Thalattoarchon is a genus of large, Middle Triassic predatory ichthyosaur from North America, containing the single species T. saurophagis. The taxon was described in 2013 from a single partial skeleton discovered in Nevada. The generic name, meaning "ruler of the seas", refers to its status as an apex predator, while the specific epithet, meaning "lizard eater", alludes to its carnivorous diet. As an ichthyosaur, Thalattoarchon had flippers for limbs and a fin on the tail. It is a large ichthyosaur, with an estimated length between 8 and 9 meters (26 and 30 ft). The animal has a long, slender body but with a proportionally large head and a straight, elongated tail. The jaws of Thalattoarchon feature large teeth with two cutting edges of up to 12 centimeters (0.39 ft). Although the fossils are incomplete, the animal would have had a skeleton comprising at least 60 presacral vertebrae. The classification of this genus within the ichthyosaurs is much debated, being either classified within the clade Merriamosauria or in the more basal family Cymbospondylidae. As its scientific name indicates, its sharp, cutting teeth suggest that it could easily have preyed upon other marine reptiles of its time, indicating that it most likely occupied the highest position in the food chain. Its behavior as an apex predator is sometimes compared to that of orcas, which have a similar lifestyle. Thalattoarchon is known from the Favret Formation, which during the Anisian represented a coastal oceanic region inhabited by a wide variety of aquatic life, including numerous other ichthyosaurs. The different ichthyosaurs in this area likely adopted distinct feeding strategies to avoid competition.
Discovery and naming The only specimen of Thalattoarchon was discovered in 1997 by the paleontologist Jim Holstein during a field expedition in the Augusta Mountains, located in Nevada, United States. It was with the support and a grant from the National Geographic Society that the fossils were completely exhumed in 2008, after a period of work of three weeks. The recovered material was transported by a helicopter and a truck out of the field. The fossils were excavated in the Taylori Zone of the Fossil Hill Member in the Favret Formation, dating to approximately 244.6 million years ago, from the beginning of the late Anisian of the Middle Triassic. The specimen, cataloged as FMNH PR 3032, consists of a major part of the skull and the axial skeleton, including parts of the pelvic girdle and rear flippers. It was in 2013 that this specimen was designated as the holotype of a new genus and species of ichthyosaurs by Nadia B. Fröbisch, Jörg Fröbisch, P. Martin Sander, Lars Schmitz and Olivier Rieppel, under the name of Thalattoarchon saurophagis in the scientific journal Proceedings of the National Academy of Sciences. The genus name comes from Ancient Greek θάλαττα (thálatta, "sea"), and ἄρχον (árchon, "ruler"), all meaning "ruler of the seas", directly referencing the animal's imposing size and position as an apex predator of the Triassic seas. The specific name also comes from Ancient Greek and is derived from the words σαῦρος (saûros, "lizard") and φᾰγεῖν (phageîn, "to eat"), meaning "lizard-eater", in reference to its carnivorous diet.
Description
The describing team estimated the size of Thalattoarchon at over 8.6 meters (28 ft) long, making it a particularly large ichthyosaur. Its size and morphology are similar to some species of the contemporary genus Cymbospondylus. However, due to some missing parts of the holotype specimen, the authors mentioned that the estimate could be revised when more is known about the animal. In 2021, Paul Martin Sander and colleagues proposed that the animal would reach a body mass of 4.3 metric tons (4.7 short tons). In a 2023 book, Darren Naish slightly increased the estimated length of the animal, bringing it to 9 meters (30 ft). Like all other ichthyosaurs, Thalattoarchon has four flippers, but unlike the more derived (advanced-diverging) representatives dating from the Jurassic and Cretaceous, it has a long, poorly developed caudal fin, unlike the two caudal lobes present in later ichthyosaurs. The animal's head is remarkably large in proportion to its body, a trait rarely observed in representatives of the group. The size of the skull is estimated at 1.2 meters (3.9 ft) long. The skull of Thalattoarchon is proportionally shorter but more robust than those of other ichthyosaurs.
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