Thalattoarchon Temporal range: Middle Triassic, Late Anisian
Skull and tooth from the holotype (FMNH PR 3032)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	†Ichthyosauria
Genus:	†Thalattoarchon Fröbisch et al., 2013
Type species
†Thalattoarchon saurophagis Fröbisch et al., 2013

Thalattoarchon is an extinct genus of large predatory ichthyosaurs that lived in the Middle Triassic (Anisian) in what is now North America, between 244.6 to 242 millions years ago. The only known species is T. saurophagis, described in 2013 from a single specimen discovered in the Favret Formation, Nevada (United States). 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, its size being estimated at around 8.6 meters (28 ft) long, although the authors are doubtful about the exact measurements. 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 of up to 12 centimeters (0.39 ft), which would have been well suited for attacking large prey. Although the fossils are incomplete, it is estimated that 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. Thalattoarchon certainly occupied the highest place in the food web within the Favret Formation, its sharp and cutting teeth suggesting that it would have mainly preyed on contemporary marine reptiles. Its position as a apex predator is generally compared to that of orcas, which have a similar lifestyle.

Discovery and naming

The only currently known 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.^[1] It was with the support and a grant from the National Geographic Society that the fossils were completely exhumed in 2008,^{[1][2][lower-alpha 1]} after a period of work of three weeks. The imposing measurements of the fossils led paleontologists to transport the recovered material by using a helicopter and a truck out of the field.^[3] 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. After preparation, 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 swimming paddles.^[1]

It was in 2013 that this specimen was designated as the holotype of a new genus and species of ichthyosaurs, under the name of Thalattoarchon saurophagis in the scientific journal Proceedings of the National Academy of Sciences. The genus name comes from Ancient Greek θάλασσα (thálassa, "sea"), and ἀρχός arkhós, "ruler"), all literally meaning "rulers of the seas",^[1] directly referencing the animal's imposing size and position as an apex predator of the Triassic seas.^[3] The specific epithet also comes from Ancient Greek and is derived from the words σαῦρος (saûros, "lizard") and φᾰγεῖν (phageîn, "to eat"), the two terms together literally meaning "lizard eater", in reference to its carnivorous diet . Due to the limited volume, the anatomy of the animal is only briefly described in the main article by Nadia B. Fröbisch and colleagues,^[1] with additional anatomical descriptions being published in a secondary article.^[4]

Description

Life restoration and size comparison of a T. saurophagis with a human

Although the only known specimen of Thalattoarchon is partially preserved, estimates of its size peg it at over 8.6 meters (28 ft) long, making it one of the largest known ichthyosaurs, having a size and morphology similar to some species of the contemporary genus Cymbospondylus.^[1][2][3] However, due to some missing parts of this same specimen, the authors must await ongoing preparations and future discoveries to refine the estimates.^[1] In 2021, Sander and colleagues propose that the animal would reach a body mass of 4.3 metric tons (4.7 short tons).^[5] Like all other ichthyosaurs, Thalattoarchon has four flippers, but unlike the more derived representatives dating from the Jurassic and Cretaceous, the latter has a long, poorly developed caudal fin, unlike the two caudal lobes present in later ichthyosaurs. A trait rarely observed in representatives of the group, the animal's head is remarkably large in proportion to its body,^[1] the size of the skull being estimated at 1.2 meters (3.9 ft) long.^[4] The skull of Thalattoarchon is proportionally shorter but more robust than those of other ichthyosaurs, a characteristic suggesting that it fed on large prey.^[6]

Skull

Reconstructed skull

Although the skull of the only known specimen of Thalattoarchon is partially preserved, missing the entire rostrum and the front of the mandible, the latter is still well preserved and allows it to be viewed as a whole. The eye sockets are elongated, reaching up to 29 cm (11 in), and each have sclerotic rings. The superior temporal fenestrae are large and oval, sharing a similar shape to those of Shastasaurus. The maxillae extend well below the eye sockets and carry large teeth all the way to their posterior ends.^[1] The nasal bones are very extensive and come into contact with the postorbital bones, but do not extend as far as the superior temporal fenestrae. The frontal bones are smaller, surrounding the anteromedial edge of the superior temporal fenestrae, but entirely surrounding the pineal foramen. In front of the pineal foramen there is a small sagittal crest which takes on the shape of a large plate further back. The parietal bones are smaller than the frontal bones and medially form the superior temporal fenestrae. The lacrimal bones and jugal bones are narrow and slender in shape.^[4]

The palate, although partially preserved, appears to be dominated by the large pterygoid bones. In the lower jaw, the dentary bone extends to the level of the maxillae but apparently lacks teeth when it reaches the latter. This means that at least the fifth or sixth tooth in the maxillae would be missing a front tooth in the mandible. The surangular has a distinct coronoid process located posterior to the eye sockets.^[4] Although the only known teeth of Thalattoarchon are located at the posterior level of the maxillae, it is possible that their size would increase towards the middle of the jaws. This morphological trend is also observed in many ichthyosaurs and other now extinct marine reptiles, such as mosasaurs, thalattosuchians and pliosaurs. The largest preserved tooth is at least 12 cm (4.7 in) long, with the crown alone measuring 5 cm (2.0 in). The teeth represent one of the main autapomorphies of Thalattoarchon, being large and thin, having two sharp edges each and a fairly smooth crown. The related genus Himalayasaurus has very similar dentition, but the latter differs from Thalattoarchon by the presence of longitudinal grooves on the crown.^[1] The teeth of Thalattoarchon would also have been adapted for cutting prey.^[4]

Classification

In the first classifications published by Fröbisch and colleagues in 2013, Thalattoarchon is classified as a basal representative of the Merriamosauria clade,^[1] being seen as more derived than Cymbospondylus and the Mixosauridae, but recovered in an unresolved polytomy with Besanosaurus, Californosaurus and Toretocnemus.^[4] However, in 2015, a phylogenetic revision carried out on the ichthyopterygians by Cheng Ji and his colleagues moved Thalattoarchon within the Cymbospondylidae, i.e. in a more basal position than the Merriamosauria as proposed two years earlier.^[7] This classification was generally accepted and retained in several phylogenetic analyzes concerning ichthyosaurs published from 2016 until 2021.^{[8][9][10][11]} In 2017, Benjamin C. Moon provisionally reclassified Thalattoarchon in the Merriamosauria, but his study proposing several cladograms for the genus, including a certain number classifying it again in the Cymbospondylidae, its positioning is then seen as not definitive.^[9] In 2021, Paul Martin Sander and his colleagues unanimously reclassified Thalattoarchon within the Merriamosauria, but this time in a position situating it as a sister taxon of the Shastasauridae.^[12][5]

The following cladogram shows the position of Thalattoarchon within the Ichthyosauria after Sander et al., (2021):^[12][5]

Paleobiology

Due to its teeth, Thalattoarchon is recognized as having been an apex predator, the abundant presence of marine reptiles within the Favret Formation suggesting that the animal would have regularly attacked the latter. Its position as a apex predator is also compared to that of current orcas, because its cutting teeth would have been adapted to attack prey of considerable size, and possibly even larger than Thalattoarchon itself, in the same way as killer whales.^[1][2][3][6] However, large ichthyosaurs like Cymbospondylus would not have been the animal's primary target, generally preferring to prey on medium-sized marine reptiles or juveniles.^[4]

Palaeoecology

Life restoration of a Cymbospondylus swimming near a group of Phalarodon, two ichthyosaurs contemporary with Thalattoarchon

Thalattoarchon is known from the Favret Formation, which, along with the Prida Formation, constitutes one of two recognized geological formations of the Star Peak Group, located in Nevada. These two formations are linked by a single member, known as the Fossil Hill Member. In the Prida Formation, this member outcrops west of the Humboldt Range, and extends to the Favret Formation, outcropping the Augusta Mountains,^[13] where it reaches up to more than 300 m wide.^[14][5] Although they are neighbors, the two formations do not share precisely the same age, the Prida one dating from the Middle Anisian, while Favret dates from the Late Anisian,^[14] between approximately 244 and 242 million years ago.^[5] The Anisian is the first of two stratigraphic stages of the Middle Triassic, spanning 247.2 to 242 million years ago.^[15]

The significant presence of marine reptiles, ammonites and other invertebrates in the Fossil Hill Member indicates that the surface waters were well aerated,^[13] but there is however little animal presence in the benthic zones, with the notable exception of bivalves of the Halobiidae family. The fossils found show that the rock unit was once a pelagic ecosystem with a stable food web. Bony fish are little known and have currently only been discovered in the Favret Formation. Among the fish discovered, we find the actinopterygians Saurichthys and an undetermined representative,^[16] while among the sarcopterygians, numerous specimens of indeterminate coelacanthids are known.^[5] The most abundant marine reptiles of the Fossil Hill Member are the ichthyosaurs, including Thalattoarchon itself, Phalarodon, Omphalosaurus and the large Cymbospondylus. Few other marine reptiles are known from the Fossil Hill Member, the only clearly identified being the sauropterygian Augustasaurus.^{[1][4][12][5]}

See also

• List of ichthyosaurs
• Timeline of ichthyosaur research

Notes

References

External links

Wikimedia Commons has media related to Thalattoarchon.

Wikispecies has information related to Thalattoarchon.

• Raúl Martín. "Life restoration of Thalattoarchon".