Myliobatis, Cuvier, 1816

Cicimurri, David J., Ebersole, Jun A., Stringer, Gary L., Starnes, James E. & Phillips, George E., 2025, Late Oligocene fishes (Chondrichthyes and Osteichthyes) from the Catahoula Formation in Wayne County, Mississippi, USA, European Journal of Taxonomy 984 (1), pp. 1-131 : 64-66

“ Myliobatis ” sp.

Fig. 19A–L

Material examined

UNITED STATES OF AMERICA – Mississippi • 170 isolated teeth; Catahoula Formation ; MMNS VP-12061 (21 teeth), MMNS VP-12062 ( Fig. 19A–C ), SC 2013.28.363 ( Fig. 19D–F ), SC 2013.28.364 ( Fig. 19G–I ), SC 2013.28.365 to 28.370 , SC 2013.28.371 (3 teeth), SC 2013.28.372 (2 teeth), SC 2013.28.373 (3 teeth), SC 2013.28.374 (2 teeth), SC 2013.28.375 , SC 2013.28.376 , SC 2013.28.377 (13 teeth), SC 2013.28.378 , SC 2013.28.379 , SC 2013.28.380 (18 teeth), SC 2013.28.381 (40 teeth), SC 2013.28.382 (54 teeth) • 1 dentition; Catahoula Formation; MMNS VP-12063 ( Fig. 19J–L ).

Description

This sample contains teeth that are much wider (mesio-distally) than long (labio-lingually). In occlusal view, the crown is six-sided, with somewhat rounded lateral angles that are located closer to the labial margin, and the overall shape ranges from arcuate (i.e., the labial margin concave and lingual margin convex) to straight ( Fig. 19A, D, G ). Other teeth have a four-sided, squared appearance in occlusal view due to the significantly reduced area of the labial and lingual crown faces. Width and length dimensions of these latter teeth are roughly equal. In profile view, the labial and lingual faces of all teeth are lingually inclined, although one large specimen demonstrates a very thick crown with a concave labial (and convex lingual) crown foot transitioning to a more vertical face. The labial face of relatively unworn crowns bears a reticulated network of ridges near the crown foot, which transitions to irregular vertical ridges towards the apex ( Fig. 19B ). The lingual face is tuberculated basally but otherwise exhibits irregular vertical ridges towards the apex. The labial crown foot may be formed into a thin ridge-like projection that overhangs the root. The lingual crown foot bears a thin, shelf-like transverse ridge that further distinguishes the crown from the root. The crown also overhangs the root on the mesial and distal sides, but lingually the root extends a short distance beyond the transverse ridge. The root is low and may have a straight or convex basal attachment surface, depending on tooth position. The labial face of the root is weakly lingually inclined. In basal view, the root is differentiated into numerous thin, closely spaced, parallel lamellae by nutritive grooves ( Fig. 19C, F, I ).

Remarks

Monognathic, dignathic, and ontogenetic heterodonty are evident in our sample.Monognathic heterodonty is expressed as a drastic transition (disjunct heterodonty) from very wide teeth of symphyseal files to more symmetrical, roughly diamond-shaped teeth in lateral files, the exact number of which in the dentition of this ray is unknown. Upper symphyseal teeth can be identified by their convex occlusal outline and straight basal attachment surface ( Fig. 19B, H ). In contrast, lower symphyseal teeth have a straight occlusal outline and convex basal attachment surface ( Fig. 19E ). We could not identify a feature among the fossil specimens that would allow the upper lateral teeth to be differentiated from those in the lower files. Ontogenetic variation is apparent based on the morphological criteria noted for “ Rhinoptera ” sp. (see below). MMNS VP-12063 ( Fig. 19J–L ) is an ablated upper dentition consisting of fused symphyseal and lateral teeth. The specimen shows that the upper dentition was convex both labio-lingually and mesio-distally.

The teeth we identify as “ Myliobatis ” sp. differ from those of “ Rhinoptera ” sp. by their basally reticulated to apically ridged labial crown faces, basally tuberculated to apically ridged lingual crown faces, lateral angles that are located closer to the labial crown margin, thin and shelf-like lingual transverse ridge at the crown foot, and root lamellae that extend beyond the lingual crown foot. Although these teeth exhibit morphological similarities to those of extant Myliobatis , molecular divergence estimates indicate that most extant myliobatid genera diverged from one another at sometime during the Early-to-Middle Miocene ( Villalobos-Segura & Underwood 2020). This, in turn, indicates that the early Chattian teeth in our sample likely belong to a genus that is ancestral to extant Myliobatis . Therefore, herein we refer these teeth to “ Myliobatis ” with the understanding that future studies may assign this morphology to a new stem genus within the Myliobatis lineage.

Figures

Fig. 19. “Myliobatis” sp. (A–L) and “Aetomylaeus” sp. (M–DD), teeth. A–C. MMNS VP-12062, “Myliobatis” sp., upper symphyseal tooth. A. Occlusal view. B. Labial view. C. Basal view. D–F. SC2013.28.363, “Myliobatis” sp., lower symphyseal tooth. D. Occlusal view. E. Labial view. F. Basal view. G–I. SC2013.28.364, “Myliobatis” sp., upper symphyseal tooth. G. Occlusal view. H. Labial view. I. Basal view. J–L. MMNS VP-12063, “Myliobatis” sp., upper tooth plate. J. Occlusal view. K. Basal view. L. Left profile view. M–N. MMNS VP-12064.1, “Aetomylaeus” sp., lower tooth plate. M. Occlusal view. N. Basal view. O–Q. MMNS VP-12065, “Aetomylaeus” sp., fragmentary upper tooth plate. O. Occlusal view. P. Labial view. Q. Basal view. R–V. MMNS VP-12066, “Aetomylaeus” sp., lateral tooth. R. Occlusal view. S. Distal view. T. Labial view. U. Mesial view. V. Basal view. W–Z. SC2013.28.383, “Aetomylaeus” sp., partial symphyseal tooth. W. Occlusal view. X. Lingual view. Y. Labial view. Z. Basal view. AA–DD. SC2013.28.384, “Aetomylaeus” sp., partial symphyseal tooth. AA. Occlusal view. BB. Lingual view. CC. Labial view. DD. Basal view. Scale bars: A–L, O–DD = 1 cm; M–N = 2 cm.