Finding a place in the sun: First cladistic analysis investigating the affinities of the pleurodire turtle Apodichelys lucianoi

BOGADO, J.P.; DE LA FUENTE, M.S.; PAGLARELLI BERGQVIST, L.; GOMES DA COSTA PEREIRA, P.V.L.

Simposio; XII Simposio Brasileiro de Paleontología de Vertebrados; 2023

Apodichelys lucianoi is a fossil turtle from the Upper Cretaceous marine limestones of the Jandaíra Formation, Potiguar Basin, known only by its holotype, an internal cast of a shell. It can be confidently identified as a pleurodire due to the sutural contact between its pelvic girdle and shell, and as a pelomedusoid due to impressions of small, hexagonal mesoplastra in the cast of the plastron. However, the scarcity of anatomical information revealed by the available material makes the attribution of this turtle to a less inclusive taxon very challenging, and only few studies concerning its taxonomic affinities have been made. One hypothesis suggests that its shell morphology indicates it was a close relative of the genus Bothremys and a member of the Bothremydidae, an extinct pelomedusoid family. However, this proposal has fallen out of favour, as more recent contributions regarded A. lucianoi as an incertae sedis pelomedusoid, too poorly known to be included in cladistics analysis. However, a recent reassessment of the holotype, including a CT-scan analysis of the cast, coupled with advances in the phylogenetic studies of pleurodiran turtles and the addition of new shell characters to phylogenetic matrices, have led us to challenge this idea, performing the first cladistics analysis including Apodichelys lucianoi as a terminal taxon. We ran a traditional search in TNT 1.5 (1.000 replicates, TBR, hold 20, implied weighting with k=12), with a matrix of 105 terminals and 268 characters, 40 of which were ordered. The analysis recovered 81 most parsimonious trees, from which a single strict consensus tree was generated (CI=0.277; RI=0.726). Apodichelys lucianoi was recovered in a basal position within the Bothremydini, a clade within the Bothremydidae. This placement is supported by two synapomorphies shared with the remaining Bothremydini: a short anterior plastral lobe that does not reach the anterior carapace margin (character 215, state 1) and the presence of a nuchal notch (character 169, state 1). This result allows us to have a better understanding of the relationships of Apodichelys with other pelomedusoids, while also resurrecting an old proposal that had since fallen out of usage.