Yesterday we looked at a new “platypus of the Triassic,” the basal thalattosaur, Concavispina and noted some traits shared with Hupehsuchus (Fig. 1), another “platypus of the Triassic,” by convergence.
Paleontologists have had trouble figuring out what Hupehsuchus (Middle Triassic) was. This marine reptile nearly stymied Carroll and Dong (1991) who found it shared 32 derived traits with ichthyosaurs and 29 with the completely unrelated mosasaurs, 26 with plesiosaurs and 22 with nothosaurs. Mesosaurs were not included, but they are further removed than ichthyosaurs. Wumengosaurus, the current outgroup taxon, was unknown at the time.
Figure 1. Click to enlarge. Hupesuchus, a close relative of Concavispina and ichthyosaurs, derived from Wumengosaurus and Stereosternum.
Wikipedia (Feb. 2013) reported, “It is unknown exactly what Hupehsuchus is related to. It is fairly clear that it shares a close relationship with Nanchangosaurus, but other relations are unknown. Many features, including the discovery of polydactyly, suggest that Hupehsuchus is related to the ichthyosaurs, but the fact that Hupehsuchus’ extra digits include more bones in the hand, rather than just the fingers as in the ichthyosaurs, may discredit that theory. It along withNanchangosaurus seem to be so different from any other reptile that a new order has been constructed for the two genera called Hupehsuchia.”
Motani (1999) correctly nested Hupehsuchus at the base of the Ichthyosauria, but nothing beyond the base, leaving that a great unknown.
This is lunacy. Or lethargy.
All it takes is a phylogenetic analysis to figure out what Hupehsuchus is and where it nests. The large reptile tree nested Hupehsuchus at the base of the Ichthyosauria. Both taxa were sisters to Thalattosauria. All three were derived from a sister to Wumengosaurus and the mesosaur, Stereosternum. You can trace the lineage all the way back to the first tetrapods, but we’ll stop here with Petrolacosaurus (Fig. 2).
One of the big problems that the large reptile tree overcame
was the nesting of mesosaurs within the Diapsida. Mesosaurs largely, but not completely, roofed over their temporal fenestrae, which caused them to be seriously mislabeled with the pareiasaurs and millerettids. By focusing on a single trait (temporal fenestrae) while ignoring a similar absence of a lateral temporal fenestra in a known diapsid, Araeoscelis, AND ignoring a suite of other traits, paleontologists essentially painted themselves into a corner they then could not escape from.
(The same sort of academic blindfold also exists with poposaur ankles.)
Figure 2. Click to enlarge. The origin of ichthyosaurs and thalattosaurs from basal diapsids and basal mesosaurs. Relationships are rather apparent when seen in this context.
Permian marine reptiles
Evidently there was a huge and rapid diversification of reptiles following the return of mesosaurs to the water. We’re just now getting twigs from that bush. That’s why Hupehsuchus looks so different form Wumengosaurus and Utatsusaurus. Then again, it looks even _less_ like anything else on the large reptile tree, and that tells the tale.
So what was behind that increase in marine diversification?
Were mesosaur descendants competing with any other tetrapods in the water? Some mesosaurs were able to handle hyper-saline waters. Living amphibians like fresh waters. But temnospondyls, the big amphibians of the Carboniferous, Permian and Triassic, are found in both fresh and coastal marine sediments. So the amphibians were there first and they were bigger.
Mesosaurs, plesiosaurs and ichythyosaurs were all live bearers, so that may have been a factor. Amphibians were all still laying eggs in water. So mother mesosaurs protected her embryos until birth, but the young were fewer in number as amphibians typically produce large amounts of eggs.
Mesosaurs, plesiosaurs and ichythyosaurs are not found in coal deposits, but in sediments that once formed sea floors. So perhaps more open seas further from shore gave early Permo/Triassic marine reptiles a playing field in which to evolve quickly and successfully, away from the Early and Middle Triassic amphibians.
Of course the great Permo-Triassic extinction event might have helped.
Remember, when we find a fossil taxon, it can be millions of years older than the original specimen of that species, having spread and multiplied, thereby multiplying our chances of finding it. Then again, the species and its fossil could be just a flash in the pan, of its own time only. Phylogenetic analysis helps in this regard, finding specimens millions of years younger than their phylogenetic descendants, or not, helps determine the longevity of a species. But I digress.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
By the way the Feb. 5 blog on phylogenetic analysis was hugely popular despite its farcical nature.
Carroll RL and Dong Z-M 1991. Hupehsuchus, an enigmatic aquatic reptile from the Triassic of China, and the problem of establishing relationships. Philosophical Transactions of the Royal Society London B 28 331:131-153.
Motani R 1999. Phylogeny of the Ichthyopterygia. Journal of Vertebrate Paleontology 19(3):473-496.
Young C-C and Dong Z-M 1972. On the aquatic reptiles of the Triassic in China. Vertebrate Paleontology Memoirs. 9-1-34.