Northern Eurasian rhinocerotines (Mammalia, Perissodactyla) by the Pliocene-Pleistocene transition: phylogeny and historical biogeography

摘要

Pliocene and earliest Pleistocene Northern Eurasian rhinocerotines are poorly documented and understudied in comparison to Pleistocene and Miocene ones. However, they represent a key-group of species for understanding the phylogeny and historical biogeography of their Pleistocene relatives. In the present paper, we revise the abundant material from the late Pliocene locality of Kvabebi, Georgia from a systematic, phylogenetic and palaeobiogeographical perspective. The specimens from Kvabebi are documented by two partially preserved skulls, one mandible and several postcranial remains. Morphological and morphometric comparison with the type material assigned to Pliocene and earliest Pleistocene Northern Eurasian Rhinocerotinae reveal that the specimens from Kvabebi have close affinities with the poorly known Dicerorhinus miguelcrusafonti Guerin & Santafe-Llopis, 1978, described from Layna (Spain). The latter species, represented by scanty remains from the Iberian Peninsula, is usually excluded from morphological and morphometrical comparisons and no findings were reported after the 1990s. Pliocene rhinocerotine species have monotonous dental and postcranial morphologies and only a few features allow us to discern the different species. The material from Layna and Kvabebi is somewhat smaller than that of other Pliocene taxa, except for the largest representatives of Stephanorhinus etruscus (Falconer, 1868). Accordingly, the earliest specimens assigned to S. etruscus on morphometric grounds should be revised in the light of the new data here presented. A cladistic analysis performed on 280 characters and 30 species suggests that the emblematic early Pliocene European species, 'Dihoplus' megarhinus (de Christol, 1834), is sister taxon to the Layna and Kvabebi rhinoceroses. Accordingly, both species are here assigned to a new genus named Pliorhinus gen. nov. Although distinct, this clade has close affinities with the paraphyletic genus Stephanorhinus, therefore suggesting the co-occurrence of at least two distinct rhinocerotine lineages raised in the late Miocene interval in Northern Eurasia.