Duchesneodus thyboi ( Bjork, 1967 )

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 : 354-355

Duchesneodus thyboi ( Bjork, 1967)

HOLOTYPE: SDSM 63689, a partial skull with right C, P3–M2, left I2 (?), C, P3–M2, M3 (unerupted).

TYPE LOCALITY: SDSM locality V582, Slim Buttes Formation, Harding County, South Dakota.

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

REFERRED SPECIMENS: (See Bjork [1967] for a complete list.) Bjork (1967) referred numerous teeth and other fragments to Duchesneodus thyboi . Among these specimens is SDSM 63690, a partial rostrum with a complete incisor row, right C, and P2–P4 from SDSM locality V6244.

DETERMINATION: Nomen dubium, holotype is an indeterminate juvenile specimen.

REMARKS

Bjork (1967) described Teleodus thyboi from a partial juvenile skull (SDSM 63689), several cranial fragments, and isolated teeth. After demonstrating Teleodus to be an invalid genus, Lucas and Schoch (1982) reassigned this species to Duchesneodus . Subsequently, they considered it a junior synonym of D. uintensis ( Lucas and Schoch, 1989b) . However, the material from which this species was described is neither consistent with D. uintensis , nor does the material sufficiently indicate a distinct taxon.

The holotype specimen (SDSM 63689) (fig. 182b) indicates a brontothere with small globular upper incisors, a large canine, a distinct postcanine diastema similar in length to P2, upper premolars with small preprotocristae, and distinct premolar hypocones that are separated from the protocones by a lingual connecting crest. Another important specimen, SDSM 63690 (fig. 182a), a rostral fragment, is consistent with the holotype and further indicates a reduced number of upper incisors (two pairs). Other specimens have an occasionally strongly developed P4 mesostyle ( Bjork, 1967).

The relatively long postcanine diastema differentiates Duchesneodus thyboi from D. uintensis and Megacerops coloradensis but several other species, such as Eubrontotherium clarnoensis , Protitanops curryi , and Notiotitanops mississippiensis , possess similar postcanine diastemata.

Bjork (1967) noted that the postorbital process of the jugal of SDSM 63689 was located above the metacone of the M2, thus indicating a shortened face that is characteristic of typical Chadronian brontotheres (i.e., Megacerops ). In comparison, the orbits of Eubrontotherium , Notiotitanops , and Protitanops are more posteriorly positioned. However, SDSM 63689 is a subadult specimen with an unerupted M3. Although brontothere cranial ontogeny has not been carefully studied (juvenile skulls are very rare), it is fairly obvious that, as the teeth erupt, there would have been ontogenetic changes in the position of the orbit with respect to the teeth. Therefore, this character does not really differentiate Duchesneodus thyboi from other species. From what can be discerned from the available material Duchesneodus thyboi is potentially synonymous with Eubrontotherium clarnoensis , Protitanops curryi , and Notiotitanops mississippiensis and it is therefore a nomen dubium.