Incisor enamel microstructure of hystricognathous and anomaluroid rodents from the earliest Oligocene of Dakhla, Atlantic Sahara (Morocco)

International audienceSeven 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 microstructureof thirty associated isolated fragments of incisors. Among these specimens, only three display an early stage ofuniserial Hunter-Schreger bands (HSBs), with mostly a single prism per band, but also occasionally two prisms perband (in two specimens), and a thin interprismatic matrix (IPM) that runs parallel to the prism direction, therebydocumenting incisors of anomaluroids. All other sampled incisors display an enamel with multiserial HSBs, therebydocumenting hystricognaths. For these latter, we recorded primarily an IPM crystallite arrangement describing thesubtype 2 of multiserial HSBs, but with variation including a wide amplitude in the angle (acute) formed betweenthe crystallites of IPM and those of the prisms, some variations in the frequency of the IPM sheet anastomoses, inthe number of prisms per HSBs, and variations in the inclination of the HSBs. The absence of the subtypes 2–3 and3 of multiserial HSBs in DAK C2 suggests that African hystricognathous rodents had still not achieved these mostresistant multiserial HSBs at that time. The drier, cooler climatic regime of the early Oligocene, having increased thefragmentation and opening of habitats, might have played a role in the subsequent selection of taxa having acquireda more resistant incisor enamel