COMPUTED MICROTOMOGRAPHY REVEALS NOVEL MORPHOLOGICAL TRAITS OF THE UPPER CRETACEOUS TURTLE Apodichelys lucianoi

BOGADO, J.P.; DE LA FUENTE, M. S.; MACHADO, A. S.; ARAUJO, O.M.; LOPES, R.T.; BERGQVIST, L.P; PEREIRA, P.V.

Trelew

Jornada; XXXV Jornadas Argentinas de Paleontología de Vertebrados; 2022

MEF

Apodichelys lucianoi is a pelomedusoid turtle from the Upper Cretaceous Jandaíra Formation of Brazil. Described in the 1950’s, it is known from a single specimen housed at the Museu de Ciências da Terra (MCT) in Rio de Janeiro – MCT.R.418, a steinkern preserving an imprint of a shell and some fragmentary bone remains. Owing to the paucity of known material and the limited anatomical information available, the knowledge about Apodichelys is very poor. To address this issue, we subjected the holotype to a computed microtomography, aiming to uncover new data about its internal anatomy. The analysis was able to clearly distinguish remaining bone fragments from the carbonate matrix, and the resulting tomographical slices were processed with the software CT Analyzer to create a tridimensional model of the steinkern. Preserved bony elements include portions of the left epiplastron, axillary and inguinal processes, dorsal vertebrae, dorsal rib heads and pelvic girdles, most of them concealed in external view. The axillary and inguinal processes are verticalized and advance far medially into the shell, forming a strongly developed bridge. Remains of the dorsal sequence include dorsal vertebrae 1–10, but only vertebrae 2–7 are preserved intact. These vertebrae are shaped like an elongate spool in dorsal view, with the cranial and caudal extremities being wider than the middle section. The ventral surface of the vertebrae ends in an acute keel. The rib heads located more cranially in the shell appear as short, horizontally oriented cylindrical processes, with an intervertebral connection to the dorsal sequence. Parts of all three pelvic bones are preserved, but the ilia are more fragmentary than the pubes and ischia. The pubis bears a thin, dorso-ventrally flattened process which projects from its lateral process and extends cranio-medially. The suture with the xiphiplastron is elliptical in shape. The ventral process of the ischium would have been connected to the xiphiplastron by a roughly triangular suture. The sutural area is adjacent to a small elongate process, which runs cranio-medially and connects at the midline with a matching process from the other ischium. Both ilia are very fragmented but judging from the shape of the remaining cavity where the bones used to be, the dorsal process would have had a roughly triangular shape, with concave sides. This new morphological information expands our knowledge about the little known Apodichelys lucianoi, the only turtle species formally described from the Upper Cretaceous of Northeastern Brazil.