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

Abstract

ABSTRACT Craniodental ontogenetic analyses on notoungulates are scarce. Several well-preserved specimens of the Eocene 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. Cranial ontogeny indicates negative allometry for most of the variables related to the brain and sense organs, with thicker 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 length and 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 posterior root shifted dorsally accompanying the modification of the basicranial angle. Cranial growth of C. bondi is similar to that of Nesodon imbricatus, sharing four coefficients with close values. Molar eruption sequence is anteroposterior, whereas the deciduous premolars are substituted posteroanteriorly. In the juvenile, dentine in M2 is already exposed on the occlusal surface before wear, but the enamel covers the entire crown in m2 of the advanced juvenile.