Sunday, March 12, 2017

Xingxiulong

Two dinosaurs were published on February 16, 2017. One of them was Isaberrysaura mollensis, which has gotten a lot of press because it's a weird basal ornithischian with gut contents. The other was Xingxiulong chengi, which hasn't gotten as much attention, although the Wikipedia article is pretty extensive. Xingxiulong is among what we used to call "prosauropods", now known as basal sauropodomorphs. It is represented by most of the skeleton, excepting the tips of the jaws, most of the hands, and the coracoids and sternal elements. It also provides me a half-point on my prediction for "prosauropods", which I'll take because it's been kind of a slow year so far.

Genus and species: Xingxiulong chengi; "Xingxiu" literally means "constellation", but in this case it refers to the historic Xingxiu Bridge of Lufeng County. "-long", of course, is "dragon". "chengi" honors the late geologist and biostratigrapher Zheng-Wu Cheng.
Citation: Wang, Y.-M., H.-L. You, and T. Wang. 2017. A new basal sauropodiform dinosaur from the Lower Jurassic of Yunnan Province, China. Scientific Reports 73, article number 41881. doi:10.1038/srep41881.
Stratigraphy and geography: basal Shawan Member of the Lufeng Formation (Lower Jurassic) (a.k.a. the Dull Purplish Beds, thought to be of Hettangian age), "Sankeshu (Three Trees) Village, Jinshan Town, Lufeng County, Chuxiong Yi Autonomous Prefecture, Yunnan Province, southwestern China".
Holotype: LFGT-D0002 [Bureau of Land and Resources of Lufeng County, Lufeng], which includes a partial skull and jaws, the atlas and axis vertebrae, three more posterior neck vertebrae, seven back vertebrae, the hip vertebrae, 35 tail vertebrae, the usual bits of ribs and chevrons, most of the left hip, part of the right pubis and ischium, and most of the hind legs. Two additional partial skeletons, LFGT-D0001 and D0003, fill in most of the rest of the body, including the neck, shoulders, and arms.

The skeleton of Xingxiulong, from Figure 2 of Wang et al. (2017) (see for full caption).

If someone was to just walk up to me in the street, slap me in the face with the skeletal restoration, and ask me what it was, I'd probably guess a massospondylid, and to be fair it doesn't look all that different from a typical massospondylid, being a bit chunkier and not as elongate in the neck and tail. Wang et al. classified Xingxiulong as a basal sauropodiform most closely related to Jingshanosaurus xinwaensis, but found it wasn't too hard to place it slightly more derived or basal than this position, or just basal to Massospondylidae+Sauropodiformes in Massopoda. The names require a moment of digression. Figure 5 from the paper will provide assistance:

Figure 5. The colored bits concern the hip vertebrae (sacrals), but for the moment we're mostly interested in the relationships and the labels.

You can see Sauropodiformes as the next label after Massospondylidae. Sauropodiformes is one of several groups that have been designated within the sauropodomorphs since 2000. Massopoda, which is also labeled here, is another of these. Massopoda is the group that includes all sauropodomorphs more derived than Plateosaurus. Sauropodiformes is a bit more complicated. It was originally defined as the node that includes Mussasaurus, the titanosaur Saltasaurus, and everything between, but because Mussaurus was poorly defined at the time, later authors changed the definition to the stem that includes all sauropodomorphs more defined than Massospondylus, which is the definition used in this publication. Of course Mussaurus was restudied and became easier to place, but the definition hasn't migrated back. In fact, in this tree original Sauropodiformes would be the same as another group, Anchisauria, which not surprisingly uses Anchisaurus as an anchor.

Xingxiulong chengi is not screamingly different from sister taxon Jingshanosaurus xinwaensis. Differences mentioned by the authors include (deep breath): some details of the skull, proportions of the neck vertebrae (proportionally longer centra [main bodies] in J), the spines of the back vertebrae being scooped in along the rear margin for X but straight in J, a concave margin to the ventral margin of the postacetabular process of the ilium in X and a convex margin for J, the lesser trochanter of the femur (thigh bone) being lower in X, different shapes to the distal end (ankle end) of the tibia (shin bone), and a sacrum (fused hip vertebrae) of three vertebrae in J but four in X (a tail vertebra has been added). Given the sample sizes we're dealing with (three skeletons for X, one for J), it doesn't seem entirely out of the question to me that these are more along the lines of species differences, with some individual variation, sexual differences, age-related differences, etc. thrown in.

At any rate, the Lufeng Formation is simply crawling with sauropodomorphs of various persuasions. Aside from Xingxiulong and Jingshanosaurus, the authors also brought up Lufengosaurus huenei, L. magnus, Yunnanosaurus huangi, Y. robustus, "Gyposaurus" sinensis, Chuxiongosaurus lufengensis, and sauropod material (apparently Kunmingosaurus wudingensis, which is from a higher part of the formation), although they note that "Gyposaurus" is not universally accepted and Chuxiongosaurus could be Jingshanosaurus. This also omits four nonavian dinosaurs in the lowest tier of obscurity: Fulengia youngi, Pachysuchus imperfectus, Tawasaurus minor, and Xixiposaurus suni, although to be fair Fulengia and Tawasaurus are universally accepted as juveniles (usually of Lufengosaurus, but sometimes these juveniles turn out to be someone else's kids) and Pachysuchus is definitely imperfectus. Xingxiulong's thing seems to have been having a relatively heavy and sauropod-like hind end while still being a biped. The authors suggest that it had relatively large guts to go along with the hip and hind limb specializations.

References:

Wang, Y.-M., H.-L. You, and T. Wang. 2017. A new basal sauropodiform dinosaur from the Lower Jurassic of Yunnan Province, China. Scientific Reports 7:41881.

5 comments:

  1. Thanks for linking to my blogpost, but you forgot the second 'n' in wudingensis. Would be cool to get redescriptions of classic Lufeng sauropodomorphs past Barrett's cranial osteologies, to see where magnus, robustus and sinensis truly lie. I had actually forgotten that Pachysuchus was found to be a dinosaur by Barrett and Xu (2012). One of those rare occasions a non-dino archosaur turns out to be a dinosaur instead of visa versa.

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    1. I fixed the typo—thank you! Lots of questions still there to go around, that's for sure!

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  2. Sort of irrelevant to this post, but, are you are going to make a post or something on Ornithoscelida? I'm still in shock from reading about it earlier today.

    Would definitely love to hear your thoughts!

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    1. Well, I'll think about it, but I'm generally of the camp that prefers to wait before committing to anything. If nothing else this just goes to show how things start looking the same near the splits.

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    2. That's exactly what I was thinking, I was going to wait for more information. This is such a huge thing... and I'm still freaking out about it haha.

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