OBSERVATIONS ON THE MORPHOLOGICAL SUPPORT FOR ORNITHOSCELIDA HUXLEY, 1870

Early dinosaur radiation has been a controversial topic for years, and new discoveries constantly change the understanding 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. The main objective of this work is to review the putative diagnostic features of Ornithoscelida Huxley, 1870, what shall be done with the redefinition and rescoring of the characters recovered as synapomorphic for the group, aiming at a better understanding of their distribution. 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. I.e., 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. As for the femoral anterior trochanter, the plesiomorphic anatomy is unclear, as the element is absent in lagerpetids. Its blade-like shape seen in ornithischians and neotheropods may not be homologous, as the structure in taxa such as Tawa differs not only from those modified forms, but also from sauropodomorphs, which have a diminutive possibly apomorphic trochanter. Moreover, once the so far modified scorings are included in the data matrices, Ornithoscelida is not recovered, with a large polytomy uniting the dinosaur major groups being found instead. We hope that, with more compressive observations and scorings, the actual distribution of characteristics will be elucidated, and their phylogenetic signal made clear. [FAPESP 2018/19178-8, 2019/02167-6, & 2014/03825-3]