Osteological description of the fossil and extant Chelus (Pleurodira: Chelidae)

Testudines is a clade consisting of two sister lineages, Cryptodira and Pleurodira, both with living and fossil representatives, which differ mainly by the way the neck retracts into the shell: vertically and laterally, respectively. The Solimões Formation, in the Acre Basin, which outcrops between the states of Acre and Amazonas, contains the most diverse fossil fauna of neogene amniotes in Brazil. The outcrops are mainly found on the erosive margins of the Purus, Acre, and Juruá rivers, among others, since the 19th century. So far, as recorded in other neogene deposits in northern South America, most of the fossil turtles of the Brazilian Amazon are represented mainly by genera attributed to the families Chelidae, Podocnemididae, and Testudinidae. The genus Chelus (Chelidae) - popularly known as matamata represented by two recent species, Chelus fimbriata and Chelus orinocensis, traditionally also includes two extinct species: Chelus colombiana and Chelus lewisi. In this dissertation, we describe and review fossils of the genus Chelus that are deposited in the collection of the Laboratory of Paleontological Research of the Federal University of Acre (Rio Branco campus). In addition, we performed a detailed osteological description of the species Chelus orinocensis, using 3D models created from tomographies performed on a small sample of specimens. The description of the fossil material, using as a means of morphological comparison specimens of Chelus fimbriata, allowed us to observe that certain characteristics that occur as intraspecific variation in living species can also occur in fossils. Morphological comparisons and analyses of intraspecific variations allowed us to confirm that only the fossil species Chelus colombiana is valid for the genus. The detailed osteological description of the species Chelus orinocensis allowed us to create an anatomical atlas for the species, which we hope will assist in anatomical comparisons in future work. (AU)