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Chaoyangsaurus
Chaoyangsaurus
Chaoyangsaurus
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Chaoyangsaurus
Temporal range: Late Jurassic,
148 Ma
↓
Skeletal restoration showing known material
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Dinosauria
Clade: †Ornithischia
Clade: †Ceratopsia
Family: †Chaoyangsauridae
Genus: †Chaoyangsaurus
Zhao, Cheng, & Xu, 1999
Species:
†C. youngi
Binomial name
†Chaoyangsaurus youngi
Zhao, Cheng, & Xu, 1999

Chaoyangsaurus ("Chaoyang lizard") is a genus of marginocephalian dinosaur from the Late Jurassic (about between 150.8 and 145.5 million years ago) of China.[1] Chaoyangsaurus belonged to the Ceratopsia (Greek for "horned faces"). Chaoyangsaurus, like all ceratopsians, was primarily a herbivore.[2][3]

Discovery and naming

[edit]
Life restoration

In 1976, the remains of Chaoyangsaurus were found by Cheng Zhengwu at Ershijiazi, in the Chaoyang area of Liaoning Province in northeastern China. The fossil was added to a travelling exhibition.

Unlike many other dinosaurs, Chaoyangsaurus had been discussed in a number of sources before its official publication. As a result of this, several different spellings of its name have come and gone as invalid nomina nuda ("naked names", names with no formal description behind them). The first name to see print was "Chaoyoungosaurus", which appeared in the guidebook to a Japanese museum exhibit, and was the result of an incorrect transliteration from the Chinese into the Latin alphabet. Zhao Xijin in 1983 also used this spelling when he first discussed the species,[4] again lacking a description so it is technically a nomen nudum. Two years later, Zhao again used this early spelling when he assigned a type specimen and species name, "Chaoyoungosaurus liaosiensis".[5]

According to Dong Zhiming in 1992, the name Chaoyoungosaurus had been officially described in a separate paper by Zhao and Cheng in 1983, but no cite for this paper was given, and later Cheng and Zhao themselves did not treat this name as valid, perhaps because the paper had not actually been published. Dong, in his 1992 book on Chinese dinosaur faunae, also emended the name to the "correct" spelling of "Chaoyangosaurus" (note the extra letter "o").[6] However, since this renaming was not accompanied by a formal description of the dinosaur, "Chaoyangosaurus" must also be considered a nomen nudum.

It was not until 1999 that the dinosaur finally received an official name. Paul Sereno in 1999 used the name "Chaoyangsaurus" in an overview of dinosaurian evolution.[7] Once again, that name was a nomen nudum. However, in December of that year, Cheng, Zhao, and Xu Xing published an official description using the name Chaoyangsaurus youngi, and as the first name for this genus that is not a nomen nudum, it has official priority over all other spellings that have been used. The generic name refers to Chaoyang. The specific name honours the Chinese paleontologist Yang Zhongjian ("C. C. Young") as the founder of Chinese vertebrate paleontology.[8]

The holotype, IGCAGS V371, has been found in a layer of the Tuchengzi Formation that was in 1999 dated to the late Tithonian. It consists of a partial skeleton with skull. It contains the lower part and braincase of the skull, the lower jaws, seven neck vertebrae, the right shoulder blade and the right humerus. It represents an adult individual. The fossil was prepared by Ding Jinzhao and Wang Haijun.[8]

Description

[edit]

In 2010, Gregory S. Paul estimated the length of Chaoyangsaurus at 1 metre (3.3 ft) and the weight at 6 kilograms (13 lb).[9]

The describing authors indicated some distinguishing traits. The boss on the jugal bone is only weakly developed and has a smooth surface. The convex quadratojugal bone overlaps the rear of the shaft of the quadrate bone. The quadrate lacks a wide side surface, not being expanded to the front. The rear lower edge of the quadrate is convex. The coronoid process of the lower jaw is low with a flat top. The planes of the side surface and the underside of the angular bone are separated by a ridge. The last five traits were considered autapomorphies.[8]

The skull is fourteen centimetres long. Per side, the appending lower cutting edge of the rostral bone, in life probably covered by horn, bears four crenulations. More to the rear, there are two conical premaxillary teeth, jutting out to below. The transversely flattened maxillary teeth number eight or nine, while there are eleven dentary teeth in the lower jaw for a maximal total of forty-four teeth for the head as a whole. The cheek teeth are imbricated and vertically rather long. Both inner and outer sides are covered by enamel.[8]

Phylogeny

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In 1999, Chaoyangosaurus was placed in the Ceratopsia, in a basal position. There were doubts whether it was outside or inside the Neoceratopia.[8]

In 2006, Zhao, et al defined a Chaoyangsauridae, as the most basal ceratopian clade.[10]

See also

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References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Chaoyangsaurus

View on Grokipedia
from Grokipedia
Chaoyangsaurus youngi is a of small, basal ceratopsian that lived during the epoch in what is now Province, northeastern . Known primarily from a single incomplete specimen consisting of a partial and paired lower jaws, it represents one of the earliest diverging members of the ceratopsian group, characterized by primitive features such as a relatively large for its body size, simple dentition, and a keeled predentary . The , housed as IGCAGS V371, measures approximately 14 cm in length from the tip of the to the rear of the quadrate, indicating an overall body length estimated at around 1 meter and a lightweight build suitable for a bipedal or facultatively quadrupedal . Discovered in the Tuchengzi Formation, dated to the stage of the (approximately 150 million years ago), Chaoyangsaurus significantly extends the temporal range of ceratopsians backward from the , highlighting early diversification of the clade in . As a basal neoceratopsian or closely related to the base of , Chaoyangsaurus youngi exhibits transitional anatomical traits, including a quadrate with a convex posterior margin and a ridge on the angular , distinguishing it from more derived ceratopsians like . Its dental system, analyzed through computed , reveals nine functional maxillary teeth per side with ovate crowns bearing primary ridges, alongside 3–5 replacement teeth, suggesting a slow replacement rate and a diet likely consisting of tough plant material processed via low-angled occlusion and possibly gastroliths. Phylogenetically, it forms the basis of the family Chaoyangsauridae, a stem-based group including other early Asian ceratopsians, and underscores the origins of key ceratopsian innovations such as beak-like premaxillary teeth and reduced retroarticular processes in the . The discovery of Chaoyangsaurus in 1999 has been pivotal in reshaping understandings of ceratopsian evolution, proposing close affinities with heterodontosaurids and emphasizing as a cradle for marginocephalian diversity during the .

Discovery and naming

Discovery

The fossils of Chaoyangsaurus were first discovered in 1976 by paleontologist Cheng Zhengwu during fieldwork conducted by the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in the Ershijiazi area near Chaoyang City, Province, northeastern . The discoveries occurred in outcrops of the Tuchengzi Formation, a unit dated to the , approximately 160–145 million years ago. The specimen (IGCAGS V371) consists of the dorsal part of the and a nearly complete of a small individual, with the preserved measuring approximately 14 cm in length.

Naming and etymology

Chaoyangsaurus was formally named and described in 1999 by the Chinese paleontologists Zhao Xijin, Cheng Zhengwu, and Xu Xing, based on a partial and lower jaws collected from the Tuchengzi Formation in the Chaoyang area of Province, , during excavations in the mid-1970s. The is Chaoyangsaurus youngi, with the designated as specimen IGCAGS V371, consisting of the dorsal part of the and nearly complete lower jaws. The genus name Chaoyangsaurus derives from "Chaoyang," the location of discovery in Province, combined with the sauros meaning "lizard" or "reptile." The specific epithet youngi honors the pioneering Chinese Clifford C. Young (Yang Zhongjian), recognized as the founder of in for his extensive contributions to the study of fossils in the region. In the original description, Chaoyangsaurus youngi was classified as a primitive , diagnosed by features such as the quadratojugal overlapping the posterior shaft of the quadrate, absence of a broad lateral surface on the quadrate, a convex posterior margin of the quadrate, and a ridge on the angular bone separating its lateral and ventral surfaces. Subsequent phylogenetic analyses in the early , incorporating additional basal ceratopsian taxa, have upheld the validity of Chaoyangsaurus as a distinct with no proposed synonyms, consistently positioning it near the base of .

Description

Skull

The skull of Chaoyangsaurus youngi measures approximately 14 cm in preserved length, extending from the tip of the snout to the back of the quadrate, representing a small, low-profile cranium typical of basal ceratopsians. The rostrum is long and narrow, formed by the premaxillae and that lack any bossing or ornamentation, contributing to a primitive marginocephalian morphology without the expansive structures seen in later ceratopsians. A small preorbital is present anterior to the , indicating retention of an ancestral ornithischian trait, though reduced in size relative to more basal ornithopods. The posterior region features an incipient frill-like structure, where the jugal and squamosal bones extend laterally and posteriorly, with the squamosal overhanging the quadrate articulation to form a modest shelf. This early frill is narrow and lacks the broad, fenestrated elaboration of derived ceratopsians, and the postorbital and parietal bones show incomplete fusion, consistent with the holotype's but not fully mature state. The braincase is robust yet compact, with the basioccipital and exoccipitals contributing to a short, rounded occipital condyle, while the palate includes a secondary bony formed by the pterygoids and ectopterygoids, supporting a primitive masticatory apparatus. Dentition is characterized by a low tooth count and simple morphology suited to a herbivorous diet. The premaxillae bear two small, conical teeth with ellipsoidal cross-sections and unworn crowns lacking mesiodistal expansion at the base. Cheek teeth in the maxilla are low-crowned and ovate in outline, with leaf-shaped forms featuring a distally positioned primary ridge, a basal ridge spanning over 70% of the crown height, and marginal denticles; lingual surfaces are concave, and roots are elongated and lingually inclined. Dentary teeth are similar in overall shape but lack primary ridges and marginal denticles, with concave labial surfaces. Each dentary side holds 9–11 functional teeth, with up to 5 visible replacements per side showing severe root resorption, indicative of continuous but low-rate tooth replacement in this early ceratopsian. Computed tomographic analysis confirms a slow replacement rate, suggesting a diet of tough plant material. These features, combined with unfused cranial elements in the holotype, underscore juvenile-like traits despite an overall adult body size estimated at around 1 meter in length.

Postcrania

The postcranial skeleton of Chaoyangsaurus youngi is known from fragmentary remains preserved in the specimen (IGCAGS V371), which includes seven , several dorsal vertebrae, a partial , fragments of caudal vertebrae, elements of the and pelvic girdles, and portions of the fore- and hindlimbs. These elements indicate a small-bodied basal ceratopsian with an estimated total body length of about 1 meter and a weight of approximately 6 kg, comparable to the size of a small Psittacosaurus. The features short neural spines on the preserved cervical and dorsal vertebrae, typical of primitive ornithischians, with the cervical series suggesting a flexible for browsing low vegetation. The partial provides a robust base for the pelvic girdle, while the caudal fragments imply a tapering contributing to balance in locomotion. Limb girdle elements include a robust and forming the shoulder, an elongated ilium, and slender pubis and in the , supporting a facultatively quadrupedal or primarily bipedal stance, with pillar-like hindlimbs. The is represented by a robust with a prominent deltopectoral crest, partial and , and metacarpals indicating 4-5 digits in the manus; the includes a straight , , , and elements of the pes with three main digits, suggesting slightly longer hindlimbs than forelimbs for agile movement as a small . Overall proportions reflect primitive ceratopsian morphology, with no specialized adaptations beyond basic bipedality or facultative quadrupedality.

Classification and phylogeny

Taxonomic history

Chaoyangsaurus was initially mentioned by Zhao Xijin in 1983 as Chaoyangosaurus, a nomen nudum, and classified within the newly proposed ornithischian superfamily Chaoyoungosauroidea, emphasizing its role as a transitional form in ornithischian evolution based on preliminary skull material from Liaoning Province, . This placement positioned it among early marginocephalians, distinct from more derived ceratopsians, though the name remained informal without a full . By 1985, Zhao revised the classification, assigning Chaoyangsaurus to Psittacosauridae within , identifying it as a basal ceratopsian characterized by features like a robust jugal arch and low-crowned, trilobate teeth, which suggested affinities with early beak-bearing ornithischians but predated typical psittacosaurid forms. This reassessment marked a shift toward recognizing its ceratopsian heritage, though it remained undescribed formally. In 1992, Dong Zhiming erected the Chaoyangsauridae for the genus, treating it as a to accommodate its primitive traits separate from , thereby distinguishing it as an early, independent lineage within basal . The genus received its formal description in 1999 by Zhao, Cheng, and Xu, who confirmed Chaoyangsaurus youngi as the earliest known and upheld its placement in Chaoyangsauridae, emphasizing differences from such as a shorter frill precursor and unique squamosal morphology that precluded synonymy. Throughout the 1990s and early 2000s, re-evaluations in ceratopsian phylogenies reinforced this basal position within , often as a sister to more advanced neoceratopsians, with no significant synonymy debates emerging due to the resolved . In recent decades, up to the , literature has consistently validated Chaoyangsaurus as a distinct basal ceratopsian, with Chaoyangsauridae recognized as a including other early Asian ceratopsians such as Yinlong and Hualianceratops, underscoring its foundational role in ceratopsian origins.

Phylogenetic position

Chaoyangsaurus is consistently positioned as a basal ceratopsian in cladistic analyses, typically as the sister taxon to all other ceratopsians or within a basal outside Neoceratopsia, based on primitive features such as a relatively small and retention of ornithischian-like cranial proportions. This placement underscores its role as one of the earliest diverging members of , highlighting shared primitive traits with non-ceratopsian ornithischians while exhibiting early ceratopsian synapomorphies like the presence of a rostral bone and a thickened squamosal. A key phylogenetic study by You and Dodson (2004) utilized a character matrix of basal ceratopsians and recovered Chaoyangsaurus as the to all remaining ceratopsians, supporting its basal status within the with moderate resolution in the consensus tree. Subsequent analyses, such as Xu et al. (2006), confirmed its affinity within —grouping it with pachycephalosaurs as the sister to more derived ceratopsians—with bootstrap support ranging from 50% to 70% for the marginocephalian node, though Yinlong was positioned slightly more basally in that dataset. This basal positioning implies that ceratopsians began diversifying in during the , with Chaoyangsaurus representing an early offshoot that retained bipedal locomotion and modest body size before the emergence of more specialized forms in the . While some alternative hypotheses suggest closer relations to other basal taxa like Yinlong within a paraphyletic Chaoyangsauridae, the consensus across recent phylogenies affirms its position as the most basal ceratopsian or part of a stem-grade basal group.

Paleoecology

Geological context

The Tuchengzi Formation represents a key stratigraphic unit in the western Basin of northeastern , where fossils of Chaoyangsaurus were discovered. This formation is characterized by a sequence of clastic sediments, including tuffaceous s, silty s, s, conglomerates, and sandstones, divided into three members that reflect varying depositional conditions. The lower member (Member 1) predominantly features tuffaceous sandstone, conglomerate, and fine-grained silty mudstone to , while the middle member (Member 2) includes gravel conglomerates and lithic sandstones, and the upper member (Member 3) consists of aeolian sandstones and sandstones. Depositional environments within the Tuchengzi Formation indicate a terrestrial setting dominated by alluvial fans, lakeside to shallow lacustrine zones in the lower sections, and aeolian influences higher up, with evidence of traction currents, , ripple lamination, and occasional subaerial exposure features such as desiccation cracks. Fossils of Chaoyangsaurus, recovered from fine-grained sediments in the lower portions, suggest preservation in low-energy environments like lake margins or protected fluvial channels, where sedimentation rates favored the burial of remains. Radiometric dating using zircon U-Pb analysis on tuffaceous layers within the formation yields ages ranging from 153.8 Ma to 137.16 Ma, constraining the Tuchengzi Formation to the (Kimmeridgian-Tithonian) through (Berriasian), with the lower sections—containing Chaoyangsaurus—firmly in the around 154–145 Ma. This age predates the major diversification of ceratopsians in the period, highlighting the formation's role in documenting early ceratopsian evolution. Taphonomic evidence points to disarticulated skeletal elements, attributable to post-mortem transport in fluvial systems, accompanied by limited mineralization due to the clastic, low-carbonate nature of the enclosing sediments.

Associated fauna and environment

Chaoyangsaurus inhabited the Tuchengzi Formation, a fluvio-lacustrine depositional system of inland basins characterized by rivers, shallow lakes, and floodplains influenced by volcanic activity, under a hot, arid paleoclimate with seasonal drought. Plant fossils, including silicified wood of the conifer Xenoxylon latiporosum and horsetails such as Neocalamites sp., indicate a adapted to semi-arid conditions, with no evidence of early angiosperms in this formation. The associated fauna was depauperate in skeletal remains but enriched by ichnofossils, reflecting an ecosystem with low vertebrate diversity transitional between Jurassic and Cretaceous biotas. Known skeletal fossils include a brachiosaurid sauropod forefoot, and the contemporaneous basal ceratopsian Xuanhuaceratops niei from the nearby Houcheng Formation, highlighting limited ceratopsian diversity during the early stages of their radiation. Dinosaur tracksites reveal a more diverse community, dominated by theropod footprints (small Grallator-like forms 5.5–15 cm long and larger Eubrontes-type tracks up to 30 cm), sauropod tracks (Brontopodus and Parabrontopodus types, 11.9–80 cm), and rare possible ornithopod impressions, suggesting co-occurrence with small to medium-sized carnivores and larger herbivores. As a small-bodied basal ceratopsian approximately 1–1.5 m long, Chaoyangsaurus functioned as a low-browser , its leaf-shaped teeth and short neck adapted for cropping low-lying such as ferns and cycads in settings. In this predator-scarce environment, where small theropods represented the primary carnivores based on track evidence, Chaoyangsaurus likely experienced relatively low predation pressure, occupying a basal niche amid sparse and infrequent large herbivores.
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