Trilophosaurus and Ophthalmothule

For this post, we highlight two extinct reptiles that have been the subjects of publications in the past few days. Other than that, they don't have much in common. One is a short-necked terrestrial herbivore less than three meters long that lived during the Late Triassic in what is now Arizona, the other is a long-necked marine carnivore between five and six meters long that lived at about the Jurassic–Cretaceous boundary at what is now an Arctic island (not quite so arctic at the time).

Trilophosaurus phasmalophos

Trilophosaurids are to date a relatively small clade of reptiles that showed up for the "anything goes" reptile party that was the Triassic and then seem to have left the party before the end of the period. Their closest relatives appear to have been the azendohsaurids, another Triassic soap bubble, and the clade including the two groups is known as Allokotosauria, the "strange lizards". If you're old enough, you'll remember Azendohsaurus for its able filling of a couple of lines of text in dinosaur encyclopedias, as something like Anchisaurus or Thecodontosaurus. Later it turned out they just shared the same fashion sense.

Despite being a "strange lizard", Trilophosaurus itself is fairly tame as Triassic reptiles go. Overall the skeleton suggests something along the lines of a 2 m (7 ft)-long iguana with a noticeable neck and a smaller, less graceful head. Its particular curiosity is its characteristic teeth: the name "Trilophosaurus" means "three crest lizard", in reference to the tricuspid teeth. Three species had been named before 2020: T. buettneri, T. dornorum, and T. jacobsi, from the Chinle Formation and Dockum Group of the southwest United States (and of course no one is completely satisfied about the placements of the species). A new fourth species has just been added: T. phasmalophos (Kligman et al. 2020). So far this species is known exclusively from teeth, which, oxymoronically for Trilophosaurus, only have two cusps. This is alluded to in the species name, which means "phantom crest", in honor of the crest that isn't there.

A skeletal reconstruction of T. buettneri, from Wikimedia Commons (photo by Daderot).

T. phasmalophos has another trick up its sleeve: it is the most recent trilophosaurid species in the Chinle Formation as well. The biostratigraphy of Upper Triassic vertebrates in the Chinle/Dockum rocks has been worked out in some detail, and includes four biozones. Previously, trilophosaurids had only been known from the lower two zones, the Otischalkian and overlying Adamanian, with something of a traffic jam of trilophosaurids in the Adamanian but nothing in the next zone up, the Revueltian. T. phasmalophos, from the lower Sonsela Member of the Chinle, extends the trilophosaurs into the Revueltian (just barely). Of note is that there is evidence for yet another trilophosaur in the Adamanian of the Chinle, as well as an undescribed azendohsaur (Kligman et al. 2020).

(Finally, I would be remiss if I didn't point out that the type specimen of T. phasmalophos came from Petrified Forest National Park!)

Ophthalmothule cryostea

"Cryptoclididae" may not ring any bells for you, but if you saw a cryptoclidid swimming by, it would be easily recognizable as a classic long-necked plesiosaur. Chronologically, they took up the banner of long-necked plesiosaurs in the Middle Jurassic, flourished in the Late Jurassic, and then bowed out partway through the Early Cretaceous, their roles to be filled by the even more neck-happy elasmosaurids. (With the elasmosaurids extinct for 66 million years, there's a long-standing opening for long-necked marine predators of small prey. Any takers?)

It turns out that one of the better places to find cryptoclidid fossils is the island of Spitsbergen, above the Arctic Circle, where the Agardhfjellet Formation has produced fossils of several genera and species from the very end of the Jurassic. The newest of these is Ophthalmothule cryostea, which is known from the majority of an articulated to associated skeleton, including skull, vertebrae and ribs to the pelvis, pectoral girdle, and parts of the limbs. The genus name combines "ophthalmo" for eye and "thule" for farthest north, referring to the animal's large eyes and the geographic location. "Cryostea" is another reference to geography ("frozen bones", which is quite appropriate for Spitsbergen) (Roberts et al. 2020).

The type skeleton of Ophthalmothule cryostea, as found (A) and as reconstructed (B). Figure 2 in Roberts et al. (2020). CC-BY-4.0.

Cryptoclidids are not known for their great skull material, so O. cryostea having a largely complete head is significant. As mentioned, the eyes are proportionally large. (This somewhat makes up for the skull itself being proportionally small, even among cryptoclidids.) The orbits are positioned such that they face primarily up; this wasn't an animal that could look at you down the tip of its snout. Despite the completeness of the skull, no active teeth were present in their sockets, but a few dislocated teeth were associated with the skeleton. These are elongate and pointed, slightly curved, and have a cross-section flattened to a D shape. They would have stuck out strongly laterally. A quantity of gravelly material was present in the gut region, but the gravel is much smaller than known elasmosaurid gastroliths. Instead, this sediment may represent ground material accidentally ingested from picking up prey from the sea floor (Roberts et al. 2020). This would be like the sediment-packed coprolites with crustacean bits we described from the Late Cretaceous of Devon Island (Chin et al. 2008). A diet of small prey would also go with the spiky dentition and slender jaws with relatively little room for strong muscles (Roberts et al. 2020).

The reconstructed cranium of O. cryostea, showing where the large, dorsally facing eyes would be. Figure 5 in Roberts et al. 2020. CC-BY-4.0.

References

Chin, K., J. Bloch, A. Sweet, J.Tweet, J. Eberle, S. Cumbaa, J. Witkowski, and D. Harwood. 2008. Life in a temperate Polar sea: a unique taphonomic window on the structure of a Late Cretaceous Arctic marine ecosystem. Proceedings of the Royal Society B 275(1652):2675–2685.

Kligman, B. T., A. D. Marsh, S. J. Nesbitt, W. G. Parker, and M. R. Stocker. 2020. New trilophosaurid species demonstrates a decline in allokotosaur diversity across the Adamanian-Revueltian boundary in the Late Triassic of western North America. Palaeodiversity 13(1):25–37. doi:10.18476/pale.v13.a3.

Roberts, A. J., P. S. Druckenmiller, B. Cordonnier, L. L. Delsett, and J. H. Hurum. 2020. A new plesiosaurian from the Jurassic–Cretaceous transitional interval of the Slottsmøya Member (Volgian), with insights into the cranial anatomy of cryptoclidids using computed tomography. PeerJ 8:e8652. doi:10.7717/peerj.8652.