The enamel microstructure in teeth of Manidens condorensis: assessing the evolution of enamel in Ornithischia

BECERRA, MARCOS G.,; POL, DIEGO; RAUHUT, OLIVER W.M.

San Luis (Capital)

Congreso; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2017

Asociación Paleontológica Argentina

The dentition of Manidens condorensis Pol, Rauhut and Becerra, 2011, from the Cañadón Asfalto Formation (Early Jurassic) is unique among ornithischians, with clear differences between maxillary and dentary teeth. This work describes the dental enamel microstructure of this species (specimens MPEF-PV 10862, 10863, 10865, 3821, 10823 and 10864), the most basal and oldest Ornithischia for which enamel microstructure has been studied. Its enamel lacks a basal unit layer, being composed by parallel to divergent crystallites with discontinuous incrementing lines at the crystallite level, and incipient divergent columnar units at the module level, representing the most abundant structure. Enamel of maxillary and dentary teeth differs to each other, but shows the simplest structure found in Ornithischia, more similar to that in Plateosaurus von Meyer, 1837 than to Coelophysis Cope, 1989 outside Ornithischia, and similar to enamel in pachycephalosaurids and, to a lesser extent, to ankylosaurs. Optimization of enamel characters in a supertree comprising ornithischians with known enamel and M. condorensis allows us to propose new ancestral states for the internal nodes of the major lineages and to highlight evolutionary tendencies: absence of a basal unit layer and presence of incipient divergent columnar units and parallel crystallites might be the ancestral state for Dinosauria; the wavy enamel of Dryomorpha might derived from the ancestral incipient divergent columnar units optimized in Ornithopoda; the shared presence of incipient divergent columnar units in Pachycephalosauria and Thyreophora represents a retention of the plesiomorphic state; and enamel thickness and asymmetry increases in derived ornithischian lineages.