Hexinlusaurus, Yinlong and Stenopelix

Earlier we looked at the skull of the basal ornithischian, Hexinlusaurus. Today we’ll look at the post-crania. A reconstruction (Fig. 1) was created by simply putting the original drawings (He and Cai 1983) together to the same scale.

The post-crania of Hexinlusaurus reveals it to be a small-skull taxon with long running legs.

Figure 1. The post-crania of Hexinlusaurus reveals it to be a small-skull taxon with long running legs. A likely biped, the long neck permitted the skull to reach the substrate for water while maintaining a bipedal configuration, but just as easily the forelimbs could have steadied this dinosaur. The wide caudal transverse processes are also found in pachycephalosaurs.

Hexinlusaurus multidens ZDM T6001 (He and Cai 1983, Barrett, Butler and Knoll 2005) middle Jurassic is a long-legged ornithischian with a rather long neck and small skull. Ironically, this is hardly what one would expect at the base of the short-necked, large skull ceratopsians like Yinlong (Figs. 2,3), yet that is where the large reptile tree (dinosaur focus) nests these two. Hexinlusaurus is primitive enough to also be basal to pachycephalosaurs, the thick-headed dinosaurs, long known to share a common ancestor with ceratopsians, and to Ornithopoda, the iguanadontid and duckbill dinosaurs represented here by Dryosaurus.

Figure 2. The skull of Yinlong a basal certatopsian. The marked concavity in the postorbital of Hexinlusaurus is accented in Yinlong.

Figure 2. The skull of Yinlong a basal certatopsian. The marked concavity in the postorbital of Hexinlusaurus is accented in Yinlong.

The marked concavity noted in the postorbital of Hexinlusaurus is accented in Yinlong. The temporal region in Yinlong is larger and longer, probably to house larger jaw muscles working on tougher plant materials. We can suppose that Hexinlusaurus had large premaxillary fangs due to phylogenetic bracketing.

Figure 3. Yinlong overall. This basal ceratopsian had a larger skull, shorter neck and shorter tail than Hexinlusaurus, its phylogenetic predecessor.

Figure 3. Yinlong overall. This basal ceratopsian had a larger skull, shorter neck and shorter tail than Hexinlusaurus, its phylogenetic predecessor. The marked concavity in the postorbital of Hexinlusaurus is accented in Yinlong.

The Stenopelix connection
A skull-less fossil sharing several traits with Yinlong was described by Meyer (1857) and named Stenopelix (Fig. 4). Yes, they do look quite similar, don’t they?

Figur 4. Known since 1857, Stenopelix appears to be a sister to Yinlong. Ischia are color coded here.

Figur 4. Known since 1857, Stenopelix (Meyer 1857) appears to be a sister to Yinlong. Ischia are color coded here. Click to enlarge.

References
Barrett PM, Butler RJ and Knoll F 2005. Small-bodied ornithischian dinosaurs from the Middle Jurassic of Sichuan, China. Journal of Vertebrate Paleontology 25: 823-834.
He X-L and Cai K-J 1983. A new species of Yandusaurus (hypsilophodont dinosaur) from the Middle Jurassic of Dashanpu, Zigong, Sichuan. Journal of Chengdu College of Geology, Supplement 1:5-14.
Meyer H von 1857. Beiträge zur näheren Kenntis fossiler Reptilien. Neues Jahrbuch für Mineralogie, Geologie und Paläontologie 1857: 532–543.
Xu X, Forster CA, Clark J M and Mo J 2006. A basal ceratopsian with transitional features from the Late Jurassic of northwestern China. Proceedings of the Royal Society B: Biological Sciences. First Cite Early Online Publishing. online pdf

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