BRONTOTHERIINAE Marsh 1873

Mader, Bryn J., 2008, A species level revision of Bridgerian and Uintan brontotheres (Mammalia, Perissodactyla) exclusive of Palaeosyops, Zootaxa 1837 (1), pp. 1-85 : 49-52

Subfamily BRONTOTHERIINAE Marsh 1873

(Includes Telmatheriinae Osborn 1914; Manteoceratinae Osborn 1914; Diplacodontinae Osborn 1914; Brontopinae Osborn 1914; and Megaceropinae Osborn 1914)

(= Telmatheriinae, sensu Mader 1989)

Diagnosis. Brontotheres distinguished from all others by the presence of a widened cranial vertex, long, triangular projection of the frontal overlapping the nasal, and a bony prominence or horn over the facial concavity.

Discussion. Mader (1989) selected Telmatheriinae ( Osborn 1914) as the most logical name for this subfamily because Telmatherium is this group's most primitive member. In 1995, however, Mader and Alexander pointed out that the valid subfamily name is Brontotheriinae Marsh (1873) based on the Principle of Coordination (Article 36, International Code of Zoological Nomenclature, Ride et al. 1999).

As noted in the diagnosis, above, brontotheriine brontotheres are distinguished from all others by the presence of a widened cranial vertex. Figure 17 illustrates the cranial vertex of the brontothere genera Palaeosyops , Mesatirhinus , and Telmatherium . Palaeosyops and Mesatirhinus exhibit the plesiomorphic cranial vertex morphology in which there is a well developed sagittal crest. In Telmatherium , however, the cranial vertex adjacent to the temporal fossa is widened and the sagittal crest is lost in this region. Further posteriorly, however, the sagittal crest is still present but is relatively wide compared to that of non-brontotheriine brontotheres. The sagittal crest of most brontothere genera has a distinct median cleft but, in Telmatherium , instead of a median cleft there is a deep pit-like fossa. Telmatherium is the most plesiomorphic brontotheriine genus and in all other brontotheriine genera the cranial vertex is greatly widened and the sagittal crest is completely lost.

Brontotheriine brontotheres are further distinguished by having a long, triangular projection of the frontal that overlaps the nasal ( Fig. 18D ), while in other brontotheres the frontal-nasal contact is usually rather straight ( Fig. 18A, B , and C). In one specimen of the dolichorhinine brontothere Metarhinus (UCM 44939), however, there is a very slight triangular projection of the frontal overlapping the nasal. The frontal-nasal suture in the Brontotheriinae is located over the facial concavity and is incorporated into the horn.

Within the Brontotheriinae there are two monophyletic subsets: the diplacodonts (= diplacodonts sensu lato of Mader 1989; 1998) and eubrontotheres. These names are informal taxonomic terms but useful in discussions of brontothere phylogeny.

The diplacodonts are synonymous with the "horned brontotheres" mentioned by Mader (1989; 2000; present paper). The group is defined by the presence of "true" horns (that is, the presence of a distinct bony projection over the facial concavity rather than a slightly thickened bony prominence), vertically enlarged external nares, concave lateral cranial profile, facial region of the skull 40 % or less of the entire skull length, and complete loss of the sagittal crest due to widening of the cranial vertex. Most diplacodonts also have partially molarized premolars in which there are two lingual cusps on the third and fourth upper premolars. The most plesiomorphic diplacodont taxon, however, has only a single lingual cusp present on the third and fourth upper premolar in some individuals. This taxon is a Protitanotherium -like brontothere and includes the type specimen of " Sthenodectes " australis Wilson (see Discussion section for Protitanotherium ).

The term "eubrontothere" was first used by Schoch and Lucas (1985) for Duchesneodus and typical Chadronian brontotheres such as Brontops . Eubrontotheres (not revised in the present paper) are defined by the presence of a greatly widened cranial vertex, well developed hypocone that is consistently present on M3, and the loss of the diastema.

Figures

FIGURE 17. Skulls of three brontotheres in dorsal view showing the width of the cranial vertex. A, Palaeosyops; B, Mesatirhinus; C, Telmatherium.

FIGURE 18. Suture pattern between the frontal (dark gray) and nasal (light gray) in A, Palaeosyops; B, Mesatirhinus; C, Sphenocoelus; and D, Telmatherium. Figures modified from Osborn, 1929, and not to scale.