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Hupehwhat? Finding a home for some unusually odd marine reptiles

A more edible version of a hupehsuchian. Photo by TheBusyBrain, CC-BY
A more edible modern version of a hupehsuchian. Photo by TheBusyBrain, CC-BY 2.0

“Swimming sausage topped with armored mustard” is probably the best way to describe a hupehsuchian. These marine reptiles, known only from 248 million year old rocks in east-central China, were odd-balls at a time when a lot of odd-balls (by modern standards) roamed our planet. Hupehsuchians had tiny toothless heads, flippers with one or two extra embedded digits, and an elongated body swathed in close-knit ribs and armor plates. This heavily modified body makes hupehsuchians interesting as well as frustrating, because it has been tough for paleontologists to reach a consensus on how the group is related to other organisms.

Skeleton of Nanchangosaurus
Skeleton of the hupehsuchian Nanchangosaurus (click for a closer look). The head (incomplete in this fossil) is to the left. Modified from Chen et al., 2014, CC-BY 4.0.

Over the years, researchers have proposed any number of relationships for hupehsuchians, with the group being posited as closely allied with eosuchians (historically a junk bin for “primitive” lizard-like animals), archosaurs (the group including birds, crocodiles, and T. rex), or icthyosaurs (“fish lizards”). The superficially dolphin-like icthyosaurs have achieved broadest recognition in recent years as the sister group to hupehsuchians, but was this just the result of common adaptations for an aquatic lifestyle? Study of hupehsuchians has been hampered by poor access to specimens, minimal description in the literature, obscured anatomy for important features, and an overall scarcity of fossils.

Hupehsuchus, in silhouette. Public domain image by Neil Kelley via PhyloPic.
Hupehsuchus, in silhouette. Public domain image by Neil Kelley via PhyloPic.

In a paper published a few days ago in PLOS ONE, Xiao-hong Chen and colleagues use previously unpublished as well as reexamined hupehsuchian fossils to test the group’s relationships. The researchers focused in particular on Nanchangosaurus suni, which was the first hupehsuchian known to science when it was named over 50 years ago. For decades, only the original (holotype) specimen was around, and it was minimally studied. Fieldwork finally turned up another really nice skeleton in 2011, which forms the core of the new paper.

With two specimens in hand, Chen and co-authors assembled a ton of new data on Nanchangosaurus. Many previously unrecognized sutures between skull bones could be mapped (important for understanding evolutionary relationships), along with the forms of vertebrae and limb bones. Despite its historic importance, Nanchangosaurus had never been analyzed in a rigorous phylogenetic analysis. Thus, this animal could provide some critical evolutionary information.

Skull of Nanchangosaurus
Nanchangosaurus partial skull; the snout points to the left. Note in particular the lack of teeth in the impressions of the jaws. Modified from Chen et al., 2014. CC-BY 4.0.

The researchers ran numerous iterations of a phylogenetic analysis to reconstruct the evolutionary relationships of hupehsuchians and other extinct groups, utilizing 213 different anatomical features in 41 species. To explore the influence of convergent adaptations for the water, they ran one set of analyses with purported aquatic adaptations removed. In all cases, icthyosaurs were solidly identified as the closest relatives of hupehsuchians.

Of course, this doesn’t mean that hupehsuchians were ancestral to icthyosaurs–both are uniquely specialized in their own ways, and the oldest members of each group lived at approximately the same time. Either way, they aren’t far separated in time from their closest common ancestor. The analysis will probably get some criticism for its relatively small scale within the standards of contemporary phylogenetic analysis, as well as for the strategy of excluding perceived functional characters, but it certainly represents a unique contribution. Additionally, the greatly bolstered comparative data for Nanchangosaurus will pay big dividends for researchers in the future. Most interestingly, the relationships between all of the various marine and non-marine reptiles are still fairly poorly understood; more fossils and more detailed analyses are our best bet for sorting things out.

Cousin? The early icthyosaur Grippia. Image by Dmitry Bogdanov, CC-BY 3.0.

Chen X-h, Motani R, Cheng L, Jiang D-y, Rieppel O (2014) The enigmatic marine reptile Nanchangosaurus from the Lower Triassic of Hubei, China and the phylogenetic affinities of Hupehsuchia. PLoS ONE 9(7): e102361. doi:10.1371/journal.pone.0102361

Want to read more about hupehsuchians? Check out this old post on their bony body tubes.

Updated to correct typo in number of characters & taxa in the phylogenetic analysis (thanks, Nick Gardner!).

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