Enigmatic Triassic Hellasaur Friday – Hupehsuchus redux

16 May 2008

After a week of small accomplishments and a roughly commensurate handful of minor setbacks here we are, just where we left of last week…props to Zach Miller for contributing original artwork to this piece!!!

Every time I begin to feel marginally comfortable with the rampant absurdity of the biological world, nature always seems to have one more joker up her sleeve. In the Triassic, she seems to have been playing with a trick deck anyway. Yet even among the motley crew of enigmatic Triassic hellasaurs, Hupehsuchus stands out as one of the weirdest of the weird.

But, for all its appealing novelty, Hupehsuchus isn’t exactly a household word. Here’s the entire Wikipedia entry on Hupehsuchus:

Hupehsuchus is an extinct genus of ichthyopterygian marine reptile from the Middle Triassic of China. The type species is H. nanchangensis.

Spencer Lucas provides a more informative account in Chinese Fossil Vertebrates:

More unusual Middle Triassic marine reptiles from China are Nanchangosaurus and Hupehsuchus. These animals are known from complete and incomplete skeletons found in the Middle Triassic Daye Formation of Hubei. Nanchangosaurus and Hupehsuchus retain many characteristics of terrestrial precursors to the ichthyosaurs and may be a “missing link” between them and their non-aquatic ancestors. (Lucas 2001)

Similarly, Nanchangosaurus (a Hupehsuchian closely related to Hupehsuchus) gets a brief plug in this diagrammatic explanation of ichthyosaur evolution which appeared in a recent New Scientist article by Don Prothero, excerpted from his book Evolution: What the Fossils Say and Why It Matters.

Both authors see the hupehsuchians as “missing-links” between ichthyosaurs and their terrestrial forebears. However, despite what Wikipedia would have you believe Hupehsuchus is probably not an ichthyopterygian sensu stricto (an error on Wikipedia! shock, horror), but may well be the sister taxon to this group (Motani 1999). Inconveniently however, Hupehsuchus is probably a few million years too young to be a direct ancestor to the ichthyosaurs, and—given its suite of unique characteristics—its role value as a “missing-link” is perhaps limited.

The most thorough description of Hupehsuchus, and indeed one of the only scientific papers on the animal, was written by Robert Carroll and Dong Zhi-ming (1991). Their publication is the source of the majority of the info presented here—aside from my own speculative ramblings—and is highly recommended to anyone whose interest is sparked by this brief discussion.

As mentioned by my brief preview last week, Hupehsuchus possesses a bizarre assemblage of unique and convergent adaptations—so much so that Carroll and Dong subtitled their paper “the problem of establishing relationships.” Perhaps the strangest part of Hupehsuchus is its long, toothless skull which bears a striking (but almost certainly superficial) resemblance to a bird skull. In other aspects it more closely resembles other marine reptiles; like the ichthyosaurs it has a deep, laterally flattened body almost certainly reflecting an adaptation to aquatic existence. Likewise the fore- and hind-limbs are broad and flattened, an adaptation seen in many secondarily marine tetrapods including sea-turtles, icthyosaurs, plesiosaurs, mosasaurs, whales and manatees among others.

A row of bony plates (osteoderms) lines the back of Hupehsuchus vaguely calling to mind the armored placodonts, although the armor is not nearly as extensively developed in Hupehsuchus. Just below the armor, the neural arches of the vertebrae are bizarrely “bipartite” divided into upper and lower sections. It’s tempting to speculate that these “jointed” vertebral processes may have somehow augmented the protection offered by the dorsal armor, but, given that no similar structures are known in any living or other fossil vertebrates, understanding the functional significance of this feature is challenging. Like the back, the underside of Hupehsuchus was “armored,” protected by a combination of robust ribs and overlapping gastralia.

The osteoderms of Hupehsuchus would presumably have served a defensive role, but we don’t quite know what against. The recently described marine archosaur Qianosuchus (Li et al. 2006), a three-meter long predator with serrated teeth, comes from slightly younger sediments than those that bear Hupehsuchus, but might be an attractive model for a hypothetical hupehsuchivore. The robust ribs and gastralia may have also helped to make Hupehsuchus negatively buoyant, similar adaptations are seen in many aquatic tetrapods.

The unusal “beak-like” skull of Hupehsuchus invites the obvious question of what the heck this thing ate. Toothlessness itself is not a very helpful clue: penguins, baleen whales, turtles, and the extinct sirenian Hydrodamalis all lack teeth but differ strongly in their diets and feeding strategies. Perhaps surprisingly, Carroll and Dong suggest that Hupehsuchus may have possessed “material resembling baleen”! They note however that the relatively narrow skull,and flexible neck don’t seem to accord well with the notion that Hupehsuchus was a swimming suspension feeder.

On the other hand, a flexible neck and relatively slender narrow, toothless skull might be a good tool for probing for benthic invertebrates hiding in the sediment. Perhaps Hupehsuchus sculled along the bottom, arcing its beak to and fro through the mud and snapping up prey in a manner vaguely reminiscent of spoonbills or platypodes? A nice image, but probably impossible to prove without the serendipitous discovery of some feeding traces—something to keep your eyes peeled for the next time you find yourself poking around the Jialingjiang Formation…

One final anatomical quirk comes from an undescribed (to my knowledge) hupehsuchian that briefly made the news a few years ago. Wu Xiao-Chun and colleagues (2003) published a brief communication in Nature which announced the discovery of a polydactylous nanchangosaurid. That digits are frequently lost over evolutionary but almost never gained has long puzzled scientists, especially since heritable mutations which cause the development of extra digits are found in many animals, including humans. One exception to this rule are the ichthyosaurs, some lineages of which multiple both the number of finger bones and the total number of digits over their evolutionary history. The extra-fingered hupehsuchian might therefore suggest again a close relationship between these two very different marine reptile groups. Unfortunately, the fossil, which appears to be largely complete, only gets a very brief description in the half-page Nature write-up.

The Carrol and Dong paper uses Hupehsuchus to launch into a fairly strident critique of over adherence to the principle of parsimony. While this debate is far beyond the scope of this brief (well, it was supposed to be brief) post. Suffice to say, Hupehsuchus is a fairly clear testament that nature has a sense of humor, that, or a drinking problem.

(Complete refs list and maybe some additional images, coming soon!)

REFS:

Carroll and Dong 1991

Lucas 2001

Wu 2003

13 Responses to “Enigmatic Triassic Hellasaur Friday – Hupehsuchus redux”

  1. Zach Miller Says:

    Right on, brother! Great writeup of what’s quickly becoming my favorite marine reptile.


  2. I agree, engaging write-up! And nice work by Zach too.

    Thanks to Zach for introducing me to Microecos. Excuse me while I look around. :-)


  3. More on Hupeh and its place in reptile phylogeny when John Merck gets around to publishing his dissertation work…

  4. neil Says:

    Thanks to all for the comments, and thanks especially to Zach for the great illustrations.

    And thanks for the tip doctor Holtz, I look forward to seeing some more work published on this amazing animal!

  5. johannes Says:

    > Perhaps the strangest part of Hupehsuchus is its long,
    > toothless skull which bears a striking (but almost certainly
    > superficial) resemblance to a bird skull.

    Another non-dinosaurian bird ancestor for Feduccia and the BANDits? ;-)

  6. Neil Says:

    In fact, Feduccia has trotted out Hupehsuchus, though not as a bird ancestor per se. Following Carroll and Dong, he suggests that Hupeh proves that parsimony can’t be used for establishing phylogenetic relationships when major convergence is at play…and ergo, BIRDS ARE NOT DINOSAURS!!!

    Feduccia, A. (1999) 1,2,3 = 2,3,4: Accommodating the cladogram
    Proceedings of the National Academy of Sciences 96: 4740-4742.

  7. David Marjanović Says:

    More on Hupeh and its place in reptile phylogeny when John Merck gets around to publishing his dissertation work…

    …or should I say “if”. I talked to him at the SVP congress in October, and he wasn’t optimistic. Seems to be over his head in administrative work.

  8. David Marjanović Says:

    an error on Wikipedia! shock, horror

    It is immoral to complain about Wikipedia. Go click on “edit” and fix it. :-)

  9. David Peters Says:

    Hupehsuchus has a new sister in Wumengosaurus, which is not a sauropterygian, but derived from a sister to the mesosaur, Stereosternum. So, that makes Hupehsuchus a derived mesosaur as well, doesn’t it? Neural arch dorsal ossifications are only part of the long list of synapomorphies. Nice to see that clade expanding. It also includes thalattosaurs and ichthyosaurs.

    See more at reptileevolution.com/hupehsuchus.htm

    • Neil Says:

      Thanks for stopping by David. Interesting to see all of these aquatic reptiles nesting together, do you think homoplasy could be influencing your phylogenetic hypotheses at all? Having seen Wumengosaurus and Hupehsuchus first-hand, a sister-group relationship between these two taxa strikes me as a surprising result.


  10. I leave it to the software, Neil. Given the opportunity of nesting in any one of 326 other points on the tree, the numbers point to the base of the Ichthyosauria, as Motani 1999 intimated. Since this is science, you’re welcome to test these results yourself. Should come up the same.

    • Neil Says:

      I’m happy enough with that topology not that it particularly matters what I think. I’ve run some of the available matrices, so I’ve got a sense for where things “like” to go. But a character matrix is just a coarse model of an already incomplete picture. The software is a useful tool, but it’s not omniscient. I know that there is a good deal of additional hupehsuchian material that has yet to be adequately prepared, described and studied. Given the preservation of the type material I think the safe bet is to accept a sister relationship between those groups as a plausible but tenable hypothesis pending future analysis. New and better fossils have a way of dashing cherished pet hypotheses about phylogeny – as you well know.


  11. Neil, sure things could change with more data and you can bet on unknown future outcomes, but I prefer to let Science hold sway and let the data speak for itself in the present day with presently known data. If you don’t “like” these sister taxa, which ones are better candidates? You’ll be hard pressed to find any. But please, let me know if you do. I encourage you and any other scientists to follow your own data and recover your own trees, then report what you find. What you’re doing above is throwing peanuts on the playing field. Science is for everyone. Get on the field and play. A character matrix is indeed a coarse model, but it’s the only game we know for fossil taxa. And it’s a generally accepted method by almost all paleontologists, as you know.


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