Thalattosauria
Thalattosauria Temporal range: Middle-Late Triassic,
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A collage of thalattosaur fossils. Clockwise from upper left: Thalattosaurus alexandrae (a thalattosauroid)
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Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Reptilia |
Clade: | Neodiapsida |
Order: | †Thalattosauria Merriam , 1904
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Superfamilies | |
Synonyms | |
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Thalattosauria (
Description
Thalattosaurs have moderate adaptations to marine lifestyles, including long, paddle-like tails and slender bodies with more than 20 dorsal vertebrae. There are few unique traits of the postcranial skeleton shared by all thalattosaurs, but the skeleton is still useful for distinguishing between askeptosauroids and thalattosauroids. Askeptosauroids are characterized by elongated necks with short neural spines and at least 11 vertebrae, while thalattosauroids have shorter necks sometimes involving as few as four vertebrae. Thalattosauroids also have tall neural spines on their neck, back, and especially the tail vertebrae, increasing the surface area for swimming via lateral undulation. Thalattosauroids additionally possess short, wide limb bones poorly adapted for movement on land. In this superfamily, the humerus is widest near the shoulder, the femur is widest near the knee, the radius is reniform ("kidney-shaped"), and phalanges are long and plate-like. Askeptosauroids retain hourglass-shaped limb bones like land reptiles, but even they share specializations with thalattosauroids such as a short tibia and fibula, with the latter expanding near the ankle.[1][9][2]
Skull
Thalattosaurs are diapsid reptiles, meaning that they have temporal fenestrae, two holes in the head behind the orbit (eye socket). However, many thalattosaurs have a vestigial upper temporal fenestra which is slit-like, and some have it fully closed up by surrounding bones.[10] Thalattosaurs lack a quadratojugal bone, leaving the lower temporal fenestra open from below. They also lack postparietal and tabular bones, while the squamosal bone is small, the supratemporal bone is extensive, and the quadrate bone is large. When seen from above, the rear edge of the skull bears a large, triangular embayment that reaches further forwards than the quadrates.[5]
Thalattosaurs have a rostrum (snout) significantly longer than the portion of the skull behind the eyes. A majority of this length is formed from the premaxillary bones, and the nares (nostril holes) are shifted back close to the eyes. The premaxillae stretch back very far and are incised into the frontal bones. This leads to an unusual trait that is characteristic of thalattosaurs, where the left and right nasal bones are separated from each other and restricted to a small portion of the snout near the nares. The lacrimal bone is typically lost or fused to the large crescent-shaped prefrontal bone in front of the orbit, mirroring the postfrontal bone which is usually fused to the three-pronged postorbital bone behind the orbit.[10][11][5]
Paleobiology
Thalattosaurs are only known from marine deposits, indicating that they were all primarily aquatic reptiles. The retracted nostrils and long, paddle-shaped tail are further evidence for aquatic habits. Thalattosauroids seemingly spent all of their time in the water, with short, wide limbs, poorly developed wrist and ankle bones, and tall vertebrae adapted for swimming via
Thalattosaurs had diverse diets, though they probably all involved marine animals in one way or another.
Distribution
It is not certain where thalattosaurs originated from. During the Triassic period, the earth had one giant supercontinent, Pangaea, which was surrounded by the superocean Panthalassa. The eastern portion of Pangaea was incised by a massive tropical inland sea, the Tethys Ocean, which extended all the way from China to Western Europe. While thalattosauroids are known from worldwide Triassic marine deposits, askeptosauroids are only known in Tethyan deposits. Assuming Endennasaurus and Askeptosaurus were the most basal askeptosauroids, Askeptosauroidea would have originated in the Western Tethys Ocean, now the Alpine region of Europe.[1] However, if Miodentosaurus is more basal, a Western Tethys (European) origin would be significantly less likely.[19] Although the sister group to Thalattosauria is still debated, one possibility, the icthyosauromorphs, seemingly evolved in the Eastern Tethys (China) during the early Triassic or earlier.[8]
The oldest known thalattosauroids (Thalattosaurus, Paralonectes, and Agkistrognathus of British Columbia's Sulphur Mountain Formation) lived in eastern Panthalassa, along what is now the western coast of North America. Müller (2005, 2007) argued that at least one branch of thalattosauroids had managed to spread worldwide early in their evolution.[1][20] However, this is based on the hypothesis that Nectosaurus (from California), Xinpusaurus (from China), and an unnamed species from Austria formed a clade basal to other thalattosaurs, a classification scheme which contrasts with many other studies.[9] The worldwide distribution of Thalattosauroidea is intriguing considering that thalattosaurs are considered to be poorly adapted for traversing open oceans, which would have been a necessity for spreading between the eastern coast of Panthalassa and the Tethys Ocean.[20] Coastal "refuges" such as volcanic island arcs and guyots may have facilitated the ability of thalattosaurs to spread between ocean basins.[10] Hescheleria-like forms were previously only reported from North America and Europe,[21] but in 2021 a Hescheleria-like snout fragment was reported from China, indicating that they also had a widespread distribution.[22] Trans-Panthalassa connections are also observed in other Triassic marine life such as pistosaurs and ammonites.[10] Evidently thalattosaurs were capable of dispersing throughout major marine regions multiple times before the group's extinction, with thalattosauroids likely more prolific at spreading than askeptosauroids due to their greater aquatic adaptations.[2]
Classification
Early hypotheses
When first named by Merriam in 1904, Thalattosauria was only known by the species Thalattosaurus alexandrae. Based primarily on the overall skull shape, it was hypothesized to have been close to the reptile order Rhynchocephalia, which includes Sphenodon (the living tuatara). Nevertheless, Thalattosaurus was recognized as distinct enough to be given its own order, and was tentatively grouped along with Rhynchocephalia in the group Diaptosauria, a collection of various "primitive" reptiles now known to be polyphyletic. Within Diaptosauria, thalattosaurs were also considered very closely related to choristoderes and "Proganosauria" (parareptiles). Comparisons were also made with Parasuchia (phytosaurs), Lacertilia (lizards), and Proterosuchus, but dismissed as incompatible with proposed evolutionary schemes.[23]
Further discussion by Merriam (1905) considered a relationship with
Modern classification and external relationships
The rising popularity of
An analysis by
Internal relationships
One of the first phylogenetic analyses specifically focusing on thalattosaurs was part of Nicholls (1999)'s reevaluation of Thalattosaurus and Nectosaurus. She used a restricted definition of Thalattosauria which referred to a clade including all reptiles more closely related to Nectosaurus and Hescheleria than to Endennasaurus or Askeptosaurus. The more inclusive group including Askeptosaurus, Endennasaurus, and traditional thalattosaurs was given the name Thalattosauriformes.[14][1][20]
However, most studies focusing on the group have preferred to retain a broader definition of Thalattosauria equivalent to Nicholls' Thalattosauriformes clade, including reptiles close to both Askeptosaurus and Thalattosaurus. In these studies, Thalattosauria is divided into two branches, one leading to relatives of Askeptosaurus and the other leading to relatives of Thalattosaurus. The clade containing reptiles closer to Thalattosaurus than to askeptosaurids is given the name Thalattosauroidea (and sometimes called Thalattosauridea[9][19]). Meanwhile, the clade containing reptiles closer to askeptosaurids is termed Askeptosauroidea[10][13][2] or Askeptosauridea.[9][19]
Subsequent studies since Nicholls (1999) started to include more taxa, including newly described Chinese taxa such as Anshunsaurus and Xinpusaurus.[10][27] However, uncertainty over Endennasaurus's thalattosaurian ancestry led to it being excluded from these analyses. After Müller et al. (2005) re-affirmed that Endennasaurus was closely related to Askeptosaurus,[12] all thalattosaurs known at the time were finally combined into phylogenetic analyses.[9][20] Studies by Rieppel, Liu, Cheng, Wu, and others continued to identify new Chinese taxa such as Miodentosaurus and various species of Anshunsaurus and Xinpusaurus, though homoplasy in these new taxa has led to little resolution in the structure of the two major branches of Thalattosauria.[13][28] In an attempt to remedy this problem, new phylogenetic analyses were developed by Liu et al. (2013) during the description of Concavispina,[19] and Druckenmiller et al. (2020) during the description of Gunakadeit.[2]
The internal relationships of thalattosaurs is still considered tentative and inconclusive, although the fundamental structure of the group (a monophyletic Thalattosauria clade split into askeptosauroids and thalattosauroids) is very stable. Some paleontologists have attempted to divide thalattosaurs into families. One family, Askeptosauridae, is typically considered to include Askeptosaurus and Anshunsaurus,[9] with a few studies also placing Miodentosaurus[13] or Endennasaurus[12] within it. Another family, Thalattosauridae, was originally used to group Thalattosaurus and Nectosaurus,[3] was later redefined to exclude Nectosaurus,[14] and later still encompassed practically all thalattosauroids.[19] Many thalattosaur-focused paleontologists avoid using family names due to their inconsistent usage and questionable validity.
The cladogram presented here is based on the largest and most recent analysis of thalattosaur ingroup relations, Druckenmiller et al. (2020). It shows all thalattosaur genera except for the fragmentary Agkistrognathus.[2]
Thalattosauria | |
List of genera
Other thalattosaurs include unnamed or indeterminate species from the
Name | Year | Formation | Location | Notes | Images |
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Agkistrognathus | 1993 | Sulphur Mountain Formation (Middle Triassic?) | Canada ( British Columbia) | A poorly-known thalattosauroid with strong jaws | |
Anshunsaurus | 1999 | Zhuganpo Formation, Xiaowa Formation (Middle Triassic-Late Triassic, Ladinian?-Carnian?) | China | A large askeptosauroid known from three species. One of the few thalattosaurs for which a growth series is known | |
Askeptosaurus | 1925 | Grenzbitumenzone (Middle Triassic, Anisian ?)
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Switzerland, Italy | The namesake of Askeptosauroidea and one of the most well-known European thalattosaurs | |
Clarazia | 1936 | Grenzbitumenzone (Middle Triassic, Anisian ?)
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Switzerland | A thalattosauroid related to Hescheleria | |
Concavispina | 2013 | Xiaowa Formation (Late Triassic, Carnian?) | China | The largest known thalattosauroid, a close relative of Xinpusaurus | |
Endennasaurus | 1984 | Zorzino Limestone (Late Triassic, Norian )
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Italy | An unusual askeptosauroid with a pointed, toothless snout | |
Gunakadeit | 2020 | Hound Island Volcanics (Late Triassic, Norian) | United States ( Alaska) | A basal thalattosauroid, the most well-preserved specimen from North America | |
Hescheleria | 1936 | Grenzbitumenzone (Middle Triassic, Anisian ?)
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Switzerland | A hook-snouted thalattosauroid | |
Miodentosaurus | 2007 | Xiaowa Formation (Late Triassic, Carnian?) | China | A very large askeptosauroid with a short snout | |
Nectosaurus | 1905 | Hosselkus Limestone (Late Triassic) | United States ( California) | One of the first thalattosaurs to be described, along with Thalattosaurus | |
Paralonectes | 1993 | Sulphur Mountain Formation (Middle Triassic?) | Canada ( British Columbia) | A poorly-known thalattosauroid with a downcurved snout | |
Wapitisaurus | 1988 | Sulphur Mountain Formation (Early Triassic) | Canada ( British Columbia) | A thalattosauroid initially described as a weigeltisaurid.[35]
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Wayaosaurus | 2000 | " Wayao Member" (Late Triassic, Carnian ?)
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China | A large askeptosauroid similar to Miodentosaurus. Initially described as a pachypleurosaur.[36] | |
Thalattosaurus | 1904 | Hosselkus Limestone, Sulphur Mountain Formation (Middle Triassic-Late Triassic) | United States ( California), Canada ( British Columbia) | The namesake of Thalattosauria and the first genus to be described. Known from at least two species | |
Xinpusaurus | 2000 | Zhuganpo Formation, Xiaowa Formation (Middle Triassic-Late Triassic, Ladinian?-Carnian?) | China | A thalattosauroid with an unusual notched skull. Known from four species, though not all may be valid |
References
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- ^ a b c d Merriam, John C. (1905). "The Thalattosauria: a group of marine reptiles from the Triassic of California". Memoirs of the California Academy of Sciences. 5 (1): 1–52.
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- ^ a b c d e Evans, Susan E. (1988). "The early history and relationships of the Diapsida". In Benton, M. J. (ed.). The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds. Oxford: Clarendon Press. pp. 221–260.
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- ^ a b c d e Wu, Xiao-Chun; Cheng, Yen-Nien; Sato, Tamaki; Shan, Hsi-Yin (2009). "Miodentosaurus brevis Cheng et al. 2007 (Diapsida: Thalattosauria): Its postcranial skeleton and phylogenetic relationships". Vertebrata PalAsiatica. 47 (1): 1–20.
- ^ a b c d e Nicholls, Elizabeth L. (April 15, 1999). "A reexamination of Thalattosaurus and Nectosaurus and the relationships of the Thalattosauria (Reptilia: Diapsida)". PaleoBios. 19 (1): 1–29.
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- ^ Metz, Eric T.; Druckenmiller, Patrick S.; Carr, Gregory (2015). "A new thalattosaur from the Vester Formation (Carnian) of Central Oregon, USA". SVP 75th Annual Meeting Abstracts of Papers.
- ^ "Triassic Reptile Skewered Clams with Teeth on Roof of Its Mouth". Live Science. 13 November 2015.
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