A second specimen of Citipati osmolskae associated with a nest of eggs from Ukhaa Tolgod, Omnogov Aimag, Mongolia

AMERICAN MUSEUM NOVITATES

Number 3899, 44 pp.

April 26, 2018

A Second Specimen of Citipati osmolskae
Associated with a Nest of Eggs from Ukhaa Tolgod,
Omnogov Aimag, Mongolia

MARK A. NORELL, 1 ' 2 AMY M. BALANOFF, 1 ' 3 DANIEL E. BARTA, 1 ' 2
AND GREGORY M. ERICKSON 1 ' 4

ABSTRACT

Adult dinosaurs preserved attending their nests in brooding positions are among the rarest
vertebrate fossils. By far the most common occurrences are members of the dinosaur group
Oviraptorosauria. The first finds of these were specimens recovered from the Djadokhta Forma¬
tion at the Mongolian locality of Ukhaa Tolgod and the Chinese locality of Bayan Mandahu.
Since the initial discovery of these specimens, a few more occurrences of nesting oviraptors
have been found at other Asian localities.

Here we report on a second nesting oviraptorid specimen (IGM 100/1004) sitting in a
brooding position atop a nest of eggs from Ukhaa Tolgod, Omnogov, Mongolia. This is a large
specimen of the ubiquitous Ukhaa Tolgod taxon Citipati osmolskae. It is approximately 11%
larger based on humeral length than the original Ukhaa Tolgod nesting Citipati osmolskae
specimen (IGM 100/979), yet eggshell structure and egg arrangement are identical. No evidence
for colonial breeding of these animals has been recovered.

Reexamination of another “nesting” oviraptorosaur, the holotype of Oviraptor philoceratops
(AMNH FARB 6517) indicates that in addition to the numerous partial eggs associated with
the original skeleton that originally led to its referral as a protoceratopsian predator, there are
the remains of a tiny theropod. This hind limb can be provisionally assigned to Oviraptoridae.
It is thus at least possible that some of the eggs associated with the holotype had hatched and
the perinates had not left the nest.

1 Division of Paleontology, American Museum of Natural History.

2 Richard Gilder Graduate School, American Museum of Natural History

3 Center for Functional Anatomy and Evolution, Johns Hopkins University.

4 Department of Biological Science, Florida State University, Tallahassee.

Copyright © American Museum of Natural History 2018

ISSN 0003-0082

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INTRODUCTION

Adult dinosaur remains definitively associated with nests of eggs are among the rarest
vertebrate fossils (Varricchio et al., 2008). Interestingly, the first such combination to be
found was the holotype of Oviraptor philoceratops (AMNH FARB 6517) (Osborn, 1924) (fig.
1). This association was incorrectly interpreted over 90 years ago as a case in which O. philo¬
ceratops was preying upon the eggs of Protoceratopos andrewsi. In 1994 it was discovered that
the egg type associated with the holotype was an oviraptorid (Norell et al., 1994). AMNH
FARB 6517 (the holotype of O. philoceratops ) was therefore a parent rather than a predator
(Norell et al., 1994, 1995, 2001). Although fossil remains of nesting dinosaurs have become
more commonplace (Dong and Currie, 1996; Fanti et al., 2012), the most complete speci¬
mens are found in the bright red, unstructured sandstone deposits of Ukhaa Tolgod, Omno-
gov Aimag, Mongolia (Dashzeveg et al., 1995; Dingus et al., 2008). In 1995 our research
group reported on the first of these specimens, IGM 100/979 (Norell et al., 1995), which was
excavated during the 1993 expedition (figs. 2, 3). Later, after more detailed examination, this
specimen was referred to the ubiquitous Ukhaa Tolgod taxon Citipati osmolskae (Clark et al.,
1999, 2001). Here we report on a second dinosaur nest attended by an adult oviraptorid
(IGM 100/1004) from Ukhaa Tolgod.

LOCALITY AND GEOLOGICAL SETTING

IGM 100/1004 (fig. 4) was discovered during the 1995 installment of the American
Museum of Natural History-Mongolian Academy of Sciences Paleontological Expedition.
IGM 100/1004 was found on the face of the Camel’s Humps amphitheater (fig. 5), at the
southern terminus of the Death Row sublocality. Dinosaur nests are very common at Ukhaa
Tolgod, and there is strong geological evidence that they were preserved as sequential event
horizons caused by rapidly collapsing sand dunes (Dingus et al., 2008). This Camel’s Hump
fossiliferous horizon is the result of one such catastrophic event that preserved several dino-
saurian taxa in life positions. These include Pinacosaurus grangeri (IGM 100/3186, IGM
100/1014) (Hill et al., 2003; 2015) and Shuvuuia deserti (IGM 100/977) (Chiappe et al., 1998;
Schweitzer et al., 1999). The specimen was excavated over a number of days (fig. 6). Some of
this excavation was filmed and photographed, appearing as part of a magazine story (Web¬
ster, 1996) and documentary on the 1995 expedition (Truitt, 1996). Because of the steepness
of the exposure the specimen had to be carefully rigged to lower it down the escarpment.
Because of the excellent preservation of specimens at this locality and the great deal of expo¬
sure, the absence of closely packed nests makes it unlikely that it was a colonial or group
nesting site for Citipati osmolskae.

Some of these fossils, including dinosaur nests such as the one described here, have been exca¬
vated in accordance with Mongolian law by professionals and are part of the Mongolian Academy
of Sciences Institute of Paleontology collection (fig. 7). At Ukhaa Tolgod we have excavated several
of these occurrences; sadly, others have been illegally poached (M.A.N., personal obs.) (fig. 8).

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Fig. 8. Portions of left fore limb, also left and right manna of Otnraptor phUoceraiops. One-fourth natural size.

It. =intfj-elsviclc. St. =atupulu. H “humeraS. I, II. Ill, bright minus with phalanges supine; left manna with phalanges prone.

FIGURE 1. AMNH FARB 6517 the type specimen of Oviraptor philoceratops found associated with a nest of
eggs. From Osborn (1924).

PREPARATION

IGM 100/1004 was purposely left incompletely prepared from the matrix so as to preserve the
relationship between the skeleton and the underlying nest (figs. 4, 8, 10-12). Because the Citipati
osmolskae type specimen (IGM 100/978) is remarkably preserved (Clark et al., 2001; Clark et al.,
2002), it is more important to keep this specimen in context than to remove the individual bones
and eggs from the matrix. In addition to the photographs and illustrations found herein, 3D surface
scanning of the specimen was conducted using a Space Spider scanner (Artec, Luxemborg). An .stl
(stereolithography) file of the entire block is available from the senior author.

OSTEOLOGICAL DESCRIPTION

The specimen is an incomplete skeleton of a large adult Citipati osmolskae (table 1) sitting
atop a nest of eggs (figs. 4, 9-11). Much of the skeleton including the skull, tail, and parts of
the hind limbs had eroded prior to its discovery in 1995. However, because IGM 100/1004 was
most likely buried alive (Dingus et al., 2008), these elements were probably present at the time
of burial. The skeleton is the largest Citipati osmolskae specimen yet reported. Based on the
length of the humerus it is 11% larger than the other Ukhaa Tolgod nester (IGM100/979) and
about 6% larger than the Citipati osmolskae holotype (IGM 100/978) (table 1).

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FIGURE 2. IGM 100/979. The nesting Citipati osmolskae as it was first found at Ukhaa Tolgod in 1993. Left
Amy Davidson, right Louis Chiappe.

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FIGURE 3. IGM 100/979 in dorsal view after preparation.

In what appears to be the stereotypical nesting posture for the taxon, the forelimbs extend
from the torso, so that the humeri lie near perpendicular to the body and the distal limb ele¬
ments (radii, ulnae, and both manus) lie nearly parallel to the nest, with the palmar surfaces
of the manus directed toward the torso. The neck is arched back beside the torso, suggesting
that the head, which is missing, was nestled next to the body. This posture may reflect the
stereotypical resting position of derived theropods (including modern birds) (Xu and Norell,
2004). A similar position of the head and neck is inferred for a nesting specimen of the ovi-
raptorid, Nemegtomaia barsboldi (Fanti et al., 2012).

The referral of IGM 100/1004 to Citipati osmolskae is based on a number of characters
unique to selective subsets of oviraptorids and a combination of characters present in the
holotype IGM 100/978 (Clark et al., 2001). These include: (1) fusion of the greater and lesser
trochanters into a trochanteric crest on the femur (Balanoff and Norell, 2012); (2) elongate
cervical vertebrae that are at least twice as long as wide (Clark et al., 2001), the longest relative

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NORELL ET AL.: SECOND SPECIMEN OF CITIPATI OSMOLSKAE

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FIGURE 4. IGM 100/1004. An adult Citipati osmolskae collected in 1995 from the Death Row sublocality at
Ukhaa Tolgod, Omnogov Aimag, Mongolia, in dorsal view (opposite page and above).

AMERICAN MUSEUM NOVITATES

NO. 3899

FIGURE 5. Death Row sub locality of the Camels Humps. Arrow signifies where IGM 100/1004 was
excavated.

length-to-width ratio for any oviraptorid; (3) ischia that form a symphysis distally (Clark et ah,
2001); and (4) a U-shaped furcula with an elongate hypocleidium (Nesbitt et ah, 2009).

Axial Skeleton

Cervical Vertebrae: Eleven cervical vertebrae are present in IGM 100/1004, excluding
the atlas and axis, which are not preserved. This corresponds to the 12 or 13 cervical vertebrae
typically found in oviraptorosaurs (Osmolska et ah, 2004). Because the vertebral centra remain
encased in matrix, only the neural arches are visible in dorsal view. In dorsal view (fig. 13), the
vertebrae display the characteristic X shape seen in other maniraptorans (Makovicky and Sues,
1998), however as in the Citipati osmolskae holotype (IGM 100/978) the cervicals are more
elongate. The anterior vertebrae are heavily weathered, but low neural spines can be discerned
on the more posterior ones. The spines are centered on the neural arches as they are in other
oviraptorosaurs. The postzygapophyses do not diverge significantly from the midline, even in
the more posterior vertebrae, thereby differing from the morphology present in Conchoraptor
gracilis and Khaan mckennai (fig. 14) (Balanoff and Norell, 2012). The condition in Oviraptor

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FIGURE 6. Michael Novacek (left) and Mark Norell (right) excavating IGM 100/1004. Courtesy of Louis
Psihoyos.

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FIGURE 7. A dinosaur nest being excavated at Ukhaa Tolgod in July 2013. Left to right: Suzann Goldberg,
Maraal Bayra, and Jian-Ye Chen.

philoceratops is difficult to determine, but it appears to be similar to the Citipati osmolskae
holotype (IGM 100/978) and IGM 100/979. The posterior cervical ribs are fused to the verte¬
brae as they are in IGM 100/978, Avimimus portentosus, Anzu wyliei, Heyuannia huangi, Apa-
toraptorpennatus (Funston and Currie, 2016) and Khaan mckennai (Balanoff and Norell, 2012;
Lamanna et al., 2014).

Dorsal Vertebrae: Ten trunk vertebrae are present in IGM 100/1004, including the
cervicodorsal vertebra (fig. 4, and accompanying .stl file available as an online supplement
at https://doi.org/10.5531/sd.sp.30), which is recognized by its expanded, fan-shaped trans¬
verse processes (Osmolska et ah, 2004). The Citipati osmolskae holotype (IGM 100/978) pre¬
serves only seven trunk vertebrae as two were inadvertently destroyed during collection of
the specimen. The neural spines become taller in the more posterior regions of the trunk
series where they are approximately as elongate dorsoventrally as anteroposteriorly as in the
type specimen. The neural spines, however, are not preserved in the last five vertebrae of this
series in IGM 100/1004. Similar to other oviraptorids, the transverse processes of the trunk
vertebrae are as wide as long, are square in dorsal view, and extend horizontally from the
neural arch. Ten dorsal ribs are preserved in IGM 100/1004, which are wide and flattened to

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FIGURE 8. An unsuccessful poaching attempt that resulted in the destruction of an oviraptorid nest, summer
of 2014. Fragments of bone suggest that an adult may have been associated. Apparently the nest fell out of a
jacket as it was being flipped.

a greater degree than in the holotype. Uncinate processes are preserved on the right side of
this specimen (fig. 15). Similar processes also are present in a variety of derived theropod
taxa including IGM 100/978 and the oviraptorids Conchoraptor gracilis, Heyuannia huangi,
Apatoraptor pennatus, and Caudiptery (Ji et al., 1998; Clark et ah, 2001; Lii, 2002; Funston
and Currie, 2016; Codd et ah, 2007). The uncinate processes span two ribs, have expanded
heads at their anterior contact, and taper posteriorly as is typical of derived theropod dino¬
saurs (Codd et ah, 2007). They articulate just proximal to the angle of the rib. The second
uncinate process (associated with trunk vertebrae 3 and 4) is the largest, and the fourth
process is the smallest. Overall the uncinate processes are relatively larger than in Conchorap¬
tor gracilis, and more comparable in size to those of the Citipati osmolskae holotype (fig. 16)
but slightly more gracile than in IGM 100/979.

Sacral Vertebrae: The sacral vertebrae are too heavily weathered to discern much mor¬
phology. At least four are present, but we suspect there were five as seen in the holotype. The
sacral ribs expand where they contact the ilia.

Caudal Vertebrae: No caudal vertebrae or chevrons are preserved, whose morphology
some contend can be used to determine the sex of the individual (Persons et al., 2015).

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TABLE 1. IGM 100/1004 measurements (mm).

Scapula (right):

292.83

Humerus (right):

230.11

Radius (left): 212.44

212.44

Ulna (left): 211.46

211.46

Manus (total length):

284.26

MC II (left):

103.52

Phalanx 1-1 (left):

92.92

Phalanx 1-2 (left):

84.05

Phalanx II-1 (left): 68.3

68.3

Phalanx II-2 (left): 75.56

75.56

Phalanx II-3 (left) (two pieces): 53.0 (distal), 13.5 (proximal)

53.0 (distal)

13.5 (proximal)

Phalanx III-l (left)

44.5

Phalanx III-2 (left)

48.2

Phalanx III-3 (left)

53.65

Phalanx III-4 (left)

57.8

Ilium anteroposterior length (left)

-253.0

Femur (left)

-402.4

Tibia (right, two pieces)

183 (distal)

270 (proximal)

MT I (right)

42.43

Phalanx 1-1 (right)

31.92

Phalanx 1-2 (right)

35.7

Phalanx II-1 (right)

53.15

Phalanx II-2 (right)

36.98

Phalanx IV-1 (right)

38.55

Phalanx IV-2 (right)

35.66

Phalanx IV-3 (right)

31.61

Phalanx IV-4 (right)

28.98

Phalanx IV-5 (right)

49.7

Forelimb and Pectoral Girdle

Scapulocoracoid: The scapula and coracoid are fused into a single element in IGM
100/1004 as in most oviraptorids outside of Caudipteryx, Conchoraptor gracilis, and Jiangxis-
aurus ganzhouensis (Lamanna et al., 2014). Although matrix and the surrounding bones largely
obscure the region of the coracoid, the scapulocoracoid appears to form a gentle arc as it does
in most other derived theropods. This arc is not as extreme as seen in more advanced mani-
raptorans, such as Velociraptor mongoliensis, where the scapulocoracoid shows an L-shaped

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FIGURE 9. IGM 100/1004 in right lateral view. Anterior is to the right.

configuration contributing to a reorientation of the glenoid (Norell and Makovicky, 1997,
1999). The glenoid fossa, formed equally by these two elements, faces laterally. The pectoral
girdle is preserved in articulation; therefore, the scapular blade can be seen extending posteri¬
orly, perpendicular to the rib shafts. The scapular blade expands only weakly at its distal end.
This condition is difficult to compare to IGM 100/978, which has a heavily weathered distal
scapula. The acromion process of the scapula extends anteriorly and is in line with the dorsal
surface of the scapular blade, as in other oviraptorids except Ajancingenia yanshini, in which
this extension is more laterally directed.

Furcula: The articulated furcula (fig. 17) lies flush with the dorsal edge of the acromion
and scapular blade on the more medial side of the scapula as in Velociraptor mongoliensis
(Norell et al, 1997). The morphology of the furcula resembles that of IGM 100/978 in being
U-shaped with tapering epicleidial processes (Nesbitt et al., 2009). A swelling is present along
the ramus between the symphysis and the epicleidial process, which also resembles that of IGM
100/978. Too little of this element is preserved in IGM 100/979 to establish a similar swelling.
In IGM 100/1004 and 100/978, the hypocleidium is elongate and tapers distally. IGM 100/1004
does not possess a midline keel on the anterior surface of the furcula as is present in Oviraptor
philoceratops , but the lateral processes are more expansive anteroposteriorly than mediolater-
ally. The hypocleidium articulates with the sternum as has been suggested for O. philoceratops
and Heyuannia huangi (Barsbold, 1983; Lii, 2002).

Sternum: Only the posterior surface of the right side of the sternum is visible (fig.
18), thus whether this element is paired, as in most oviraptorids (and other basal paravi-
ans), or fused, as in Ajancingenia yanshini, cannot be determined. The visible surface is

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FIGURE 10. IGM 100/1004 in left lateral view. Anterior is to the left.

featureless. The anterior margin of the sternum has a sigmoidal rim resembling both IGM
100/978 and 100/979. The lateral margin bears two processes—a distally tapering cranial
process and a larger, more rounded caudal (xiphoid) process. The caudal process has a
distal cranial extension that is not present in other specimens of Citipati osmolskae (IGM
100/978, IGM 100/979) where this feature can be observed. The sternum is emarginated
between these processes for the costal articulations. Three sternal ribs are preserved near
this articulation in IGM 100/1004—the same number found in IGM 100/978 and 100/979
(fig. 18).

Humerus: Both humeri are preserved in articulation with the radii and ulnae and are
similar in overall morphology to other oviraptorids (fig. 19). They have a sigmoidal shape with
a large deltopectoral crest (107 mm) spanning almost 50% of the total length of the element
(fig. 4). The lateral margin of the crest is rugose and likely served as an attachment site for the
deltoid musculature. It is not present in Oviraptor philoceratops (fig. 20). The proximal articular
surface is highly eroded on both sides but appears to have been mediolaterally elongate as in
all oviraptorids. The distal articulation is discernible only on the left side of IGM 100/1004 (fig.
21). This region is wider than the humeral shaft and has an anterior articulation with the radius
and ulna.

Radius: The radius and ulna (fig. 21) are approximately the same length and slightly
shorter than the humerus (table 1), yet the radius extends slightly further distally than the
ulna. The proximal articulation with the humerus is largely obscured by the overlying
humerus and ulna, but the distal articular surface is mediolaterally compressed and spatu-
late in form (fig. 22).

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FIGURE 11. IGM 100/1004 in anterior view.

Ulna: The ulna is best preserved on the left side of IGM 100/1004. Similar to other ovi-
raptorids (and most maniraptorans) other than Heyuannia huangi and Gigantoraptor erlianen-
sis, the shaft is bowed posteriorly (Gauthier, 1986). Proximally, a small olecranon process is
present (~12 mm tall). As in other oviraptorids except for Apatoraptor pennatus (Funston and
Currie 2016), there is no indication of feather quill knobs (sensu Turner et al. 2007). A large
foramen found on the lateral side of the olecranon process appears to be the result of weather¬
ing, preparation, or a large insect cavity The last mentioned are found in many other Gobi
Desert specimens, on the sternal plates, ilium, and pubis of Velociraptor mongoliensis (IGM
100/985) (Norell and Makovicky, 1997: figs. 3, 9, 14; Fanti et al., 2012; Clark et at., 2001: fig.
2). Just distal to the proximal end, the ulnar shaft is mediolaterally compressed, however, the
distal end is compressed anteroposteriorly and expanded mediolaterally.

Manus: Only the left manus of IGM 100/1004 is preserved. It is 284.3 mm long and
makes up approximately 40% of the total forelimb length. Similar to other oviraptorids,
digits II and III are approximately the same length and both exceed the length of digit I.

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FIGURE 12. IGM 100/1004 in posterior view.

Digit I (fig. 23) is large and robust relative to the rest of the digits that have a large curved
ungual. It is similar in relative size to the same digit in IGM 100/978, Khaan mckennai ,
Conchoraptor gracilis, and Machairasaurus leptonychus (Longrich et al., 2010; Balanoff and
Norell, 2012), but does not achieve the level of robustness seen in Ajancingenia yanshini
(Barsbold, 1981).

Carpals: The carpals and metacarpals are preserved on the left side of the specimen. The
semilunate carpal can be distinguished and covers the proximal ends of MC II and III as in
other oviraptorids. An additional small carpal, likely the radiale, is present at the distal end of
the radius, just proximal to the semilunate carpal. This attribution is in accord with Zanno and
Sampson (2005).

Metacarpals: MC I is missing from the left manus, but MCs II and III are approximately
the same length as in other oviraptorids with the exception of Hagryphus giganteus (Zanno and
Sampson, 2005). The ulna sits on top of the proximal end of MC III, so that its length cannot
be measured with certainty. The metacarpals do not fuse proximally. Both preserved metacar¬
pals have distal ginglymoid articulations. MC III appears to be mediolaterally compressed
towards the proximal end as in all known oviraptorids.

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FIGURE 13. Close up of cervical vertebrae of IGM 100/1004.

Manual Phalanges: IGM 100/1004 retains the plesiomorphic 2-3-4 phalangeal formula of
maniraptors. The ventral surfaces of the phalanges are relatively straight in lateral view. Phalanx

I- 1 is similar to that of Khaan mckennai and other specimens of Citipati osmolskae, in that it is
robust compared with the other two digits and has a ginglymoid distal articulation. The collateral
ligament pits are deep, tear shaped and situated dorsally. In phalanx 1-2, the ungual (fig. 23) is
highly curved along its ventral margin. This element lacks an upturned dorsal lip on its dorso-
ventrally elongate articulation surface, differing from Chirostenotes per gracilis, Elmisaurus rarus ,
Hagryphus giganteus, and Machairasaurus leptonychus (Osmolska, 1981; Currie and Russell, 1988;
Zanno and Sampson, 2005; Longrich et al., 2010). The large flexor tubercle is separated from this
surface by a small space. A deep groove runs along the medial surface.

Digits II and III are approximately the same size and equally robust. Phalanx II-1 has
a large dorsal lip on its proximal articulation surface that is not present in IGM 100/978
and symmetrical extensor tubercles at its distal articulation (fig. 24). The collateral liga¬
ment pits are deep, round and centrally positioned. Phalanx II-2 is broken and very little
morphology can be discerned on this element (fig. 21). The ungual of digit II (phalanx

II- 3) is broken into two pieces, exaggerating its ventral curvature. Phalanx III-1 is short
with a relatively shallow, circular collateral ligament pit that is centrally placed. Phalanx

III- 2 is subequal in length to III-1. It has an unusually tall dorsal lip (fig. 24) on the proxi¬
mal articular surface, which is expressed to a greater degree than in IGM 100/978. The
collateral ligament pits are shallow and not easily discerned. Phalanx III-3 is slightly longer
and straighter than III-2, but similar in morphology to the more proximal phalanx. Pha¬
lanx III-4 (ungual) differs little from the unguals of the other two digits. It has a dorso-
ventrally elongate articular surface with a large, anteriorly placed flexor tubercle, and a
dorsal lip as in the holotype and in Oviraptor philoceratops. The lateral surface similarly
bears a deep groove.

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FIGURE 14. Close up of cer¬
vical vertebrae of: A. the Citi-
pati osmolskae holotype (IGM
100/978); B. Conchoraptor
gracilis (IGM 100/1203); and
C, the Oviraptor philocera-
tops holotype (AMNH FARB
6517). Note how long the ver¬
tebrae of Citipati are relative
to their widths compared to
the other taxa.

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FIGURE 15. Ribs and uncinate processes on the right side of IGM 100/1004.

Hind Limb and Pelvis

Ilium: The three pelvic bones are not completely fused, as is the condition in all ovirap-
torids excluding Avimimus portentosus (Kurzanov 1983). Only the lateral surface of the left
ilium is visible in IGM 100/1004. It is dorsoventrally concave (fig. 25). The dorsal margin of
the midportion and posterior edge of the element is missing, but enough remains to show
that the proportions of the preacetabular and postacetabular processes are roughly equal.
Although this is the condition typically found in oviraptorids, these proportions can vary
more widely within the more inclusive clade Oviraptorosauria (Osmolska et al., 2004). The
preacetabular margin is hooked, but it does not extend ventral to the acetabulum as it does
in Caudipteryx zoui and caenagnathids (Ji et al., 1998; Osmolska et al., 2004; Lamanna et al.,
2014). As in other oviraptorids, the cuppedicus fossa is evident as a flat shelf on the ventral
surface of the preacetabular process. The brevis fossa is not visible. The pubic peduncle
extends ventrally indicating that the pubis also extended ventrally. The ischial peduncle is
not preserved.

Pubis: The proximal region of the right pubis is the only region remaining in IGM 100/1004.
This portion shows the typical oviraptorid condition in that it is anteriorly concave and projects
vertically, and so is nearly perpendicular to the long axis of the ilium (Osmolska et al., 2004).

Ischium: Almost the entire left ischium is visible in IGM 100/1004, although its proximal
articulation with the ilium is obscured by matrix. The distal portion of the right ischium is also

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FIGURE 16. The uncinate processes of: A. Conchoraptor gracilis (IGM 100/1203) and B. Citipati osmolskae
(IGM 100/978).

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FIGURE 17. Right oblique view of the pectoral region of IGM 100/1004.

preserved. Only the internal surfaces of these elements are observable, but it can be discerned
that the ischia contact each other along their distal margin as they do in IGM 100/978 (fig 26).
Similar to other oviraptorids, the obturator process is situated at approximately midshaft. Alter¬
ation from weathering has given the process a more rounded appearance than the triangular
shape present in IGM 100/978. A tubercle, which has not been described for other oviraptorids,
is present on the internal surface of both ischia near the level of obturator process. The poste¬
rior edge of the ischium is straight distally, but concave posteriorly.

Femur: Both femora are preserved. However, only the distal end of the right femur is
present. Much of the proximal end of the left femur is obscured as it is still articulated with
the ilium. The shaft of the femur is long and straight. The lesser and greater trochanters
appear to be fused into a single trochanteric crest as is found in IGM 100/978 and many
other oviraptorids like Gigantoraptor erlianensis, but not Khaan mckennai or Conchoraptor
gracilis (Balanoff and Norell, 2012). Due to heavy weathering, however, the presence of this
feature cannot be definitely confirmed. As in other oviraptorids, a small ridge runs along the

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FIGURE 18. The right sternal region of IGM 100/1004.

shaft of the femur proximolaterally to distomedially from the lesser trochanter and ends just
above the medial condyle.

Tibia: Both tibiae are preserved, although the right element is broken into two pieces. The
proximal end is not exposed on either side as the left side is covered by matrix and the right is
covered by its own distal region (fig. 27). A small portion of the quadrangular-shaped fibular crest
is exposed and extends from the lateral surface and appears again just below the level of the
proximal head (fig. 27). The tibia is roughly circular in cross section at midshaft. Distally, the
astragalus is not fused to the tibia and they contact along a strong horizontal suture, (fig. 28).

Fibula: Both fibulae are present, however much the morphology is distorted. The proximal
and distal ends cannot be delineated on the left side and are not preserved on the right side.
A large tubercle is present approximately 1/3 down the length of the shaft. The fibula tapers
distally and is attenuated, but still appears to nearly reach the tarsals. The proximal fibula is
distinctly bowed laterally.

Pes: The left foot is almost completely obscured under the torso. The right foot is partially
preserved from the midpoint of the metatarsals, but most of the phalanges are obscured by the
overlying tibia and fibula.

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FIGURE 19. Detail of the anterior right humerus of IGM 100/1004.

Tarsals: Only the posterior surfaces of both astragali are visible (fig. 29). The articular
surface is simple and smooth and similar to other oviraptorosaurs. No distal tarsals are visible
in IGM 100/1004.

Metatarsals: The metatarsals are visible on the right side of the specimen. MT II,
III, and IV are eroded at their midpoints and only the unfused distal portions remain
intact. The distal portion of the metatarsus is unfused. MT I is complete. As in all ovirap-
torids, it is reduced to a small pyramidal bone that articulates with the distal end of MT
II. The distal articulation of MT I is gingylmoid. MT II and III are flattened in a dorso-
plantar direction. Although only partially preserved, MT III is similar to that of IGM
100/978 and does not appear to significantly taper proximally on its dorsal surface. MT
IV is more slender than MT III and laterally diverges from the other metatarsals. A deep,
oblong ligament pit is visible on its lateral surface. The distal articulation of the MT IV is
ovoid, enabling a large range of motion (Currie and Russell, 1988). MT V is not
preserved.

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FIGURE 20. The humerus of the Oviraptor philoceratops holotype (AMNH FARB 6517).

Phalanges: The phalanges are not all exposed (fig. 27). The incomplete phalangeal for¬
mula for the foot is 2-?-?-5. The ungual of digit I has a curved ventral margin and a deep
lateral groove as in the holotype IGM 100/978. Only the medial surfaces of the phalanges of
digit II are visible. The collateral ligament pits are well developed and dorsally positioned.
Digit III is not visible. The phalanges of digit IV are dorsoventrally flattened, possibly a result
of postmortem distortion. Deep, elongate collateral ligament pits are present in this digit.
The ungual of digit IV has a deep lateral groove and highly curved ventral margin. Pro¬
nounced dorsal lips are found on all of the pedal unguals.

EGGS AND EGGSHELL

IGM 100/1004 lies over the remains of 12 exposed partial to nearly complete eggs
arranged in a ring. Presumably if preparation were continued, more eggs would be discov-

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FIGURE 21. The left arm and manus of IGM 100/1004.

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FIGURE 22. The left wrist of IGM 100/1004.

ered. No traces of developing embryos are present. One egg is represented only by a small
mass of eggshell fragments beneath the right pectoral girdle. The eggs are grouped into five
pairs within the ring. One egg and the mass of fragments lack partners. Assuming these two
eggs were also paired with corresponding eggs, there would have been at least 14 eggs in the
clutch. The eggs appear to have shifted vertically with respect to one another around the ring,
but no observable eggs directly overlie one another. This suggests that only a single layer of
eggs is preserved. This stands in contrast to the clutch of IGM 100/979, which has stacked
eggs exposed in one portion of the clutch (Norell et al., 1995; Clark et al., 1999). This differ¬
ence might represent an artifact of incomplete preparation, a taphonomic difference between
the two specimens, individual variation in the arrangement of the clutch, or clutches cap¬
tured at different stages of laying by the female. Nevertheless, in some of the video and
images taken during the excavation of the specimen there appears to be several eggs below
the level of the primary layer of eggs, especially just posterior to the right pes. These are
hidden in the supporting field jacket, which could not be removed without compromising
the integrity of the specimen.

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FIGURE 23. The ungual of IGM 100/1004.

Only one egg of IGM 100/1004 is nearly completely exposed. It partially underlies the right
pes and is separated from it by matrix. The egg measures approximately 181 mm long. This
elongate egg appears slightly asymmetric, with the blunt pole pointing inward to the center of
the clutch. The other pole is more crushed; therefore, it remains unclear whether the egg would
have been asymmetric in life. The average width of all adequately exposed eggs (n = 6) is 66.8
mm. This provides an elongation index (egg length:width) of 2.7. All eggs show a fine lineartu-
berculate external ornamentation similar to “variant 2” of Mikhailov (1991: fig. 8). There are
eight to 11 ridges per centimeter, except for the poles, which are smooth. Eggs of this sort are
very common at the Ukhaa Tolgod locality and they often compose entire or partial clutches
(fig. 30), and on occasion are found as unassociated pairs (fig. 31). As mentioned above, even
though eggs and nests of the oviraptorid type (as well as other dinosaur taxa) are ubiquitous
at this locality, the nests are not found in direct proximity, which suggests that the animals were
not communal or associative nesters.

The near-completely exposed egg described above was sampled for microstructural exami¬
nation under a scanning electron microscope (SEM) (Zeiss EVO 60 Variable Pressure, Zeiss
Inc., Jena, Germany) and a petrographic microscope (Leitz Laborlux 11 POL S; Leitz Inc.,
Wetzlar, Germany). Radial thin sections were ground until transparent. Eggshell thickness and
microstructural dimensions were measured with software (ImageJ, NIH, Bethesda, Maryland)
from both thin-section photomicrographs (fig. 32A) and SEM images (fig. 32B). The eggshell
measures 0.71-1.3 mm thick. The equator and blunt pole of the egg have a similar range of
thickness. The eggshell is composed of two structural calcite layers separated by an abrupt,

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FIGURE 24. Phalanges on digits 2 and 3 in IGM 100/1004.

straight boundary. The inner is the mammillary layer (ML) composed of mammillary cones,
each with a radiating crystal fabric; and the outer is the continuous or cryptoprismatic (Jin et
al., 2007) layer (CL) that contains a squamatic structure typical of many nonavian theropod
and avian eggs. The surface is diagenetically eroded in many places. The ML averages 0.19 and
0.26 mm thick at the blunt pole and equator, respectively. The CL measures 0.53-0.72 mm thick
at the blunt pole and 0.50-1.0 mm thick at the equator. These ranges reflect differences among
measured individual fragments and whether thickness is measured below the raised ornamen¬
tation or in the “valleys” between ridges. The CL:ML ratio ranges, on average, from 2.2 in the
“valleys” at the equator to 3.7 beneath the ornamentation at the equator and blunt pole. No
crystal splaying (Jin et al., 2007) is evident along the ML-CL boundary. Accretion lines are
visible in thin section throughout the CL and their undulations mirror those of the eggshell
surface. They are also visible under SEM in the outer third of the eggshell (fig. 32B). The pore
canals are straight, narrow tubes (angusticanaliculate pore system) that vary slightly in diam¬
eter along their width (fig. 32B).

The above macro- and microstructural characters allow assignment to the oofamily Elon-
gatoolithidae, as is the case for all other eggs associated with oviraptorosaur skeletal remains
(Norell et al., 1994, 1995; Dong and Currie, 1996; Sato et al., 2005; Cheng et al., 2008; Weisham-
pel et al., 2008; Fanti et al., 2012; Pu et al., 2017; Wang et al., 2016). The eggs of IGM 100/1004
are nearly identical in size, shape, and microstructure to other confirmed Citipati osmolskae

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FIGURE 25. The left ilium of IGM 100/1004.

eggs (IGM 100/971 [Norell et al., 1994, 2001] and IGM 100/979 [Norell et al., 1995; Clark et
al., 1999]). Citipati osmolskae eggs blur the distinction between the oogenera Elongatoolithus
and Macroolithus, as their length overlaps the range for Macroolithus eggs reported by Mikhailov
(1994), but they correspond more closely in surface ornamentation and microstructure to
Elongatoolithus.

Given the overlapping nature and susceptibility to intraspecific variation and/or tapho-
nomic alteration of some characters used to distinguish Elongatoolithus oospecies (e.g., eggshell
thickness, surface ornamentation), caution is warranted when attempting to assign a given egg
to any oospecies. We do not attempt to make a definitive ootaxonomic characterization of
Citipati osmolskae eggs, as this would likely require an extensive review of Elongatoolithidae,
which is outside the scope of this paper. Nevertheless, we offer detailed comparisons to existing
oospecies below.

As stated by Mikhailov (2014), Citipati osmolskae eggs are most similar to Elongatoolithus
frustrabilis (Mikhailov, 1994), also from the Djadokhta Formation of Mongolia. They are also
similar in size and eggshell thickness to those of E. sigillarius, but lack the short transverse
ridges and nodes along the equator of that oospecies (Mikhailov, 1994). Citipati osmolskae eggs
similarly overlap the eggshell thickness range of eggs of E. subtitectorius, but this oospecies is
known solely from fragments, hindering further comparisons (Mikhailov, 1994). Citipati
osmolskae eggs, at about 180-190 mm long, exceed the size range given for E. frustrabilis by
Mikhailov (1994) (140-170 mm). However, they closely resemble this oospecies in other char¬
acteristic features, presenting partially overlapping total eggshell thicknesses, CL:ML thickness

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ratios, and ridge densities of the lineartuberculate ornamentation. Similar eggs other than those
described by Mikhailov (1994) are known from the Ukhaa Tolgod (IGM 100/1125 [Grellet-
Tinner et al., 2006]) and Bayn Dzak (AMNH FARB 6633, AMNH FARB 6509 [Carpenter et
al., 1994]) localities.

As noted by Clark et al. (1999), Citipati osmolskae eggs are longer than those of both Ovi-
raptor philoceratops from the Djadokhta Formation of Mongolia (Osborn, 1924) and the nest¬
ing oviraptorid from the Bayan Mandahu redbeds of Inner Mongolia, China (Dong and Currie,
1996). Citipati osmolskae eggs may also be longer than the estimated 140 to 160 mm long eggs
of a nesting specimen of Nemegtomaia barsboldi from the geologically younger Nemegt Forma¬
tion, but these eggs are too incompletely preserved to make more confident macrostructural
or microstructural comparisons (Fanti et al., 2012).

Partial eggs containing oviraptorid embryos from the Bugin Tsav locality of the Nemegt
Formation of Mongolia are most similar to Elongatoolithus andrewsi or E. elongatus, though
the lack of complete eggs makes such assignments tentative (Weishampel et al., 2008). As in
Citipati osmolskae eggs, these eggs exhibit a straight ML-CL contact. Their CL:ML thickness
ratio is 2:3, within the range for that of C. osmolskae eggs. Weishampel et al. (2008) note that
the Bugin Tsav eggs differ from a Citipati osmolskae egg (IGM 100/971) in possessing more
variability in mammillary layer thickness within an egg.

Previous authors describe associations of Macroolithus yaotunensis (or similar) eggs with
oviraptorosaur skeletal remains from the Upper Cretaceous Nanxiong Formation in Jiangxi
Province, China. These include eggs inside the pelvis of a female oviraptorid (Sato et al.,
2005) and eggs containing oviraptorid embryos (Cheng et al., 2008; Wang et al., 2016). These
eggs all differ from Citipati osmolskae eggs by their undulating boundary between the ML
and CL and coarser lineartuberculate to ramotuberculate ornamentation. Other Macroolithus
oospecies described by Mikhailov (1994) differ from Citipati osmolskae eggs as well in pos¬
sessing thicker eggshell overall and coarser lineartuberculate ornamentation with six to eight
ridges per centimeter.

Wang et al. (2016) describe a Citipati osmolskae egg (IGM 100/971) as most similar to those
of Elongatoolithus elongatus , but according to Mikhailov (1994), eggs of this oospecies are
substantially shorter (at 115-131 mm) than the complete Citipati osmolskae eggs of IGM
100/979 and IGM 100/1004. Thus, apart from being slightly more elongate, Citipati osmolskae
eggs are most similar to E. frustrabilis among currently described Elongatoolithus.

HISTOLOGICAL REPORT ON IGM 100/1004

Midshaft diaphyseal samples of a femur, dorsal rib, and fibula of IGM 100/1004 were
sampled for petrographic histological analysis. The histological make-up of each of these
elements was characterized. Given that the specimen is very likely a reproductively active
adult, as is IGM 100/979, which was also found on a nest, the medullar cavity was examined

for the presence of medullar bone indicative of female oviposition and hence, sex (Schweitzer
< -

FIGURE 26. The ischiac symphysis of IGM 100/1004.

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FIGURE 27. The left
legofIGM 100/1004.

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FIGURE 28. The right astragalus of IGM 100/1004.

et al., 2005). Counts of lines of arrested growth (LAGS) and annuli were made in each ele¬
ment and back-calculated to infer age (Erickson et al., 2007). In a previous study, spacing
between the LAGs was used to develop a percentage of adult size to age growth curve from
which the developmental stage of the specimen was inferred (Erickson et al, 2007). It was
determined that the animal was somatically mature. Here additional histological details sup¬
porting that interpretation are presented.

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FIGURE 29. The right pes of IGM 100/1004.

The histological characterization (fig. 33A) shows that the diaphysis of the femur is
composed almost entirely of woven bone with plexiform vascularization (Francillon-Vieil-
lott et ah, 1990). Negligible Haversian remodeling is present near the endosteal border of
the element and sporadically within the cortex (Francillon-Vieillott et ah, 1990). Osteo¬
clastic erosion spanning the entire endosteal border with a thin veneer of lamellar endos¬
teal bone is present. The endosteal border of the cortex shows a thick veneer of lamellar
bone and lacks medullar bone as found in the femur of IGM 100/979 (Varricchio et ah,
2008). Nine definitive LAGs are present, two of which are within the EFS (external funda¬
mental system, sensu Cormack, 1987). The spacing between the growth lines diminishes

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FIGURE 30. IGM 100/3505, a clutch of oviraptorid eggs from Ukhaa Tolgod.

toward the outer cortex becoming attenuated in the EFS. Back-calculation to the center of
the medullar cavity based on the average width of the innermost three growth zones, is
continued to where the addition of an average zone width cannot be encompassed without
exceeding the center point and the remaining bone assumed to approximate the hatchling
bone radius, suggests four growth lines had been resorbed prior to death. The diminishing
growth-line spacing culminating in an EFS indicates the animal had reached advanced
somatic maturity. This coupled with the growth curve generated from it showing the speci¬
men reached a somatic growth plateau suggests the animal died at or near full adult size
(Erickson et al., 2007). The idea that the animal was a somatic adult is also reinforced by
the fusion of the neural arches onto the centra in the dorsal vertebrae and the partial
fusion of the astragali to the distal tibiae.

The dorsal rib section is poorly preserved histologically (fig. 33B). Much of the original
osseous matrix is effaced by fungal intrusion. Furthermore, the element is highly fragmented
where it was sampled for histological analysis. What can be gleaned is that the medullar region
is expansive, with only thin remnants of the cortex. The bulk of the primary matrix appears to
be woven fibered bone with longitudinal vascularization. Very large osteoclast erosion rooms

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FIGURE 31. IGM 100/1125, an unassociated pair of oviraptorid eggs from Ukhaa Tolgod.

with moderate Haversian infilling make up most of the inner cortex. The outermost cortex
shows up to six LAGS, one or two of which compose what appears to be an EFS. As with the
femur, these results suggest IGM 100/1004 is somatically mature. Estimation of the absolute
age of the specimen from this element was not possible owing to the samples fragmented
nature, which had made identification of its center uncertain.

The fibula (fig. 33C) is primarily composed of woven bone with longitudinal vasculariza¬
tion that grades into parallel-fibered matrix and then EFS structuring. Nevertheless, one of the
outermost growth zones is partially composed of woven bone with reticular vascularization.
Haversian remodeling is prevalent through the inner two-thirds of the cortex and sporadically
in the outermost zones. Ten prevalent growth lines are present. The inner three are annuli

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FIGURE 32. A. Radial thin section of IGM 100/1004 eggshell under plain polarized light. Scale bar equals 0.5
mm. B. Radial view of IGM 100/1004 eggshell under SEM. Scale bar equals 0.1 mm. The double-headed arrow
indicates a pore. Accretion lines are visible to either side of the pore in the upper third of the eggshell (marked
with triangles).

whereas the remainders are LAGS. The zone spacing diminishes toward the periosteal surface
of the element. Three to four LAGs are encompassed in the EFS. Back-calculated age, based on
the mean width of the inner three growth zones, suggests IGM 100/1004 was approximately
13 years of age at the time of its demise (the same estimate derived from the femur; see above).
Again the histological indices suggest the animal was somatically mature.

Discussion

As explained above, fossilized nests of dinosaurs with attending adults are incredibly rare.
Yet in addition to IGM 100/1004, four other oviraptorids have been discovered associated with
nests of eggs. First was Oviraptor philoceratops, AMNH FARB 6517 (Osborn, 1924). This speci¬
men is quite poorly preserved (see Clark et al., 2002), and its misinterpreted association with
a group of eggs led to the original misdirected nomenclatural moniker. But an additional ele¬
ment of this specimen has never been reported. Associated with the AMNH FARB 6517 skel¬
eton are the remains of a juvenile oviraptorid (now numbered AMNH FARB 33092) that is
presumably from the same taxon (fig. 34). The tibia of this animal is just 58.7 mm long and
likely represents a perinate (i.e., an embryo or hatchling) within the nest. These bones will be
described in detail in another paper. Notably there are also several known multi-individual
associations of oviraptorosaur specimens both published (Funston et al., 2016) and unpub¬
lished. The unpublished specimens include Khaan mckennai (IGM 100/3616) from Ukhaa
Tolgod, an undescribed oviraptorid from Udan Sayr (IGM MAE 16-08), and a Conchoraptor
gracilis specimen (IGM 199/1275) from Khulsan. All of these show at least one and usually
more adult or near-adult specimens accompanied by groups of juveniles. Collectively these
finds suggest oviraptorids were social animals throughout their lives.

Comparisons with other nesting specimens (Dong and Currie, 1996; Clark et al., 1999;
Fanti et al., 2012) also show a remarkable conservation in the posture of these animals sitting
on the nest in a stereotypical fashion. The bulkiest part of the torso sits in the middle of the
nest in a hollow in the center of a ring of tiered eggs. The forelimbs are extended away from
the body at the perimeter of the nest much in the same fashion as extant Aves. While it is easy

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FIGURE 33. Histological sections of IGM/1004. Specimens are viewed using polarized petrographic light
microscopy under oil- immersion. White circles denote growth lines.

to homologize this brooding position among these taxa, some caution must be exercised.
Except for extant Aves and a putative occurrence of the dromaeosaurid Deinonychus anthir-
ropus (Grellet-Tinner and Makovicky, 2006), oviraptorids are unique among theropods in
showing multiple occurrences of individuals brooding their eggs. Some other associations of
theropods with eggs have been reported such as Troodon formosus (Varricchio et al., 2002), a
troodontid (IGM 100/1129) (Erickson et al., 2007), and a non-ornithuromorph bird (Varric¬
chio and Barta, 2015). While not definitive without a more extensive taxonomic sample a
preliminary hypothesis of homology regarding brooding position can be made.

CONCLUSIONS

Discoveries of theropod dinosaur nests are increasingly common. However, those pre¬
served with attending adults remain incredibly rare. Curiously, the overwhelming preponder¬
ance of such specimens are oviraptorids (Erickson et al., 2007). The same holds true for most
of the maniraptoran group assemblages that have been discovered (e.g., Funston et al., 2016)—
these include unpublished specimens collected by American Museum of Natural History-Mon¬
golian Academy of Sciences Expeditions and a specimen that was looted from the Gobi Desert
and repatriated to Mongolia. Taken together, these occurrences suggest that oviraptorid dino¬
saurs were very social animals and that this sociality extended to their early days in the nest.
This nesting behavior, as is still seen in living birds, most likely has its origin at some level near
the base of Maniraptora. This once again pushes what was thought to be an “avian” character¬
istic back into the evolutionary history of nonavian dinosaurs (e.g., Norell and Xu, 2005; Bala-
nofF et al., 2013; Nesbitt et al., 2009).

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FIGURE 34. The hind limb of a small oviraptorid (AMNH FARB 33092) associated with the Oviraptor philo-
ceratops holotype (AMNH FARB 6517).

ACKNOWLEDGMENTS

Great thanks to the 1995 American Museum of Natural History-Mongolian Academy of
Sciences field crew. We thank D. Lawyer and D.J. Simon for helpful comments on the manu¬
script. B. Goldoff and D. Ebel provided access to equipment and assistance with sectioning of
eggshell in the AMNH Department of Earth and Planetary Sciences. M. Eklund helped with
petrographic microscopy of the eggshell. H. Towbin assisted with scanning electron micros¬
copy in the AMNH Microscopy and Imaging Facility. Many thanks are due Mick Ellison for
the figures and Lynn Merrill for support and scanning of the specimen. We thank Louis Psi-
hoyos for permission to use the image in figure 6. Marilyn Fox prepared the specimen. Greg
Funston and an anonymous reviewer greatly improved the manuscript. The 1995 expedition
was in part supported by the National Geographic Society. The histological analyses were sup¬
ported by a National Science Foundation grant (EAR 0207744 to GE and MN). D.E.B. is sup-

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ported by a Richard Gilder Graduate School Graduate Fellowship and M.A.N. by the Macaulay
Family endowment.

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APPENDIX 1

Abbreviations

Anatomical Abbreviations

ac

acromion

CL

continuous layer

cv

cervical vertebra

dpc

deltopectoral crest

Dv-1

dorsal vertebra 1

Dv-2

dorsal vertebra 2

egg

egg

f

furcula

fit

fibular tubercle

ht

humeral tubercle

hyp

hypocleidium

lac

left astragalo-calcaneum

If

left femur

lfi

left fibula

lh

left humerus

lhr

lateral humeral rugosity

li

left ischium

lil

left ilium

lr

left radius

Is

left scapula

It

left tibia

lu

left ulna

mc-2

metacarpal 2

mc-3

metacarpal 3

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ML

mammillary layer

mpl-1

manual digit 1-phalanx 1

mpl-2

manual digit 1-phalanx 2

mp2-l

manual digit2-phalanx 1

mp2-2

manual digit2-phalanx 2

mp2-3

manual digit2-phalanx 3

mp3-l

manual digit3-phalanx 1

mp3-2

manual digit3-phalanx 2

mp3-3

manual digit3-phalanx 3

mp3-4

manual digit3-phalanx 4

pp4-l

pedal digit 4- phalanx 1

pp4-2

pedal digit 4- phalanx 2

pp4-3

pedal digit 4- phalanx 3

pp4-4

pedal digit 4- phalanx 4

pp4-5

pedal digit 4- phalanx 5

r

rib

rac

right astragalo-calcaneum

rc

right coracoid

rf

right femur

rfi

right fibula

rh

right humerus

ri

right ischium

rmt-2

right metatarsal 2

rmt-3

right metatarsal 3

rmt-4

right metatarsal 4

rp

right pubis

rr

right radius

rs

right scapula

rt

right tibia

ru

right ulna

si

semilunate

stp

sternal plate

str

sternal rib

unc

uncinate process

Institutional Abbreviations

IGM

AMNH FARB

Institute of Paleontology, Mongolian Academy of Sciences

American Museum of Natural History, Fossil amphibians, reptiles and birds