A Large Accumulation of Avian Eggs from the Late Cretaceous of Patagonia (Argentina) Reveals a Novel Nesting Strategy in Mesozoic Birds

Abstract

WE REPORT THE FIRST EVIDENCE FOR A NESTING COLONY OF MESOZOIC BIRDS ON GONDWANA: a fossil accumulation in Late Cretaceous rocks mapped and collected from within the campus of the National University of Comahue, Neuquén City, Patagonia (Argentina). Here, Cretaceous ornithothoracine birds, almost certainly Enanthiornithes, nested in an arid, shallow basinal environment among sand dunes close to an ephemeral water-course. We mapped and collected 65 complete, near-complete, and broken eggs across an area of more than 55 m(2). These eggs were laid either singly, or occasionally in pairs, onto a sandy substrate. All eggs were found apparently in, or close to, their original nest site; they all occur within the same bedding plane and may represent the product of a single nesting season or a short series of nesting attempts. Although there is no evidence for nesting structures, all but one of the Comahue eggs were half-buried upright in the sand with their pointed end downwards, a position that would have exposed the pole containing the air cell and precluded egg turning. This egg position is not seen in living birds, with the exception of the basal galliform megapodes who place their eggs within mounds of vegetation or burrows. This accumulation reveals a novel nesting behaviour in Mesozoic Aves that was perhaps shared with the non-avian and phylogenetically more basal troodontid theropods.

Full text

A Large Accumulation of Avian Eggs from the Late
Cretaceous of Patagonia (Argentina) Reveals a Novel
Nesting Strategy in Mesozoic Birds
Mariela S. Ferna
´
ndez
1
, Rodolfo A. Garcı
´
a
2
, Lucas Fiorelli
3
, Alejandro Scolaro
4
, Rodrigo B. Salvador
5
,
Carlos N. Cotaro
6
, Gary W. Kaiser
7
, Gareth J. Dyke
8
*
1 Instituto de Investigaciones en Biodiversidad y Medioambiente, INIBIOMA - CONICET, San Carlos de Bariloche, Rı
´
o Negro, Argentina , 2 Instituto de Investigacio
´
nde
Paleontologı
´
a y Geologı
´
a, Museo ‘‘Carlos Ameghino’’, Universidad Nacional d e Rı
´
o Negro, Cipolletti, Rı
´
o Negro, Argentina, 3 Centro Regional de Investigaciones Cientı
´
ficas
y Transferencia Tecnolo
´
gica, CRILAR-CONICET, Anillaco, La Rioja, Argentina, 4 Ca
´
tedra de Ecologı
´
a, Universidad Nacional de la Patagonia San Juan Bosco y CENPAT-
CONICET, Puerto Madryn, Chubut, Argentina, 5 Museu de Zoologia, Universidade de Sa
˜
o Paulo, Sa
˜
o Paulo, Sa
˜
o Paulo, Brazil, 6 Caracterizacio
´
n de Materiales, Centro
Ato
´
mico Bariloche, San Carlos de Bariloche, Rı
´
o Negro, Argentina, 7 Natural History, Royal British Columbia Museum, Victoria, British Columbia, Canada, 8 Ocean and Earth
Science, National Oceanography Centre, University of Southampton, Southampton, United Kingdom
Abstract
We report the first evidence for a nesting colony of Mesozoic birds on Gondwana: a fossil accumulation in Late Cretaceous
rocks mapped and collected from within the campus of the National University of Comahue, Neuque
´
n City, Patagonia
(Argentina). Here, Cretaceous ornithothoracine birds, almost certainly Enanthiornithes, nested in an arid, shallow basinal
environment among sand dunes close to an ephemeral water-course. We mapped and collected 65 complete, near-
complete, and broken eggs across an area of more than 55 m
2
. These eggs were laid either singly, or occasionally in pairs,
onto a sandy substrate. All eggs were found apparently in, or close to, their original nest site; they all occur within the same
bedding plane and may represent the product of a single nesting season or a short series of nesting attempts. Although
there is no evidence for nesting structures, all but one of the Comahue eggs were half-buried upright in the sand with their
pointed end downwards, a position that would have exposed the pole containing the air cell and precluded egg turning.
This egg position is not seen in living birds, with the exception of the basal galliform megapodes who place their eggs
within mounds of vegetation or burrows. This accumulation reveals a novel nesting behaviour in Mesozoic Aves that was
perhaps shared with the non-avian and phylogenetically more basal troodontid theropods.
Citation: Ferna
´
ndez MS, Garcı
´
a RA, Fiorelli L, Scolaro A, Salvador RB, et al. (2013) A Large Accumulation of Avian Eggs from the Late Cretaceous of Patagonia
(Argentina) Reveals a Novel Nesting Strategy in Mesozoic Birds. PLoS ONE 8(4): e61030. doi:10.1371/journal.pone.0061030
Editor: Andrew A. Farke, Raymond M. Alf Museum of Paleontology, United States of America
Received October 18, 2012; Accepted March 5, 2013; Published April 17, 2013
Copyright: ß 2013 Ferna
´
ndez et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: Funding was provided by the Jurassic Foundation (to RAG), the Secretarı
´
a de Gobierno de La Rioja and the Consejo Federal de Ciencia y Tecnologı
´
a
(COFECYT) (SCTIP Nu1198/06 – Proyecto LR02/06). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the
manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: gareth.dyke@soton.ac.uk
Introduction
In the 1980s a team from the National University of Comahue
(Patagonia: Argentina) collected a large number of eggshell
fragments along with some intact whole eggs from the late
Cretaceous Bajo de la Carpa Formation in Neuque´n City,
Patagonia (Figure 1). Part of this fossil collection was later
described by Schweitzer et al. [1] who reported that some of the
eggs contained embryonic bone fragments (MUCPv 284, 305,
306) and one an articulated embryo (MUCPv 284). Schweitzer et
al. [1] assigned these fossil remains to basal birds, showing, on the
basis of preserved embryonic anatomy, that they were certainly
ornithothoracines, and most likely enantiornithines. This report
[1] was the first to associate the anatomy of a Cretaceous bird with
preserved eggshell morphology and was soon followed by others
[2,3]. Later, Grellet-Tinner et al. [4] studied eggs from this
collection and interpreted the loss of the polar and immediately
adjacent regions as evidence of hatching and thereby a specific
hatching strategy typical of modern birds. Most recently, Dyke et
al. [5] described a fossil association of jumbled eggshell, adult and
juvenile bones and complete eggs (lacking embryonic remains)
from the Late Cretaceous of Transylvania (Romania) that they
interpreted as the remains of a nesting colony.
Here, we significantly augment the known fossil record of
Cretaceous birds by presenting the first known concentration of
contemporaneous and complete avian eggs preserved in their laid
positions. Unlike the jumbled and broken accumulation reported
by Dyke et al. [5], the positions of these Argentine fossil eggs allow
collection of spatial information bearing on nesting and hatching
behaviour. Further, many of these eggs contain isolated broken
bones, some partially ossified. Estimates of shell water vapour
conductance (G
H2O
) enable us to establish palaeoecological
context and infer the likely palaeobiology of this Argentine
Cretaceous bird breeding colony.
Materials and Methods
Institutional Abbreviations
CRILAR, Centro Regional de Investigaciones Cientı
´
ficas y
Transferencia Tecnolo´gica La Rioja, La Rioja Province,
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Argentina; MUCP, Museo de Geologı
´
a y Paleontologı
´
a, Uni-
versidad Nacional del Comahue, Neuque´n, Argentina.
In Situ Eggs and Eggshells
We mapped 65 eggs within the campus of the Universidad
Nacional del Comahue, North of Neuque´n city, Argentina
(Figure 1) and obtained permission from the MUCP to access
their collections and to research this fossil material. No permits
were required for this research. Among the eggs we collected,
many contain embryonic remains (MUCPv 1354 to 1358) while
some are almost complete but lack bony remains (MUCPv 37,
189, 235, 238, 283/1, 283/2, 285, 286, 307, 1239, 1240). Other
eggs are partially preserved (MUCPv 36, 236/1, 236/2, 237/1,
237/2, 260, 1241–1249, 1251–1257, 1271) and some are just large
shell pieces (MUCPv 1258–1270, 1272, 1359–1366). In addition,
some eggs lack complete shells so are preserved only as endocasts
(MUCPv 235, 286). We made thin section preparations of
MUCPv 1258, 1259 and 1260 and used SEM to reveal the
ultrastructure in some broken egg pieces and complete eggs
(MUCPv 1258, 1259 and 1260). Schweitzer et al. [1] studied
MUCPv 305, 306, 350–355 and the embryo MUCPv 284.
For comparisons, we also include one well-preserved and two
poorly preserved eggs collected in 2006 from the eastern Comahue
campus and loaned to one of us (LEF) by Dr. L. Salgado. These
specimens are housed in the CRILAR collections (CRILAR-Pv
410a, 410b and 410c).
To map this fossil accumulation we placed a 0.25-m
2
grid over
an area of 45.25 m
2
(Figure 2), an accurate approach because
bedding in this area is horizontal with little or no dip (see
Geological Setting). Eggs present in each grid square were counted
and their distribution analyzed using a x
2
test in SigmaStar 3.5.
modelling the eggs using a Poisson distribution. We were able to
determine whether their distribution on the ground was random,
continuous or uniform [6] and calculated x
2
for two degrees of
freedom. We examined the microscopic details of the shells using
SEM and prepared thin sections of eggshell using standard
methods [7].
To calculate egg volume we made a silicone mold of one
complete egg (MUCPv 1240) and calculated its displaced volume.
We then verified this estimate mathematically using the volumetric
formula for ellipsoids: V = 4/3 p.a.b.c (a: length/2;b: width/2; c:
width/2).
Water Vapour Conductance
The structural and functional properties of eggshell are
paramount determinants of the incubation and hatching success
of reptile and bird embryos [8]. One of the main physiological
properties of an egg is shell permeability, or conductance to both
respiratory gases and water vapour. Gas diffusion through the
eggshell pores can be quantified as water vapour conductance
(G
H2O
). This measure is commonly obtained experimentally for
modern bird and reptile eggs [9], but has rarely been estimated for
fossils. From fossil eggs, however, G
H2O
can be determined by
simple equations and thus represents a valuable proxy for assessing
moisture content in archosaurian nesting environments and
potentially can provide additional information on parental nesting
strategies [10,11]. G
H2O
estimates for a number of dinosaur eggs
have been published [11–13] but this parameter has never been
computed for a Cretaceous fossil bird egg other than the
enantiornithine Gobipteryx [11,14].
In order to obtain G
H2O
estimates for the Comahue fossil eggs,
we used a well-established equation for extant birds [15]. Two
oological parameters are required for this calculation, egg density
and egg radius (Table 1), and we approximated the shape of the
fossil eggs as ellipsoids (prolate spheroids). Thus, there are two
radii: Equatorial radius (a) and polar radius (b). Egg density was
inferred from extant bird eggs following Paganelli et al. [16]; for
comparative purposes we have also tabulated predicted G
H2O
values for bird and non-avian theropod eggs from other published
sources, including the Mongolian Gobipteryx minuta (Table 2). Note
that these G
H2O
estimates were calculated from eggshell thin
sections, not the equation-based approach we use here (see
Discussion).
Geological Setting
This accumulation of fossil bird eggs was found on exposed beds
that have been referred to the Bajo de la Carpa Formation (Rı
´
o
Colorado Subgroup, Neuque´n Group; Middle-Upper Santonian)
[17–19] (Figure 3). The Bajo de la Carpa Formation rests
conformably on the Plottier Formation and is capped by deposits
of the Anacleto Formation (Figure 3) [19]. Unlike other regions of
the Neuque´n Basin, the Santonian rocks of this formation were
deposited by fluvial and aeolian systems [20] as well as extensive
flood plains [18,19] (Figure 3).
The yellow quartz-rich sandstone of the Bajo de la Carpa
Formation contains poorly sorted, subangular-to-subrounded
grains of low sphericity; these generally monocrystalline quartz
grains range between 0.1 to 0.5 mm in diameter, producing a fine-
to-medium sandstone that does not contain feldspars, mica
fragments or any associated lithics (see [19,20]). It has a clay
matrix and calcareous cement – a microspar– that is ferric and
light in colour. The isopachous carbonate cement is secondary
(diagenetic) [19,20], formed in a waterlogged environment [18].
Regionally, this system formed in an arid and dry continental
climate via aeolian deposition [18,19]; there is clear variation from
fluvial systems to distal floodplains across the sequence with
increasing participation of aeolian sediments [19]. The palaeoeon-
viroment inferred for the university campus area consists of
aeolian deposition that created large dunes and inter-dune lagoon
basins skirted by fluvial deposits, criss-crossed by streams and
seasonal or ephemeral water bodies [18,21].
Aeolian deposits offer little resistance to the movement of
groundwater. As a result they are subject to sudden changes in the
water table when distant rains raise the level of local streams or
raise the level of the local ground water. In a relatively flat area,
such as Comahue, relatively small quantities of water at the surface
could cover a large area.
Palaeontological Context
The Comahue eggs come from the same stratigraphic level as
some parts of a rich, associated paleofauna (fossils collected from
beds throughout the Bajo de la Carpa Formation), dominated by
crocodyliforms [18,22,23] (Figures 2, 3). Indeed, the Bajo de la
Carpa crocodilian fauna is well-known and includes the
plesiomorphic crocodyliform Neuquensuchus universitas [24,25],
notosuchians Notosuchus terrestris [26] and Comahuesuchus brachybucca-
lis [22], baurusuchids Cynodontosuchus rothi [26] and Wargosuchus
Figure 1. Site location. Counter-clockwise from top right: map of Argentina and the Comahue region (Neuque
´
n and Rı
´
o Negro Provinces); map of
Neuque
´
n Province; map of Neuque
´
n City; close up of Neuque
´
n City to show the University of Comahue (UNC) campus, and the location of the fossil
bird nesting colony (shaded box).
doi:10.1371/journal.pone.0061030.g001
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Figure 2. Close-up of site location and
in situ
egg map. Top: overview of the Universitary campus showing the location of several paleofaunal
elements and the grid corresponding to the nesting colony (shaded purple box). Bottom: grid showing the location of each mapped egg; circles
represent upright eggs, ovals represent eggs slightly inclined vertically and the oval that lies with its long axis parallel to the substrate represents an
egg found in that position.
doi:10.1371/journal.pone.0061030.g002
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australis [27], as well as a peirosaurid crocodyliform of uncertain
phylogenetic position [18]. The remainder of the fauna includes
skeletal material referred to the basal snake Dinilysia patagonica
[28,29], the abelisauroid theropod Velocisaurus unicus [22,30], the
alvarezsaurid theropod Alvarezsaurus calvoi [22], and the enantior-
nithine and basal ornithuromorph birds Neuquenornis volans [31]
and Patagopteryx deferrariisi [32] (Figure 2).
Results
Egg Mo rphology and Contents
Although a small number of the fossil eggs from the Comahue
campus have been reported before [1,4], previous work focused on
isolated specimens examined outside the context of this remark-
able accumulation of in situ eggs (Figures 2, 4). Indeed, based on
preserved microstructure and embryonic anatomy, we concur with
previous workers that the Comahue eggs were laid by ornithothor-
acine birds. Embryonic bones inside the eggs include strut-like
coracoids and wide ulnae associated with small, narrow radii [1],
synapomorphies of Ornithothoraces [33–35]. As noted above,
adult fossil remains of two ornithothoracine birds, the enantior-
nithine Neuquenornis [31] and the more basal Patagopteryx [32] are
known from the same area.
Many of the partial eggs in the collection (n = 22) appear to lack
their larger pole, suggesting that they had hatched prior to any
flooding event [4]. However, the peculiar vertical posture of these
eggs (Figure 4) would also have exposed the large ends to a greater
risk of damage from accident or small predators and scavengers.
We mapped and collected all but one of the most complete eggs
from this site. These specimens (n = 46) are between 41 and
47 mm (60.01) in length and have equatorial diameters that range
between 26 and 29 mm (Figures 4, 5, 6). External eggshell surfaces
are smooth and cream-coloured and complete eggs are ellipsoid
with only one axis of symmetry (Figures 4, 5, 6). Like the eggs of
most modern birds the specimens lack surface ornamentation (as
noted previously by Schweitzer et al. [1] and Grellet-Tinner et al.
[4] (Figure 6). Many exhibit diagenetic surface textures as a result
of abrasion, corrosion and dissolution. Average shell thickness is
Table 1. Summary of all parameters, units, equations and results for the Comahue eggs (N = 65).
Parameters Unit Formula/Method
N. volans
a Equatorial radius cm data from observation 1.35
b Polar diameter cm data from observation 2.25
e Angular eccentricity of elipse - e = arccos (a/b) 0.93
As Egg surface area cm
2
As = 2 p+[a
2
+(abe/sin (e))] 33.57
V Egg volume Cm
3
V = (4/3)?pa
2
b 17.18
r Egg density g/cm
3
Assumed from avian egg data 1.08
m Egg mass g m = r?V 19.96
G
H2O
Water vapor conductance mg
H2O
/day?Torr G
H2O
= 0.384?m
0.814
4.39
Calculations in this table use formulae from [15].
doi:10.1371/journal.pone.0061030.t001
Table 2. Predicted G
H2O
values (in mgH20/day?Torr) for the Comahue eggs (based on Table 1 and formulae in [15]) alongside
those for other taxa from previous studies (i.e., G
H2O
values estimated using pore counts from egg shell thin sections).
Source Locality Egg G
H2O
This study Neuque
´
n, Argentina Neuquenornis volans (enantiornithine bird) 4.39
Sabath (1991) Gobi Desert, Mongolia Gobipteryx minuta (enantiornithine bird) 2.7
Deeming (2006) Portugal Lourinhanosaurus antunesi (theropod) 541
Deeming (2006) China Macroelognatoolithus xixianensis ( theropod) 600
Deeming (2006) Canada Prismatoolithus levis (troodontid theropod) 39
Deeming (2006) Gobi Desert, Mongolia Gobipteryx minuta (enantiornithine bird) 2.5
Ar et al. (1974) Africa Struthio camelus (ostrich) 105
Ar et al. (1974) Australia Dromiceius novaehollandiae (emu) 51.8
Ar et al. (1974) Holartic Larus argentatus (herring gull) 16.5
Ar et al. (1974) Cosmopolitan Gallus gallus (domestic chiken) 14.4
Ar et al. (1974) Asia Phasianus colchicus (ring-necked phaesant) 6.6
Ar et al. (1974) East Asia Coturnix coturnix (Japanese quail) 3.1
Ar et al. (1974) North America Quisculus quiscula (common grackle) 2.3
Ar et al. (1974) Cosmopolitan Passer domesticus (house sparrow) 0.9
Note that: (1) because different approaches were used to predict G
H2O
values (regressions versus pore counts from thin sections) they may not be comparable; and (2)
value shown for G. minuta is that predicted when compared to an avian egg of similar size, not G
H2O
, which has been estimated to range from 63.9 [11] to 22.4 [14]. See
[11] for further discussion.
doi:10.1371/journal.pone.0061030.t002
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180 mm(62,5 mm); volume was estimated for complete eggs
MUCPv 1240, MUC-Pv 307 and CRILAR-Pv 410a as 19.5,
18.77, and 17.18 cm
3
, respectively. These estimates are similar to
data for living plovers (e.g. Vanellus chilensis (18.37 cm
3
), Arenaria
interpes (19.36 cm
3
) and other Charadriiformes [36,37]. There is
only a weak correlation between egg size and body size. V. chilensis
is slightly larger (35–37 cm total body length) but very much
heavier (327 g) than A. interpes (21–25 cm and 136 g) [36–38].
Nonetheless, the egg volumes suggest an association between the
Comahue eggs and the enantiornithine Neuquenornis in agreement
with Schweitzer et al. [1]. This enantiornithine was much smaller
than examples of Patagopteryx from the same locality [32,33].
Spatial Distribution of Eggs
We mapped 65 complete and partial eggs in situ (Figure 2).
Almost all are separated by just over their own length from their
neighbour as is typical of exceptionally dense avian colonies [39].
We found only a single pair of eggs and one group of three
(Figure 2). All eggs occur in a broad band that is oriented north-
south and that certainly extended further to the north than we are
now able to explore (Figure 2).
Once mapped in two dimensions (Figure 2), all eggs (apart from
one) were collected. The majority were buried vertically in the
sediment with their polar region pointing downwards (Figure 4a–
c); only in a few rare cases were eggs found lying horizontally on
the ground (Figure 4d), presumably because of disturbance after
burial. A chi-squared test (x
2
= 348; N = 182; P,0.001) shows that
this egg concentration (Figure 2) is non-random in distribution and
Figure 3. Stratigraphic profile. Bajo de la Carpa Formation, Neuque
´
n Group [17,31] to illustrate the diverse biota spread through the horizons
within this sequence. Layer containing the eggs discussed in this paper is indicated. Abbreviations: An, Anacleto Formation; Pl, Plottier Formation.
doi:10.1371/journal.pone.0061030.g003
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strongly suggests that birds congregated in this area to breed. Our
interpretation of a nesting colony is supported by the fact that the
embryonic remains identified inside several eggs are all in a similar
state of fairly advanced development, have hatched [4] or are
broken and lack bones completely. This mixture of hatched and
developing eggs is a characteristic of modern avian nesting
colonies [39–41].
Water Vapour Conductance
We calculated an average G
H2O
of 4.14 mg
H2O
/day?Torr for
the Comahue eggs (Table 1) which lies well within the lower end of
the known distribution for modern bird eggs [9] (Table 2). Such a
low G
H2O
value indicates relatively little water vapour loss from
the Comahue eggs and implies an ability to use a dry nesting area
with low relative humidity, such as the paleoenvironment inferred
for the Bajo de la Carpa Formation [18–20].
Discussion
Descriptions and our map of the Comahue eggs (Figure 2)
strongly support interpretation of this fossil accumulation as the
remains of a Cretaceous bird nesting colony. The in situ
preservation of eggs, in combination with well-preserved surface
textures (Figures 4, 5, 6) demonstrate minimal (if any) taphonomic
disturbance prior to burial [42]. Field observations are consistent
with a high degree of synchronicity [42], typical of other amniote
egg and nest fossils interpreted as representing colonies from the
Cretaceous of Romania [5], Asia (Gobi Desert) [14] and from the
Sanagasta neosauropod nesting site in La Rioja, Argentina [43].
Egg and nest associations will be quickly disturbed and disartic-
ulated by post-burial processes, even in inferred low energy
environments (enantiornithine eggs and nests, for example, from
Romania [5]). Low degrees of taphonomic disturbance are also
characteristic of the other fossil vertebrates from the Universitary
Campus area [18]. Indeed, we argue that the fossils from this area
comprise a ‘‘census assemblage’’ (Model I) (sensu [18]): A very high
proportion of articulated remains often with surfaces in pristine
condition and some –if not most–in their positions at the moment
of death [44,45]. Moreover, because the concentration of the
Comahue eggs is ‘‘intrinsic’’ (sensu [46]), this association could only
have been produced by the gregarious behaviour of colonial
organisms [45].
The ellipsoidal shape of the eggs at Comahue is typical of
modern eggs laid in a clutches of 3 to 8 and may be related to
incubation efficiency [47]. Except in special situations such as the
placement of nest sites on cliff edges, nearly spherical eggs are the
most efficient shape for single-egg clutches. In this case, the vertical
posture allows the egg to comply with the prediction of Barta and
Sze´kely [47] by exposing a spherical surface to the incubating
adult.
If the partial eggs at Comahue share internal morphology with
modern birds, their missing poles would have held the air cell [48]
while the disappearance of the shell from that part of the partial
eggs implies that enantionithine birds had already adopted a
hatching behaviour favoured by modern neornithine birds [4].
However, the vertical placement of solitary eggs in an open nest is
unknown among modern birds. Only the megapode, a basal
galliform, deposits its eggs vertically but then only in clutches
Figure 4. Several
in-situ
complete and fragmentary eggs from the Comahue campus nesting colony. These eggs demonstrate spatial
arrangement in vertical (a and c), subvertical (d) and horizontal (b) positions.
doi:10.1371/journal.pone.0061030.g004
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buried in a heap of decomposing vegetation, tree roots or burrows.
Its eggs are not asymmetrical and numerous small air cells are
scattered around the embryo [49]. The Comahue eggs resemble
those of some troodontids that placed their eggs vertically and
appear to have hatched by breaking out through the upper pole
[50]. The north-south linear arrangement of this accumulation is
also significant; such linearity is often characteristic of extant bird
colonies established along the edge of a stream or cliff [39,41].
Among extant neornithine birds, the most similar nesting
strategy is use of a simple ‘‘scrape’’ [49]. A scrape is typically just a
shallow depression with enough of a rim to keep eggs from rolling
away [49]. At Comahue, a nest structures appears to be just
sufficient to prevent the eggs toppling over. Use of these simple
scrapes is seen in several paleognaths [51] and many neognaths,
including members of the Galloanserinae, Charadriiformes,
Falconiformes, Caprimulgiformes, Otidae, and Pteroclidae [37].
Some charadriiforms, in particular terns (Sternidae), breed in
colonies broadly comparable to the Comahue accumulation, often
on a sandbar or a beach where their scrapes are situated on barren
or sparsely vegetated areas near water [52].
Our low prediction for the water vapour conductance (G
H2O
)of
the Comahue eggs (Table 1) is consistent with geological
observations. An arid and dry environment for the site, also
inferred by calculated G
H2O
values, is corroborated by the local
sedimentology [18,20]. Further, the absence of any nesting
structures and the fact that the Comahue eggs were all half-
buried in situ suggests that the upper portion of these eggs were
exposed on the surface after laying and would thus require an
attending brooding parent [11]. A similar strategy has been
implied for the enantiornithine Gobypterix minuta [53] for which we
Figure 5.
In situ
eggs within the Comahue campus. In situ association in bedding plane (a), inset of single egg in position (b), egg half-buried in
sediment (typical for almost all eggs collected) (c), close up lateral view of same egg showing degree of asymmetry (d).
doi:10.1371/journal.pone.0061030.g005
Patagonian Cretaceous Bird Eggs and Nesting
PLOS ONE | www.plosone.org 8 April 2013 | Volume 8 | Issue 4 | e61030

predict an even lower G
H2O
value, 2.5–2.7 mg
H2O
/day?Torr
(Table 2).
Corresponding with observations on living birds [11,15], these
relatively low values of G
H2O
may have allowed the Comahue
birds to exploit drier patches of habitat. Indeed, generally much
higher G
H2O
values that have been predicted for the phylogenet-
ically more basal non-avian theropods [11] (Table 2) (egg size
notwithstanding) are an order of magnitude greater than for any
living birds and may suggest that these taxa required more humid
nesting environments or had more elaborate nests [11,13].
Comparisons of similar-sized eggs (once found and collected from
the fossil record) between birds and non-avian theropods will,
however, be required to corroborate our speculation. Nevertheless,
predicted G
H2O
values are significantly lower for some small, non-
avian theropods that are considered phylogenetically close to
birds, including troodontids [11,54]. These fall well within the
extant avian range yet are still higher than those predicted for
Cretaceous fossil birds (Table 2): low values are consistent with the
suggestion that nests of the North American Troodon formosus were
attended by a brooding parent [54]. Parental care has also already
been well-established in the closely-related oviraptorid Oviraptor
philoceratops [55].
We conclude that the Comahue fossil bird eggs present an
interesting mixture of primitive and advanced traits. On the one
hand, it appears likely that embryos were ventilated by a single,
large air chamber and used a strategy considered distinctively
avian for exiting the egg [1,4] (although this has also been
proposed to have been the case for troodontids [11,54]), while on
the other eggs were laid vertically and could not have been turned
by the parent. Egg turning is widespread in extant birds and has
been intensively studied in galliforms for the poultry industry [56].
This behaviour is believed to place the embryo in an opportune
position and allow effective functioning of the connections between
the embryo and the yolk sac: the unturned eggs of domestic fowl
have a much higher mortality rate (85 percent) and take seven
hours longer to hatch [56]. The incubation period for vertically-
placed megapode eggs (that are buried and cannot be turned) is
four or five times that of domestic fowl [57]. We speculate that the
vertical nesting strategy evidenced by the Comahue eggs was
abandoned by later lineages because it was not competitive with
the greater incubation success and reduced incubation time of
turned eggs. Among modern birds, even basal-most lineages
(including some palaeognaths) contain at least some species that
turn their eggs [58,59,60].
Acknowledgments
We thank the Centro Ato´mico Bariloche for access to SEM FEG, the staff
of the Departamento de Geologı
´
a, Universidad Nacional de San Luis for
help with preparing thin sections, Mauro Cocco (Instituto Balseiro) for help
with egg volume calculations and Sergio de la Vega (CRILAR, Argentina),
Gerald Grellet-Tinner (Journey Museum, USA) and Leonardo Salgado
(INIBIOMA-CONICET, Argentina) for loaning fossils and for comments
on the manuscript. We are very grateful to Frankie Jackson and a second
anonymous reviewer for their valuable comments on the manuscript as
well as to Andrew Farke for his editorial assistance and patience.
Author Contributions
Conceived and designed the experiments: MSF RAG LF AS RBS CNC
GWK GJD. Performed the experiments: MSF RAG LF AS RBS CNC
GWK GJD. Analyzed the data: MSF RAG LF AS RBS CNC GWK GJD.
Contributed reagents/materials/analysis tools: MSF RAG LF AS RBS
CNC GWK GJD. Wrote the paper: MSF RAG LF AS RBS CNC GWK
GJD. Found, collected, prepared and imaged the fossils: MSF RAG LF AS
RBS CNC.
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