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Paludicola 5(4):255-261 June 2006
by the Rochester Institute of Vertebrate Paleontology
255
A JUVENILE PLESIOSAUR (REPTILIA: SAUROPTERYGIA) ASSEMBLAGE FROM THE SUNDANCE
FORMATION (JURASSIC), NATRONA COUNTY, WYOMING
William R. Wahl
Wyoming Dinosaur Center, Big Horn Basin Foundation, 110 Carter Ranch Road, Thermopolis, WY. 82443
wwahl2@aol.com
ABSTRACT
The predominance of juveniles from one taxon is rarely found in faunal studies. However, the presence of seven juveniles in
a sample of ten cryptocleidoid plesiosaurs from the Redwater Shale Member of the Sundance Formation of Natrona County,
Wyoming may be such a paleocommunity. Juvenile characters are recognized by the lack of facets and ossification on the distal
ends of the propodials and by cross-sections of the limbs. Juveniles have dense pachyosteosclerotic bone structures whereas
adults have more spongy, osteoprotic bone. The dense bone of the juveniles suggests a difference in environmental preference
between juvenile and adult plesiosaurs.
INTRODUCTION
The Sundance Formation (Bajocian-Oxfordian)
was the last and most extensive transgressive sequence
of the Jurassic in North America (Kvale et al., 2001).
The majority of the vertebrate fossils have been
collected from the Redwater Shale Member. The
presence of the small cardiocerid ammonite,
Quenstedtoceras colleri, establishes the lower
Redwater Shale as latest Callovian. This Callovian age
was further confirmed by the identification of the
coleoid belemnite, Pachyteuthis densa, and the
pelecypods Camptenectes bellestrius and Ostrea
strigilecula (Kvale et al., 2001).
The upper Redwater Shale Member is Oxfordian
in age (Kvale et al., 2001). It represents a shallow,
open shelf environment dominated by silty to shaley
mudstone, occasional bioturbated shale, and ripple-
dominated, glauconitic fine-grained calcareous
sandstone (Figure 1; Andersson, 1979; Specht and
Brenner, 1979; Kvale et al., 2001). It has been
compared to the Callovian lower Oxford Clay of
England (Wahl, 1997, 1999). The Sundance Seaway
was affected by the Arctic or Boreal Seaway that
connected to the Tethys Seaway of Europe, of which
the Oxford Clay was included (Doyle, 1987; Martill,
1991). This connection is indicated by the
identification of the coleiod family Cylindroteuthidae
(belemnites), notably the species Pachyteuthis densa,
which exhibited provincialism with notable migrations
southwards related to sea-level change or possible
seasonal migration (Doyle, 1987, 1995). The presence
of belemnites in various sizes may indicate seasonality
during deposition of the Redwater Shale (Imlay, 1980,
Kvale et al., 2001; Wahl 1998).
The water depth during the Redwater Shale
sequence was estimated to be 40 m (Specht and
Brenner, 1979). The relatively shallow depth made
storm action on paleocommunities very destructive
(Tang and Bottjer, 1996). The presence of glauconitic
grains and siltstone rip-up clasts is evidence of a high-
energy environment (Specht and Brenner, 1979). The
presence of storm damaged bioherms consisting of bits
of fragmented Camptonectes and Gryphaea and
winnowed sandstones are further evidence of a rough
depositional environment in the Redwater Shale
(Specht and Brenner, 1979).
Remains of fish are extremely rare in the
Sundance paleoenvironment (Schaeffer and Patterson,
1981). The marine reptile fauna is dominated by
ichthyosaurs. Specimens collected in recent years
suggest a fauna comprised of 80 % ichthyosaurs, 18 %
plesiosaurs and only 2 % pliosaurs. Currently, a single
species of ichthyosaur, Opthalmosaurus natans is
recognized from the Sundance (McGowan and
Montani, 2003). A single specimen of the giant
pliosaur (13m), Megalneusaurus rex as well as two
genera of cryptocleidoid plesiosaurs, Pantosaurus
striatus and Tatenecktes laramiensis have also been
reported (O’Keefe and Wahl 2003a, b). This is the first
report of a plesiosaur fauna from one locality in the
Sundance Formation where a majority of the specimens
are juveniles.
Institutional Abbreviations—LEIUG, Leicester
University, Department of Geology, Leicester,
England; UW, University of Wyoming, Laramie, WY;
WDC, Wyoming Dinosaur Center, Big Horn Basin
PALUDICOLA, VOL. 5, NO. 4, 2006
256
Foundation, Thermopolis, WY; YPM, Yale Peabody
Museum, New Haven, CT.
FIGURE 1. Sea level changes during Redwater Shale deposition,
with generalized lithostratigraphy. Modified from Specht and
Brenner, 1979.
_______________________________________________________
JUVENILE PLESIOSAUR OSTEOLOGY
Juvenile plesiosaurs in large numbers are found in
Cretaceous marine formations in Antarctica, New
Zealand, and Australia as well as elsewhere in the
United States (Martin et al., 1994; Wiffen et al, 1995).
Diagnostic characteristics of juvenile plesiosaurs have
been debated by several authors (Brown, 1981; Wiffen
et al., 1995; O’Keefe and Wahl 2003b). Smaller sizes
of particular elements are not always a good parameter
in recognition of juveniles. Evidence of delayed
ossification such as differential fusion in the vertebral
column, is a better indicator in plesiosaurs. The
vertebral column may also show some bone
remodeling with a lack of fusion of the centrum to the
neural arch, another good indicator of a juvenile
specimen (Wiffen et al, 1995).
Poorly ossified and faceted epipodials are also a
common occurrence in juveniles. Features such as
lack of ossification in cartilage contact and incomplete
articulation on several portions of the distal surface of
propodials are thought to be another indicator of the
juvenile stage of plesiosaurs (Figure 2A; Andrews,
1910; Wiffen et al, 1995). Adult plesiosaurs are
identified based on the rigidly faceted distal portions of
the limbs and the neural arches fused with the vertebral
centra. In adults, these facets were most likely used to
rigidly support the epipodials to make a stiff flipper for
swimming (Brown, 1981).
However, as Wiffen et al. (1995) noted, juvenile
plesiosaur material can also be identified by general
aspects of bone cross-sections, with thick dense cortex
bone structures in juveniles and open cancellous bone
structures in adults (Figure 2B). In contrast to the
spongy, woven cancellous bone in an adult propodial,
the juvenile propodial will have up to 65% compact
bone in the radius with a gap region of highly
compacted bone structures (Figure 2B; Wiffen et al.,
1995). The end of the propodials in juveniles contains
periosteal cortices that are generally compact bone with
only moderate vascularization (Figure 2C). The
pachyosteosclerotic condition of the juveniles,
compared to the osteoprotic-like condition of the
adults, had an effect on the mass of the limb bones
(Wiffen et al., 1995). The rapid periosteal accretion
visible on the limbs of plesiosaurs from the New
Zealand Cretaceous suggests a high, sustained growth
rate in the juveniles. Adult plesiosaurs, however, are
described as having skeletons with cancellous bone
displaying abundant, intense remodeling by repeated
cycles of reabsorption and accretion (Wiffen et al,
1995).
MATERIALS
Between 1992 and 2003 articulated specimens
and isolated limb elements of plesiosaurs were
collected from surface finds in both lower and upper
Redwater Shale member, Sundance Formation,
Natrona County, WY. (UW locality V-95010).
Specimens from the same locality, although not
necessarily from the same stratigraphic horizon, were
examined for this study. The majority of the material
was found disarticulated and mostly in the form of
isolated propodials and vertebral centra. The few
articulated specimens were found in fine-grained
limestone and limey mudstone layers. The Redwater
Shale plesiosaur specimens do not appear to have been
buried quickly as there is some shell encrustation on
the surface of several limb elements.
From this collection 7 out of 10 individual
specimens were identified as juveniles based on the
lack of defined facets on the distal edges of the
propodials. Several limb bones also lack any deltaic
WAHL-- JUVENILE PLESIOSAUR ASSEMBLAGE 257
FIGURE 2. Illustration of adult and juvenile plesiosaur propodials. Lower diagram of each pair is the adult; upper diagram is the juvenile. A,
Propodials. Note facets present for epipodials on adult. B, Cross-section through radius. C, Polished photomicrograph of cross-section of radius.
Figure modified from Wiffen, et al., 1995.
________________________________________________________________________________________________________________________
ridge definitions on the proximal end of the propodials.
Cross-sections show dense pachyosteosclerotic bone
and a demarcation between the core and the outer bone
(Figure 3). I consider specimens UW 24217, 24219,
24236, 24239, 24240, 24243 and 24244 to be juveniles
(Figure 4; Table 1). In addition to what is in the
photograph, UW 24219 also includes several isolated
neural arches found with the limb. The limb elements
of the seven specimens could not be determined to be
femora or humeri as the ridges are rounded off and
would have been supported by a cartilage sheath in
juveniles. Adult plesiosaurs from this location are
cryptocleidoid plesiosaurs (O’Keefe and Wahl, 2003a,
b), so it is assumed that the juveniles are also. The
specific taxon cannot be determined. Although not
from this locality, the holotype of Pantosaurus striatus
(YPM 543), a partial skeleton that includes articulated
vertebrae, ribs, pectoral girdle, and limb fragments, is
also a juvenile (O’Keefe and Wahl, 2003a). There is
no evidence of juvenile plesiosaurs from the Sundance
Formation other than those of the Redwater Shale.
PALEOBIOLOGY OF JUVENILE PLESIOSAURS
Why is there a high proportion of juveniles
relative to adults at this location? A reliable food
source for juveniles in the form of coleoid cephalopods
would be one possibility. The Arctic Boreal and
Atlantic Callovian-Oxfordian realm had achieved a
maximum diversity and areas such as shelf seas and
provincial interior seaways preserve mass
accumulations of belemnite rostra indicating they were
relatively common (Imlay, 1980; Kvale et al., 2001).
Predation on coleoids by marine reptiles has been well
documented (Pollard, 1968; Ulrichs et al, 1994). A
seasonal migration of belemnites might be an important
food source for both plesiosaurs juveniles and adults in
a lagoonal, provincial seaway such as that of the
Redwater Shale. Coleiod hooklets have been found as
gastric contents in several adult plesiosaurs (Martill,
1992; Wahl, 1998) including some from the Sundance
Formation: Pantosaurus striatus (UW 24215),
Tatenecktes laramiensis (UW 24801) and in a juvenile
(WDC SS01; Figure 5). Coleiod hooklets were also
found in a Sundance ichthyosaur (UW 34653; Massare
and Young, 2005). Thus belemnites were a significant
source of food for the marine reptiles in the Redwater
Shale.
Juveniles co-occurring, but in larger proportions
to adult specimens, may identify a Jurassic nursery
paleoenvironment. The survival rate of juveniles may
have been greatly increased by association with adults
as some juveniles are found with evidence of predation.
UW 24219 has numerous bite marks in the bone
surface at the distal end of the propodial (Figure 6A).
A plesiosaur propodial from the Oxford Clay has also
been found with similar bite marks (Figure 6B; Martill
et al, 1994). Although cannibalism could have been a
danger, no gastric evidence has been found to support
this hypothesis. Social structure and group association
for protection and perhaps coordinated feeding occur in
some extant groups of marine mammals, specifically
PALUDICOLA, VOL. 5, NO. 4, 2006
258
FIGURE 3. Cross-sections of Sundance plesiosaur limb material. A, Dense juvenile bone (UW 24239) showing delineation of core from outer bone;
B, Spongy adult bone with no delineation (UW 24801). Scale bar equals 3 cm.
________________________________________________________________________________________________________________________
FIGURE 4. Juvenile plesiosaur limb material A, UW 24217. Note rounded unfaceted ends of metapodials. B, UW 24244. C, Linear cross-section of
juvenile limb UW 24240, D, UW 24243. E, UW 24236. F,UW 24239. G, UW 24219. Scale bar in cm.
________________________________________________________________________________________________________________________
seals and sea lions (Young, 1972; Clarke and
Trillmich, 1980; Nilssen et al, 2002; Craig and Ragen,
1999). Such behavior could have occurred in
plesiosaurs.
Martin (1994) suggested that extremely small
juvenile plesiosaurs may indicate evidence of live
birth. However, the material found in the Sundance
Formation is not embryonic, nor small enough to
suggest that the Jurassic environment was a plesiosaur
breeding ground. Extremely small juveniles have been
reported from the Cretaceous Pierre Shale. These may
indicate some aspect of parental care because the
chances of survival of such a small animal so far from
shore would have been increased by the presence of
associated adults (Martin, 1994). However, although
the juvenile material in the Sundance Formation is
found associated with adults, this does not provide any
evidence of parental care.
WAHL-- JUVENILE PLESIOSAUR ASSEMBLAGE 259
FIGURE 5. Juvenile plesiosaur gastric contents including coleiod hooklets, WDC SS-01. Scale bar equals 1 cm.
_______________________________________________________________________________________________________________________
FIGURE 6. A, Bite marks on distal end of Redwater Shale plesiosaur propodial UW 24219. Scale bar in cm. B, plesiosaur propodial from Oxford
Clay, LEIUG 114205 from Martill et al, (1994). Scale bar equals 10cm.
________________________________________________________________________________________________________________________
Another possibility was that the differences in the
bone mass density between juveniles and adults, as
previously mentioned, might have had consequences
on their swimming abilities and respective
paleobiology. Wiffen et al. (1995) suggested that
juveniles and adults lived in separate environments.
Taphonomic processes can produce size or shape
biases, but rarely ontogenetic systematic biases (Wiffen
et al, 1995). Discoveries of juveniles from non-marine
deposits thought to be estuaries suggest a shallow
water habitat separation for juveniles or perhaps a
nursery site for plesiosaurs (Sato et al.,2005). The
shallow water of the Sundance Formation may have
been such a habitat.
PALUDICOLA, VOL. 5, NO. 4, 2006
260
Table 1. Plesiosaur specimens identified as juveniles.
Measurements in cm.
Specimen
number
Material Preserved Width (cm)
UW 24217
vertebra, neural arch
pieces, epipodials and
phalanges
UW 24219
distal end of propodial
9.8
UW 24236
distal end of propodial
6.0
UW 24239
distal end of propodial
4.7
UW 24240
broken propodial
5.7
UW 24243
proximal and distal
ends of propodial
4.0 (proximal)
10.0 (distal)
UW 24244
proximal and distal
ends of propodial
5.0 (proximal)
9.7 (distal)
CONCLUSION
The presence of a large number of juvenile
plesiosaurs is unusual especially as adult plesiosaurs
are not as common as the juveniles in the study area.
Unfortunately, known material is not sufficiently
complete to identify which taxa are represented.
Chances of survival of juveniles may have been
improved by association with adults as there are
several juveniles found to every adult. Alternately, the
predominance of juvenile plesiosaurs in the provincial
seaway could be explained by environmental controls
such as a food source in the form of belemnite
migrations into transitional lagoonal areas.
Another possibility is that juvenile plesiosaurs
may have been limited to lagoonal or shoreline shelf
areas whereas the adults would have been more
adapted to the open sea. The dense bone of juveniles
made the skeleton displace more mass, limiting
swimming speed and capabilities of rapid maneuvers
for small dense bodies (Wiffen et al, 1995).
Depositional evidence of rough, shallow water in the
Redwater Shale Member of the Sundance Formation,
may have made the bone mass difference useful for
stabilization in the wave-agitated water.
ACKNOWLEDGEMENTS
I would like to thank the people of the Bureau of
Land Management and the University of Wyoming, for
their cooperation and patience in the collection and
preparation of these specimens. I had useful
discussions with D. Martill (Portsmouth University,
Portsmouth, England). Thanks also to M. Ross for the
field work, B. Schumacher for a review, and J. Massare
(SUNY Brockport) for review and advice.
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