Three-dimensionally preserved Dinosaur remains from a new Jehol Biota outcrop in Inner Mongolia.

The Jehol Biota produces a wide range of exquisitely preserved Early Cretaceous fossils from locations across Liaoning, Inner Mongolia, and Hebei provinces in northeastern China. However, in almost all cases these are two-dimensionally preserved (i.e. flattened), with only a single site, an outcrop of the the Lujiatun Unit of the Yixian Formation in western Liaoning, producing fossils in three dimensions. The fossils from this location are predominantly Dinosaurs, with some Lizards and Mammals, and show signs of representing an area with an area with a different faunal composition and ecology to the rest of the Jehol Biota, as well as having a different taphonomy (mode of preservation).

In a paper published in the journal Acta Palaeontologica Polonica on 20 June 2022, Honggang Zhang of the College of Earth Science and Engineering, and Paleontological Institute at Shandong University of Science & Technology, Dongxiang Yu, also of the College of Earth Science and Engineering at Shandong University of Science & Technology, Yuhui Feng, also of the Paleontological Institute at Shandong University of Science & Technology, Rui Pei of the Key Laboratory of Evolutionary Systematics of Vertebrates at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, and Chang-Fu Zhou, again of the College of Earth Science and Engineering at Shandong University of Science & Technology, describe a second outcrop producing three-dimensionally preserved fossils from the Jehol Biota, this time located in the Ningcheng Basin of Inner Mongolia.

The new outcrop, at a site called Xidayingzi in Ningcheng County, comprises an outcrop of interbedded volcanic and sedimentary rocks about 80 m thick. The volcanic rocks here are predominantly basaltic andesites, rhyolitic breccia lavas and andesitic tuffs, while the sedimentary units are gravel-rich siltstones, sandstones, and conglomerates, all containing volcanically derived material.

Map of the Inner Mongolia, Liaoning and Hebei (A) showing location of the Xidayingzi site and the outcrops of the Lujiatun Unit of the Yixian Formation of the Jehol Biota in Beipiao, western Liaoning Province. Stratigraphic column (B) and corresponding photographic image (C) of the Early Cretaceous Xidayingzi site. Zhang et al. (2022).

Initial investigations at this site uncovered the remains of Dinosaurs from four different groups, plus Lizard and Mammal remains, all preserved in three dimensions and many in a semi-articulated state, although most material is individual bones.

Fossils found in the Xidayingzi site from the Early Cretaceous Ningcheng Basin, Inner Mongolia. (A) Sinovenator-like Troodontid Dinosaur (SDUST-V1062), the left pes is exposed in medial view. (B) Ceratopsian Dinosaur Psittacosaurus sp. (PMOL-AD00163), the maxilary teeth are loosely arranged and exposed in lateral view. (C) Neornithischian Dinosaur Jeholosaurus sp. (SDUST-V1063), the scapulocoracoid, humerus, ulna, and radius in lateral view. (D) Euhelopus-like Sauropod (SDUST-V1064), digital image of the tooth crown in medial view. (E) Indeterminated Lizard (PMOLAR00268), digital image of the fragmentary mandible in lingual view. (F) Symmetrodont-like Mammal (PMOL-AM00036), digital image of the mandible in lingual view. Zhang et al. (2022).

The most notable specimen from the site (SDUST-V1062) is the articulated post-cranial skeleton of a Troodontid Dinosaur, possibly a specimen of Sinovenator; the specimen resembles Sinovenator changii in most regards, but differs in having having a round anterior margin of the preacetabular process of the ilium and a moderately developed ambiens process of the pubis, and may therefore represent a second species in the genus.

Three Sauropod teeth were found at the site, two of them well preserved and similar to those of Euhelopus, a Titanosauriform of similar age to the Jehol Biota, but not previously reported from there.

Eight Neornithischian Dinosaur fossils were found at the site, with four being semi-articulated partial postcranial skeletons. These all resemble  Jeholosaurus shangyuanensis, a small Neornithischian previously only known from the Lujiatun Unit of the Jehol Biota.

Six specimens of Ceratopsian Dinosaurs were found, although these were rather fragmentary in nature. All appear similar to the early-diverging Psittacosaurus, often used as an index fossil for the Lower Cretaceous in East Asia, and common in the Jehol Biota. Curiously, the maxillary teeth of the new specimens appear to be spaced apart rather than being imbricated as in all other known Psittacosaurs, which may again indicate they belong to a new species.

A single fragment of a Lizard mandible (PMOL-AR00268) was found. This has pleurodont, conical, single-cusped, and closely packed teeth, which, are features common in a range of Mesozoic Lizards, and not helpful diagnostically.

Mammals are also represented by a single mandible (PMOL-AM00036), which has one premolar, five molars, and eight empty alveoli; the molars have acutely-triangulated molar cusps, which allows the specimen to be referred to the Symetrodonts, although this is a polyphyletic assemblage of Mammals, made up of members of different groups which converged on a similar morphology, possibly as a response to some Mesozoic ecological condition.

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