Protoceratops

Protoceratops andrewsi inhabited the semi-arid deserts of Central Asia during the Campanian, between 75 and 71 million years ago. The Djadochta Formation, its primary preserved environment, was an aeolian dune desert with moist interdune areas and occasional ephemeral water sheets. The climate was hot and dry, with strong seasonal winds forming large sand dunes. Vegetation was sparse, composed mainly of xerophytic shrubs, primitive flowering plants, and possibly dwarf conifers. The associated fauna included Velociraptor mongoliensis, oviraptorids, ankylosaurids, lizards, and multituberculate mammals.

Protoceratops andrewsi was a low-browsing herbivore specialized in ground-level vegetation. Its sharp horned beak functioned like an efficient scissors for cutting leaves, thin branches, and possibly fruits of xerophytic plants. Posterior teeth formed compact dental batteries capable of grinding hard and fibrous plant material such as roots and stems. Jaw musculature was particularly robust for an animal of its size, generating relatively high bite forces. Adults were obligate quadrupeds, limiting vertical reach to low vegetation browsing, while juveniles could rear up on hindlimbs to access higher levels.

The fossil record of Protoceratops andrewsi points to developed social behavior. The abundance of specimens at different age stages frequently preserved in groups suggests gregarious behavior. Nest MPC-D 100/530 with fifteen post-hatching hatchlings implies nest fidelity and parental care in early developmental stages. The parieto-squamosal frill, which grows disproportionately during ontogeny (Hone et al. 2016) and shows high morphological variance consistent with selection (Knapp et al. 2021), suggests use in intraspecific social signaling, probably species recognition, hierarchical dominance, or mate choice. The predatory relationship with Velociraptor is well documented both by the Fighting Dinosaurs fossil and by bite mark evidence (Hone et al. 2010).

Bone histology of Protoceratops andrewsi (Fostowicz-Frelik & Slowiak 2018) reveals mesothermic physiology, with rapid juvenile growth and progressive deceleration at maturity. Fibrolamellar tissue with abundant vascularization in juveniles is characteristic of relatively elevated metabolism, while adult lamellar bone indicates stabilization. The incubation period calculated by Erickson et al. (2017) of at least 83 days is reptilian-grade, slower than comparably sized birds, suggesting that elevated body temperature was not maintained during embryogenesis or that eggs were incubated by substrate temperature (like turtles). Confirmation of soft-shelled eggs by Norell et al. (2020) indicates they were buried in dune sand for passive incubation, similar to modern crocodilians.

The founding paper of the species, published by paleontologists Walter Granger and William King Gregory following the AMNH 3rd Central Asiatic Expedition to the Flaming Cliffs (Bayan Dzak). The holotype AMNH 6466, collected on September 2, 1922 by photographer J.B. Shackelford, is a nearly complete skull. Granger and Gregory characterize a primitive ceratopsian with horned beak, developed parieto-squamosal frill, and absence of true orbital and nasal horns, distinguishing it from derived ceratopsids known from North America. The name honors Roy Chapman Andrews, leader of the AMNH Asian expeditions. This work establishes Protoceratopsidae as a distinct family within Ceratopsia and opens the debate on the Asian origin of horned ceratopsians.

Holotype skull of Protoceratops andrewsi (AMNH 6251) at the American Museum of Natural History, New York, the same institution that funded the Granger and Gregory expeditions in 1922.

Skull of Protoceratops andrewsi specimen AMNH 6466, collected at the Flaming Cliffs in 1922 and described by Granger and Gregory in the founding paper of the species.

Classic osteological monograph by Barnum Brown and Erich Schlaikjer describing P. andrewsi anatomy from multiple AMNH specimens across ontogenetic stages. The authors document dramatic shape changes during growth: the parietal frill starts small and nearly round in juveniles and becomes large and fan-shaped in adults; nasal bones progressively elongate; eye sockets decrease in relative size with age. Brown and Schlaikjer also report a specimen (AMNH 6418) with a hardened layer over the skull interpreted as possible skin impression. This work establishes the first formal growth series for a ceratopsian and remains the primary anatomical reference for the species for decades.

Protoceratops andrewsi specimen AMNH 6418 showing possible skin impressions over the skull, reported by Brown and Schlaikjer (1940). The authenticity of the impression remains debated.

Skeletal comparison between juvenile and adult Protoceratops andrewsi, documenting the profound shape differences through growth studied by Brown and Schlaikjer (1940).

Fastovsky and colleagues describe an extraordinary nest: a circular depression containing fifteen juvenile Protoceratops andrewsi all of similar size and developmental stage, indicating they belong to the same clutch. The nest was collected from the Djadochta Formation at Tugriken Shireh, Mongolia. The hatchlings are not neonates: they have functional dentition and well-ossified bones, suggesting they remained in the nest for weeks or months after hatching. This implies parental care at least during early post-natal development. The work provides the first direct evidence of basal social behavior in ceratopsians. Data on hatchling size and nest geometry suggest the parent was 1.5 to 2 meters long. This is the first formally confirmed Protoceratops nest and places this Asian genus alongside hadrosaurs and oviraptorids as examples of dinosaurian parental care.

Line drawing of Protoceratops andrewsi nest MPC-D 100/530, showing the fifteen articulated juveniles within the circular depression as described by Fastovsky et al. (2011).

Protoceratops andrewsi hatchling at the American Museum of Natural History. Specimens like this demonstrate the advanced developmental state of hatchlings at birth, consistent with Fastovsky et al. (2011).

Farke and colleagues describe a new ceratopsian from the Cloverly Formation (Montana, USA) and insert it into a comprehensive phylogenetic analysis of Neoceratopsia. The result is one of the reference phylogenies for the group, positioning Protoceratops andrewsi as a basal coronosaur near the base of the large-horned ceratopsians. The biogeographic analysis traces multiple dispersal events between Asia and North America, placing the protoceratopsid lineage as ancestral to derived North American forms. The work includes one of the most complete open-access phylogenies of Ceratopsia, with node diagnoses and synapomorphy lists. For Protoceratops andrewsi specifically, it confirms its position in clade Coronosauria, defined as the smallest clade containing Protoceratops andrewsi and Triceratops horridus.

Global distribution map of ceratopsians, showing the biogeographic pattern studied by Farke et al. (2014). Protoceratops andrewsi is one of the key taxa used as calibration in the phylogenetic analysis.

Size comparison of Protoceratopsidae members: Protoceratops andrewsi, P. hellenikorhinus, Bagaceratops, and Breviceratops. Farke et al. (2014) analyzed the phylogenetic relationships of this Asian family.

Maiorino and colleagues apply two-dimensional geometric morphometrics to 29 Protoceratops andrewsi skulls, systematically testing the sexual dimorphism hypothesis that had been suggested by earlier studies based on differences in frill and skull size. Principal component analyses and non-parametric MANOVAs do not recover clear separation between hypothetical males and females in cranial morphospace, in either lateral or dorsal view. The only character with a potentially dimorphic signal is nasal horn height, but even this result is statistically weak. The central conclusion is that male and female P. andrewsi have similar cranial morphologies, which has deep implications for interpreting the frill as a sexually selected signal.

Morphological variation diagram for Protoceratops specimens, showing individual differences in skull shape analyzed by Maiorino et al. (2015) in their geometric morphometrics study.

Diagram of Protoceratops andrewsi skull in lateral view, showing structures measured by geometric morphometrics in studies such as Maiorino et al. (2015).

Hone, Wood, and Knell analyze 37 Protoceratops andrewsi specimens spanning four distinct size classes and demonstrate that the parieto-squamosal frill shows positive allometry during ontogeny: it grows disproportionately larger relative to body size as the animal ages. Jugals also show a relative increase trend. By contrast, other cranial structures do not show this exaggerated allometry. The pattern is consistent with the socio-sexual selection hypothesis: ornaments subject to competitive selection among conspecifics grow faster than the rest of the body. This was the first formal multi-specimen allometric analysis of P. andrewsi and the first direct evidence of selection on the frill in a non-ceratopsid taxon.

Comparison of Protoceratops andrewsi skulls at different ontogenetic stages, demonstrating the disproportionate frill growth documented by Hone, Wood, and Knell (2016).

Size comparison between P. andrewsi and P. hellenikorhinus with human reference figure. Hone et al. (2016) studied frill allometry in 37 P. andrewsi specimens.

Erickson and colleagues use an innovative technique to directly determine dinosaur incubation periods: counting Von Ebner growth lines (incremental daily growth lines formed during tooth mineralization) in embryonic teeth of Protoceratops andrewsi and Hypacrosaurus stebingeri. For P. andrewsi, the mean embryonic tooth replacement period was 30.68 days, and the calculated minimum incubation period was 83.16 days, two to three times slower than bird eggs of comparable size. This reptilian-grade incubation rate is remarkable because it contrasts with the high-level endothermic metabolism inferred for dinosaurs. The authors propose that long incubation periods may have contributed to extinction vulnerability at the end of the Cretaceous.

Protoceratops andrewsi specimen at CosmoCaixa Barcelona. Incubation studies such as Erickson et al. (2017) were possible thanks to the abundance of well-preserved embryos of this species.

Size comparison between Protoceratops andrewsi and a human. The animal's small size is reflected in the small egg size studied by Erickson et al. (2017), approximately 12 cm long.

Fostowicz-Frelik and Slowiak present the first comprehensive bone histology study of Protoceratops andrewsi, analyzing sections of long bones, parietal frill, and ribs from specimens at different ontogenetic stages of the Djadochta Formation. Bone microstructure reveals fibrolamellar bone with Sharpey fibers and longitudinal vascular canals in juveniles, indicating rapid growth. Adults show extensive remodeling and parallel lamellar bone deposition characteristic of slower growth. Growth rings allow age estimation and identification of growth tempo changes. Results are consistent with mesothermic physiology, intermediate between ectothermic reptiles and endothermic birds.

Protoceratops andrewsi skeleton in museum. The histological study by Fostowicz-Frelik and Slowiak (2018) analyzed sections of long bones such as the femur and tibia to determine the species growth rate.

Protoceratopsidae diversity: Bagaceratops (top) and Protoceratops (bottom). The 2018 histological study compared P. andrewsi with other family members to establish phylogenetic growth patterns.

Slowiak, Tereshchenko, and Fostowicz-Frelik describe the appendicular skeleton (limbs) of Protoceratops andrewsi based on a new nearly complete and articulated subadult. Despite decades of study, the post-cranial anatomy of P. andrewsi had never been described in detail: this paper fills this gap with a bone-by-bone description of all fore and hindlimb elements, compared to other ceratopsians. Ontogenetic analysis reveals that juveniles have limb proportions compatible with facultative bipedal locomotion, while adults are obligate quadrupeds. Analysis of forelimb morphology and range of motion confirms semi-erect posture, not sprawling.

Skeletal reconstruction of adult Protoceratops andrewsi by Scott Hartman, showing the prominent parieto-squamosal frill and quadrupedal posture confirmed by Slowiak et al. (2019).

Historic Protoceratops andrewsi specimen from the AMNH (6417). Slowiak et al. (2019) used articulated specimens as reference to describe for the first time the limb anatomy of this species in detail.

Knapp, Knell, and Hone bring Protoceratops frill study to a new methodological level: 3D landmark-based geometric morphometrics using digitized skull surfaces. They test three predictions of the socio-sexual signaling hypothesis: (1) low morphological integration of the frill with the rest of the skull; (2) significantly higher ontogenetic shape change rate in the frill than other cranial modules; (3) higher morphological variance in the frill. All three predictions are supported by the data, providing strong evidence that the frill was under selection distinct from the rest of the skull. Notably, sexual dimorphism in cranial shape is not detected, suggesting signaling may have been for mutual mate choice or hierarchical social signals, not just male-to-female.

Protoceratops andrewsi specimen MPC-D 100/519, representative of skulls analyzed by Knapp, Knell, and Hone (2021) with 3D geometric morphometrics to test the socio-sexual signaling hypothesis of the frill.

Protoceratops fossil from the Bayan Mandahu Formation (China). Knapp et al. (2021) demonstrated that the frill shows disproportionate morphological variance relative to the rest of the skull, consistent with a social signaling structure.

Norell and colleagues (including Jasmina Wiemann and Matteo Fabbri from Yale) apply in situ Raman spectroscopy and petrographic microscopy to embryo-bearing eggs of Protoceratops andrewsi and Mussaurus patagonicus. Results reveal chemically preserved proteinaceous eggshell membrane residues typical of soft-shelled eggs. This is the first evidence that ceratopsians laid soft-shelled eggs, explaining why ceratopsian nests were so rare in the fossil record: soft shells decompose easily. The discovery has deep evolutionary implications: hard calcified eggs were not ancestral in dinosaurs but derived, having evolved independently at least three times.

Block of four juvenile Protoceratops andrewsi specimens (MPC-D 100/526 A-D), buried together in the Djadochta Formation. Nests and groups like this provide paleoecological context for Norell et al. (2020)'s discovery of soft-shelled eggs in the species.

Protoceratops andrewsi specimen at Tugriken Shireh, Gobi, discovered in 1971. The Djadochta Formation at this locality provided the embryo-bearing eggs chemically studied by Norell et al. (2020).

Hone and colleagues report a set of Protoceratops bones from the Bayan Mandahu Formation (Inner Mongolia, China) with dromaeosaurid tooth marks, accompanied by shed teeth likely belonging to Velociraptor. At least eight bone fragments show unambiguous feeding marks: surface grooves, deeper punctures, and one fragment with marks on both sides. This material is independent of the famous Fighting Dinosaurs specimen (MPC-D 100/512) and was preserved in the same Formation but at a different locality. The marks are interpreted as late-stage carcass consumption, whether through predation followed by prolonged feeding or scavenging. The work geographically expands the evidence of the Velociraptor-Protoceratops predatory relationship.

Replica of the Fighting Dinosaurs (MPC-D 100/512 and 100/25), showing Protoceratops andrewsi with its jaws on Velociraptor mongoliensis's forearm. Hone et al. (2010) found additional evidence of this predatory relationship in the Bayan Mandahu Formation.

Caudal vertebrae and tail spines of Protoceratops andrewsi. Hone et al. (2010) documented Velociraptor tooth marks on bones of this species, confirming the predatory interaction between the two animals.

Tereshchenko examines cervical vertebrae from multiple Protoceratops andrewsi specimens and identifies at least four distinct morphotypes based on shape differences: vertebral body proportions, neural arch pedicle inclination, diapophysis orientation, and long axis inclination. The available sample contains representatives of four morphotypes, whose origin (sexual, ontogenetic, or individual variation) is still debated. The work contributes to understanding intraspecific variation in P. andrewsi and raises questions about the genus taxonomy: are some morphotypes individual variants or representatives of distinct taxa?

Right hindlimb of Protoceratops andrewsi. Tereshchenko (2018) analyzed morphological variation in cervical vertebrae of multiple specimens like this one, identifying at least four distinct morphotypes in the species.

Sternal plates of Protoceratops andrewsi, described in detail by Slowiak et al. (2019) and related to the intraspecific polymorphism documented by Tereshchenko (2018) in cervical vertebrae.

Jerzykiewicz and colleagues conduct comprehensive sedimentological and stratigraphic analysis of the Djadochta Formation and its correlatives in Inner Mongolia (China), comparing them with the type locality at Bayan Dzak. Five lithological facies are described: aeolian dune deposits with cross-stratified sandstones (E-1), aeolian dunes modified by calcitic cementation (E-2), sand avalanche deposits (S), basin-margin conglomerates (C), and interdune mudstones from ephemeral lakes and ponds (M). The paleoenvironment is a warm semi-arid desert with large sand dunes, strong wind conditions, and episodic water availability. Protoceratops andrewsi is the most abundant macrovertebrate in both localities, and the authors discuss how its abundance in such an arid environment implies sufficient shrub vegetation to sustain populations.

Scapula of Protoceratops andrewsi subadult specimen ZPAL MgD-II/3 from the Djadochta Formation. Jerzykiewicz et al. (1993) described rapid burial conditions by sand avalanches that allow the preservation of articulated specimens like this one.

Foot morphology of Protoceratops andrewsi at different ontogenetic stages, from hatchling to adult. The robust feet were adapted for locomotion in the sands of the Djadochta Formation, whose desert paleoenvironment was analyzed by Jerzykiewicz et al. (1993).

Slowiak and Fostowicz-Frelik describe the axial skeleton of a subadult Protoceratops andrewsi from the Djadokhta Formation, complementing the 2019 appendicular skeleton work. The study provides detailed description of cervical, dorsal, sacral, and caudal vertebrae, filling a significant gap in knowledge of the post-cranial anatomy of this species. Cervical vertebrae show diagnostic characters distinct from other basal ceratopsians, including specific centrum proportions and transverse process orientation. Subadult material allows ontogenetic comparisons revealing how the vertebral column changes shape during growth.

Hands of Protoceratops andrewsi, part of the appendicular skeleton described by Slowiak et al. (2019) and whose knowledge was complemented by the axial skeleton study published by Slowiak and Fostowicz-Frelik in 2022.

Right femur of subadult Protoceratops andrewsi specimen ZPAL MgD-II/3, specimen used in Slowiak and colleagues' post-cranial anatomy studies, including the 2022 work on the axial skeleton.

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