Phylogeny of the Notoungulata (Mammalia) based on cranial and dental characters

The Notoungulata is the richest order of South American endemic placentals, but phylogenetic relationships within this order are unclear. This work provides short descriptions of new cranial characters useful for phylogenetic research on notoungulates, argues for a redefinition of some characters on the dental cristae, and provides a long overdue treatment of notoungulate relationships - the most complete thus far - via a cladistic phylogenetic analysis comprising 50 notoungulate genera and 133 morphological characters of the skull and teeth. The monophyly of the Notoungulata (including Pyrotherium) is well supported by numerous cranial and dental apomorphies. The validity of most traditional notoungulates suprageneric taxa is tested, and the monophyly of the two traditional notoungulate suborders Toxodontia and Typotheria is supported. These two taxa are united in a clade supported by an original character on the morphology of the ectopterygoid crests. The Henricosborniidae, Isotemnidae and Oldfieldthomasiidae are paraphyletic, reflecting the fact that these families are mostly defined by plesiomorphic characters. The monophyly of Notohippidae is questioned, as well as the possibility of a close relationship between leontiniids and toxodontids on the basis of cranial arguments. It is observed that two major subclades diverge early within the Typotheria: the Interatheriidae and the clade Archaeohyracidae + Mesotheriidae + Hegetotheriidae. This new phylogeny of the notoungulates from cranial and dental anatomy is an essential step towards reconstructing the ancestral morphotype of the Notoungulata, and is crucial for research on the origin and phylogenetic affinities of these South American ungulates within the Placentalia.