Concealed morphological diversity revealed by new fossils of Chelus (Testudines, Chelidae) from the Upper Miocene of the Acre Basin, Brazil

Abstract

The Solimões Formation, in the Acre Basin, which outcrops between the states of Acre and Amazonas, northern Brazil, contains the country's most diverse fossil fauna of Neogene amniotes. 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 the clades Chelidae, Podocnemididae, and Testudinidae. The genus Chelus (Chelidae) — popularly known as mata-mata —, represented by two extant species, Chelus fimbriata and Chelus orinocensis, traditionally also includes two extinct species: Chelus colombiana and Chelus lewisi. In Brazil, fossils attributed to the genus Chelus have been reported since the late 19th century. Still, the first valid identifications were made in the 1980s and 1990s, and later in the 2000s, with the proposed presence of C. colombiana and C. lewisi observed in Miocene strata of the Brazilian Amazon. Here, we describe recently discovered fossils housed at the Laboratório de Pesquisas Paleontológicas at the Universidade Federal do Acre (Campus Rio Branco) and assign them to Chelus. Our comparative analyses show that features traditionally and recently used to diagnose the two extinct taxa are present in the same individual. This overlap leads us to question the consistency of these characteristics and consequently the validity of C. lewisi.