Evolution of incisor enamel microstructure in Theridomyidae (Rodentia)

Abstract

ABSTRACT The earliest representatives of Theridomyidae (belonging to the subfamilies Issiodoromyinae, Theridomyinae, Sciuroidinae, Pseudosciurinae, and Remyinae) from the Middle to Late Eocene (MP 13–17) have a plesiomorphic pauciserial incisor schmelzmuster. Within Issiodoromyinae at least since the Early Oligocene (MP 22) a fully uniserial schmelzmuster is developed. Pseudoltinomys phosphoricus (Issiodoromyinae, MP 17) exhibits a schmelzmuster with irregular decussation of prisms most probably derived from pauciserial Hunter-Schreger bands (HSB). Within Theridomyinae, the genus Theridomys retains the primitive pauciserial schmelzmuster with somewhat thinned HSB. In the genera Blainvillimys and Archaeomys, convergent to Hystricognathi and derived Ctenodactyloidea, a schmelzmuster with pseudo-multiserial HSB was developed (MP 23–30). This condition includes a number of clear differences to the schmelzmuster with multiserial HSB found in the incisors of hystricognath rodents: lower HSB thickness, lower angle of interprismatic matrix (IPM) in the portio interna (PI), and a much thicker portio externa (PE). As in this pseudo-multiserial enamel, the uniserial schmelzmuster in the Theridomyidae remains comparatively primitive, and in its HSB only a stage with acute angular IPM is achieved. Patriotheridomys (Theridomyinae) and Ectropomys (Oltinomyinae) have vertically oriented HSB. There exists a close correspondence between the grade of hypsodonty of the cheek teeth and the development of uniserial and pseudo-multiserial HSB in the incisor enamel from the plesiomorphic pauciserial condition. Theridomyidae split off from basal rodent clades (Ischyromyoidea or Ctenodactyloidea).