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.
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!)