Myliobatis sp. 2, Cuvier, 1816

Ebersole, Jun A., Cicimurri, David J. & Stringer, Gary L., 2019, Taxonomy and biostratigraphy of the elasmobranchs and bony fishes (Chondrichthyes and Osteichthyes) of the lower-to-middle Eocene (Ypresian to Bartonian) Claiborne Group in Alabama, USA, including an analysis of otoliths, European Journal of Taxonomy 585, pp. 1-274 : 130-133

Myliobatis sp. 2

Figs 47 K–T, 48

Myliobatis toliapicus – Clayton et al. 2013 : fig. 4.g – i.

Ariidae – Maisch et al. 2016: fig. 3.1–2.

Material examined

UNITED STATES OF AMERICA – Alabama • 287 isolated teeth; Claiborne Group ; ANSP 23427 , MSC 35007, MSC 35008, MSC 35944 (7 specimens), MSC 35945, MSC 37103, MSC 37152, MSC 37290, MSC 37337 (12 specimens), MSC 37560, MSC 37656, MSC 38640, MSC 38790 (6 specimens), MSC 38795 (5 specimens), MSC 38802 (3 specimens), MSC 38804, MSC 38809 (19 specimens), MSC 38818 (8 specimens), MSC 38828 (101 specimens), MSC 38837, MSC 38841 (20 specimens), MSC 38846 (2 specimens), MSC 38849 (2 specimens), MSC 38855 (3 specimens), MSC 38871 (5 specimens), SC 2012.47.220, SC 2012.47.221, SC 2012.47.222 (3 specimens), SC 2012.47.223, SC 2012.47.224 (2 specimens), SC 2012.47.225 (52 specimens), SC 2012.47.226 (10 specimens), SC 2012.47.227 specimens), WSU 12 , WSU 5004 .

Description

Tooth crown on median teeth wider than long, six-sided, has sharp lateral angles. Upper median teeth have convex crown, straight crown foot. Lower teeth relatively flat, have an undulating crown foot (convex medially, straight laterally). Labial face conspicuously overhangs the root, is weakly concave, may be vertical or slightly lingually inclined. Lingual face is correspondingly convex, may be vertical or slightly inclined. Labial crown face bears a network of fine reticulated ridges on the lower part, grading apically into longitudinal ridges that anastomose and become finer towards the occlusal surface. Ornamentation on lingual face similar, but stronger. Shelf-like and rounded basal transverse ridge at the lingual crown foot. Root polyaulocorhize, on unworn teeth it constitutes one-third to one-half the total tooth height. Labial root face oblique; lingual side of root extends past the crown base.

Upper and lower lateral teeth six-sided, nearly equidimensional, have highly irregular occlusal outline. The labial and lingual faces comparable in shape and ornament to median teeth. Root subdivided into three lobes by two nutritive grooves.

Remarks

This species is represented in our sample by two partial dentitions. MSC 35008 ( Fig. 48 A–F) is the right half of an upper dentition consisting of six incomplete but articulated median teeth, and a single lateral row (articulated with the third through sixth median teeth). Overall, the occlusal surface is convex mesiodistally, and the dentition is convex labiolingually. The two anterior-most median teeth are concave due to their having formed part of the triturating surface.

MSC 35007 ( Fig. 48 G–L) is a partial lower dentition consisting of five sutured median teeth and a single left lateral tooth between the fourth and fifth medians (the second and third medians had become detached but reconnected with B- 76 in ethanol). The sutured median teeth show that the lower dentition was flat mesiodistally and labiolingually. The distal ends of the lateral teeth on both MSC 35007 and MSC 35008 are ablated and it is not entirely clear if other lateral rows were present. However, the fact the distal ends are angular indicates that there was at least one additional row of lateral teeth.

The heavily wrinkled labial and lingual faces of the crown helps to articulate the teeth tightly together, and in oral view the crowns are heavily sutured. The articulation of individual teeth within the dentition is enhanced by the labial crown foot fitting into a lingual furrow of the preceding tooth, as well as overlap of the labial crown foot with the lingual transverse ridge. The labial face of the root also fits tightly against the sloping lingual face of the preceding tooth.

Myliobatis sp. 2 is easily distinguished from other similar taxa occurring within the Tallahatta/ Lisbon contact zone, the Lisbon Formation, and Gosport Sand. The median teeth of Lisbon Formation Leidybatis bear an unusual, highly convex area flanked by thinner margins, and the occlusal surface has a thick and tuberculated enameloid covering, both features that are lacking in Myliobatis . Additionally, the root of Myliobatis is very high, with an oblique labial face and lamellae that conspicuously extend past the lingual crown foot. This contrasts with the roots of both Leidybatis and Rhinoptera , which are typically very low compared to crown thickness, the labial faces are vertical, and the lamellae do not extend past the lingual crown foot. The labial and lingual crown faces on the teeth of Leidybatis , Aetobatis , and Rhinoptera bear vertical wrinkling of differing coarseness, whereas the lower part on Myliobatis sp. 2 tooth crowns bear reticulated ridges that grade apically into longitudinal ridges. The median teeth of Aetomylaeus are similar to Myliobatis sp. 2 but can be differentiated by their pitted labial face and beaded appearance on the lingual face. Additionally, the lateral teeth are wider than long (nearly diamond-shaped) and not nearly as hexagonal as on Myliobatis sp. 2. Lastly, the lateral angles on Aetomylaeus teeth are more oblique than on Myliobatis sp. 2, and the margins of Aetobatus median teeth are straight.

This morphology was identified as Myliobatis toliapicus by Clayton et al. (2013), but examination of the larger sample of teeth available to us, in addition to the partial dentitions, leads us to believe that the species is similar, if not conspecific, with M. latidens as illustrated by Hovestadt & Hovestadt- Euler (2013: pl. 25, figs 12–13). This is based on the labiolingual narrowness of the crown and the highly serrated occlusal outline. However, we refrain from a more specific identification until direct comparisons to the M. latidens type material can be made.

Stratigraphic and geographic range in Alabama

The specimens in our sample were collected from the contact of the Tallahatta and Lisbon formations at sites ACon-6 and ACov-11, the basal Lisbon Formation at site ACov-11, the “upper” Lisbon Formation at site ACl-3, the basal Gosport Sand at site ACl-4, and the Gosport Sand at sites ACh-21, ACl-15, and AMo-4. Middle Lutetian to middle Bartonian, zones NP15 to NP17.

Figures

Fig. 47. Myliobatis Cuvier, 1816, teeth. A–J. Myliobatis sp. 1. A–E. MSC 38937, median tooth, lower Tallahatta Formation. A. Oral view. B. Basal view. C. Lingual view. D. Labial view. E. Profile view. F–J. MSC 33293, median tooth, lower Tallahatta Formation. F. Oral view. G. Basal view. H. Lingual view. I. Labial view. J. Profile view. — K–T. Myliobatis sp. 2. K–O. MSC 38640, median tooth, Gosport Sand. K. Oral view. L. Basal view. M. Lingual view. N. Labial view. O. Profile view. P–T. MSC 38849.1, lateral tooth, basal Gosport Sand. P. Oral view. Q. Basal view. R. Labial view. S. Lingual view. T. Profile view. Labial at top in oral and basal views. Scale bars: A–O =5 mm; P–T=2 mm.

Fig. 48. Myliobatis sp. 2, tooth plates. A–F. MSC 35008, upper tooth plate, Tallahatta/Lisbon formation contact zone. A. Left lateral view. B. Oral view. C. Basal view. D. Right lateral view. E. Lingual view. F. Labial view. G–L. MSC 35007, lower tooth plate, Tallahatta/Lisbon formation contact zone. G. Right lateral view. H. Oral view. I. Basal view. J. Left lateral view. K. Lingual view. L. Labial view. Labial at top in oral, basal, and lateral views. Scale bars = 1 cm.