DESCRIPTION OF AN ONTOGENETIC SERIES IN COQUENIA BONDI DERACO ET AL., 2008 (LEONTINIIDAE)

DERACO, M. VIRGINIA; GARCÍA LÓPEZ, DANIEL A.

San Juan

Congreso; IV Congreso Latinoamericano de Paleontología de Vertebrados; 2011

Ontogenetic analyses focused on paleogene notoungulates are scarce. In this contribution we present a comparison of three almost complete skulls of the Eocene species Coquenia bondi Deraco et al., 2008 (Leontiniidae Ameghino, 1895) which represent three different ontogenetic stages, from juvenile to adult. The juvenil specimen PVL S-IV-1 shows all its deciduous teeth plus its first and second upper molar. The first molar is erupted and presents a low wear degree, while the second molar is not erupted. The second specimen PVL S-VI-2a is here considered as an advanced juvenile. Only the anterior deciduous premolars are still present and, within permanent dentition, incisors, posterior premolars, and first and second upper molars are erupted. The upper canine and second permanent premolar are not erupted although these teeth are visible at the sides of the maxilla. The third upper molar is absent. In the adult individual (PVL 5853), all permanent teeth are present showing some wear degree, which is higher in the first upper premolar and the first upper molar. A notable feature derived from the observation of the juvenile individual is that, in unerupted molars, exposed dentine is present on the occlusal surface. Besides these evident changes in the dentition, striking modifications in skull structures were observed along the series. The skull length increases about 50% from the juvenile to the adult. The postorbital constriction appears thicker in the juvenile but becomes progressively narrower with age. Tympanic bullae and epitympanic sinuses became relatively smaller. Other structures are small or slender in the juvenile, but are considerably well developed in the adult, such as the sagittal crest, the paraoccipital process, the narial process, the postorbital process, the exit of the Eustachian tube, and the meatal crest. Moreover, the basioccipital surface became strongly crested in the adult, the rostrum is higher, and the narial apertures are wider. Other evident differences are observed in the basicranial angle, which is progressively more open, and the zygomatic arch, which becomes stouter and sigmoid, with its posterior root shifted dorsally in response to the mentioned modification of the basicranial angle. Finally, the external auditory meatus, which exhibits a subhorizontal orientation in the juvenile, becomes more vertical in the adult. The availability of several well-preserved specimens of C. bondi represents a unique opportunity for the study of morphological trends present in the ontogenetic series of a Paleogene notoungulate recorded previously to the “modernization” of the ‘ungulate’ faunas in South America (Croft et al., 2008). This is useful to assess the phylogenetic or taxonomic weight of potential characters observed in isolated remains. Moreover, this study provides an important source of information for morphofunctional and paleobiological analyses.