Myledaphus bipartitus was erected by Cope (1876) for a tooth-design common in the Judith River Fm (Campanian) of Montana. These teeth were generally deemed to be freshwater stingrays until Cappetta (1987:140-41) judged them ; Cappetta's conclusion was validated (he's so damn good) when in 1999 a complete skeleton was found in Alberta (Neuman & Brinkman, 2005:168). Based on reported locations and tooth-design, Myledaphus bipartitus was likely a fresh / brackish water taxon with a crushing dentition feeding on invertebrates.
The crowns are high, usually hexagonal and broader than deep; the occlusal surface bear a transverse ridge with apico-basal folds extending down the labial & lingual faces. The roots are high with a nutrient groove separating rather triangular lobes with multiple small foramina; the marginal root face has scattered foramina near the crown with a larger one lingually. Cappetta (1987: 140-41) noted that they are up to 7 mm wide and a second nutrient groove may be present.
Reports of this taxon are largely confined to the Late Cretaceous of the Western Interior Seaway (New Mexico & Utah to Alberta and Saskatchewan); but as Cappetta (1987) noted, they are also known the Late Cretaceous of Uzbekistan (Nessov 1981) and Lower Paleogene of NA (which he questioned).
At least one other unnammed Myledaphus-like tooth-design appears to be present in the Western Interior Seaway and/or Gulf Coast.
Meyer (1974: 158-59, fig. 49) included as ?Hypolophus sp. a Campanian tooth-design from Mississippi with a high rhombic crown and high roots, similar to design of Figures and below.
Peng et al (2001: 8-10) in reporting on the Judith River Group (Milk River & Foremost Fms., Campanian) of Alberta, included the typical M. bipartitus tooth as such (pl.2, figs. 1-3), but included a specimen which appears to be a cross between Pseudohypolophus and Myledaphus as Myledaphus sp (pl2, fig 4-6).
In two GSA poster abstracts, Thompson (2004 & 05) discussed two distinct Myledaphus tooth-designs from Utah; one Santonian (blocky, less-deep crown) and the other Campanian (rounder, core bulbous crown). Lacking figures, it will be interesting to see final determinations if this research is ever published.
Bourdon et al (2011) reported from the Pt. Lookout Fm. (E-M Santonian) of New Mexico Myledaphus sp. and noted the similarity to M. bipartitus but the lack of vertical folds of the crown face and no evidence of a transverse ridge.
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