Friday, July 29, 2016

Hupehsuchians 2: The Return

A gorgeous new specimen of Hupehsuchus (ZMNH M8127).
When last we discussed Hupehsuchians, I ran down the known genera and went a bit into their lifestyle and phylogenetic relationships. The short version: these are small marine reptiles that are allied with ichthyosaurs in a monophyletic Ichthyosauriforms. They are long-snounted, toothless, filter-feeding creatures with closely-knit ribs and gastralia that form various degrees of “bony body tubes” across genera. They all lived around the same time, in the same place. Although they are taxonomically diverse, they are morphologically conservative.

A new taxon was briefly reported in Carrol & Dong (1991) but they noted that since the specimen (IVPP V4070) lacked any solid bones—only natural casts of the skeleton—it would not do as a holotype. In 2010, a second specimen (WGSC V26020) of this mystery taxon was discovered and prepared. Thankfully, it consists largely of a bony skeleton. Nonetheless, it is clearly a second individual of Carrol & Dong’s mystery animal. In 2015, Chen et al. named it Eretmorhipis carroldongi.

That's WGSC V26020 on top and IVPP V4070 on the bottom.
 Eretmorhipis exhibits some transitional features between Hupehsuchus and Parahupehsuchus. Only the front of the trunk is exceptionally stiff and features double-headed ribs. The stiffness of the trunk eases up farther back, with more space between the ribs, which have single articulations. The belly ribs (gastralia) are larger, proportionately, than in other hupehsuchians but there are fewer of them. The “stacked” neural spines return, but in a different configuration than in other hupehsuchians. Eretmorhipis also exhibits a “false first finger” (and toe) in that there seems to be an extra metacarpal and metatarsal in front of the first finger and toe—I wonder if those extra bones anchored non-ossified digits.

While the two specimens differ in some minor ways, the authors still confidently consider them to be the same species. Additionally, Eretmorhipis differs in significant ways from Parahupehsuchus, so they are not synonymous. Further, Chen et al. did not refer the still-undescribed “polydacylous hupehsuchian” (briefly described in my last post) to Eretmorhipis based largely on the fact that, despite being ontogenetically younger, it has two full extra ossified digits in the hand.

The foreflippers of Eretmorhipis.
Eretmorhipis must have swam and lived quite differently from its relatives. Unlike the flipper-like forelimb of Parahupehsuchus, that of Eretmorhipis was broad and fan-shaped. “The variation in forelimb shape,” write the authors, “suggests that hupehsuchians had species-dependent behavior adaptations, probably to different types of habitats.” Chen et al. are a bit puzzled by the “half-tube” of Eretmorhipis, which stiffens the front of the body while allowing the back half to be more flexible. Whether the full tube of Parahupehsuchus is a derived or basal condition remains to be seen, but the authors posit that the body tube may have originated as “an adaptation that rigidified the pectoral region…as in turtles and basal sauropterygians.”

Now, we still need somebody to fully describe SSTM 5025—the “polydactylous” hupehsuchian. I cannot fathom why this hasn't happen yet, given all the other hupehsuchians being described lately.
That's SSTM 5025's forelimb on the far left. Freaky.
In other news, Wu et al. (2016) described a new specimen of Hupehsuchus nanchangensis in the pages of Historical Biology. For the most part, it just reaffirms what we know about that animal but also restates how it differs from Eohupehsuchus, Parahupehsuchus, and Nanchangosaurus. I wish the pictures in this paper were better—there are composite photos of the entire skeleton and skull, but they are small and poorly figured and there’s no supplementary information. As I have yet to see a life restoration of a hupehsuchian in any descriptive paper, I have pulled Ethan Kocak in to illustrate Hupehsuchus!
The Triassic was a wild time for all of us.
On to a slightly different topic: you may recall that Motani et al. (2014), in their description of short-snouted ichthyosaur Cartorhynchus, erected a new taxon—Ichthyosauromorpha—to include the last common ancestor and descendants of Ichthyosaurus and Hupehsuchus. In other words, hupehsuchians are ichthyosauromorphs. Basal ichthyosaurs and hupehsuchians share a number of subtle features that I won’t get into here.

Here's Cartorhynchus, who totally has a snub-nose.
There’s a new basal ichthyosauriform on the block: Sclerocormus Jiang, et al. (2016), another short-snouted critter and sister to Cartorhychus. Its gastral basket, especially, shows some similarities with hupehsuchians. For me, the real takeaway is that Cartorhynchus and Sclerocormus (together the “Nasorostra”)* are the basalmost Ichthyosauriformes, and together with hupehsuchians, diversified during the Early Triassic before being replaced by more advanced ichthyosaurs and sauropterygians. As hupehsuchians and nasorostrans employed different prey-capture strategies than later ichthyosaurs and sauropterygians (lunge feeding and suction feeding, respectively), one wonders why they went extinct.
Which means this must be Sclerocormus. Note the very long tail.
Sander et al. (2011) surprisingly (in hindsight), interpret Shastasaurus and Shonisaurus, both very large ichthyosaurs, as toothless, short-snouted suction-feeders. Sounds a lot like Cartorhynchus and Sclerocormus, doesn't it? Is that straight convergence or is a closer relationship possible? The authors compare these ichythosaurs to zipiid (beaked) whales to some degree. A couple years later, however, Motani et al. (2013) respond directly, noting that, morphologically, this comparison just doesn't hold up, and in fact Shastasaurus and Shonisaurus were very likely ram-feeders (pursuing prey) like every other ichthyosaur. Interestingly, they also note that the absence of suction-feeding ichthyosaurs may indicate that slow-moving, bottom-living, soft-bodied coleoid prey did not exist during the Late Triassic.

It could also just mean that ichthyosaurs hadn't evolved deep-diving capabilities yet, which they eventually did in the Late Jurassic.

So I wonder what CartorhynchusSclerocormus, and hepehsuchians were eating, and why did they go extinct?

Certainly, Hupehsuchus and its cousins weren’t being subjected to intense competitive pressure by other lunge-feeders--even if Shatasaurus and Shonisaurus were filter-feeders, they were not contemporaries. Maybe they experienced an ecological shift that affected their favored prey? Certainly, hupehsuchians must have been vulnerable, as they all lived at the same time and in the same place. Snuffing out the entire group might not have been too difficult.

*Why not Cartorhychidae?

1 comment:

  1. In general what little there seems to be of filter-feeding sauropsids seems to have been largely isolated, coastoal species. Henodus, stomatosuchids and Mourasuchus are all within this range.

    Maybe ammonites and pachycormids just proved too efficient at being the top filter-feeders