Modern Amphibians all belong to a single group, the Lisamphibians, which are quite closely related to the Amniotes (Reptiles, Birds, and Mammals). However, in the Carboniferous other Amphibian groups, such as the Temnospondyls and Ichthyostegids, which were more distantly related to the Amniotes. Although considered to be 'primitive' compared to modern Tetrapods, some Temnospondyls independently acquired Reptile-like traits such as the scaley skin and the ability to live away from water. The majority of Temnospondyls perished in the End Permian Extinction, but a few groups survived into the Mesozoic, and persisted as long as the Cretaceous. One of these groups was the Chigutisauridae, an exclusively Gondwanan group of wide-headed predatory, aquatic Temnospondyls, which first appeared in the Early Triassic of Australia and persisted across the southern continents until the mid-Cretaceous. However, despite having originated in Australia, the Australian fossil record of Chigutisaurids is very sparse, with three known species from Queensland, one from the Early Triassic, one from the Early Jurassic, and one from the Early Cretaceous, plus a possible species from the Late Triassic of South Australia.
In a paper published in the Journal of Vertebrate Paleontology on 3 August 2023, Lachlan Hart of the Earth and Sustainability Science Research Centre at the University of New South Wales, and the Australian Museum Research Institute, Bryan Gee of the Burke Museum and Department of Biology at the University of Washington, Patrick Smith, also of the Australian Museum Research Institute, and of the Department of Biological Sciences at Macquarie University, and Matthew McCurry, also of the Earth and Sustainability Science Research Centre at the University of New South Wales, and the Australian Museum Research Institute, and of the Department of Paleobiology at the Smithsonian National Museum of Natural History, describe a new species of Chigutisaurid Temnospondyl from the Early–Middle Triassic Terrigal Formation of New South Wales.
The new species is described from a single specimen, AM F125866, which was found among rocks obtained from Kincumber Quarry, approximately 90 km to the north of Sydney on the Central Coast of New South Wales, for the purpose of building a wall on a private property, and donated to the Australian Museum in the mid 1990s. The specimen is preserved in ventral aspect (belly-up) in a piece of sandstone, with the forward part of the skeleton and the articulated skull preserved, as well as a soft-tissue outline of the body. Only the part (body fossil) of the specimen is known, the counterpart (would of the fossil) was not found, and was apparently lost during the quarrying process. The fossil proved to be impossible to further separate from the matrix without damaging it, and an attempt to X-ray the specimen using a large cargo scanner used by the Australian Border Force proved unsuccessful, as there was to little contrast between the fossil and the matrix.
The new species is named Arenaerpeton supinatus, where 'Arenaerpeton' means 'thing that creeps in the sand' a reference to the sandstone matrix in which the fossil was found, and 'supinatus' means 'on its back' a reference to the position in which the fossil was found. The specimen from which it is described comprises elements of the pre-maxilla, maxilla, vomers, parasphenoid, exoccipital, pterygoid, quadrate, quadratojugal, mandible, and possible remnants of the ectopterygoid, the onterclavicle and clavicle, a possible scapula, a humerus, radius, ulna, and metacarpal, and a near complete precaudal vertebral series with associated ribs. A small piece of what might be the pelvis is present, but no other parts of the pelvic girdle, hindlimbs, or tail.
Despite a small number of known fossils, Chigutisaurids are the only group of Temnospondyls which are known to have survived the End Triassic Extinction in Australia. Arenaerpeton supinatus is the second known Triassic species from Australia, along with Keratobrachyops australis, from the Early Triassic of Queensland (another Late Triassic probable Chigutisaurid is known from the South Australia, but this is poorly preserved and has not formally been described). Also known from Queensland are the Early Jurassic Siderops kehli, and the Early Cretaceous Koolasuchus cleelandi. Both of the post-Triassic species were large for Temnospondyls, with Siderops kehli having a total estimated length of 2.6 m, while Koolasuchus cleelandi reached about 3 m, while the Early Triassic Keratobrachyops australis was much smaller; its length cannot be reconstructed from the available material, but the largest known specimen has a maximum skull width of 14.5 cm. Arenaerpeton supinatus appears to be intermediate in size, with a maximum skull width of 23.6 cm, and a bodylength of 94 cm excluding the tail (which was not preserved, comparable with Triassic Chigutisaurids from India and South America.
Arenaerpeton supinatus is the second known Temnospondyl from the Terrigal Formation of the Sydney Basin, the other being the Brachyopid Platycepsion wilkinsoni. This is only the second known co-occurrence of Chigutisaurid and Brachyopid Temnospondyls (the only two Temnospondyl groups which survived the End Triassic Extinction), with the two groups otherwise tending to be found in different settings, possibly implying different ecological specializations.
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