Brachyodonty and hypsodontyin some Palaeogene Eurasiatic Lagomorphs

Abstract

Palaeogene Eurasiatic lagomorphs show more or less different degrees and kinds of brachiodonty and hypsodonty in the cheek teeth. No fully brachyodont lagomorphs are known up to the present. The probably Paleocene Mimolagus from Western Kansu shows the less advanced stage against partial hypsodonty in the upper cheek teeth. Partial hypsodonty is more advanced in the late Eocene Mongolian genera Shamolagus, Gobiolagus and Lushilagus. Roots, however, are still well developed in the cheek teeth. Some recent discoveries from earlier Oligocene sites in France and Germany seem to represent a comparable degree of hypsodonty. The Asiatic Oligocene Desmatolagus is partiallyhypsodont with root formation at later age. Compared with the Eocene taxa partial hypsodonty is, however, somewhat more advanced so far reduction of roots and extension of unilateral enamel covering is concerned. Lagomorphs with a similar degree of partialhypsodonty are known from the Lower Miocene of Kasachstan ( Agispelagus) and the later Miocene of Anatolia and the Mongolian Peoples Republic. In Europe partial hypsodont lagomorphs similar to but not identical with the Desmatolagus level and more advenced than the earlier Oligocene specimens appear at the late Stampian and have a continuous, structurally rather stable record up into the Vallesian. The earliest nearly complete hypsodont lagomorph in Eurasia is the central Asiatic medial and late Oligocene Sinolagomys. Primary structures disappear after some wear in the upper teeth, the hypostria is short, root formation is vestigial or absent. Decendants of Sinolagomys may be represented by some Miocene Asiatic ochotonids ( Bellatona Alloptox). European late Palaeogene lagomorphs demonstrate severalways in the acquisition of hypsodonty. Examples are the Marcuinomys-Lagopsis lineage, in hypsodonty near to Sinolagomys, the Amphilagus-Titanomys lineage and the Piezodus-Prolagus lineage. The two latter are independent from the Sinolagomys type of hypsodonty. Thus, evolution towards hypsodonty, one of the very characteristics in lagomorphous evolution, is not a one way phenomenon but shows a rather diversified pattern with differend levels of completion.