Shunosaurus

Chatterjee, Sankar & Zheng, Zhong, 2002, Cranial anatomy of Shunosaurus, a basal sauropod dinosaur from the Middle Jurassic of China, Zoological Journal of the Linnean Society 136 (1), pp. 145-169 : 165-166

AFFINITY OF SHUNOSAURUS

Ever since Seeley (1887) established the Order Saurischia on the basis of pelvic structure, nearly all authors have assumed that Sauropoda was a welldefined, coherent group ( Huene, 1927, 1932; Romer, 1956). Currently, there are about 90 genera of sauropods known, but most taxa are based on fragmentary postcranial material and are not diagnostic at lower taxonomic levels. So far, cranial material is limited to a few genera. Recent cladistic analyses ( Gauthier, 1986; Russell & Zheng, 1993; Upchurch, 1995, 1998, 1999; Zheng, 1996; Salgado, Coria & Calvo, 1997; Wilson & Sereno, 1998) confirm that the Sauropoda is a monophyletic clade and shed new insights on the interrelationships of several genera. However, there is a great deal of disagreement about the placement of the Chinese sauropod taxa. Upchurch (1998) grouped Shunosaurus with Omeisaurus , Mamenchisaurus and Euhelopus into a basal clade, Euhelopodidae . Wilson & Sereno (1998), on the other hand, put Euhelopus as a highly derived member, a sister taxon of Titanosauria. Unfortunately, Sauropoda cannot be defined cladistically on the basis of cranial characters ( Wilson & Sereno, 1998; Upchurch, 1998). The inclusion of Shunosaurus is based entirely on postcranial characters, because the cranial anatomy of basal members of sauropods such as Vulcanodon ( Raath, 1972; Cooper, 1984) and Barapasaurus ( Jain et al., 1975) is unknown. As Shunosaurus is the earliest sauropod for which cranial remains are known, it provides the critical morphology of the basal sauropods. It also provides insights about the evolution of the sauropod skull from the prosauropod condition. Using prosauropods as a sister clade to Sauropoda ( Galton, 1990) , Shunosaurus shares the following synapomorphies with Eusauropoda Upchurch 1995:

1. Premaxilla, stepped rostral margin ( Wilson & Sereno, 1998).

2. Maxillary border of external naris long ( Gauthier, 1986).

3. External naris retracted upward and backward ( McIntosh, 1990b).

4. Antorbital fenestra smaller than external naris.

5. Antorbital fossa absent ( Wilson & Sereno, 1998). 6. Quadratojugal, rostral process is expanded dorsoventrally at its tip and is longer than the dorsal process ( Gauthier, 1986).

7. Quadrate, caudal fossa present ( Wilson & Sereno, 1998).

8. External mandibular fenestra highly reduced ( Russell & Zheng, 1993).

9. dentary, rostral end increases in depth towards symphysis ( Upchurch, 1998).

10. Tooth crown lacks ridges.

11. Tooth enamel with wrinkled texture ( Wilson & Sereno, 1998).

12. Tooth row ends below the antorbital fenestra ( Gauthier, 1986).

13. Precise crown to crown occlusion ( Wilson & Sereno, 1998).

14. Rostral end of snout is broadly rounded in dorsal view ( Upchurch, 1998).

15. Ventral border of external naris lies higher than that of orbit ( McIntosh, 1990b).

16. Lacrimal head slopes backward relative to its base.

17. Jugal length reduced so that antorbital and lateral temporal fenestra are crowded and approach each other ( Upchurch, 1995).

18. Quadratojugal, ascending process fails to reach the descending process of the squamosal ( Gauthier, 1986).

19. Frontal excluded from supratemporal fossa ( Wilson & Sereno, 1998).

20. Supratemporal fenestra, long axis orientated transversely ( Wilson & Sereno, 1998).

21. Supraoccipital platform prominent.

22. Premaxillary dental count 4.

23. Maxillary teeth less than 20 ( Gauthier, 1986).

24. Tooth row terminates rostral to antorbital fenestra ( Gauthier, 1986).

25. Teeth are procumbent, sloping forward ( Gauthier, 1986).

26. Constriction between tooth crown and root.

27. Palatine, lateral ramus narrow ( Wilson & Sereno, 1998).