Friday 3 November 2023

Amanasaurus nesbitti: A new species of Silesaurid Dinosauromorph from the Late Triassic of Brazil.

Silesaurids were a group of Triassic Archosaurs which typically possessed slender limbs and a beak-like projection from the tip of the lower jaw. They are known from the Middle Triassic of Brazil, Tanzania, and Zambia, supporting a Gondwanan origin, with Late Triassic specimens known from both Gondwana and Laurasia. No Jurassic or later Silesaurid has ever been discovered, implying that they died out in the End Triassic Extinction. The exact phylogenetic position of the Silesaurids is unclear, with suggestions having been made that they are the closest relatives and sister group to the Dinosaurs, or that they are Ornithischian Dinosaurs, ever being a distinct clade which forms a sister group to all other members of the Ornithischia, or as a series of early-branching clades forming a stem-group to the 'core Ornithischians'.

In a paper published in the journal Scientific Reports on 11 April 2023, Rodrigo Müller and Maurício Garcia of the Centro de Apoio à Pesquisa Paleontológica da Quarta Colônia and Programa de Pós‑Graduação em Biodiversidade Animal at the Universidade Federal de Santa Maria, describe a new species of Silesaurid from the Carnian (earliest Late Triassic) of Rio Grande do Sul State, Brazil. 

The new species is named Amanasaurus nesbitti, where 'Amanasaurus' means 'rain-Lizard' in reference to the Carnian Pluvial Episode, a period of climatic change at the onset of the Late Triassic associated with significant species turnover and the emergence of a number of significant groups, during which this species would have lived, and 'nesbitti' honours palaeontologist Sterling Nesbitt, an expert on Triassic Archosaurs, who first described the Silesauridae. It is described on the basis of the proximal portion of a right femur from the lower portion of the Candelária Sequence between the municipalities of Restinga Sêca and São João do Polêsine. A distal portion of a left femur from an individual slightly larger than the holotype and excavated from the same locality is also referred to the species.

Provenance of Amanasaurus nesbitti. (a) Surface distribution of the geologic units in the area depicting the location of the Pivetta site. (b) General view of the Pivetta site. (c) Hypothetical reconstruction of the skeleton of Amanasaurus nesbitti depicting (in orange) the preserved portions. (d) CAPPA/UFSM 0374 (holotype), a proximal portion of a right femur in anterior view. (e) CAPPA/UFSM 0375 (referred specimen), a distal portion of a lef femur in anterior view. Müller & Garcia (2023).

Although this material is extremely fragmentary, The proximal femur portion is well preserved and possesses typical traits of Silesaurs, such as the presence of a notch between the ventral transition from the femoral head to the femoral shaft and a straight medial articular facet of the proximal portion in proximal view, making Müller and Garcia confident in the assignment of the specimen to the Silesauridae, as well as a number of unique features, including the absence of a posteromedial tuber of the femoral head, the ventral margin of the anteromedial tuber exceeding the femoral head margin, the presence of a fossa trochanterica, the absence of a raised anterolateral scar, the ; presence of a semi-circular scar on the posterodorsal surface of the femoral head, and a cleft between the proximal tip of the anterior trochanter and the femoral shaft.

Holotype and referred specimen of Amanasaurus nesbitti from the Candelária Sequence (mid-to-late Carnian) of the Santa Maria Super-sequence, southern Brazil. Holotype (CAPPA/UFSM 0374) in anterior (a), lateral (b), proximal (c), medial (d), and posterior (e) views. Referred specimen (CAPPA/UFSM 0375) in anterior (f), lateral (g), posterior (h), and distal (i) views. Abbreviations: alt, anterolateral tuber; amt, anteromedial tuber; at, anterior trochanter; cl, cleft; ctf, crista tibiofbularis; dlt, dorsolateral trochanter; dltp, posterior portion of the dorsolateral trochanter; fo, foramen; fot, fossa trochanterica; gt, greater trochanter; lc, lateral condyle; lia, linea intermuscularis cranialis; mc, medial condyle; ms, muscle scar; no, notch; pg, proximal groove; pof, popliteal fossa; scs, sub-circular scar. Müller & Garcia (2023).

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