Xuanhanosaurus qilixiaensis Dong, 1984

Rauhut, Oliver W. M., 2003, The interrelationships and evolution of basal theropod dinosaurs, Special papers in palaeontology 69, pp. 1-213 : 22

Xuanhanosaurus qilixiaensis Dong, 1984

Age . Bathonian-Callovian.

Occurrence. Xiashaximiao Formation, Sichuan, China.

Diagnosis. Glenoid articular facet of humerus forms a raised horizontal ridge that overhangs the humeral shaft posteriorly.

Remarks. Xuanhanosaurus is based on fragmentary material from the Middle Jurassic of the Xiashaxiamo Formation of China ( Dong 1984). However, the specimen is remarkable since it includes one of the best preserved forelimbs for any Middle Jurassic theropod. Despite the fragmentary nature of the holotype, it seems to be clearly different from all other Middle Jurassic theropods with the possible exception of " Szechuanosaurus " zigongensis. Apart from the possible autapomorphy given in the diagnosis, the most striking character of Xuanhanosaurus is the very robust humerus which has strongly expanded proximal and distal ends and is reminiscent of the humeri in Torvosaurus (Galton and Jensen 1979) and baryonychids (MNN GDF 500, BMNH R 9951; Charig and Milner 1997). A further similarity to these taxa is the very strongly keeled anterior dorsal vertebrae.

Some comments on the anatomy of the animal might be added. The element identified as a sternum by Dong (1984, fig. 2) is a part of the right coracoid that is only preserved as an impression in the sediment. Thus the coracoid is much larger than figured by Dong and it is high oval in shape, with a moderate, rounded ventral process anteriorly.

In contrast to a statement by Molnar (1990, p. 317), carpal and phalangeal joints of the manus do not differ significantly from those of other theropods. An enlarged distal carpal overlaps the proximal ends of metacarpal I and parts of metacarpal II, and although this carpal does not show a semilunate morphology, its shape and development is comparable with other basal tetanurans like Afrovenator . In particular the distal articular end of metacarpal II indicates that a great degree of extension and flexion was possible at the basis of the second digit. Whereas metacarpal I is closely appressed to the basal half of metacarpal II, phalanx 1-1 is directed slightly medially, indicating that the first digit was somewhat opposable, as in many other theropods (e.g. Galton 1971). Unfortunately, the phalanges and metacarpals are in articulation and the manus is still contained within a slab of matrix, so that no detailed analysis of the morphology can be carried out. The ungual of the first digit is unusual for theropod manual unguals in being rather broad and not very strongly curved. However, despite the robustness of the forelimb, it seems rather unlikely that it was used in locomotion as argued by Dong (1984), since the humerus is rather short when compared to the size of the vertebrae.