Description of a juvenile specimen of Baurusuchidae (Crocododyliformes: Mesoeucrocodylia) from the Bauru Group (Late Cretaceous): preliminary considerations on ontogeny

Abstract

Among taxons of Crocodyliformes from the Bauru Group (Late Cretaceous), a large number of morphotypes, including cranial and post-cranial materials, have been described associated with Baurusuchus pachecoi. However, the lack of ontogenetic studies, similar to what has been done with Mariliasuchus amarali, has led to the allocation of new genera and species to the few complete skulls found. The present contribution provides the description of a juvenile specimen of Baurusuchidae, housed in the collection of the "Museu de Ciências da Terra", Rio de Janeiro, under the number MCT 1724 - R. It is composed of rostrum and mandible, associated and in occlusion, with the left side better preserved than the right one, with extremely reduced ziphodont dentition. The fossil is 125.3 mm in preserved length from the anterior edge of the premaxilla to the posterior end of the dentary; 117.5 mm in preserved length from premaxilla to palatines; and 51.4 mm in lateral height. Among the ontogenetic aspects observed are faint ornamentation, consisting of very separated, large vermiform grooves, a straighter maxillary ventral line, dentary slightly dorsally inclined, mandibular symphysis less vertical than in other Baurusuchidae of large size. The greater part of the diagnostic characters of Baurusuchus pachecoi were identified in MCT 1724 - R: high rostrum and compressed lateral, and ziphodont dentition with strong dental reduction, culminating in four premaxillary and five maxillary teeth. In conclusion, as the morphotype cannot be separated morphologically from the type species of the genus and because it has been found in outcrops of the Adamantina Formation, it is possible to identify it as a juvenile Baurusuchus cf. pachecoi. Changes which can be observed gradually from smaller to larger morphotypes, thus being of ontogenetic nature include: an increase in skull ornamentation; the median/posterior portion of the ventral edge of the maxilla passing from straight to concave; and the verticalization of the mandibular symphysis line to close to 45º.