 1 0 7 7 56.53 S10
NMT RB229 0 1 ? 0 1 1 1 0 1 1 1 7 8 56.63 S13
NMT RB223 1 1 1 1 1 1 1 1 1 1 0 10 10 56.79 S19
NMT RB219 1 1 ? 1 1 1 1 0 1 1 0 8 9 58.83 S16
NMT RB228 1 1 1 0 1 1 1 0 1 1 1 9 9 60.77 S17
NMT RB211* 1 1 1 1 1 1 1 0 1 1 0 9 9 61.32 S16
NMT RB218 1 1 1 1 1 1 1 0 1 1 1 10 10 61.9 S20
NMT RB112 1 1 1 1 1 1 1 0 1 1 1 10 10 63.26 S20
NMT RB212* 1 1 1 1 1 1 1 1 1 0 0 9 9 64.21 S18
NMT RB222 1 1 1 1 1 1 1 0 1 0 0 8 8 65.19 S14
NMT RB102 0 0 0 0 0 0 1 0 1 1 0 3 3 67.02 S3
NMT RB217 1 1 1 0 1 1 1 0 1 1 1 9 9 68.05 S17
NMT RB215 1 1 1 1 1 1 1 0 1 1 0 9 9 68.08 S16
NMT RB216 1 1 1 1 1 1 1 1 1 1 1 11 11 68.13 S22
NMT RB213* 1 1 1 1 1 1 1 1 1 1 0 10 10 68.71 S19
NMT RB179 1 ? ? 0 1 ? 1 ? ? ? ? 3 5, 6, 9, 10 74.09 S6, S7, S9, S17, S21
NMT RB171 1 1 1 1 1 1 1 1 1 1 0 10 10 77.4 S19
NMT RB159 [R] 1 1 1 1 1 1 1 0 1 1 1 10 10 81.27 S20
NMT RB159 [L] 1 1 1 1 1 1 1 0 1 1 1 10 10 81.48 S20
NMT RB226* 1 ? ? 1 1 1 1 1 ? ? ? 6 9, 10, 11 88.4 S18, S19, S22
SAM-PK-10598 1 ? 1 0 1 1 1 1 1 1 1 9 10 100 S21
Asterisk (*) on specimen number indicates a thin-sectioned specimen. 0 Dcharacter absent, 1 Dcharacter present, ? Dlocation of character absent or too damaged to determine absence or
presence of character. The fourth trochanter is present in all specimens, and occurs as either a ridge-like (bladed) or rounded morph (0 Dridge-like, 1 Drounded). Observed maturity score is
determined by summing all character transformations (0–1) that have occurred in the specimen, whereas reconstructed maturity score is given by optimization of missing data by ontogenetic
sequence analysis (OSA). Number in parentheses next to character abbreviation is the OSA character number. The OSA semaphoront(s) represented by each specimen are listed. Abbreviations
follow Figures 2 and 3.
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developmental events that have occurred in that individual) and
connected by developmental events to other semaphoronts, illus-
trating the number of potential developmental sequences leading
from the least mature to the most mature state. The recon-
structed maturity score for each specimen may differ from the
observed maturity score because of different optimizations of
missing data, and hereafter we refer to the reconstructed matu-
rity score when referencing maturity scores. To determine the
modal sequence (the sequence that appears to be most fre-
quently utilized by the specimens representing the sample), we
assigned each semaphoront a specimen frequency support weight
based on the number of specimens represented by the semaphor-
ont. Because of different optimizations of missing data, multiple
semaphoronts sometimes represented the same specimen, and in
these cases the weight of the specimen was divided evenly
between all the semaphoronts representing it; for example, a
semaphoront representing three specimens, one of which is also
represented by another semaphoront, would be assigned a fre-
quency support weight of 2.5. Additionally, OSA sometimes pre-
dicts semaphoronts that do not occur in the sample, and these
are included in the analysis but given a frequency support of
zero (0). We added the specimen frequency weights of all the
semaphoronts in every sequence, and the modal sequence was
that sequence with the greatest total frequency weight, repre-
senting the highest number of specimens in our sample. Addi-
tionally, we found the mean sequence position of each
developmental character and used this to order a hypothetical
mean OSA developmental sequence, although it is important to
note that this sequence is not one that was returned by OSA,
unlike the modal sequence.
In order to investigate the relationship between femoral size
and developmental maturity in our sample, we used R to con-
struct two linear regressions of our entire sample, with femur
length (in mm) as the independent variable and observed and
reconstructed maturity score (0–11) as the dependent variable,
respectively. Additionally, to illustrate the relationships between
maturity score, sample size, and size variability, we used R to
plot maturity score (0–11) against both the total specimen fre-
quency support of all semaphoronts for each reconstructed matu-
rity score and the size range (the length of smallest relevant
femur subtracted from the length of the largest relevant femur)
of each maturity score.
DESCRIPTION
The femur of Asilisaurus is sigmoidal in lateral and anterior
views, and the proximal portion (Fig. 2) is relatively simple, with
no distinct neck between the femoral head and the femoral shaft.
The femoral head is medially flattened and deflected, ventral
and medial to the femoral head is the ‘notch’ apomorphic for
Silesauridae (Nesbitt et al., 2010), and the femur also possesses a
straight sulcus on the proximal surface. The proximal end of the
femur also bears three tubera, anterolateral, anteromedial, and
posteromedial. Well-preserved bone scars are present along the
length of the femur, but most are restricted to the proximal half
of the element (Fig. 3; see below). The distal condyles of the
FIGURE 2. Drawing of bone scars and tro-
chanters of the proximal end of the right
femur (NMT RB159) of Asilisaurus kongwe
in anterolateral (A) and posteromedial (B)
views. Anterior trochanter and trochanteric
shelf are not fused. The rounded morph of
fourth trochanter is present. Abbreviations:
4th, fourth trochanter; als, anterolateral scar;
at, anterior trochanter; cfb, M. caudofemora-
lis brevis insertion scar; dlp, distolateral pro-
trusion on the fourth trochanter; dlta,
anterior portion of the dorsolateral trochan-
ter; dltp, posterior portion of the dorsolateral
trochanter; lia, linea intermuscularis cranialis;
lip, linea intermuscularis caudalis; ts, trochan-
teric shelf. Scale bar equals 1 cm.
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FIGURE 3. The proximal ends of femora of Asilisaurus kongwe showing variation in the presence, absence, and morphology of bone scars, in
descending order of size. A, NMT RB159 in anterolateral and B, posteromedial views. Robust fourth trochanter is present. C, NMT RB216 in antero-
lateral and D, posteromedial views. Robust fourth trochanter is present. E, NMT RB102 in anterolateral and F, posteromedial views. Gracile fourth
trochanter is present. The most proximal part of the femur is missing. G, NMT RB218 in anterolateral and H, posteromedial views (left femur).
Robust fourth trochanter is present. I, NMT RB185 in anterolateral and J, posteromedial views. Fourth trochanter is too damaged to determine
morph. K, NMT RB221 in anterolateral and L, posteromedial views (left femur). Gracile fourth trochanter is present. M, NMT RB169 in anterolateral
and N, posteromedial views (left femur). Gracile fourth trochanter is present. Labeling of bone scars indicates presence in that specimen; absent scars
are not labeled. Left femora have been horizontally mirrored for ease of visual comparison with right femora. Abbreviations:at Cts, fusion of the
anterior trochanter and trochanteric shelf; all other abbreviations follow Figure 2. Scale bar equals 1 cm.
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femur are poorly differentiated from the shaft and are simply
rounded. The middle part of the distal surface bears a deep con-
cavity that is roughly circular in distal view.
Dorsolateral Trochanter
We hypothesize that the thin scar located on the anterior dis-
tolateral surface of the ‘greater trochanter’ in Asilisaurus kongwe
(Fig. 2) is homologous to the dorsolateral trochanter (Bonaparte
et al., 1999), a flange-like extension of the ‘greater trochanter’
present in early ornithischians (e.g., Lesothosaurus diagnosticus,
Sereno, 1991; Eocursor parvus, Butler, 2010), early saurischians
(e.g., Herrerasaurus ischigualastensis, Novas, 1993:fig. 7; Saturna-
lia tupiniquim, Langer, 2003) including neotheropods (Coelophy-
sis bauri, Liliensternus liliensterni, Nesbitt, 2011), and the
silesaurids Sacisaurus agudoensis (Ferigolo and Langer, 2006;
Langer and Ferigolo, 2013), Eucoelophysis baldwini (Nesbitt
et al., 2007), Silesaurus opolensis (Nesbitt, 2011; although it is
absent in smaller S. opolensis specimens [Nesbitt et al., 2007;
Piechowski et al., 2014]), and an unnamed silesaurid (TMM
31100-1303). Nesbitt (2011) considered the dorsolateral trochan-
ter to be a synapomorphy of the clade Silesauridae CDinosauria.
The dorsolateral trochanter was hypothesized by Rowe (1986) to
correspond to an attachment of a branch of the Mm. iliotrochan-
terici in Aves (hypothesized by Hutchinson [2001] to be homolo-
gous to M. pubo-ischio-femoralis internus 2 in Crocodylia).
Given that a similar scar is absent in lagerpetids (e.g., Dromo-
meron gregorii, Nesbitt et al., 2009), it is clear from phylogenetic
analyses (Langer and Benton, 2006; Irmis et al., 2007; Nesbitt
et al., 2010) that the presence of a dorsolateral trochanter is
derived for dinosauriforms. Although this scar is absent in most
of the smaller femora (NMT RB169, 220, 185, 109, 19, 229;
Fig. 3I–N), it becomes common with increasing size early in the
series of A. kongwe femora (NMT RB223), initially only present
on the anterior distolateral surface of the ‘greater trochanter’
(Fig. 2A). As size increases, the scar extends to the posterior dis-
tolateral face of the ‘greater trochanter’ (Fig. 2B), a feature that
is not present in the dorsolateral trochanter of any other dino-
sauriforms we have examined.
Insertion Scar of the M. Caudofemoralis Brevis
The insertion scar of the crocodilian M. caudofemoralis brevis
(CFB; hypothesized by to be homologous to M. caudofemoralis
pars pelvica in Aves [Hutchinson, 2001; Schachner et al., 2011])
is slightly proximal and lateral to the fourth trochanter in A.
kongwe, bordered anterolaterally by the linea intermuscularis
cranialis (Fig. 2B). The CFB (along with the M. caudofemoralis
longus) is present in crown group saurians (e.g., Varanus, Hutch-
inson, 2001:fig. 1; Iguana, Schachner et al., 2011:fig. 2B), but in
early archosauriforms it is shifted distally along with the M. cau-
dofemoralis longus as the internal trochanter of non-archosauri-
form archosauromorphs transitioned into the fourth trochanter
of archosauriforms. This attachment scar is present in the sec-
ond-smallest femur of our sample (NMT RB221; Fig. 3L) and is
present in femora of all sizes, absent in only three (NMT RB169,
the smallest, in addition to NMT RB19 and 102; Fig. 3F, N). This
scar is therefore one of the most common in our hypothesized
ontogenetic series, and we consider it to be one of the first scars
to form during ontogeny.
Fourth Trochanter
The fourth trochanter is the insertion of the M. caudofemoralis
longus (CFL; hypothesized to be homologous to M. caudofemor-
alis pars caudalis in Aves [Hutchinson, 2001; Schachner et al.,
2011]), along with an associated medial depression that is present
in many taxa (e.g., Hutchinson, 2001; Langer, 2003). The fourth
trochanter is an archosauriform novelty, and some debate has
surrounded its evolutionary history: Gregory and Camp
(1918:524; cited in Hutchinson, 2001) hypothesized that the
internal trochanter of basal Archosauromorpha is homologous
with the fourth trochanter, whereas other studies have asserted
that the fourth trochanter is a novel structure arising indepen-
dently of the internal trochanter (Parrish, 1983, 1992). Although
the latter hypothesis was ostensibly supported by the suggestion
that Erythrosuchus possessed both an internal trochanter and
fourth trochanter (Parrish, 1992), the hypothesized ‘fourth
trochanter’ of Parrish was the attachment point for the M. iliofe-
moralis, not the M. caudofemoralis (Gower, 2003). The internal
trochanter of lizards and the fourth trochanter of archosaurs are
both the attachment point for the M. caudofemoralis (Nesbitt,
2011), and we follow Gregory and Camp (1918), Gower (2003),
and Nesbitt (2011) in homologizing the two structures.
In A. kongwe, the fourth trochanter is a large crest elongated
in a proximal to distal orientation on the posteromedial face of
the proximal portion of the diaphysis (Fig. 2B). This scar is pres-
ent in all specimens in our sample, and there is a medial rugose
pit closely associated with the fourth trochanter in all specimens.
A small protrusion on the distolateral face of the fourth trochan-
ter is not present in the smallest specimens (NMT RB169, 172;
Fig. 3F, N) but is present in the majority of the larger ones. In
smaller specimens, the fourth trochanter is ridge-like, especially
the most proximal portion of the ridge, whereas in many larger
specimens (e.g., NMT RB159 [left and right elements], NMT
RB226, SAM-PK-10598) it has a rounded, robust morphology
(Figs. 2B, 3B, D, H). Therefore, although this feature is present in
all specimens examined, its morphology is variable between speci-
mens and appears to be roughly correlated with femoral size.
Anterolateral Scar
A large, raised, roughly disc-shaped feature, which we here
name the anterolateral scar ( D‘dorsolateral ossification’ sensu
Piechowski et al., 2014), is present in nearly all specimens on the
anterolateral surface of the proximal end of the femur of Asili-
saurus kongwe, anterior to the ‘greater trochanter’ and posterior
to the femoral head (Fig. 2A). The external surface of this fea-
ture is covered in coarse bone fibers with no particular orienta-
tion, and the edges of the scar are poorly cemented to the rest of
the femur. This scar also appears in other silesaurids (e.g., Sile-
saurus opolensis, Dzik, 2003:fig. 13; Piechowski et al., 2014; the
unnamed Otis Chalk silesaurid TMM 31100-1309) and is likely a
feature only present in silesaurids given that we have not recog-
nized it in any other archosaur taxon. Although this ossification
has been hypothesized to be an extension of the dorsolateral tro-
chanter in Silesaurus opolensis (Piechowski et al., 2014), in which
this scar is closely associated with the dorsolateral trochanter. In
both A. kongwe and S. opolensis, these two scars are separate in
most individuals (Asilisaurus kongwe, Fig. 2; Silesaurus opolen-
sis, Piechowski et al., 2014:figs. 4, 10), and we hypothesize that
these two closely associated bone scars are different structures.
Given that this feature is not present in any extant reptile, the
muscle(s), if any, associated with this scar are unknown. How-
ever, the iliofemoral ligament in extant crocodiles, hypothesized
to be homologous with the pubofemoral ligament in Aves (Tsai
and Holliday, 2015), inserts into the “lateral metaphyseal surface
of the femur” (Tsai and Holliday, 2015:22), and this insertion in
Alligator mississippiensis and the anterolateral scar of A. kongwe
are broadly similar in location (compare Fig. 2A and Tsai and
Holliday, 2015:fig. 6B). Therefore, the anterolateral scar may be
the ossification of the iliofemoral ligament insertion. The antero-
lateral scar is occasionally absent in smaller Asilisaurus femora
(NMT RB169, 220, 185, 109, 19, 229; Fig. 3E, I, M), but as size
increases the scar becomes increasingly common: in femora of
length greater than 70% of the largest femur, the scar is only
absent in two (NMT RB179, SAM-PK-10598). This absence is
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FIGURE 4. Long bone histology of Asilisaurus kongwe.A, the anterolateral scar (NMT RB213, slide A30) under cross-polarized light (with quartz
wedge) viewed at 40£magnification; B, close up of anterolateral scar showing Sharpey’s fibers, indicated by arrowheads, respectively; C, cortical
bone of a femur (NMT RB210, slide C2) under cross-polarized light with quartz wedge; D,E, cortical bone of a femur (NMT RB210, slide C1) under
D, cross-polarized light (with quartz wedge) and E, plane-polarized light; F, cortical bone of a humerus (NMT RB225, slide J2) under cross-polarized
light; G, cortical bone of a tibia (NMT RB214, slide I2) under cross-polarized light with quartz wedge. Erosional line, not LAG, is visible; H, cortical
bone of a fibula (NMT RB209, slide G2) under cross-polarized light with quartz wedge. Micrograph scale bars equal 200 mm(A), 500 mm(B), and
100 mm(C–H). A. kongwe reconstruction scale bar equals 10 cm.
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especially conspicuous in SAM-PK-10598, which is the largest
femur in the sample.
To investigate the morphology of the scar further, we sec-
tioned through the feature in a serially sectioned proximal end
of the femur of Asilisaurus kongwe (NMT RB213, sections A28
and 30; Fig. 4A, B). The scar consists of very dense bony tissues
with little vascularization and is easily differentiated from the
outer cortex of the main body of the femur. The vascularization
that is preserved is concentrated in the inner portions of the
thickest portion of the feature, and the long axis of the vascular
canals are oriented parallel with the outer surface of the feature.
Sharpey’s fibers are aligned perpendicular to the outer cortex of
the femur and are arranged in clear bands that are more appar-
ent near the contact with the rest of the femur relative to that of
the outer portions of the scar.
Trochanteric Shelf
The trochanteric shelf is thought to have originated in Dino-
sauromorpha (Hutchison, 2001) and was hypothesized to be
homologous with the insertion of the M. iliofemoralis externus
in Aves (Hutchinson, 2001; this muscle was hypothesized to be
homologous to the M. iliofemoralis in Crocodylia [Hutchinson,
2001]). The trochanteric shelf is absent in two of the smaller
specimens (NMT RB169, RB220), and is generally present with
the anterior trochanter (e.g., Figs. 2A, 3), although in one speci-
men the anterior trochanter is present whereas the trochanteric
shelf is absent (NMT RB220; Fig. 3E; see below for discussion).
Anterior Trochanter
The anterior, or ‘lesser,’ trochanter is an important feature in
archosaur phylogenetics, hypothesized to have originated in
Dinosauriformes as part of the anterior side of the trochanteric
shelf (Novas, 1996; Hutchinson, 2001) and is the insertion scar of
the M. iliotrochantericus caudalis (hypothesized by Hutchinson
[2001] to be homologous to the M. iliofemoralis in crocodilians).
Anterior trochanter-like structures are present in some pseudo-
suchian archosaurs, generally in large adults (e.g., Ornithosuchi-
dae [Walker, 1964], as well as Aetosauria and Crocodylomorpha
[Hutchinson, 2001:table 2]), although these features do not
appear to represent a synapomorphic character for Archosauria
FIGURE 5. A, general order of appearance
of the bone scars of the proximal end of the
femur in the majority of Asilisaurus kongwe
individuals, scaled by relative femoral length
compared with the largest femur in the series
(SAM-PK-10598, 177 mm in length). The
shortest femur in the series (NMT RB169,
73.8 mm in length) is also indicated. Each
muscle scar abbreviation indicates the
appearance of that feature in development.
There is a zone of developmental transition
from the ridge-like to the rounded morph of
the fourth trochanter in which both morphs
are equally common; B, relative order of
appearance of bone scar characters based on
the OSA modal sequence; C, relative order
of appearance of bone scar characters based
on the mean OSA sequence. Abbreviations
follow Figures 2 and 3. Scales in Band Care
dimensionless and illustrate relative, not
absolute, timing.
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FIGURE 6. Ontogenetic sequence analysis (OSA) reticulating diagram of femoral bone scar development in Asilisaurus kongwe showing 33 unique
parsimonious developmental sequences. Semaphoronts are represented by ellipses, with width proportional to frequency support. Semaphoronts are
ranked by maturity score (0–11). Semaphoronts with a frequency support of less than 1 (those with multiple maturity scores because of different opti-
mizations) have an ellipse width equal to those with frequency support of 1 for visual clarity. Character transitions are listed with the path on which
they occur, and all character changes proceed in an irreversible direction from immature to mature (0–1). Characters are listed in Table 1.
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(Gauthier, 1986). The insertion of the M. iliofemoralis was more
distal in early-diverging archosaurs, including the extant Croco-
dylia, and moved to a more proximal position on the femur in
Dinosauriformes, forming a true anterior trochanter (Hutchin-
son, 2001). The anterior trochanter was therefore hypothesized
to be a synapomorphy for Dinosauriformes (Hutchinson, 2001);
however, the presence of an anterior trochanter in the non-dino-
sauriform dinosauromorph Dromomeron gregorii (Nesbitt et al.,
2009) indicates that an anterior trochanter was present in at least
some non-dinosauriform dinosauromorphs. Whereas many dino-
sauriforms possess both an anterior trochanter and trochanteric
shelf in the femora (Marasuchus lilloensis, Pseudolagosuchus
major, Silesaurus opolensis, Herrerasaurus ischigualastensis, and
Saturnalia tupiniquin [Nesbitt, 2009]), many individuals, but not
all, of early-diverging theropod dinosaurs possess an anterior tro-
chanter but lack a trochanteric shelf (e.g., Coelophysis bauri,
Colbert, 1990; the gracile morph of ‘Syntarsus’ rhodesiensis,
Raath, 1977, 1990), and smaller femora of Saturnalia tupiniquin
also lack a well-defined trochanteric shelf but possess an anterior
trochanter (Nesbitt et al., 2009). The anterior trochanter, along
with the trochanteric shelf, is absent in the non-dinosauriform
dinosauromorphs Lagerpeton chanarensis and Dromomeron
romeri and only appears in larger individuals of Dromomeron
gregorii (Nesbitt et al., 2009).
The anterior trochanter is present in all Asilisaurus femora but
the smallest (NMT RB169; Fig. 3M), and in one individual the
anterior trochanter is present where the trochanteric shelf is
absent (NMT RB220; Fig. 3E). Conversely, in some larger Asili-
saurus femora, the two structures cannot be differentiated and
thus they form a distinct continuous surface. This variability indi-
cates that, although the two structures share a close evolutionary
history, development and individual variation can play a role in
presence or absence of these structures in early-diverging dino-
sauromorphs and dinosauriforms. In A. kongwe, the anterior
trochanter is a roughly textured, triangular surface adjacent and
anterior to the trochanteric shelf (Fig. 2A), and in some larger
femora of A. kongwe it is fused with the trochanteric shelf, form-
ing a continuous rugose surface (e.g., NMT RB216, 213, 171, 226,
SAM-PK-10598; Fig. 3C, I, K).
Linea Intermuscularis Cranialis
The linea intermuscularis cranialis is a sharp lineated anterior
edge of the proximal end of the femur (Fig. 2A) that extends
toward the proximal tip of the femoral condyles—distal connec-
tions vary in Archosauria (Hutchinson, 2001), and in Asilisaurus
it terminates at the distal sigmoidal apex of the femur. The linea-
tion develops from the border between the M. femorotibialis
externus and M. femorotibialis internus (Crocodylia; hypothe-
sized to be homologous to the M. femorotibialis lateralis and
Mm. femorotibialis medialis and intermedius, respectively, in
Aves [Hutchinson, 2001; Schachner et al., 2011]), and Hutchin-
son (2001) proposes this feature as an archosaur synapomorphy.
This feature is present in nearly all Asilisaurus kongwe femora
included in this study, absent in only two specimens (NMT
RB169, the smallest, and NMT RB185; Fig. 3I, M).
Linea Intermuscularis Caudalis
Morphologically similar to the linea intermuscularis cranialis,
the linea intermuscular caudalis is formed from the border
between the M. femorotibialis externus and M. adductor femoris
1 and 2 (Crocodylia; hypothesized to be homologous to the M.
femorotibialis lateralis and Mm. puboischiofemorales medialis
and lateralis, respectively, in Aves [Hutchinson, 2001]; the M.
adductor femoris 1 and 2 has been hypothesized to be homolo-
gous with the M. pubo-ischio-trochantericus in Sphenodon
[Schachner et al., 2011]). Like the linea intermuscularis cranialis,
it is a plesiomorphic archosaur character, normally connecting
the base of the ‘greater trochanter’ with the proximal posterior
end of the lateral condyle (Hutchison, 2001). In A. kongwe,it
originates distal to the fourth trochanter and terminates at
roughly the same level as the linea intermuscularis cranialis,
although on the posterior end of the diaphysis (Fig. 2B). The
linea intermuscularis caudalis of A. kongwe also intersects with a
morphologically similar line that originates on the distal end of
the fourth trochanter, similar to the ridge on the distal dorsolat-
eral side of Sacisaurus agudoensis (Ferigolo and Langer, 2006).
The linea intermuscularis caudalis is not present in smaller fem-
ora that possess the linea intermuscularis cranialis, appearing
later in our qualitative ontogenetic series (first present in NMT
RB109).
Asilisaurus Histological Specimens
Femora—All of the femora selected for histological sectioning
were incompletely preserved, but their lengths were recon-
structed based on complete elements (see Fig. 1; Table S1). All
sections were taken as close to the midshaft as possible. We sam-
pled a variety of sizes, but were unable to sample the larger
specimens of Asilisaurus because of a lack of specimens. All of
the femoral specimens have undergone partial recrystallization
of the some of the bony tissues, compressive fracturing through-
out the specimens, and the medullary cavity is infilled by calcite
crystals.
Generally, all of the specimens have a similar morphology in
section throughout the length of the femur. Each specimen is
nearly circular, with a similarly shaped medullary cavity. The
medullary cavity and the inner portion of the cortex lack trabec-
ular bone at the midshaft as preserved, but as noted by Werning
(2013), the large crystals within the medullary cavity may have
destroyed these structures during fossilization. The cortex is
thickest at midshaft (Fig. 4C–E) and thins proximally and
FIGURE 7. The range of sequence positions recovered by ontogenetic
sequence analysis for each character in Asilisaurus kongwe. Upper and
lower edges of each box illustrate the upper and lower quartiles, respec-
tively; bold lines illustrate median value; squares illustrate mean value;
whiskers illustrate sequence position range for each character. Data can
be found in Tables S2 and S3. Abbreviations follow Figures 2 and 3.
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distally. The most proximally sampled sections have a very thin
cortex with trabecular bone nearly reaching the outer surface.
The trabecular bone is formed by thin, parallel-fibered endosteal
layers.
The microstructure of the cortex is nearly uniform throughout
with no discontinuities (e.g., lines of arrested growth [LAGs]).
The bone tissue is consistently woven-fibered throughout with
circumferentially oriented fibers most common. The outermost
cortex bears more circumferentially oriented fibers than the mid-
dle and inner cortex. The entire cortex is moderately to well vas-
cularized by primary osteons. Throughout the cortex, the
primary osteons are predominantly longitudinal, but they anasto-
mose throughout the cortex, particularly in the outer cortex. The
larger specimens bear a combination of longitudinal and radial
primary osteons where some of the radial primary osteons
stretch for at least half of the width of the cortex. The radius of
primary osteons is consistent with a few exceptions of larger radii
in the histological sections closer to the proximal end. Primary
osteons are encircled by thin bands of parallel-fibered bone tis-
sue and osteocyte lacunae, yet the osteocyte lacunae are not ori-
ented radially around the primary osteons. A high density of
osteocyte lacunae is present throughout the cortex, as in other
dinosauromorphs (Werning, 2013), and the density of the osteo-
cyte lacunae slightly decreases from the inner to outer cortex.
The shape and distribution of the osteocyte lacunae show no
apparent pattern. No secondary osteons were observed in any
example. Compared with the histological features of the femur
of Silesaurus opolensis (Fostowicz-Frelik and Sulej, 2010), the
relative thickness of the cortex, the distribution of longitudinal
primary osteons and osteocyte lacunae, and the woven-fibered
bone tissues throughout the cortex are very similar. In Asilisau-
rus kongwe, the densities of both the primary osteons and osteo-
cyte lacunae and the number of primary osteons that
anastomose are relatively higher than in S. opolensis.
Humerus—The histological sections from NMT RB225 were
taken as close to the midshaft as possible, but because the ele-
ment was not complete, it is not clear how close we sampled
from the midshaft. Comparisons with a complete humerus of
Asilisaurus kongwe (NMT RB159) suggest that we sampled
between 5 and 10 mm of the midshaft.
The medullary cavity bears no trabeculae, and it appears that
recrystallization did not postdepositionally remove trabeculae
because the area that is recrystallized is restricted to the inner-
most portion of the cavity. The cortex consists entirely of pri-
mary osteons of which most are longitudinal. Some of the
primary osteons clearly anastomose and form a reticulated pat-
tern with no favored orientation (Fig. 4F). A weak band of fewer
primary osteon anastomoses are present in the mid cortex rela-
tive to the inner and outer cortex, and this band is visible
throughout the section. Primary osteons reach the periosteum.
As a result, the external surface of the humerus near the mid-
shaft is decorated in proximodistally oriented grooves and ridges,
creating a fibrous surface. A similar external morphology of long
bones is a characteristic of fast-growing stages in tetrapods,
including crocodilians (Tumarkin-Deratzian et al., 2007) and
birds (Tumarkin-Deratzian et al., 2006; Erickson et al., 2009).
Like in the femur, no secondary osteons are present. Addition-
ally, like the femur, there is a high density of osteocyte lacunae
without any clear orientation, whereas some of the osteocyte
lacunae are clearly circumscribed around the primary osteons.
Comparisons between the humerus of Asilisaurus kongwe and
other archosaurs are difficult given that humeri are relatively
rarely preserved in the skeletons of early archosaurs and are par-
ticularly rare among dinosauromorphs. Overall, the tissues pres-
ent in the humerus are very similar to those of the femur of A.
kongwe.
Tibiae—Our histological sample originally consisted of three
samples, two from the distal halves of tibiae (NMT RB214, 224)
and one from the proximal third of a tibia (NMT RB209) in artic-
ulation with a fibula (also sampled; see below). Specimen NMT
RB209 is highly fractured and is of little utility, whereas the sec-
tion of NMT RB224 was taken too close to the distal end to have
much use when reconstructing the growth of A. kongwe. The
sample from the largest tibia (NMT RB214) is closest to the mid-
shaft of the tibia and is within an estimated 20% of the midshaft;
the following description relies on NMT RB214.
The medullary cavity is largely infilled by calcite crystals, but
there is clear evidence from shattered bone shards in the medul-
lary cavity that trabeculae were present prior to fossilization.
The inner cortex consists of unremodeled compact coarse cancel-
lous bone deposited during cortical drift. Here, the longitudinal
primary osteons are surrounded by either parallel-fibered or
woven bone. The woven-fibered bone in the inner cortex is sepa-
rated from the rest of the cortex by an avascular region com-
posed of parallel-fibered bone. In the middle to outer cortex, the
bone tissue changes to tissues most similar to those found in the
cortex of the femur (see above). No LAGs are present. The mid-
dle to outer cortex tissues are predominately parallel-fibered,
with longitudinal primary osteons with few anastomoses
(Fig. 4G). Primary osteon anastomoses that are present are
restricted to the outer cortex. Parallel-fibered bone becomes
more organized near the external surface of the bone. Osteocyte
lacunae are in high abundance and are oriented in laminae sur-
rounding primary osteons.
Fibula—Only one fibula was sectioned because partial fibulae
are rare in the A. kongwe sample. The fibula (NMT RB209) was
sampled at approximately one-third the length of the element
from the proximal surface. Much of the histological section of
the fibula consists of coarse cancellous bone surrounded by a
thin cortex (typically less than 15% of the radius of the fibula)
composed of more compact bone (Fig. 4H). An avascular and
compact band of bone lies between the coarse cancellous and
compact bone. Within the cortex, the bone tissue consists of par-
allel-fibered bone with longitudinal osteons. The longitudinal
osteons reach the external surface of the bone, but the external
surface of the bone did not have a fibrous texture like that of the
humerus. No LAGs are present, but one portion of the fibula is
clearly banded in the outer cortex. These bands are short and do
not circumscribe the section. There are relatively fewer osteo-
cyte lacunae than in the femur and tibia, and these osteocyte
lacunae are located in bands throughout the preserved cortex.
In comparison with other A. kongwe long bones sampled, the
degree of banding present in the fibula is atypical. Moreover, the
banding in the fibula is not typical of any other long bones of
dinosauromorphs sampled thus far. Yet, no other fibulae have
been sampled among dinosauromorphs.
RESULTS
Femoral Ontogeny
The large number of femora sampled, with femoral size being
taken as a proxy for relative age, allows a general qualitative pat-
tern of development of the bone scars of the proximal end of the
femur to be hypothesized in Asilisaurus kongwe. This allows for
the testing of this proxy (commonly assumed in paleontology)
against size-independent methods of sequence reconstruction
(i.e., OSA), as well as testing the utility of qualitatively recon-
structing growth trajectories with size. These femoral scar fea-
tures developed at different times in ontogeny, and there is a
common ontogenetic trajectory that the majority of individuals
seem to follow (Fig. 5A; Table 1). The smallest specimen exam-
ined, NMT RB169, lacks all muscle scars with the exception of a
ridge-shaped gracile fourth trochanter, which we take to be the
most immature femoral state in our sample (Fig. 3M, N). Based
on our qualitative sequence, we hypothesize that next the attach-
ment scar for the M. caudofemoralis brevis develops along with
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the linea intermuscularis cranialis and the anterior trochanter.
The trochanteric shelf then develops, followed by the protrusion
on the distolateral part of the fourth trochanter, the linea inter-
muscularis caudalis, and the anterior portion of the dorsolateral
trochanter. The anterolateral scar and the posterior side of the
dorsolateral trochanter appear next in the sequence. The
rounded, robust morph of the fourth trochanter commonly
begins appearing at this point in the series, although it does not
become the dominant morph until later. Fusion between the
anterior trochanter and the trochanteric shelf becomes common
next, and lastly the robust fourth trochanter becomes the domi-
nant fourth trochanter morph.
Despite the existence of a common ontogenetic trajectory,
there is a large amount of variability in the presence and mor-
phology of bone scars between Asilisaurus specimens of roughly
the same size in our sample. We interpret this variability as in
part signaling a lack of strict relationship between size, age, and
skeletal maturity, and in part indicating the presence of sequence
polymorphism—variability in the timing and relative sequence of
development between individuals within a population (Garn
et al., 1966), which violates the traditional idea of distinct,
ordered ontogenetic stages undergone by all individuals of a spe-
cies during ontogeny. Developmental studies with large samples
of extant taxa have revealed that sequence polymorphism is sur-
prisingly common (Cubbage and Mabee, 1996; Mabee and Tren-
dler, 1996; Sheil and Greenbaum, 2005; De Jong et al., 2009;
Morris, 2013), and recent studies have utilized ontogenetic
sequence analysis (OSA; see below) in an attempt to describe
and quantify sequence polymorphism in fossil organisms (Morris
et al., 2013; Olori, 2013). Sequence polymorphism is not only
present but also common in our sample of A. kongwe femora. As
previously mentioned, the morphology of the fourth trochanter,
although gracile in smaller specimens and robust in larger ones,
is difficult to predict based either on femoral size or on the pres-
ence/absence of other bone scars in a large portion of the ontoge-
netic series. The first occurrence of the robust fourth trochanter
is in NMT RB229 (ninth smallest in the series), and the last
occurrence of the gracile morph is in NMT RB171 (23rd largest
in the series). In this region, there appears to be no pattern to
which of the two morphs will be present in a specimen, nor does
either morph seem to be associated with the presence or absence
of other muscle scars. Specimen NMT RB102 presents another
example of polymorphism: although 17th by size in the ontoge-
netic series, this specimen has a gracile fourth trochanter and
lacks all other muscle features but the anterior trochanter and
the linea intermuscularis cranialis, appearing far less developed
than femoral size alone would suggest (Fig. 3E, F). In contrast,
NMT RB221 appears far more developed than would be
expected (Fig. 3K, L): although the second smallest in the series,
this specimen displays every mature muscle scar except a robust
fourth trochanter and the linea intermuscularis caudalis, includ-
ing fusion between the anterior trochanter and the trochanteric
shelf, which does not appear unambiguously again in the series
until NMT RB216, which is 20th by size in the series. Although
NMT RB159 is one of the largest specimens in the series, con-
taining every mature muscle feature, it lacks this trochanteric
fusion (Figs. 2A, 3A). The anterolateral scar becomes common
early in the ontogenetic series, but in two of the largest speci-
mens (SAM-PK-10598 and NMT RB179) it is conspicuously
absent.
Ontogenetic sequence analysis (OSA) returned a total of 33
unique (but often partially overlapping) equally parsimonious
ontogenetic sequences, with sequences diverging early in ontog-
eny and multiple ontogenetic changes occurring between most
semaphoronts (Fig. 6). A single modal sequence with a fre-
quency weight of 11.67 was returned (Figs. 5B, 6; Table S2),
43.2% of the total weight of the sample (27). However, because
the total frequency weight of all sequences was 225.96, the modal
sequence only represented 5.16% of the total weight of all
sequences, indicating a large amount of variation in the sample.
Most sequences contained some characters that appear to
develop simultaneously because these characters have an unre-
solved sequence order with respect to each other. This could
indicate either that these characters do develop simultaneously
in the given sequence, or that we simply lack enough ontogenetic
sample to resolve the two. Distinguishing between the two possi-
bilities for given characters in a sequence is impossible using
OSA alone. In the modal sequence, the anterior trochanter
develops first, followed by the linea intermuscularis cranialis, the
linea intermuscularis caudalis, and the distolateral protrusion on
the fourth trochanter. The anterior portion of the dorsolateral
trochanter and the trochanteric shelf develop next, unresolved
with respect to each other. Next, the anterolateral scar develops,
followed by the insertion scar of the CFB and the posterior por-
tion of the dorsolateral trochanter. Finally, the anterior trochan-
ter and trochanteric shelf fuse into one continuous scar, and the
fourth trochanter develops into the robust morph. Notably, the
anterior trochanter develops first in all sequences (Fig. 6). Seven
semaphoronts that were not observed in our sample were pre-
dicted by the analysis. The modal sequence returned by the OSA
and the qualitative sequence described previously are remark-
ably similar (Fig. 5A, B), and even the exceptions to this agree-
ment only differ by a single developmental step. The only
notable difference between the two is the insertion scar of the
CFB, which develops first in the OSA modal sequence and sev-
enth in the qualitative analysis sequence, but this is probably a
result of the low resolution inherent in the qualitative assess-
ment, along with the fact that the modal sequence only repre-
sents roughly 5% of the total weight of all sequences.
Additionally, the OSA mean sequence is very similar to the
modal sequence (Fig. 5B, C), with only four characters differing
by more than one position between the two sequences. Most
characters had a wide range of sequence positions because of the
prevalence of sequence polymorphism in the sample (Fig. 7;
Table S3). The linear regression of femoral size versus observed
maturity score showed an overall poor correlation between size
and maturity (Fig. 8A; R
2
D0.1306; P D0.06406; linear formula
returned was y D0.046x C2.26179, in which x is the length of the
femur in mm and y is the observed maturity score). This insignifi-
cant correlation is partly because of missing data, which results in
mature specimens being assigned anomalously low observed
maturity scores. A strict reading of Figure 7A may suggest that
there are two developmental sequences present in our sample:
one in which the individuals are more mature at smaller sizes,
and one in which the individuals are less mature at these same
sizes. However, this is an artifact of missing data (which in the
regression of observed maturity score is identical with character
absence). Even if all characters scored as missing really were
absent, the low-maturity individuals could still not be part of the
same developmental sequence, because the specimens making
up the hypothetical trajectory in Figure 8A (NMT RB102, 185,
220, 226, 179) have conflicting developmental characters. For
example, NMT RB220 (length D96.08 mm) only possesses the
anterior trochanter and linea intermuscularis cranialis, whereas
NMT RB179 (length D131.3 mm) only possesses the anterior
trochanter and both portions of the dorsolateral trochanter.
Thus, although it appears that the two may share the same devel-
opmental trajectory in Figure 8A, for this to be true, this trajec-
tory would have to lose character 9, then gain it back again to
reach the most mature developmental state.
Reconstructed maturity score accounts for this missing data;
however, the overall poor correlation between size and develop-
mental maturity holds even in the regression of femoral size and
reconstructed maturity score (R
2
D0.2551; P D0.001432; linear
formula returned was y D0.061x C0.605, in which x is the length
of the femur in mm and y is the reconstructed maturity score).
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Higher maturity scores tended to have larger size ranges and
therefore more variability (Fig. 8C), but this may be an artifact
of sample size, because more specimens with high rather than
low maturity scores are present in the sample, and size range fol-
lows total specimen frequency support for each maturity score
closely (Fig. 8C). With the possible exception of some of the
smallest and largest individuals, overall size has little to do with
developmental maturity in our sample, and femoral size is a poor
predictor of maturity.
Histology
The character states of the bony tissues of the long bones
(femur, tibia, fibula, humerus) of Asilisaurus kongwe are consis-
tent across element sampled. All share a relatively large medul-
lary cavity with no trabeculae, only primary osteons preserved,
longitudinal canals predominate but anastomoses are present in
the outer cortex, woven-fibered bone tissues throughout the cor-
tex, and a high density of osteocyte lacunae. None of the speci-
mens sampled show any indication of having slowed or stopped
growth before death. These characteristics of the bony tissues of
limb bones of A. kongwe are remarkably similar to those of the
Late Triassic silesaurid Silesaurus opolensis (Fostowicz-Frelik
and Sulej, 2010). In particular, the histological tissues of A.
kongwe (NMT RB210; Fig. 4C–E) and S. opolensis (ZPAL Ab
III/405; Fostowicz-Frelik and Sulej, 2010) femora are virtually
identical. The bony tissues of A. kongwe are very similar to those
of the coelophysid theropods Coelophysis bauri (AMNH unnum-
bered; tibia) and ‘Syntarsus’rhodesiensis (QG 715, femur). The
most obvious difference between A. kongwe and coelophysids is
that the coelophysids have more anastomoses throughout the
cortex. Furthermore, LAGs were recorded in the outer cortex of
‘Syntarsus’rhodesiensis (Chinsamy, 1990; Chinsamy-Turan,
2005) and Saturnalia tupiniquim (Stein, 2010), whereas no LAGs
were found in any long bone of A. kongwe sampled.
DISCUSSION
Growth of Asilisaurus
We hypothesize that our sample (n D27) represents an onto-
genetic series of femora of a single, species-level taxon because
of the large size range of femora attributable to Asilisaurus
kongwe or found in nearby bonebeds consisting almost exclu-
sively of A. kongwe material. As Asilisaurus matured, different
bone scars developed at different times. To understand the rela-
tive order of appearance of these bone scars, we utilized the
common assumption that size is correlated with relative age; i.e.,
larger femora are at a later developmental stage than smaller
femora. However, because the development of bone scars is
indicative of skeletal maturity, this assumption is challenged by
the abundant variation in bone scar presence and appearance
between femora of roughly the same size (and therefore the
same assumed developmental stage), showing that similarly sized
individuals can represent several stages of skeletal maturity.
Additionally, some smaller femora appear to be more skeletally
mature than some larger femora when presence of bone scars is
taken as indicative of skeletal maturity. This suggests that femo-
ral size and relative skeletal maturity (as measured by degree of
bone scar development) are somewhat disjunctive, although size
gives a rough estimate for skeletal maturity in our sample in the
largest and smallest individuals.
The use of size as a proxy for developmental maturity in pale-
ontology has been challenged in other studies: overall long bone
size is a poor indicator of skeletal maturity in the Late Triassic
sauropodomorph Plateosaurus engelhardti, a common early
dinosaur (Sander and Klein, 2005; Klein and Sander, 2007).
Osteohistology was used to determine skeletal maturity and
change in growth rate in this dinosaur, and skeletally mature
FIGURE 8. A, linear regression of observed maturity score (0–11) as a
function of Asilisaurus kongwe femoral length (R
2
D0.1306; P D
0.06406); B, linear regression of reconstructed maturity score (0–11) as a
function of Asilisaurus kongwe femoral length (R
2
D0.2551; P D
0.001432); C, reconstructed maturity score (0–11) plotted against both
the total frequency support for each maturity score and the size range
(length of the smallest femur of the relevant maturity score subtracted
from the length of the largest femur of the relevant maturity score).
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individuals were found across the entire size range sampled. In
addition, different stages of growth (termed ‘fast growth’ and
‘slow growth’) were variable across sizes. Rejecting sexual
dimorphism, the authors hypothesized that strong developmen-
tal plasticity, found in extant ectothermic reptiles as a response
to environmental factors, is the cause of the large variability in
size at maturity in Plateosaurus (Sander and Klein, 2005; Klein
and Sander, 2007). Brochu (1992) explored the relationship
between size and overall skeletal maturity in Alligator mississip-
piensis, finding that although an approximate relationship exists
between maturity and size, there is a considerable amount of var-
iability in developmental timing, and larger individuals cannot be
assumed to be more developmentally mature than smaller indi-
viduals. At the level of individual skeletal elements, the relation-
ship between size and maturity becomes even less clear:
although a general trend of A. mississipiensis individuals from
early stages of ontogeny always being smaller than individuals
from very late stages holds, size ranges of ontogenetic stages
overlap significantly. Most strikingly, a 50-mm-length alligator
femur was found to be several developmental stages more
mature than a 100-mm-length femur, and femora of a single
length may represent up to four different ontogenetic stages
(Brochu, 1992). Similarly, a study of the developmental pattern
of ossification of the humerus, ulna, and femur in relation to size
in Ophiacodon and Dimetrodon found that size correlates poorly
with developmental stage in these animals (Brinkman, 1988).
Noting the similarities between the nature of early-diverging syn-
apsid and crocodylomorph archosaur ontogeny, Brochu (1992)
suggested that the pattern of poor correlation between size and
maturity may be plesiomorphic to amniotes. The poorly under-
stood relationship between overall size and developmental matu-
rity is not simple and adds a complication to studying the
ontogeny of fossil organisms, especially if developmental varia-
tion is characteristic of a species or clade.
The closure of neurocentral sutures has been used as an indica-
tor of ontogenetic stage in archosauriforms, including phytosaurs
(Irmis, 2007) and extant crocodilians (Brochu, 1996), in both of
which the neurocentral sutures of the axial skeleton fuse in a pos-
terior-anterior pattern during ontogeny. Notably, in nearly all A.
kongwe vertebral specimens, the neurocentral sutures are open;
however, in the largest specimen with femora and vertebral ele-
ments (NMT RB159, which possesses all mature character states
but trochanteric fusion), the neurocentral sutures of the cervical,
sacral, and caudal vertebrae are closed, suggesting that this indi-
vidual is near skeletal maturity. If this specimen does represent a
skeletally mature individual in which lateral growth has almost
completely ceased, this provides a rough size at skeletal maturity
for the A. kongwe femora. In our sample of A. kongwe femora,
size seems to be somewhat correlated with relative age and devel-
opmental maturity, but there are enough exceptions to this trend
to suggest that sequence polymorphism plays a large role in deter-
mining individual variation.
The OSA method allows for the visualization and quantitative
analysis of sequence polymorphism, and the large number of
equally parsimonious developmental sequences predicted by
OSA, differing in major ways from the both the modal sequence
and the qualitative sequence, indicates that sequence polymor-
phism is prevalent in our sample, resulting in a number of anom-
alously mature and immature specimens (Figs. 6, 7). The high
level of agreement between our qualitative sequence (deter-
mined utilizing the assumption that size is a reasonable proxy for
age) and the OSA modal sequence indicates that size correlates
somewhat with developmental maturity in our sample. However,
the large amount of polymorphism indicated by the OSA, as well
as the poor correlation between femoral size and maturity score
(R
2
D0.1386; Fig. 8A), shows that this assumption of size as a
proxy for developmental maturity is tenuous for much of our
sample and cannot be relied on with any degree of certainty.
The lack of LAGs in the femora of Asilisaurus kongwe pre-
vents a direct comparison between absolute ontogenetic age
(based on annual LAGs) and development of bone scars on the
external surface of the femur (maturity scores; see below),
although the histology does not indicate a slowing or stoppage of
growth in the individuals sampled. Because of this lack of data,
we cannot test for any relationship between size and age or skel-
etal maturity and age. We histologically sampled femora with
reconstructed lengths (Fig. 1; Table S1) that spanned much of
the larger sizes and higher maturity scores observed across our
total sample, and it was clear that no LAGs were present even in
specimens with the highest maturity score or presumably from
older individuals. Comparisons between the histology of the fem-
ora sampled (reconstructed sizes; Table S1), femur lengths, and
maturity scores reveal a relationship that implies that (1) A.
kongwe did not deposit annual LAGs during ontogeny and
reached larger sizes recorded in our sample over years; or (2) A.
kongwe grew rapidly and reached the larger sizes and highest
maturity scores in our sample in less than a year.
We reject the latter hypothesis for the following reasons. For
all the femora in our sample (femoral lengths: 73.8–177.2 mm) to
come from individuals less than 1 year in age, an extremely high
rate of growth would be necessary, because the largest, most
mature femur thin-sectioned (NMT RB226) is over twice the
length of the smallest, least mature femur in the series (NMT
RB169; Fig. 6; Table S1). The similarity between the bone tis-
sues of Asilisaurus kongwe and the early theropod ‘Syntarsus’
rhodesiensis suggests that these two taxa had similar growth
rates, and the femur of ‘Syntarsus’rhodesiensis has been shown
to require multiple years of growth to reach a size comparable to
the largest femora of A. kongwe (Chinsamy, 1990), indicating
that A. kongwe would require several years of growth at a similar
rate to reach full size. Further, the absence of LAGs may be
common for silesaurids: a femur from an unnamed silesaurid
recently recovered from the Middle Triassic Ntawere Formation
of Zambia also lacks LAGs, despite its exceptionally large size
(»350 mm in length; Peecook et al., 2013).
The bone tissue present in A. kongwe provides evidence
against the presence of growth rates high enough to achieve
adult body size in 1 year. Although the sustained fibrolamellar
tissue complex present in A. kongwe indicates a relatively more
rapid rate of growth than that of extant crocodilians (Padian
et al., 2004; Huttenlocker et al., 2013), collagen fibers are more
organized and arranged circumferentially in a large portion of
the cortices of the elements thin-sectioned (Fig. 4C, D, G), indi-
cating a slower rate of growth for much of the latter part of life
in the individuals sampled (Huttenlocker et al., 2013). The vas-
cular size and level of vascularization in A. kongwe are much less
than in extant birds (Castanet et al., 1996; Horner et al., 2001;
Padian et al., 2001; Starck and Chinsamy, 2002), which com-
monly attain adult body size in under a year (Padian et al., 2004;
Stark and Chinsamy, 2004). Thus, similarly vascularized tissue
would be expected in A. kongwe if the taxon reached adult size
in the same time. Longitudinal vascular canals dominate the cor-
tices of all elements (Fig. 4), with only some slightly anastomis-
ing canals, and this vascular style is characteristic of slower
growth rates relative to strongly anastomosing, radial, and lami-
nar vascular styles (Castanet et al., 1996, 2000; Padian et al.,
2001). However, growth had not completely ceased in the indi-
viduals sampled, as evidenced by anastomosing, vascularized
periosteal surfaces (Fig. 4) and lack of external fundamental sys-
tem (EFS) in all elements, indicating that no element thin-sec-
tioned belonged to a fully skeletally mature individual in which
lateral growth had ceased. We therefore hypothesize that A.
kongwe did not deposit annual LAGs but that continuous, multi-
year growth was typical of this species, and that the majority if
not all of the elements thin-sectioned were from individuals at
least 1 year old. This hypothesis may be falsified with the
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discovery of annual LAGs in larger A. kongwe femora; however,
based on observations of neurocentral suture fusion, one of the
largest specimens included in our sample is probably near skele-
tal maturity (NMT RB159), and this would preclude the discov-
ery of femora much larger than those present in our current
sample.
Implications for Growth and Evolution of Non-Dinosaurian
Dinosauromorphs
The similarity between the femoral ontogeny of Asilisaurus
kongwe and the ontogenetic changes in the bone scars of the
proximal portion of the femur in the Late Triassic non-dinosauri-
form dinosauromorph Dromomeron gregorii (Nesbitt et al.,
2009) suggests that certain ontogenetically controlled features
may be common throughout Dinosauromorpha. Smaller speci-
mens of D. gregorii (TMM 31100-764, 1234, and 1308) do not
possess either an anterior trochanter or a trochanteric shelf, and
because of the appearance of both at the same ontogenetic stage
(appearing in the two largest specimens, TMM 3100-464 and
1306), Nesbitt et al. (2009) suggested a coupling of the anterior
trochanter and trochanteric shelf in development, similar to
what is present in A. kongwe. Both anterior trochanter and tro-
chanteric shelf are absent in all specimens of the non-dinosauri-
form dinosauromorphs Lagerpeton chanarensis and
Dromomeron romeri, and because absence of these features in
some specimens of D. gregorii was thought by Nesbitt et al.
(2009) to be ontogenetically controlled, this indicates that the
described specimens of both L. chanarensis and D. romeri may
represent a developmental stage too immature to display the
anterior trochanter and trochanteric shelf. This variability in the
presence or absence of the anterior trochanter introduces a com-
plication in the methodology of early-diverging archosaur sys-
tematics, in which it has been considered a crucial character.
Many archosaur phylogenies (Gauthier, 1986; Novas, 1992;
Langer and Benton, 2006) have followed Bakker and Galton
(1974) in using the presence of an anterior (D‘lesser’) trochanter
to separate early dinosaurs from other archosaurs. Similarly, the
trochanteric shelf, along with the anterior trochanter, have been
hypothesized to be synapomorphies of Dinosauriformes (Novas,
1996).
The trochanteric shelf and anterior trochanter are hypothe-
sized to have evolved as a single unit, as opposed to the trochan-
teric shelf appearing earlier in ornithodiran evolution than the
anterior trochanter (Nesbitt et al., 2009), following Hutchinson’s
(2001) hypothesis of the M. iliofemoralis splitting into the M.
iliofemoralis externus and the M. iliotrochanericus early in the
evolution of bird-line archosaurs. The fact that both Lagerpeton
chanarensis and Dromomeron romeri lack these muscle charac-
ters introduces a complication to this hypothesis, because this
absence could indicate that the anterior trochanter and trochan-
teric shelf of dinosauriforms have a different evolutionary his-
tory than that of D. gregorii, and the shared character is the
result of convergent evolution, not homology. However, the
anterior trochanter and trochanteric shelf could be plesiomor-
phic in Dinosauromorpha, and simply secondarily absent in L.
chanarensis and D. romeri, or the individuals known from these
species are too developmentally immature to possess these char-
acters. The clear ontogenetic variability of these features in the
early-diverging ornithodirans D. gregorii and A. kongwe indi-
cates that care should be taken in using the presence or absence
of these features as an important indicator in archosaur system-
atics, especially with the consideration that archosaurian muscle
scars increase in prominence as the individual grows (Brochu,
1992).
The fourth trochanter of D. gregorii displays ontogenetic
change in morphology similar to that described in A. kongwe.
The morphology of the fourth trochanter is mound-like with a
central rounded ridge (TMM 31100-1234), whereas larger speci-
mens of D. gregorii (TMM 31100-464 and 1306) show a bulbous,
ridgeless distal end of the fourth trochanter, although the proxi-
mal portion of the trochanter retains the morphology of smaller
specimens (Nesbitt et al., 2009). This developmental change is
similar to the progression we described in A. kongwe (see
above), because the fourth trochanter changes from a gracile,
ridged fourth trochanter in smaller specimens to a robust,
rounded shape in larger specimens. Also notable is the develop-
ment of an anteroposterior expansion of the distal end of the
fourth trochanter in the two larger specimens of D. gregorii (Nes-
bitt et al., 2009), which may be compared with the development
of the protrusion on the distolateral face of the fourth trochanter
that is present in most larger specimens of A. kongwe (NMT
RB217, RB215, RB216, RB213, RB171, RB159, RB226; SAM-
PK-10598). In both A. kongwe and D. gregorii, the fourth tro-
chanter is present in all specimens, and as such its presence is far
less variable than the anterior trochanter and trochanteric shelf.
This is unsurprising, because even hatchling Alligator individuals
possess a fourth trochanter (Brochu, 1992), and the muscle scar
seems to be a widespread archosaurian character.
The similarity between the most common ontogenetic
sequence of A. kongwe (both qualitative sequence and the size-
independent modal sequence) and the sequence described for D.
gregorii (Nesbitt et al., 2009; based on length), along with the
close phylogenetic relationship of the two taxa and the evidence
for widespread sequence polymorphism in early dinosaurs (see
below), suggests that sequence polymorphism may be present in
D. gregorii development. However, because the series of D. gre-
gorii femora lacks evidence of sequence polymorphism,
sequence polymorphism may be a synapomorphy of Dinosauri-
formes and may be absent in D. gregorii and other non-dino-
sauriform dinosauromorphs. Alternatively, this lack of evidence
may be the result of the small sample size of D. gregorii femora
(n D5) studied by Nesbitt et al. (2009). As more specimens are
recovered, the increase in sample size will allow the presence or
absence of sequence polymorphism in the ontogeny of this clade
to be more rigorously tested, in addition to allowing better reso-
lution in the relative developmental timing of different bone
scars.
Although both the linea intermuscularis cranialis and the linea
intermuscularis caudalis are present in D. gregorii, the order of
development described by Nesbitt et al. (2009) is the opposite of
that which we have qualitatively described in A. kongwe. The
linea intermuscularis cranialis is only present in the largest speci-
men of D. gregorii (TMM 31100-1306), and the linea intermuscu-
laris caudalis appears early in ontogeny, and is present in all but
the smallest femur of D. gregorii.InA. kongwe, we observed the
reverse: the linea intermuscularis cranialis is present in all but
the smallest specimen (NMT RB169) with one exception (NMT
RB185), whereas the linea intermuscularis caudalis develops
slightly later, first appearing in the sixth femur in the series
(NMT RB109); this same order holds in the OSA modal
sequence. The presence of both muscle scars in D. gregorii and
A. kongwe support Hutchinson’s (2001) hypothesis that these
intermuscular lines are archosaur synapomorphies, but this
hypothesis may require revision in the future, because the pres-
ence of these characters seems to be at least partly tied to devel-
opmental timing. Because the presence of many
phylogenetically important characters appears to be at least par-
tially ontogenetically controlled for these early-diverging dino-
sauromorphs, care should be taken in their utilization as
important characters in systematics.
A series of 14 nearly complete femora ranging from roughly 90
to 110 mm in length are known from the silesaurid Sacisaurus
agudoensis (Langer and Ferigolo, 2013); however, these femora
lack variation similar to that reported for Dromomeron gregorii
(Nesbitt et al., 2009) or that we have reported in Asilisaurus
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kongwe. All described femora of this taxon possess the same
bone scars as the most skeletally mature individuals of Asilisau-
rus except for the trochanteric shelf, the linea intermuscularis
cranialis, as well as a developed linea intermuscularis cauda-
lis—although a ridge extends distally from the fourth trochanter
along the posterolateral surface of the femur, similar to Asilisau-
rus, it does not extend to the proximal half of the femur (Langer
and Ferigolo, 2013). All S. agudoensis femora also possess a
flange-like dorsolateral trochanter similar in morphology to that
of Silesaurus opolensis (Piechowski et al., 2014) and an unnamed
Otis Chalk silesaurid (TMM 31100-1309; Nesbitt et al., 2010), as
well as Tawa hallae (GR 241; Nesbitt et al., 2009). Lack of these
three bone scars in S. agudoensis could indicate that all described
femora of this taxon belong to skeletally immature individuals or
that this taxon does not possess the same ontogenetic trajectory.
Potentially, recovery of more S. agudoensis femora across a
larger size range or histological sampling could resolve this
dilemma.
A recent study of a hypothesized growth series (n D33) of
Silesaurus opolensis femora reported a large amount of variation
in bone scars of the proximal end of the femur, quantified by
measurements of distances between several femoral landmarks
(Piechowski et al., 2014). This study reported a smaller number
of variable femoral ossifications than we have reported for Asili-
saurus, probably because of the smaller size range of femora
(roughly 140–200 mm in length) available for study; however,
the authors acknowledge that the study only compares femora of
later ontogenetic stages, and they use previously published his-
tology (Fostowicz-Frelik and Sulej, 2010) and degree of neuro-
central suture fusion (Brochu, 1996; Irmis, 2007) to estimate that
at least most of the S. opolensis femora in the sample are close to
skeletal maturity.
Piechowski et al. (2014) reported that four ossifications show
variation in the sample, with these features present in five of the
longest individuals: the anterolateral scar (D‘dorsolateral
ossification’ of Piechowski et al., 2014), the distolateral protru-
sion on the fourth trochanter (D‘fourth trochanter ossification’
of Piechowski et al., 2014), the trochanteric shelf, and an
‘overhang structure’ on the distolateral end of the femoral head
(also present in A. kongwe; Fig. 2) interpreted as a calcification
of articular cartilage. The authors report two classes of Silesaurus
femora, those lacking these ossifications and those possessing
them, and propose that the four ossifications developed simulta-
neously in ontogeny. Additionally, the lack of a posteromedial
tuber (D‘tuber’ of Ezcurra, 2006) is reported as being associated
with presence of the ossified structures. The anterior, dorsolat-
eral, and fourth trochanters are not variable, but present in all
femora figured (Piechowski et al., 2014:figs. 2, 4, 10). How
closely associated the anterior trochanter and trochanteric shelf
are in those individuals that possess both is unreported, as is the
presence/absence of the lineae intermusculares and insertion of
the M. caudofemoralis brevis.
We report that the presence/absence of all bone scars but the
fourth trochanter is variable in our sample of A. kongwe femora,
and that the scars appear to develop at different times during
ontogeny, rather than simultaneously. Because of the similarity
between the bone scars and the close relation of these taxa, we
hypothesize that the femoral bone scars of Silesaurus opolensis
also follow this pattern of sequential rather than simultaneous
development. Because only five of the S. opolensis femora were
reported to possess the variable femoral features, the sample
size over the given length range may be too small to determine
the relative timing of development of these features. The poster-
omedial tuber, which is reported to be absent in those S. opolen-
sis femora possessing the four variable femoral ossifications, is
present in Asilisaurus femora possessing these same ossifications
(e.g., NMT RB156; Fig. 2). Additionally, we report variation in
femoral features not reported to be variable in the S. opolensis
sample, such as the presence of the anterior trochanter, and this
is probably because of the lack of smaller femora of S. opolensis.
We predict that as more S. opolensis femora are recovered, vari-
ation more in line with what we have reported in Asilisaurus will
be observed, and that smaller femora will show variation in the
features now thought of as constant in Silesaurus (e.g., the ante-
rior trochanter).
In the absence of absolute ontogenetic age data, femoral size
was taken as an approximation of ontogenetic age in Silesaurus
opolensis, although the lack of strict correspondence between
size and age was acknowledged (Piechowski et al., 2014).
Although the five femora with the most bone scars and ossifica-
tions are some of the largest, Piechowski et al. state that “size
ranges of Silesaurus opolensis femora with and without [the four
additional] ossifications overlap strongly ... [and] the range of
variability ... seems to be largest among specimens close to the
mean of the sample” (2014:1391). They do not interpret this vari-
ation as intraspecific variation but as sexual dimorphism, drawing
on the apparently bimodal split between those similarly sized
individuals that lack extra ossifications and possess the postero-
medial tuber and those that lack the posteromedial tuber but
possess extra ossifications. Piechowski et al. (2014) follow Raath
(1990) in hypothesizing that the more robust femora in the popu-
lation represent females.
Following our interpretation of variation in femoral scars in
Asilisaurus individuals of similar size, we interpret similar varia-
tion in Silesaurus opolensis femora as intraspecific variation in
growth trajectories, and not sexual dimorphism. Although Pie-
chowski et al. used 33 femora in a principal component analysis
(PCA), only 12 femora (plus one very incomplete femur) were
used to chart the bimodal ossification patterns that were used to
hypothesize sexual dimorphism (2014:fig. 10), limiting the oppor-
tunity to observe any intrapopulation variation in bone scars.
Because only five ossification structures were variable, and were
always present together, there is currently no evidence of
sequence polymorphism in this species; however, the reported
variability is consistent with (1) size as a poor correlate for age in
this size range, or (2) size as a poor correlate for skeletal matu-
rity as measured by bone scar development in this size range, or
(3) a combination of both of these factors. As in A. kongwe, vari-
ation in bone scar development between similarly sized individu-
als could indicate that similarly sized individuals are of different
ages, that bone scars develop at different ages in different indi-
viduals, or that both of these factors are complicating the ontoge-
netic signal, even if sequence polymorphism in relative timing of
bone scars is completely absent in S. opolensis. We hypothesize
that the recovery of more S. opolensis specimens will eliminate
the bimodal distribution of presence/absence of the four variable
bone scars. Further histological investigation could provide a
control on ontogenetic age in this population, because LAGs
have been reported in the tissues of S. opolensis long bone ele-
ments (Fostowicz-Frelik and Sulej, 2010).
Implications for Growth in Early Dinosaurs
Asilisaurus, being both one of the earliest known bird-line
archosaurs and an early-diverging non-dinosaurian dinosauri-
form (Nesbitt et al., 2010), is in an excellent temporal and phylo-
genetic position to guide understanding of developmental traits
plesiomorphic for dinosaurs. Whereas differences in bone fusion
and appearance and morphology of bone scars have been used
as evidence for sexual dimorphism in some early dinosaurs
(‘Syntarsus’rhodesienis, Raath, 1977, 1990; Coelophysis bauri,
Colbert, 1990; Thecodontosaurus antiquus, Benton et al., 2000),
our data from Asilisaurus kongwe suggest that this variability
can also be explained by understanding the differences in the
developmental sequences between individuals in a population,
especially sequence polymorphism. The existence of a common
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developmental trajectory taken by most individuals of A.
kongwe, with variation resulting from a deviation from this tra-
jectory, indicates that sequence polymorphism is the major cause
of variation observed in the bone scars of the proximal portion
of the femur. Depending on the feature in question, size alone
may be a poor indicator of maturity or muscular development in
A. kongwe, with some individuals maturing at both different
rates and in different sequences than the majority. The relatively
large sample size (n D27) across a wide size range (73.8–
177.2 mm) of femora utilized in this study, as well as the large
number of muscle features analyzed (n D11, including the fusion
observed between the anterior trochanter and the trochanteric
shelf), allows a clearer picture of early ornithodiran ontogeny
than previously available, especially of patterns that may be ple-
siomorphic for dinosaurs or more inclusive clades (e.g., Dino-
sauriformes, Dinosauromorpha). Because ontogenetic series of
early dinosaurs and other bird-line archosaurs are rare, the
ontogeny of A. kongwe can serve to help interpret developmen-
tal patterns and variation found in closely related animals, espe-
cially early dinosaurs.
Morphological variation similar to that which we described for
Asilisaurus kongwe has been described in various elements of
the postcranial skeleton of the Early Jurassic coelophysoid the-
ropod ‘Syntarsus’(DMegapnosaurus)rhodesiensis (Raath, 1977,
1990). These studies also focused on variation in femoral muscle
scars, and although most variation is interpreted as a function of
age, Raath found “unequivocal evidence of bimodal variation”
(1990:96) in the muscle scars and trochanters of the femur, with
robust morphs possessing a more highly developed musculature
than the gracile morphs. Features of the robust morph described
by Raath (1977, 1990) include a bulbous and rugose ‘greater
trochanter,’ a broad lesser trochanter, the presence of an obtura-
tor ridge sensu Raath (1977, 1990), a posterior femorotibialis
region outlined by heavy scarring, a rugose and sharply rimmed
insertion pit for the CFB, and a rugose distal patellar ridge. In
contrast, the gracile morph is described with a flat and smooth
‘greater trochanter,’ a narrow lesser trochanter, the absence of
an ‘obturator ridge,’ lack of scarring in the posterior femorotibia-
lis region, the insertion pit for the CFB smooth and not sharply
rimmed, and a smooth distal patellar ridge.
In ‘Syntarsus’rhodesiensis, robust femoral features are only
found in specimens of a certain size (quantified by maximum
width of femur head), and all specimens below this ostensible
ontogenetic stage, as well as some above it, exhibit gracile fea-
tures (Raath, 1977, 1990). The apparent bimodal distribution
found in these muscle features, along with the presence of both
morphs in individuals of similar size, led the author to conclude
that the variation found in ‘Syntarsus’rhodesiensis is the result of
sexual dimorphism, with the point where robust features first
appear in ontogeny as the hypothesized age of sexual maturity.
All 10 of the robust femora are at or above this hypothesized
stage of development, and of the eight gracile morphs four are
above this size.
A robust/gracile dichotomy has also been reported in the
femur of the Triassic coelophysoid Coelophysis bauri, with the
robust morph differentiated from the gracile by an enlarged tro-
chanteric shelf, and this along with proportional differences has
been suggested to be indicative of sexual dimorphism in this spe-
cies (Gauthier, 1984), although the Coelophysis material was not
actually examined for Gauthier’s (1984) study. Ostensible
bimodal variation in proportional differences in C. bauri individ-
uals, also termed ‘robust’ and ‘gracile,’ has been described
between two individuals of roughly the same size (Colbert,
1990), although these terms referred to differences in postcranial
element proportions and sacral fusion patterns, not femoral
morphs as in ‘Syntarsus’rhodesiensis (Raath, 1977, 1990). These
proportional differences were also interpreted as sexual dimor-
phism and not intraspecific variation (Colbert, 1990), and others
have followed this interpretation for variation in cranial and
postcranial proportions in the Ghost Ranch population of C.
bauri (Gay, 2005; Smith and Merrel, 2006; Rhinehart et al.,
2009). However, the claim that bimodal variation is present in
this population has been disputed (Genin, 1992).
The noasaurid theropod Masiakasaurus knopfleri has also
been reported to possess a robust/gracile dichotomy in the bone
scars of the femur, as well as in bone scars of the tibia and fusion
of the tibia and astragalocalcaneum, that was poorly correlated
with size (Carrano et al., 2002; Lee and O’Connor, 2013). In the
sample of 13 femora described, robust individuals possessed
prominent bone scars, whereas gracile individuals either
completely lacked scars or had very poorly developed scars; in
the 12 tibiae described, the same pattern holds, with gracile indi-
viduals also lacking fusion of the tibia and astragalocalcaneum
(Carrano et al., 2002). This variation is described as bimodal,
although a detailed description of what scars are absent or pres-
ent in each individual was not presented in this study. A recent
study of the osteohistology of M. knopfleri found that individuals
grew differently, with variation in the ages and sizes at which
maturity was reached in this taxon (Lee and O’Connor, 2013).
The authors interpret this variation as developmental plasticity,
similar to interpretations of Plateosaurus (Sander and Klein,
2005; Klein and Sander, 2007), except Lee and O’Connor (2013)
argue that this plasticity in response to environmental factors
only occurred in the earliest stages of ontogeny, after which the
growth trajectory was set. The two robust M. knopfleri individu-
als thin-sectioned in this study (one femur and one tibia) did not
have the largest asymptotic size or fastest growth rate, and there
was no clear ontogenetic trend in either morph, although the
robust morphs are both at or very close to complete cessation of
linear growth. Because of this, Lee and O’Connor (2013) inter-
pret these two morphs as two different states of maturity, with
the robust morphs as the mature state, similar to our interpreta-
tion of Asilisaurus kongwe variation.
Based on our observations of the ontogeny of A. kongwe,we
suggest that the similar variation present in the femur of
‘Syntarsus’rhodesiensis individuals of similar size is not the result
of sexual dimorphism but is another example of variation caused
by sequence polymorphism. The lack of robust morphs in smaller
specimens indicates that the development of robust musculature
is at least partially tied to developmental timing, and we hypoth-
esize that the four larger gracile specimens are simply less devel-
opmentally mature than their more robust counterparts. It
should be noted that all Raath’s (1990) gracile specimens above
the hypothesized age of sexual maturity are still smaller than the
mean robust morph: whereas individuals at a late stage of ontog-
eny are clearly large and robust, and those at an early stage are
smaller and gracile, there is an area of overlap between the two
extremes, with developmental maturity being mostly unrelated
to size during a significant portion of development. Because Coe-
lophysis bauri is both closely related and morphologically similar
to ‘Syntarsus’rhodesiensis, this explanation of the variation in
the postcranial skeleton of ‘Syntarsus’ should be considered
when interpreting variation in C. bauri. Variation in bone fusion
and bone scars in C. bauri has been reported (Colbert, 1989,
1990) but not in detail, and the majority of these characters are
not available for study in A. kongwe, limiting our ability to com-
pare these taxa.
Whereas polymorphism is more pronounced in A. kongwe,
possibly as a result of the larger sample size and number of fea-
tures studied, bimodal variation is absent. Although we have
described the presence of less-developed, gracile morphs of
larger size and more-developed robust morphs, most muscle fea-
tures tend to be variable on a continuum and the presence or
absence of certain muscle features does not necessarily correlate
with the presence or absence of others in the same individual.
We suggest that this ontogenetic pattern of individual variability
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in the timing of muscular development is plesiomorphic for
Dinosauria and should be considered when interpreting variation
observed between individuals within a dinosaurian species, espe-
cially early-diverging dinosaur taxa. A thorough understanding
of individual and developmental variation is a vital part of prop-
erly understanding the phylogeny of early dinosaurs, particularly
coelophysoids (Tykoski, 2005). A phylogenetic study that
assumes all taxa are known from adult individuals may place
those taxa only known from immature individuals in artificially
basal positions relative to other taxa in the analysis, and simply
removing data by ignoring ontogenetically variable characters
does little to rectify this problem (Tykoski, 2005). Treating char-
acters known to be ontogenetically variable as missing data
instead of absent in specimens thought to be developmentally
immature may present a better method for dealing with this diffi-
culty (Tykoski, 2005), but more work on ontogenetic variability
in phylogenetically important characters is necessary to resolve
this difficulty.
CONCLUSION
The close relative of dinosaurs Asilisaurus kongwe is a fast-
growing dinosauriform that has similar bony tissues throughout
the long bones. Our sample of different sizes of long bones of A.
kongwe represents an ontogenetic series, but with the exception
of the smallest and largest femora, the relative ages of the speci-
mens sampled are poorly constrained because of a large amount
of intraspecific variation in bone scar development. Absolute
age is unknown because no LAGs are present within long bones.
The femur provides the best record of ontogeny in A. kongwe.
The 11 scars of the proximal end of the femur of Asilisaurus
kongwe do not appear simultaneously in ontogeny, but have a
developmental order that is fairly consistent in most individuals.
However, sequence polymorphism is common, and size is a poor
indicator of skeletal maturity in many specimens: some smaller
individuals appear anomalously mature and some larger individ-
uals appear underdeveloped, whereas some femora appear more
robust than others of the same size. This variability in develop-
ment is well known in extant vertebrates, and similar variability
has been observed in early dinosaurs and non-dinosaurian dino-
sauromorphs; however, in dinosaurs, it has often been inter-
preted as sexual dimorphism. This ontogenetically controlled
variability in the presence and appearance of femoral scars must
be accounted for when undertaking phylogenetic studies, and
although some solutions to this difficulty have been proposed,
more work on the impact of ontogenetically variable characters
on phylogenetic studies is necessary. Because sequence polymor-
phism is common in many extant animals, and the similarities
between A. kongwe development and the development of early
dinosaurs indicate that this pattern may be plesiomorphic to
dinosaurs, developmental sequence variation should be investi-
gated when studying intraspecific variation and growth in dino-
saurs and close dinosaur relatives.
ACKNOWLEDGMENTS
We were supported by the 2012 National Science Foundation
Research Experiences for Undergraduates program (NSF DBI-
1156594 to P. Sierwald and K. Angielczyk) at the Field Museum,
Chicago. We thank R. Schumacher for his assistance with R and
R. Z. Morris for his assistance with OSA. We thank M. McLain
and M. Stocker for their helpful reviews and discussion, the Vir-
ginia Tech Paleobiology Research Group for discussion, and two
anonymous reviewers, whose comments helped greatly in
improving the manuscript. We thank S. Werning for comments
and access to pictures of early archosaur histology. We also
thank Andrey Atuchin for providing the artwork that accompa-
nies this article.
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