Craniodental ontogenetic variation in the leontiniid coquenia bondi Deraco, Powell, & López, 2008 (Notoungulata, Toxodontia) from the Eocene of Northwestern Argentina

Abstract

Craniodental ontogenetic analyses on notoungulates are scarce. Several well-preserved specimens of theEocene species Coquenia bondi allow for the first ontogenetic study in a Palaeogene leontiniid notoungulate.We studied five almost complete skulls of C. bondi representing three different ontogenetic stages. Cranialontogeny indicates negative allometry for most of the variables related to the brain and sense organs, withthicker postorbital constriction and braincase breadth in juveniles. The rostrum grows isometrically in length,with positive allometry in height and negative in anterior width. Isometry is recovered for the palatal lengthand the upper postcanine length, both variables correlated with the longitudinal growth of the splanchnocranium.Other ontogenetic difference is the stouter and sigmoid zygomatic arch in adults, with its posteriorroot shifted dorsally accompanying the modification of the basicranial angle. Cranial growth of C. bondi issimilar to that of Nesodon imbricatus, sharing four coefficients with close values. Molar eruption sequence isanteroposterior, whereas the deciduous premolars are substituted posteroanteriorly. In the juvenile, dentinein M2 is already exposed on the occlusal surface before wear, but the enamel covers the entire crown in m2of the advanced juvenile.