New rodent assemblages from the Eocene Dur At-Talah escarpment (Sahara of central Libya): systematic, biochronological, and palaeobiogeographical implications

Abstract

In this paper, we describe four fossil rodent taxa from two new localities situated in the Idam Unit ('Bioturbated Unit) of the Dur At-Talah escarpment in central Libya These rodents belong to the family Phiomyidae (Hys tricognathi) and are distributed amongst three genera (Phiomys, Protophiomys, and Talahphiomys gen. nov.) that include three new species (Phiomys hammudai sp. nov., Protophiomys durattalahensis sp. nov., and Talahphiomys libycus sp. nov.). Although some of these new species are inorphologically close to certain phiomyids from the latest Eocene and Oligocene of the Jebel elQatrani Formation of the Fayum in Egypt, the Idam rodent faunas lack the abundant and somewhat morphologically derived Fayum phiomyids (such as: Metaphiomys, Gaudearnus, Paraphiornys, Phiocricetomys), thereby excluding a similar age for the Dur At-Talah rodent assem blages. More resemblance is shared with the phiomyid (Protophiomys algeriensis) of the Nementcha locality in Algeria, for which a late middle Eocene age is presently admitted. Protophiomys is a primitive representative of the phiomyid African radiation and it is represented at Dur At-Talah by a slightly more derived species (Pr. durattalahensis) than that of Nementcha, thereby suggesting a younger age for Dur AtTalah. As a result, the new rodent assemblages suggest a late middle Eocene age for the Idam ('Bioturhated) deposits of the Dur At-Talah escarpment. This age hypothesis is substantiated by other mammals (especially Proboscidea), which occur in the same sedimentological unit. Interestingly, the dental pattern of Protophiornys and that of Talahphzomys have somewhat stronger affinities with South Asian hystricognath baluchimyines than with Fayum phiomyids. It is clear that haluchimyines and phiomyids have a common ancestry, and that dispersal occurred between Asia and Africa during the middle of the Palaeogene. However, it is not clear if both groups can be strictly separated in two distinct natural groups inasmuch as some haluchimyines (e.g Lophthaluchia, Bugtirnys, Hodsahibia) appear to be phioinyid4ike, and some early members of phiomyids (e.g. Protophiomys, Talahphiomys) are baluchimyine4ike. South Asia and North Africa represent two centres of adaptive radiation of early hystricognathous rodents. The strong dental resemblances between early Asian and African forms are perhaps the result of subsequent convergent evolution after an initial dispersal from Asia, Otherwise, the systematics of these rodents has to be entirely revised, or we must consider that their historical biogeography is much more complex. (C) 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 160, 195-213.