Myledaphus bipartitus

As recorded by Stanton and Hatcher (1905, p. 68) and Lambe (1902, p. 28) isolated teeth belonging to Myledaphus bipartitus are common in late Cretaceous fish faunas of North America. Detached teeth were originally described by Cope (1876 a, p. 260) from the Judith River Formation. The present collection of teeth from the Judith River Formation localities consists of specimens that show not only positional differentiation in a jaw, but also differences between the mandibular and the maxillary dentition. The teeth, probably representing a single species, range in size from 2 mm. to 8 mm., measured in the direction of the long axis of the crown. Estes (1964) made a detailed study of Myledaphus bipartitus and was able to differentiate upper from lower teeth. The criteria that he gave for differentiating between the two sets of teeth are based on his comparison with Hypolophus sephen, a Recent dasyatid ray. The upper teeth are asymmetrical and compressed, whereas the lowers are more symmetrical and higher crowned. An upper tooth, AMNH 10105, has a hexagonal shape and the tooth is dorsoventrally compressed. AMNH 10104 is a large, lower tooth, much larger than the average dimensions of lower teeth, and possesses a robust crown which is heavily striated on the sides as well as on the slightly worn occlusal surface. AMNH 9303, which Estes has tentatively described as a marginal tooth, is similar to AMNH 10106. Both teeth are laterally compressed with a relatively longer bifid root. Myledaphus bipartitus teeth are fairly common in the Judith River Formation. They are also recorded from the Milk River Formation of southern Alberta by Russell (1935, p. 120) who stated that the Milk River specimens average smaller, but are otherwise similar to those found in the Oldman Formation. The Cretaceous distribution of Myledaphus is fairly widespread on the American continent. The genus has also been reported from the Edmonton Formation, Alberta (Sternberg, 1926, p. 14); from the Mesaverde and Lance formations, Wyoming (Estes, 1964, p. 18); from the Kirtland Formation, New Mexico (Gilmore, 1916, p. 280); and from the early Eocene Four Mile Fauna in Colorado (R. Estes, personal commun.).