Myledaphus bipartitus Cope, 1876

Brand, Nickolas A., Heckert, Andrew B., Sanchez, Israel, Foster, John R., Hunt-Foster, Rebecca K. & Eberle, Jaelyn J., 2022, New Late Cretaceous microvertebrate assemblage from the Campanian-Maastrichtian Williams Fork Formation, northwestern Colorado, USA, and its paleoenvironmental implications, Acta Palaeontologica Polonica 67 (3), pp. 579-600 : 585-586

Myledaphus bipartitus Cope, 1876

Fig. 4A, B .

Material.—MWC 8887, two teeth; MWC 9591, tooth; from Campanian–Maastrichtian, Williams Fork Formation, J&M site, northwest Colorado, USA.

Description.—Mid-to-high crowned durophagous teeth. A low, transverse ridge divides the oral surface of the crown through the center. Labiolingually oriented enamel folds are present on the oral surface, and continue as ridges onto the vertical faces of the crown ( Kirkland et al. 2013).

Remarks.—We assign batoid teeth that have a roughly hexagonal to rhomboidal shape, with both the occlusal and lateral surfaces of the crown textured, and a transverse ridge that may divide the occlusal surface of the crown to Myledaphus bipartitus . Occlusal texturing is asymmetrical across the transverse ridge. The root is often taller than the crown and may taper inwards basally. The teeth vary in crown width 1–4 mm, and combined crown plus root height of 1.5–3 mm. We have collected 19 specimens of Myledaphus bipartitus . Campanian ( M. bipartitus ) and Maastrichtian ( M. pustulosis ) species of Myledaphus bear different enamel ornamentation patterns ( Kirkland et al. 2013), thus making them potentially useful biostratigraphically. The specimens from the J&M site thus far all appear to pertain to M. bipartitus rather than M. pustulosis , suggesting a Campanian age. Some specimens appear to have ridges that are more broken up, and resemble somewhat the pustules of M. pustulosis , although this patterning is not as pronounced as that of the types of M. pustulosis . Myledaphus material is particularly abundant in the late Campanian-aged rocks, when the genus appears to have originated and begun to replace the rays Pseudomyledaphus and Cristomylus ( Kirkland et al. 2013) .

Stratigraphic and geographic range.—The uppermost Cretaceous of New Mexico, Colorado, Utah, Wyoming, South Dakota North Dakota and Montana in the Unites States, Alberta and Saskatchewan in Canada, and Mexico.

Figures

Fig. 4. Representative batoid chondrichthyan teeth from the J&M site, Colorado, USA, Williams Fork Formation, Upper Cretaceous. A, B. Myledaphus bipartitus Cope, 1876. A. MWC 8887, tooth crown and root in mesial (A1), labial (A2), and occlusal (A3) views. B. MWC 9591, tooth crown and root in mesial (B1), labial (B2), and occlusal (B3) views. C, D. Pseudomyledaphus sp. C. MWC 9589, tooth crown and root in mesial (C1) and occlusal (C2) views. D. MWC 9590, tooth crown and root in mesial (D1) and occlusal (D2) views. E, F. Cristomylus sp. E. MWC 8886, tooth crown in mesial (E1) and occlusal (E2) views. F. MWC 9588, tooth crown in mesial (F1) and occlusal (F2) views.