Yizhousaurus sunae

I originally had something else in mind for today, but it was heavily photo-dependent, and the photos weren't coming out very well (unresolved lighting and depth-of-field issues). I will have to try again some other time. Meanwhile, here comes Yizhousaurus sunae to the rescue! I'm not quite sure why "prosauropods" should have featured so frequently here (see also Bagualosaurus, Meroktenos, Xingxiulong, and two visits with Anchisaurus), but there you go.

Figure 3 from Zhang et al. (2018) (check the link for the lengthy caption). Note that the quadrate and associated bones are displaced from the back of the skull, which obscures the complete shape of the profile.

Genus and species: Yizhousaurus sunae; "Yizhou" refers to the Chuxiong Yi Autonomous Prefecture of Yunnan Province in China, "saurus" is of course "lizard", and "sunae" honors Professor Ai-Ling Sun. This gives us, roughly, "Professor Sun's lizard from Chuxiong Yi Autonomous Prefecture".
Citation: Zhang, Q.-N., H.-L. You, T. Wang, and S. Chatterjee. 2018. A new sauropodiform dinosaur with a "sauropodan" skull from the Lower Jurassic Lufeng Formation of Yunnan Province, China. Scientific Reports 8, article 13464. doi:10.1038/s41598-018-31874-9.
Stratigraphy and geography: uppermost Zhangjiaao Member of the ever-popular Lower Jurassic Lufeng Formation (a.k.a. the "dark red beds", thought to be of Sinemurian age), "near Duwafang Village, Chuanjie Town, Lufeng County, Chuxiong Yi Autonomous Prefecture, Yunnan Province, China" (Zhang et al. 2018).
Holotype: LFGT-ZLJ0033 [Bureau of Land and Resources of Lufeng County, Lufeng], a partial articulated skeleton including a skull and mandible, 9 cervical vertebrae, 14 dorsals, 3 sacrals, 5 anterior caudals, the shoulder and pelvic girdles, the forelimbs except for the wrist bones, and the femora. The whole animal is estimated as 7 m (23 ft) long.

Yizhousaurus sunae is an example of a name that showed up in preliminary form several years before it was was formally published. In this case, it was first used in a Geological Society of America abstract back in 2010 (Chatterjee et al. 2010). At the time, there was some additional publicity, including a GSA press release which is no longer available (unless you know how to use Internet Archive, or you query your search engine of choice using "Found: First complete remains of early sauropod dinosaur" to find other copies) and a video featuring Sankar Chatterjee discussing the teeth and cranial bones. At the time, Y. sunae was being referred to as a very early sauropod; almost eight years later, it has become a very derived not-quite-sauropod. This speaks to the inevitable difficulties of picking out the beginning of a group, and to the historical circumstances: "prosauropods" were particularly unstable in the early 2000s, and the membership of Sauropoda at some times included such names as Anchisaurus. Probably Y. sunae didn't so much change as did our understanding of the "prosauropod"–sauropod transition.

In basic form, Y. sunae looks the part of a large "prosauropod". The arms are still relatively short, especially the forearm bones. The ilium has the classic "prosauropod" look, being low with a short anterior (preacetabular, the acetabulum being the socket for the femur) process and a longer postacetabular process. The femur has a bit of a curve to it, which is also more "prosauropod" than sauropod. The hand is broad, short, and robust, suggesting (to me, at least) that it was not particularly well-adapted for grasping, nor did it have the weight-bearing adaptations seen in the hands of true sauropods. There is a pathology in the tail: the fourth and fifth caudals are fused (Zhang et al. 2018).

The skeleton of Yizhousaurus sunae; Figure 2 in Zhang et al. (2018). The full caption is: "Status of preservation of Yizhousaurus sunae gen. et sp. nov. (A) The reconstruction in sketch of Yizhousaurus in left lateral view (drawn by Xiao-Cong Guo), regions in red rim represent absent elements; (B), the original burial map of Yizhousaurus (drawn by Q.-N.Z)."

The head is where things get unusual. The typical "prosauropod" skull, as in Plateosaurus or Massospondylus, is relatively long, low, and narrow. The Y. sunae skull, as shown above, is short, deep, and broad. This is more like basal sauropod skulls; about the only "prosauropod" it shows much of a resemblance to is the restored skull of super-obscure Yimenosaurus youngi. It's not quite at a sauropod configuration yet, as shown by the sizes and shapes of the various openings, but it seems to be on that line. One un-sauropod-like feature is the anterior end, which comes to a blunt point; in sauropods, the bitey end of the skull is usually broadly rounded to squared-off (as with Antarctosaurus wichmannianus).

Y. sunae marks "yet another Lufeng sauropodomorph", with the profusion of Lufeng sauropodomorphs already mentioned in the Xingxiulong post. Unlike in the case of Xingxiulong, Y. sunae does not come out in phylogenetic analyses as closely related to any other well-known Lufeng sauropodomorphs (Jingshanosaurus, Lufengosaurus, Xingxiulong, Yunnanosaurus), and it appears to reasonably distinguishable from them. Zhang et al. (2018) classified it as a sauropodiform, and identified it as the most derived of the Lufeng sauropodomorphs, but did raise the point that its sauropod-like features may have been evolved independently from the sauropod line, which would make it a side branch.

References

Chatterjee, S., T. Wang, S. G. Pan, Z. Dong, X. C. Wu, and P. Upchurch. 2010. A complete skeleton of a basal sauropod dinosaur from the Early Jurassic of China and the origin of Sauropoda. Geological Society of America – Abstracts with Programs 42(5):26.

Zhang, Q.-N., H.-L. You, T. Wang, and S. Chatterjee. 2018. A new sauropodiform dinosaur with a "sauropodan" skull from the Lower Jurassic Lufeng Formation of Yunnan Province, China. Scientific Reports 8, article 13464. doi:10.1038/s41598-018-31874-9.