Aktautitan hippopotamopus Mihlbachler, Lucas, and Emry, 2004a

Mihlbachler, Matthew C., 2008, Species Taxonomy, Phylogeny, and Biogeography of the Brontotheriidae (Mammalia: Perissodactyla), Bulletin of the American Museum of Natural History 311 (1), pp. 1-475 : 202-203

Aktautitan hippopotamopus Mihlbachler, Lucas, and Emry, 2004a

HOLOTYPE: KAN N2 /875, a complete skull, mandible, and skeleton lacking only parts of the right tarsus and pes.

TYPE LOCALITY: Kyzylbulak Formation, Kyzyl Murun near Aktau Mountain, Ily Basin, Kazakstan.

AGE: Middle Eocene (Irdinmanhan land mammal ‘‘age’’).

REFERRED SPECIMENS: (From the same locality as the holotype) KAN N2 /873, a complete skull and articulated mandible, complete dentition, and fully articulated right forelimb with radius, ulna, and manus, and a partial left manus ; KAN N/2 639, anterior portion of a cranium.

DIAGNOSIS: Aktautitan hippopotamopus is a large brontothere with small frontonasal horns that are elevated high above the orbits. The nasal process and horns are elevated to the peak of a tall frontonasal process that rises anterodorsally from above the orbit at an angle of about 45 °. The nasal incision is dorsoventrally deep and it extends posteriorly to the P4. The orbit is positioned over the M2. The elevated nasal process is angled downward, not strongly rounded anteriorly, and with lateral walls that are deep proximally and shallow distally. The premaxillomaxillary rostrum deepens posteriorly and is not enclosed by bone dorsally. Other cranial characteristics include a saddle-shaped cranium, separate parasagittal ridges, nearly straight zygomatic arches, and a ventrally unconstricted and mediolaterally angled external auditory pseudomeatus. Postzygomatic processes, as seen in Metatitan , are absent.

Dentally, Aktautitan hippopotamopus has three large upper incisors including a subglobular I1, and more subcaniniform I2 and I3, a distinct postcanine diastema, a complex P1, a distinct P2 metacone, and weak preprotocristae on the P2 and P3. Premolar hypocones are absent. The upper molars have tall, lingually angled ectolophs with weak labial ribs, and thinned lingual ectoloph enamel with wedge-shaped paracones and metacones. A cingular parastyle shelf is absent. Distinct central molar fossae and anterolingual cingular cusps are present. Paraconules and metalophs are absent. The three lower incisors are large and subcaniniform. The i2 is the largest incisor. There is a distinct postcanine diastema and a small p3 metaconid. The p2 trigonid is nearly twice the length of the talonid. The lower molars have shallow basins and m3 is long and slender.

Aktautitan hippopotamopus shares with Metatitan the unique combination of having closely positioned horns and a nasal process that are elevated to the peak of a superorbital frontonasal process. However, it differs from Metatitan in its more anteriorly positioned orbits, more poorly developed p3 metaconid, postcanine diastemata, and larger and more subcaniniform incisors.

REMARKS

Aktautitan hippopotamopus is a large brontothere from the middle Eocene Kyzylbulak Formation of Kazakstan that resembles Metatitan in its elevated horns and nasal process, but differs from Metatitan in a number of respects. Preliminary reports of this new species mistakenly attributed it to the genus Protitan ( Emry et al., 1997; Emry and Lucas, 2002, 2003; Lucas and Emry, 2001). The formal description of A. hippopotamopus represents a part of this study and follows methods and a format similar to that used here but was published elsewhere ( Mihlbachler et al., 2004a).