The systematics and phylogeny of North American Eocene brontotheres (Mammalia:Perissodactyla)

Although extensively studied in the late nineteenth and early twentieth centuries, the extinct perissodactyl family Brontotheriidae has remained a poorly understood group and most recent workers have pointed out the need for a major systematic revision. The present dissertation is the first comprehensive revision of North American Eocene (Wasatchian - Duchesnean) brontothere species in over sixty years. Brontotheres (=titanotheres) are defined by the characteristic bunoselenodont pattern of the upper molars in which there is a well-developed W-shaped ectoloph and essentially isolated lingual cusps. Exclusive of the most primitive brontothere genus, Eotitanops, brontotheres are further defined by a shortening of the face; the presence of distinctive triangular-shaped, subcaniniform incisors; and a deep lateral nasal incision that is broadly bordered by the maxilla. In this dissertation thirteen North American Eocene brontothere genera are recognized as valid: Eotitanops, Palaeosyops, Telmatherium, Mesatirhinus, Metarhinus (=Rhadinorhinus), Sphenocoelus (=Dolichorhinus), Protitanotherium, Diplacodon progressum Peterson (a new generic name for this taxon will be published in the near future), Eotitanotherium, Notiotitanops, Duchesneodus, Sthenodectes, and Metatelmatherium. Within the Brontotheriidae four subfamilies are recognized: Eotitanopinae (Eotitanops), Palaeosyopinae (Palaeosyops), Dolichorhininae (Mesatirhinus, Metarhinus, and Sphenocoelus), and Brontotheriinae (Telmatherium, Protitanotherium, Diplacodon progressum, Eotitanotherium, Notiotitanops, and Duchesneodus). Diplacodonts and eubrontotheres are recognized as valid subgroups of the Brontotheriinae. An hypothesis of relationship for brontotheres is presented in the form of a cladogram. To determine whether other, equally parsimonious hypotheses of relationship may have been overlooked, the characters used to construct the cladogram were analyzed using the phylogenetic analysis program PAUP. The PAUP analysis produced three cladograms, all of which were essentially identical to the original hypothesis of relationship. It was concluded, therefore, that no other equally parsimonious cladograms had been overlooked. The computer generated cladograms had a consistency index of 100%, indicating that they are highly corroborated hypotheses of relationship