Sunday, March 22, 2015

Hupehsuchians: a Primer

How many modern marine reptiles can you name? I count two, broadly speaking: sea turtles and marine iguanas. Back in the Cretaceous, there were a few others. Many of the world’s oceans hosted the deadly mosasaurs, which were essentially marine-adapted monitor lizards. You also had the familiar but puzzling plesiosaurs; with their long necks, tiny heads, and wide rounded bodies, it’s still difficult to determine exactly how they made their living. Also still going strong were the ichthyosaurs—dolphin-shaped marine lizards with huge eyes and a taste for cephalopods. Marine iguanas weren’t a thing yet, but sea turtles were actually more diverse (and bigger) than they are today.


But even the Cretaceous is trounced by the Triassic in terms of diversity. After the Permio-Triassic extinction, many reptile groups must have decided that being terrestrial wasn’t all that great and took the plunge into Pangean seas. Here’s a taste of all the groups who decided to try a marine existence:
  1. Ichthyosaurs got their start in the Triassic and quickly exploded in diversity.
  2. Tangasaurids (or hovasaurs) were lizard-like reptiles with elongate, laterally flattened tails.
  3. Protorosaurs, like Tanystropheus, are thought to be largely aquatic.
  4. Sauropterygians include a wealth of semi-aquatic forms like pistosaurs, pachypleurosaurs, and nothosaurs. Plesiosaurs came out of this group as well.
  5. Placodonts, which are sauropterygians, deserve special mention: they went from fat, unarmored durophageous reptiles to heavily armored, turtle-like durophageous reptiles over the course of the entire Triassic.
  6. Thalattosaurs were lizard-like reptiles that were not as strongly aquatic as some of the others on this list, but did invade the water to some degree.
  7. Sinosaurosphargids were turtle-like critters with dorsal armor made up on thousands of tiny scutes and ventral armor composed of enlarged belly ribs.
  8. Hupehsuchians were long-snouted, toothless reptiles that will be the subject of this post.
Now add to this all the predatory fish and marine amphibians to the mix and it’s a miracle anyone survived the Triassic. But note that, for the most part, all of these animals were carnivores. Placodonts avoided competition by going after shellfish while simultaneously protecting themselves with ridiculous amounts of body armor—to the point where the latest-surviving placodont, Henodus, looks like a turtle. I’m not sure what sinosaurosphargids were eating. Hupehsuchians deserve special mention, though, as being potentially the strangest of all these bizarre marine reptiles. There’s nothing quite like them.

The original hupehsuchian, Nanchangosaurus suni. The holotype is on top; a new specimen is below. From Chen et al. (2014b).

Hupehsuchia (HOOP-uh-SOOK-ee-uh) is currently comprised of four genera: Hupehsuchus, Eohupehsuchus, Parahupehsuchus, and Nanchangosaurus. Oddly enough, all come from the Jialingjiang Formation of Hubei Province in China, which is probably late Early Triassic in age. Two more, undescribed specimens (V4070 & SSTM 5025) probably represent more new genera. That so many monotypic genera could exist so close together during the same time is highly unusual, but all six are clearly morphologically different from each other. SSTM 5025, especially, shows marked polydactyly—seven fingers on the manus and six in the pes. Somebody needs to describe this weirdo, pronto.

Hupehsuchus nanchangensis (Wang, 1959), from Carroll & Dong (1991).

Hupehsuchians have always been a fairly obscure group, but 2014 was the Year of Hupehsuchians, with two genera named (Eohupehsuchus and Parahupehsuchus) and Nanchangosaurus properly described for the first time. Previously, only a handful of technical papers had ever focused exclusively on hupehsuchians—arguably the most important was Carroll & Dong (1991), who redescribed Hupehsuchus and speculated on its biology, as well as tried to ferret out its closest relative. This has proven to be exceptionally difficult, as hupehsuchians are so ridiculously derived that establishing a link with other known Triassic reptiles is a fool’s errand. It’s a bit like trying to figure out what mammal group whales are related to without using fossils or molecular evidence. Good freakin’ luck.

The bizarre dorsal vertebrae & woven ribcage of Parahupehsuchus longus (Chen et al. 2014c).

What makes hupehsuchians so ridiculous? Glad you asked.

For starters, their vertebrae and ribcage anatomy is without precedent. The neural spines of the dorsal vertebrae are capped by separate dorsal outgrowths from each spine which are then, themselves, topped by small plates of interconnected dermal armor. The expanded, overlapping ribs are double-articulated, preventing movement, and they are further curtailed by a complex, weave-like articulation with the gastralia. Chen et al. (2014c) note that there is no room for intercostal muscles (just like turtles) and that the hupehsuchian torso presents an immobile “bony body tube.” They summarize the purpose of this tube thusly:

"The stiffened body trunk of Parahupehsuchus most likely had an anti-predatory function. The body tube is not a proper carpace because it does not form an outer shell of the body, exposing epaxial, pectoral, and pelvic muscles outside. However, the tube directly protects the internal organs from predators. Moreover, there were few or no intercostal muscles, so much of the trunk lacked exposed muscles that required protection. … The body plan of hupehsuchians in general is toward building a heavily ossified skeleton that would make ingestion and digestion by predators difficult."

Despite this, there is no evidence of armor apart from the row of armor scutes along the middle of the back. Other reptiles with stiff, inflexible bodies (turtles, placodonts, saurosphargids, crocodiles) are extensively armored, so what’s up with the lightly-armored hupehsuchians? It may be that further armor was unnecessary given that adult hupehsuchians were typically the largest animals in their restricted environment, and the “bony body tube” may have provided just enough protection from predators as juveniles. Who’d want to bite down on a bony tube? Carroll & Dong suggested that the armor may have provided “additional rigidity to the vertebral column.”

The very small holotype of Eohupehsuchus brevicollis from Chen et al. (2014a).

You may next ask how they could swim if they weren’t flexible? Turtles use their limbs; did hupehsuchians? The limbs may have done some work, although different genera came with surprisingly different limb anatomy. In all genera (where known) the forelimb is larger than the hindlimb. In Hupehsuchus, the shape of the manus suggests a stubby, semi-circular paddle. In Eohupehsuchus, the forelimb is a bit more flipper-shaped, but the manus is quite short. In the polydactylus SSTM 5025, the forelimb is more like Hupehsuchus, but broader. Where known, the hindlimb is always quite small and relatively flipper-shaped. Overall, the limb anatomy seems closer to ichthyosaurs and mosasaurs, neither of which practiced limb-driven swimming. Their flippers, like those of dolphins and whales, likely provided rudder control.

The holotype of Parahupehsuchus (top) compared to a specimen of Hupehsuchus (bottom) from Chen et al. (2014c). Note it's missing the head and most of the tail. Despite this, however, it's clear that Parahupehsuchus had a longer body, proportionately, than Hupehsuchus.

Thus, propulsion must have been left to the well-muscled, deep and elongated tail. While their bodies are not as tightly restricted as hupehsuchians, this is how crocodilians get along—they keep the limbs close to the body and swim with their tails.

What did they eat? That’s a very good question that nobody’s quite sure of yet. Hupehsuchians have relatively small skulls that have long, but toothless, mouths. Carroll & Dong were the first to suggest some mode of filter feeding, as the “general configuration of the rostrum resembles the pattern of modern whalebone whales. This raises the possibility that Hupehsuchus might have possessed a material resembling baleen.” They cited, however, Hupehuschus’ relatively long neck (compared to whales) as a possible knock against this theory, as the neck would be prone to lateral bending during lunge-feeding. Chen et al. (2014a) do not speculate on the feeding habits of Eohupehsuchus, but note that it has a shorter neck than its cousins. Chen et al. (2014b) similarly do not speculate on the feeding habits of Nanchangosaurus. Finally, Chen et al. (2014c) do not discuss the feeding habits of Parahupehsuchus, as the holotype lacks a skull.

Detail of the skull of a new specimen of Hupehsuchus. The skull is exposed in palatal view; the blue arrows point to "shallow groove-like depressions," suggesting the presence of a filtering structure. From Motani et al. (2015).

Motani et al. (2015) finally tackled the hupehsuchian diet problem thanks to a new specimen of Hupehsuchus which preserves the skull in palatal view. They note that “the new specimen shows that the skull and mandible of Hupehsuchus is characterized by a mixture of features resembling feeding adaptations in pelicans and rorqual whales, suggesting that the genus shared the feeding style of these two animals.” This is known as lunge-feeding. The authors realized that the long, thin mandible of Hupehsuchus would have permitted bowing as in pelicans, and rejected Carroll & Dong’s suggestion that the neck was too long for lunge-feeding, as pelicans have much longer necks and get along just fine. And remember that Eohupehsuchus has a shorter neck than Hupehsuchus. Note, also, that this new specimen preserves shallow grooves in the premaxillae which appear to be similar to the same structures in the skulls of baleen whales. Motani et al. suggest this may indicate the presence of a soft-tissue filtration system. Hupehsuchians also have large hyoid bones, so a large tongue could have been used to push water out the mouth, through the filter, capturing any small organisms that remained. 

The structure of the mandibles also implies the presence of a large gular pouch as in pelicans. The authors write that the filter could have been like that of baleen whales or the pecten of ducks. However, they point out the palate is more like that of pelicans and whales than ducks, and that, combined with the mandibular morphology, "duck-like feeding is implausible for Hupehsuchus."

Motani et al. conclude that Hupehsuchus (and, by extension, its cousins) "most likely represents the first lunge feeder in the history of life, appearing...more than 200 million years before rorqual whales or pelicans."

So these critters are clearly extremely weird, but what ARE they? That is, who are they related to?

Carroll & Dong (1991) lamented that, back when only Hupehsuchus and a bad specimen of Nanchangosaurus were known, that the "specific sister-group affiliations are potentially knowable, but they are not necessarily determinable on the basis of currently available evidence." They chose not to produce a phylogenetic tree. In their redescription of Nanachangosaurus, Chen et al. (2014b) consistently found Hupehsuchia as a sister taxon to Ichthyopterygia, a position retained in Chen et al. (2014c). The discovery of basal ichthyosauriform Cartorhynchus (Motani et al. 2014) provided evidence of a close relationship from the ichthyopterygian side. When the authors ran phylogenetic analyses with and without aquatic adaptations considered, Hupehsuchia resolved as a sister taxon to Ichthyopterygia both times. With this close relationship in mind, Motani et al. named a new clade, Ichthyosauriformes, to include both Hupehsuchia and Ichthyopterygia.

As Darren Naish has pointed out, this implies (but does not prove) that seemingly disparate groups of Triassic marine reptiles may all belong to a giant "super-clade." Time, more specimens, and more studies will ultimately tell if this hypothesis is borne out, but it's an exciting time to be interested in Triassic marine reptiles!

There you have it folks, the Hupehsuchia. I may update this post later with my own terrible art, if I ever finish it.

Notice that, with the exception of the Cartorhynchus description, all of these papers are open access!

References:

Carroll, R. L. & Dong, Z. (1991) Hupehsuchus, an enigmatic aquatic reptiles from the Triassic of China, and the problem of establishing relationships. Phil. Trans. R. Soc. Lond. B. (331), 131-153.

Chen, X., Motani, R., Cheng, L., Jiang, D. & Rieppel, O. (2014a). A small short-necked hupehsuchian from the Lower Triassic of Hubei Province, China. PLOS ONE: 16 pages.

Chen, X., Motani, R., Cheng, L., Jiang, D. & Rieppel, O. (2014b). The enigmatic marine reptiles Nanchangosaurus from the Lower Triassic of Hubei, China and the phylogenetic affinities of Hupehsuchia. PLOS ONE 9(7): 12 pages.

Chen, X., Motani, R., Cheng, L., Jiang, D. & Rieppel, O. (2014c). A carpace-like bony 'body tube' in an Early Triassic marine reptile and the onset of marine tetrapod predation. PLOS ONE 9(4): 8 pages.

Motani, R., Chen, X., Jiang, D., Cheng, L., Tintori, A. & Rieppel, O. (2015). Lunge feeding in early marine reptiles and fast evolution of marine tetrapod guilds. Scientific Reports 5: 8 pages.

Wu, X., Li, Z., Zhou, B. & Dong, Z. (2003). A polydactylous amniote from the Triassic period. Nature 426: Pg. 516.

Motani, R., Jiang, D., Chen, G., Tintory, A., Rieppel, O., Ji, C. & Huang, J. (2014). A basal ichthyosauriform with a short snout from the Lower Triassic of China. Nature (online).  

*The naming conventions for this group are gawdawful. You’ve got Hupehsuchus, then Parahupehsuchus (“near Hupehsuchus”) and Eohupehsuchus (“dawn Hupehsuchus”). Nanchangosaurus is pretty original until you realize that Hupehsuchus’ species name is nanchangensis

1 comment:

  1. with regard to the marine superclade, that was shown back in 2011 at reptileevolution.com/reptile-tree.htm and has been enhanced with more taxa since then.

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