Evolution and systematics

The classification of the Bathyergidae has long been problematic. There is now good support, using morphometric parameters and molecular phylogeny, for grouping them with the phiomorph rodents (the Old World lineage of the suborder Hystricognathi). Their closest relatives are the rock rats (Petromuridae), cane rats (Thyronomyidae), and Old World porcupines (Hystricidae).

The New and Old World Hystricognathi probably separated from their common ancestor during the Eocene, 33-49 million years ago (mya). The Bathyergidae are monophyletic, that is, all taxa can be traced back to a single common ancestor. However, fossil evidence and molecular data indicate an early divergence of Heterocephalus from the other genera in the family. Their fossil history is sparse but they most likely had an ancient African origin. The earliest fossil finds, in East Africa and Namibia, are from the early Miocene, around 25 mya.

The five modern genera are grouped into two subfamilies. The Bathyerginae contains one genus, Bathyergus, and two species, characterized by having grooved upper incisors and enlarged forefeet with strong digging-claws. The Georychi-

nae contains four genera, Georychus, Heliophobius, Cryptomys, and Heterocephalus, all with ungrooved upper incisors and forefeet and claws that are not enlarged. All the Georychinae, except Cryptomys, are currently considered to be monospecific. The number of species in the genus Cryptomys is uncertain and needs revision; furthermore, the taxa in Cryptomys can be placed in two such genetically distinct groups that they may eventually be assigned to separate genera. Three genera are solitary, one animal inhabiting each burrow system; indeed, this is the usual pattern for subterranean mammals. Ba-thyergidae are exceptional in that Cryptomys and Heterocephalus are highly social: they live in colonies with a single reproductive female, her consorts, and a number of non-breeding workers (these are closely related to the breeders). Because Heterocephalus diverged from the other Bathyergidae early in their history, this unusual type of sociality probably evolved twice in the family.

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