Incisor Enamel Microstructure of Hystricognathous and Anomaluroid Rodents from the Earliest Oligocene of Dakhla, Atlantic Sahara (Morocco)

Abstract

Seven hystricognaths and five anomaluroids have been recently described from the earliest Oligocene of the Dakhla (DAK C2) region of Morocco, based primarily on isolated cheek teeth. Here, we analyzed the enamel microstructure of thirty associated isolated fragments of incisors. Among these specimens, only three display an early stage of uniserial Hunter-Schreger bands (HSBs), with mostly a single prism per band, but also occasionally two prisms per band (in two specimens), and a thin interprismatic matrix (IPM) that runs parallel to the prism direction, thereby documenting incisors of anomaluroids. All other sampled incisors display an enamel with multiserial HSBs, thereby documenting hystricognaths. For these latter, we recorded primarily an IPM crystallite arrangement describing the subtype 2 of multiserial HSBs, but with variation including a wide amplitude in the angle (acute) formed between the crystallites of IPM and those of the prisms, some variations in the frequency of the IPM sheet anastomoses, in the number of prisms per HSBs, and variations in the inclination of the HSBs. The absence of the subtypes 2–3 and 3 of multiserial HSBs in DAK C2 suggests that African hystricognathous rodents had still not achieved these most resistant multiserial HSBs at that time. The drier, cooler climatic regime of the early Oligocene, having increased the fragmentation and opening of habitats, might have played a role in the subsequent selection of taxa having acquired a more resistant incisor enamel.