Morphological support of Ornithoscelida Huxley, 1870

• Autor(a) principal: Kirmse, João Pedro Silva
• Data de Publicação: 2021
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações da USP
• Texto Completo: https://www.teses.usp.br/teses/disponiveis/59/59139/tde-24082021-141444/
• topic: Anatomia de dinossauros; Dinosaur anatomy; Dinosauria; Dinosauria; Filogenética; Ornithoscelida; Ornithoscelida; Phylogenetics; Saurischia; Saurischia

Early dinosaur radiation has been a controversial topic for years, and new discoveries constantly change the overview of the area. Recent studies have reignited the debate on the relations of the major dinosaur groups and the anatomic traits that characterise them. The traditional Ornithischia and Saurischia (Sauropodomorpha + Theropoda + Herrerasauridae) scheme was challenged in favour of an Ornithoscelida (Ornithischia + Theropoda) and Saurischia (Sauropodomorpha + Herrerasauridae) hypothesis. There was not, however, an exercise of closer scrutiny of the characters used. The main objective of this work is to review the supposedly diagnostic features for Ornithoscelida Huxley, 1870; the recently resurfaced group. This was done through redefinition and rescoring of the characters recovered as synapomorphic for the group, to better understand their distribution. The anatomic data and basis for the recodifications were extracted from personal observations of the specimens, with the goal of assembling a sturdy database. The distribution analyses make clear the support for the clade is weak at closer inspection. One of the most consistent findings so far is that most evaluated characters do not encompass the full gamut of morphologies present in the dinosaur lineage. E.g., the acetabular wall is completely closed in forms such as Lagerpeton, has a straight margin in Saturnalia, is partially excavated in Herrerasaurus, is almost fully opened, with only a small round margin, in taxa such as Coloradisaurus, and fully opened in Eocursor. Therefore, scoring the acetabular wall simply as present or absent does not represent the full variability of the trait and misses relevant information. Some characters have unclear homology series, such as the post temporal fenestra, as it remains unclear which of the reduced apertures of modified taxa represent the large plesiomorphic element. In the end, the observed morphology does not support an unequivocal grouping of theropods and ornithischians. Moreover, once the modified scorings are included in the data matrices, Saurischia is again recovered in both. Ornithoscelida does not provide a strong enough challenge to Saurischia, but showcases a certain negligence when dealing with the characters themselves in palaeontological studies. The rigid scrutiny of the construction, distribution, and meaning of characters is found to be the main way of eliminating uncertainties from phylogenetic studies.