[Paleontology • 2023] Arenaerpeton supinatus • A New chigutisaurid (Temnospondyli: Brachyopoidea) with Soft Tissue Preservation from the Triassic Sydney Basin, New South Wales, Australia

Arenaerpeton supinatus

Hart, Gee, Smith & McCurry, 2023

DOI: 10.1080/02724634.2023.2232829 

Reconstruction by José Vitor Silva.

ABSTRACT

Compared with other Mesozoic tetrapod groups, chigutisaurid fossils from Australia are rare, with only three named taxa described from the continent. From Queensland, Keratobrachyops australis is known from the Triassic, and Siderops kehli from the Jurassic. Koolasuchus cleelandi, from the Cretaceous of Victoria, represents the youngest-known temnospondyl globally. Here we describe the first chigutisaurid from New South Wales, from the Early–Middle Triassic Terrigal Formation. The specimen (AM F125866) comprises an articulated, near-complete skeleton, presented in ventral aspect, as well as outlines of soft tissue. The new taxon preserves features that indicate an affinity with Chigutisauridae, confirming previous hypotheses on the presence of large-bodied chigutisaurids in the Triassic of Australia. The new Terrigal chigutisaurid is only the second chigutisaurid known from the Lower Triassic and fourth from Australia overall. The distribution of brachycephalic temnospondyls throughout the Mesozoic suggests specific adaptations led to the long-term survival of chigutisaurids, especially across the end-Triassic extinction event.

SYSTEMATIC PALEONTOLOGY

TEMNOSPONDYLI von Zittel, Citation1887–1890

STEREOSPONDYLI von Zittel, Citation1887–1890

BRACHYOPOIDEA Lydekker, Citation1885 (sensu Warren & Marsicano, Citation2000)

CHIGUTISAURIDAE Rusconi, Citation1951

 Arenaerpeton supinatus, AM F125866, articulated skeleton.
A, full fossil in ventral view; B, schematic interpretation.

Abbreviations: cl, clavicle; cp, cultriform process of the parasphenoid; ect, ectopterygoid; eo, exoccipital; h, humerus; ic, interclavicle; m, mandible; mca, metacarpal; mx, maxilla; p, pelvis; pmx, premaxilla; psph, parasphenoid; pt, pterygoid; q, quadrate; qj, quadratojugal ra, radius; s, scapula; ul, ulna; v, vomer, ve, vertebrae. 

Light gray elements represent exposed bone. Darker sections represent cavities in the matrix. Unfilled outlines indicate unexposed bones within the matrix. Photograph by Thomas Peachey.

ARENAERPETON SUPINATUS, gen. et sp. nov.

Holotype—AM F125866 (Fig. 2), an articulated skeleton containing cranial and postcranial elements, exposed in ventral aspect.

Locality and Horizon— The holotype derives from Kincumber Quarry, on the Central Coast of New South Wales, approximately 90 km north of Sydney, Australia (Fig. 1A). The fossil is preserved in sandstone from within the Terrigal Formation of the Lower–Middle Triassic Sydney Basin (Spathian–Anisian) (Fig. 1B, C).

Diagnosis—Arenaerpeton supinatus possesses a brachycephalic skull, as seen in all other brachyopoids and some dvinosaurs but can be excluded from Dvinosauria due to the presence of a flat cultriform process of the parasphenoid which does not expand anteriorly (Warren & Marsicano, Citation2000). Arenaerpeton supinatus is included within Chigutisauridae as it exhibits a large number of small marginal teeth on the premaxilla and maxilla (as opposed to the large and sparse teeth seen in Brachyopidae); and a postquadratojugal projection (Warren & Marsicano, Citation1998). Arenaerpeton supinatus is identified as a new chigutisaurid due to the following unique combination of characters: (1) pterygoid and exoccipital suture (shared with Compsocerops—may also be present in other chigutisaurids); (2) corpus of pterygoid and parasphenoid unornamented (shared with all chigutisaurids except Keratobrachyops); (3) parasphenoid cultriform process narrow and flat (shared with Pelorocephalus and Compsocerops); (4) vomerine depression anterior to the cultriform process (shared with Keratobrachyops and Siderops); (5) parasphenoid corpus almost as wide as long (shared with all chigutisaurids except Compsocerops); (6) ventral surface of the cultriform process of the parasphenoid lacking a keel (shared with Siderops, Compsocerops, and Kuttycephalus); (7) mandible post-glenoid area slender and elongate (shared with Keratobrachyops and Pelorocephalus); (8) adjacent intercentra contact (shared with Siderops and Pelorocephalus); (9) ossified ceratobranchials absent in adult (shared with Kuttycephalus and Compsocerops).

Etymology—The genus name is a combination of the Latin ‘arena,’ meaning ‘sand,’ in reference to the sandstone matrix in which the fossil is preserved, and ‘erpeton’ meaning ‘thing that creeps,’ commonly used for fossil amphibians and reptiles. The species name is the Latin ‘supinatus,’ meaning ‘on its back,’ in reference to the holotype specimen being preserved in ventral aspect.

 Head width and temporal span of chigutisaurids throughout the Mesozoic. Skull silhouette based on Pelorocephalus mendozensis. Silhouettes of Australia, India, and South America used under Creative Commons Zero 1.0 Public Domain License from Openclipart.org.

 Reconstruction of  Arenaerpeton supinatus, preying on Cleithrolepis granulata.

Reconstruction by José Vitor Silva.

CONCLUSIONS

Chigutisaurids are a key component in South American Triassic ecosystems (Dias-da-Silva et al., 2012; Marsicano, 2005) but are surpassed in species diversity by other temnospondyl clades in Australia (where brachyopids, mastodonsaurids, and rhytidosteids are more common). Here we describe Arenaerpeton supinatus (Fig. 8), the fourth chigutisaurid taxon from Australia and the first from the Sydney Basin. It is also only the second chigutisaurid from the Early Triassic and adds weight to the observations of Gee and Sidor (2021) on the asymmetric distributions of brachyopoids throughout the Mesozoic. Chigutisauridae is strictly a Gondwanan clade which has no records in the Middle Triassic, although constituent taxa have a temporal span from the Early Triassic through to the Early Cretaceous (Dias-da-Silva et al., 2012; Marsicano, 1999; Warren & Marsicano, 2000). However, brachyopids are known throughout the Middle Triassic (Gee & Sidor, 2021; Warren & Marsicano, 2000) in Gondwana. Chigutisaurids and brachyopids only co-occur in two localities, both in Australia: Arenaerpeton supinatus with Platycepsion wilkinsoni (from the Terrigal Formation, NSW); and Keratobrachyops australis with Xenobrachyops allos (from the Arcadia Formation, QLD). Apart from these two occurrences, brachyopids, chigutisaurids (and plagiosaurids, also a brachycephalic clade) show marked differentiation in their distribution. This may indicate that an ecological (perhaps an ability to estivate or tolerate unfavorable climatic conditions) or morphological trait (such as overall large body size, or skull shape) drove this segregation, and possibly also assisted chigutisaurids in their survival through to the Cretaceous.

 

Lachlan J. Hart, Bryan M. Gee, Patrick M. Smith and Matthew R. McCurry. 2023. A New chigutisaurid (Brachyopoidea, Temnospondyli) with Soft Tissue Preservation from the Triassic Sydney Basin, New South Wales, Australia. Journal of Vertebrate Paleontology.  e2232829. DOI: 10.1080/02724634.2023.2232829   twitter.com/Dinoh555/status/1687236171707887616