Cristomylus sp.

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 : 586-587

Cristomylus sp.

Fig. 4C, D .

Material.—MWC 8886, tooth, MWC 9588, tooth; from the Campanian–Maastrichtian, Williams Fork Formation, J&M site, northwest Colorado, USA.

Description.—Flattened teeth with rounded lateral surfaces of the crown. A transverse crest is present that divides the crown into equal halves in occlusal view ( Kirkland et al. 2013). These specimens are smaller than the teeth of species of Myledaphus and Psuedomyledaphus, and contain a pair of lateral foramina on the labial surface of the root ( Kirkland et al. 2013).

Remarks.—We assign batoid teeth with a transverse crest but that are otherwise characterized by heavily rounded lateral faces of the crown to Cristomylus . The teeth retain a slightly hexagonal shape and the root surfaces are heavily weathered. The transverse crest may be straight or follow the curvature of the tooth, if the shape of the crown is not perfectly linear. The root is always taller than the crown in our specimens. We have collected only three specimens of Cristomylus , and the genus is previously unreported from the upper Campanian, as it is otherwise only known from older rocks ( Kirkland et al. 2013). While we do not have enough specimens to provide a species level identification, it is of interest that the J&M site bears specimens of species of both Myldepahus and of Cristomylus , as Myledaphus appears to have replaced Cristomylus in many late Campanian ecosystems ( Kirkland et al. 2013).

Genus Pseudomyledaphus Kirkland, Eaton, and Brinkman, 2013

Type species: Pseudomyledaphus madseni Kirkland, Eaton, and Brinkman, 2013 ; John Henry Member of the Straight Cliffs Formation, Upper Cretaceous .

Pseudomyledaphus madseni Kirkland, Eaton, and Brinkman, 2013

Fig. 4E, F .

Material.— MWC 8868, tooth. MWC 9589, tooth. MWC 9590, tooth. MWC 9966, 17 teeth, many with roots; from the Campanian–Maastrichtian, Williams Fork Formation, J&M site, northwest Colorado, USA.

Description.—Large, flattened teeth similar to these of species of Myledaphus , other than that they lack the characteristic enamel folds of Myledaphus , except on the mesial lingual face of the crown (after Kirkland et al. 2013)

Remarks.—We assign batoid teeth similar to those of Myledaphus , but that lack the enameloid texturing on the lateral surfaces of the crown, to Pseudomyledaphus madseni . These teeth are hexagonal to rhomboidal in shape, and lack any enamel texturing on the occlusal surface of the crown. The lateral surfaces of the crown are straight and do not curve as dramatically as those seen in Cristomylus . Pseudomyledaphus is the most common chondrichthyan we recovered from the J&M site, with a total of 45 individual teeth collected. The teeth vary in width 1.5–6 mm, and height 1.5–4 mm. Pseudomyledaphus is previously unknown from the upper Campanian ( Kirkland et al. 2013). Like Crystomylus, it is believed that this genus was replaced by Myledaphus during the late Campanian ( Kirkland et al. 2013), making its co-occurrence with Myledaphus in the J&M site of interest.

Stratigraphic and geographic range.—Conician–Campanian of Utah, United States and Alberta, Canada.

Osteichthyes Huxley, 1880

Actinopterygii Klein, 1885

Amiidae Bonaparte, 1838

Vidalamiinae Grande and Bemis, 1998

Vidalamiini Grande and Bemis, 1998

Genus Melvius Bryant, 1987

Type species: Melvius thomasi Bryant, 1987 ; Hell Creek Formation , Upper Cretaceous .

cf. Melvius sp.

Fig. 5B .

Material.— MWC 8867, tooth; MWC 8888, numerous teeth (more than 100); from the Campanian–Maastrichtian, Williams Fork Formation, J&M site, northwest Colorado, USA.

Description.—Tall, lanceolate osteichthyan crowns with well-defined carinae running from crown tip to base ( Bryant 1987).

Remarks.—We assign teeth that are tall and lanceolate with oval bases and well-defined carinae present from the base of the crown to the tip to cf. Melvius sp. These teeth may be opaque to nearly translucent, and are widest at the base and get progressively narrower toward the tip. Sizes vary from 3–7 mm in height and 2–5 mm in width. The teeth are generally comparable to the crowns of both Maastrichtian Melvius thomasi Bryant, 1987 , and Campanian Melvius chauliodous Grande and Bemis, 1998 , and highly carinate tooth tips are a defining character of the Vidalamiinae and are not restricted to Melvius ( Bryant 1987; Grande and Bemis 1998; Sullivan et al. 2011). While diagnostic characters of Melvius are all in the vertebrae and skull ( Grande and Bemis 1998), Melvius is the only vidalamiine currently known from the Upper Cretaceous of North America ( Grande and Bemis 1998), so we tentatively refer the isolated teeth to cf. Melvius sp. here. These amiid teeth are the most common identifiable microvertebrate fossil at the J&M site, where we have collected over one hundred of them. Originally described from Hell Creek Formation channel deposits ( Bryant 1987), Melvius fossils are often found in close proximity to the Western Interior Seaway margin, thus suggesting that they may have been capable of inhabiting brackish or marine waters ( Bryant 1987). Other than an abstract by Hunt-Foster and Foster (2013) that mentions a large, diagnostic Melvius centrum from a different locality, this is the first report of cf. Melvius sp. from the Williams Fork Formation.

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.

Fig. 5. Representative osteichthyan teeth from the J&M site, Colorado, USA, Williams Fork Formation, Campanian–Maastrichtian, Upper Cretaceous. A, E. Paralbula casei Estes, 1969b. A. MWC 8855, tooth crown in occlusal (A1), basal (A2), and mesial (A3) views. E. MWC 8856, tooth crown in occlusal view. B. cf. Melvius sp., MWC 8867, tooth crown in labial view.C. Pycnodontiformes gen.et sp. indet., MWC 8873, tooth plate fragment in occlusal (C) view. D. Actinopterygii indet., MWC 8867, tooth crown in lateral (D) view. F. Dipnoi indet., MWC 8885, toothplate fragment in lateral view.