Monday 1 July 2013

A juvenile Coelurosaur from the Middle Jurassic of Xinjiang Province, China.

The Coelurosaurs were a diverse group of Therapod Dinosaurs, including the Tyrannosaurs, Compsognathids, Ornithomimids and Maniraptors (the group that includes the modern Birds). They are thought to have appeared and diversified in the Mid-to-Late Jurassic, though fossils from this early are rare, and usually fragmentary, often comprising only isolated teeth.

In a paper published in the Journal of Systematic Palaeontology on 3 May 2013, a team of scientists led by Jonah Choiniere of the Division of Paleontology and Richard Gilder Graduate School at the American Museum of Natural History and the Department of Biological Sciences at George Washington University, describe juvenile Coelurosaur, at most a year old, from the latest Middle Jurassic (i.e. about 161.5 million years old) of northern Xinjiang Province, China.

The new specimen is named Aorun zhaoi, after Ao Run, the Dragon King of the West in the Chinese epic Journey to the West, which featured the pilgrimage of the Buddhist monk Xuanzang (sometimes known as Tripitaka) and his companions, and Zhao Xi-jin, a distinguished Chinese palaeontologist. Aorun zhaoi is described from a partial skeleton comprising most of a skull, several vertebrae, the left ulna (one of the bones in the forearm) and manus (hand),and part of the hip and legs.

The skull of Aorun zhaoi. (A) Right lateral view; (B) line drawing of right lateral view; (C) left lateral view; (D) line drawing of left lateral view. Abbreviations: a, articular; aof, antorbital fenestra; aofo, antorbital fossa; dg, dentary groove; emf, external mandibular fenestra; en, external naris; hy, hyoid; itf, infratemporal fenestra; jg, jugal; lac, lacrimal; lang, left angular; ld, left dentary; lfp, left frontal and parietal; lmr, lateral maxillary ridge; lpa, left prearticular; lpal, left palatine; lsa, left surangular; lspl, left splenial; max, maxilla; mf, maxillary fenestra; mhf, mylohyoid foramen; na, nasal; o, orbit; pf, prefrontal; pmx, premaxilla; po, postorbital; prq, pterygoid ramus of the quadrate; q, quadrate; qj, quadratojugal; qjc, quadratojugal contact; rang, right angular; rd, right dentary; rls, right laterosphenoid; rpal, right palatine; rpt, right pterygoid; rq, right quadrate; rsa, right surangular; saf, surangular foramen; scl, sclerotic ring; sq, squamosal; v, vomer. Scale bar is 1 cm. Choiniere et al. (2013).

Overview of partially prepared block containing Aorun zhaoi. The skull was removed prior to this photograph and its approximate position is indicated by the dashed black line with white fill. The dashed line with arrows indicates the approximate position of the histological section taken for analysis. (A) Photograph of block; (B) line drawing. Abbreviations: sk, skull; rfib, right fibula; lcarp, left carpus; lmc, leftmetacarpus; ltib, left tibia; rpes, right pes; rtib, right tibia; lman, left manus; pub, pubis; rmt, right metatarsus; lpes, left pes; lmt, left metatarsus; ldt, left distal tarsals. Scale bar equals 2 cm. Choiniere et al. (2013).

Describing species from juvenile specimens is problematic, and it is usually considered preferable to use mature specimens to avoid giving different names to different growth stages of the same animal. On this occasion Choiniere et al. have concluded that the available material is distinctive enough to exclude the possibility of it being the juvenile of a previously described species of Coelurosaur, of which there are a limited supply in this time period. 

The attempt was also made to assign systematic position to Aorun zhaoi. A cladistic analysis suggests a position after the Tyrannosaurs but before the divergence of the other groups, but Choiniere et al. caution this may be a product of the young age of the specimen, and that features that would lead to it being placed within a more developed group might have developed later in its development.

Strict consensus tree showing phylogenetic position of Aorun zhaoi. Choiniere et al. (2013).

Map showing the location where Aorun zhoi was found. Choiniere et al. (2013).

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