Thalattoarchon

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Thalattoarchon
Temporal range:
Ma
↓
FMNH
PR 3032)
Scientific classification Edit this 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 a

ichthyosaur from North America, containing the single species T. saurophagis. The taxon was described in 2013 from a single specimen discovered in the Favret Formation 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. 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 an ichthyosaur, Thalattoarchon had

fin on the tail. The size of this large ichthyosaur is estimated around 8.6 meters (28 ft) long, although the authors are uncertain 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 with two cutting edges 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. Thalattoarchon would have been an apex predator, its sharp and cutting teeth suggesting that it would have mainly preyed on contemporary marine reptiles. Its position as a apex predator is generally comparable to that of orcas
, which have a similar lifestyle.

Discovery and naming

The only specimen of Thalattoarchon was discovered in 1997 by the paleontologist Jim Holstein during a field expedition in the

pelvic girdle and rear flippers.[1]

It was in 2013 that this specimen was designated as the

genus name comes from Ancient Greek θάλασσα (thálassa, "sea"), and ἄρχον (árchon, "ruler"),[1] directly referencing the animal's imposing size and position as an apex predator of the Triassic seas.[3] The specific name also comes from Ancient Greek and is derived from the words σαῦρος (saûros, "lizard") and φᾰγεῖν (phageîn, "to eat"), in reference to its carnivorous diet.[1][4]

Description

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

The describing team estimates the size of Thalattoarchon at over 8.6 meters (28 ft) long, making it one of the largest known ichthyosaurs. Its size and

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.[1] The size of the skull is 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.[6]

Skull

The skull of the only specimen of Thalattoarchon is partially preserved, missing the entire

sclerotic ring. 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, forming the anteromedial edge of the superior temporal fenestrae and surrounding the pineal foramen. In front of the pineal foramen there is a small sagittal crest that 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

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]

Postcranial skeleton

Although the total number of vertebrae is not known in the animal, it is estimated that it would have had at least 60 in the presacral

cervical vertebrae. The posterior dorsal vertebrae are the same height and length as the middle dorsal vertebrae. However, the latter are characterized for having a flattened ventral surface and articular facets for the double-headed ribs, a trait also observed in some basal ichthyosaurs and in neoichthyosaurians dating from the Late Triassic to the Cretaceous.[4]

The caudal vertebrae of Thalattoarchon are laterally flattened and are twice as tall as they are wide, gradually decreasing in length. Large facets for the haemal arches, or chevrons, are present on the anterior caudal vertebrae. The animal's tail appears to be rather straight, having visibly no curvature and not having a turned-down end.[4]

Few bones are preserved in the pelvic girdle and hind limbs, the only one fully preserved being the right ilium. The end of this bone participating in the hip socket is similar to that of Cymbospondylus, but its articulations with the sacral ribs may have been rather weak, as suggested by its pointed upper end. The femur is flattened and broad, having a slightly narrowed body and an enlarged distal end, being on the whole broader than that of Cymbospondylus. Another preserved bone is interpreted as a zeugopodial element (the part of a limb corresponding to either a forearm or a leg), but it is uncertain whether it is a tibia or a fibula. Regardless, it has a concave margin, enlarged ends, and is wider than long. The hind limbs and the pelvic girdle are proportionally small in relation to the body of the animal, with just the length of the femur being twice the height of the caudal vertebrae.[4]

Classification

In the

sister taxon to C. nichollsi.[9] In 2021, Sander and his colleagues reclassified Thalattoarchon within the Merriamosauria, but this time as the sister taxon of the Shastasauridae.[12][5]

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

Ichthyosauromorpha

Paleobiology

Due to its bicarinate, cutting teeth and its large, robust head, Thalattoarchon is recognized as having been an apex predator. The abundant presence of marine reptiles within the Favret Formation suggests that the animal would have regularly attacked them. Its position as an apex predator is comparable to that of current orcas, because its cutting teeth would have been adapted to attack prey of considerable size, 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 Thalattoarchon's primary target; the generally preferred prey would be medium-sized marine reptiles or juveniles instead.[4] In 2021, Sander and his colleagues suggested that if Thalattoarchon would have fed on Cymbospondylus, it would have preyed on it using a pack-hunting strategy, a behavior that is not at all reported in the fossil record. The authors of this study also suggest that Thalattoarchon would have only fed on smaller animals, both alive and dead, a feeding strategy comparable to that of the great white shark. Due to its apex predator adaptations, Thalattoarchon would not likely feed on ammonites, squid, or even fish.[5]

Palaeoecology

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 in the Augusta Mountains,[13] where it reaches up to more than 300 metres (980 ft) 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 significant presence of marine reptiles, ammonites and other

sauropterygian Augustasaurus.[1][4][12][5]

See also

Notes

  1. ^ For an unclear reason, some sources list the year of discovery of the fossil specimen as 2010.[3]

References

  1. ^
    PMID 23297200
    .
  2. ^ a b c Riley Black (January 7, 2013). "Sharp-Toothed Thalattoarchon Was the First Ruler of the Triassic Seas". National Geographic. Archived from the original on May 7, 2021.
  3. ^
    Field Museum (January 7, 2013). "Giant fossil predator provides insights into the rise of modern marine ecosystem structures". ScienceDaily
    .
  4. ^
    doi:10.1073/pnas.1216750110
    .
  5. ^
    doi:10.1126/science.abf5787
    .
  6. ^ a b Ryan Marek (2015). "Fossil Focus: Ichthyosaurs". Palaeontology Online. 5: 8. Archived from the original on 18 January 2021. Retrieved 13 June 2020.
  7. S2CID 85621052
    .
  8. PMID 27211319
    .
  9. ^
    S2CID 90912678
    .
  10. PMID 32385319
    .
  11. PMID 33996277
    .
  12. ^
    S2CID 245444783
    .
  13. ^
    S2CID 129115608
    .
  14. ^
    S2CID 219078178
    .
  15. S2CID 133180790
    .

External links

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