Thalattosaurus
By @stolpergeist
Etymology: Ocean reptile
First Described By: Merriam, 1904
Classification: Biota, Archaea, Proteoarchaeota, Asgardarchaeota, Eukaryota, Neokaryota, Scotokaryota, Opimoda, Podiata, Amorphea, Obazoa, Opisthokonta, Holozoa, Filozoa, Choanozoa, Animalia, Eumetazoa, Parahoxozoa, Bilateria, Nephrozoa, Deuterostomia, Chordata, Olfactores, Vertebrata, Craniata, Gnathostomata, Eugnathostomata, Osteichthyes, Sarcopterygii, Rhipidistia, Tetrapodomorpha, Eotetrapodiformes, Elpistostegalia, Stegocephalia, Tetrapoda, Reptiliomorpha, Amniota, Sauropsida, Romeriida Diapsida, Neodiapsida, Thalattosauria, Thalattosauroidea, Thalattosauridae
Referred Species: T. alexandrae, T. borealis, T.? shastaensis
Status: Extinct
Time and Place: 237 to 221 million years ago, from the Carnian to the Norian of the Late Triassic
Thalattosaurus is known from California and British Columbia.
Physical Description: Thalattosaurus was a member of the thalattosaurs, a clade of marine reptiles endemic to the Triassic. It had a long and slender body, about 2 meters (7 feet) long with a powerful, paddle-like tail. Like Askeptosaurus, which we have talked about before, Thalattosaurus’s limbs were likely webbed, but not heavily modified into paddles - they still had distinct fingers and claws.
Thalattosaurus’s skull is where things get weird. The front of the upper snout is distinctly downturned and narrower than the corresponding part of the lower jaw. The premaxilla lacked teeth, but had blunt “pseudodont” projections that resembled teeth. Actual teeth were present on the lower jaw and in the back of the jaws, which were robust. The lower jaw had pointy teeth anteriorly and both jaws had broad, bumpy teeth posteriorly. The eye sockets were large and the nares were directed somewhat dorsally.
Diet: The broad, chunky teeth in the back of the jaws suggest Thalattosaurus ate shellfish of various sorts, although I doubt it was particularly picky and wouldn’t turn down a fish or two.
Behavior: Although Thalattosaurus was a good swimmer, it likely spent a good amount of its time on the shore resting and sunning, going out to sea to feed. Its claws would have been useful for clinging onto rocks in heavy surf. It may have caught cephalopods such as ammonites in the open water, using the anterior teeth and pseudodonts to pry out the fleshy bits from the shell (or, failing that, just crush the shell with the back teeth). If I may speculate, it may have also rooted through the sandy seafloor for buried shellfish.
Ecosystem: Thalattosaurus alexandrae and T.? shastaensis are known from the Hosselkus Limestone of California. This was a coastal marine environment, and was also home to the thalattosaur Nectosaurus, the giant ichthyosaur Shastasaurus, and the sharks Hybodus and Acrodus. T. borealis is known from the slightly earlier Sulphur Mountain Formation of British Columbia, which was also home to conodonts, the thalattosaur Paralonectes, and the relatively small ichthyosaur Pharalodon.
Other: The T. alexandrae fossils were collected in the early 1900’s by a team from the University of California. The expedition was very fruitful, recovering many marine reptile remains representing at least three genera, although the area was filled with mountain lions. The recovered material is probably stored in the same building where I wrote this post (although I know at least some of the UCMP’s marine reptile material is stored in a clock tower on campus).
~ By Henry Thomas
Sources under the cut