Evolution of incisor enamel microstructure in Theridomyidae (Rodentia)

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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).