Aetobatus sp, de Blainville, 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 : 123-125

Aetobatus sp .

Fig. 44

Aetiobatis sp. cf. A. irregularis – Thurmond & Jones 1981: 75 , fig. 36b.

Aetobatis sp. – Holman & Case 1988: 328.

Aetobatus irregularis – Cappetta & Case 2016: 66 , pl. 14, fig. 1.

Material examined

UNITED STATES OF AMERICA – Alabama • 127 isolated teeth; Claiborne Group ; GSA-V699, MMNS VP-5642 (2 specimens), MMNS VP-5648, MSC 2389.1 – 11 , MSC 37150 (4 specimens), MSC 37184 (3 specimens), MSC 37244 (2 specimens), MSC 37255 (9 specimens), MSC 37303 (15 specimens), MSC 37330 (7 specimens), MSC 38806 (2 specimens), MSC 38810 (14 specimens), MSC 38830 (3 specimens), MSC 38947, NJSM 24031 , NJSM 24032 , SC 2012.47.23, SC 2012.47.24, SC 2012.47.25, SC 2012.47.26, SC 2012.47.27, SC 2012.47.28 (4 specimens), SC 2012.47.29 (2 specimens), SC 2012.47.211 (29 specimens), SC 2012.47.212, SC 2012.47.213, SC 2012.47.214, SC 2012.47.215, SC 2012.47.216, SC 2012.47.217, SC 2012.47.218, SC 2012.47.219, WSU 5012 , WSU 5023 .

Description

Upper and lower tooth plates consist of a single median file. In oral view, tooth crowns very wide but narrow. Teeth in upper dentition straight, sinuous, or weakly arcuate (convex labially) in outline. Teeth in lower dentition strongly arcuate or chevron shaped. Distal ends of crown within both dentitions are straight and perpendicular to width, basally curving and slightly lingually directed. The labial crown face overhangs the root; root extends well beyond the lingual side of the crown. Both crown faces ornamented with fine vertical wrinkles; thick and rounded transverse ridge present at lingual crown foot. Root polyaulacorhize; numerous thin lamellae present on basal and dorsal surfaces. Labial root face oblique (basiolingually directed); may bear shallow grooves that are in line with lamellae.

Remarks

Partial teeth of Aetobatus could be confused with Pseudaetobatus (see below), although there is currently no evidence that the two taxa were coeval within the Claibornian of Alabama. There are no lateral teeth in the Aetobatus dentition, and the distal ends of individual teeth are straight and perpendicular to the width, basally curving, and lingually directed. In contrast, distal ends of Pseudaetobatus median teeth are angular and form a point of articulation for lateral teeth.

Aetobatus does occur with several other Myliobatinae within Claibornian strata. Tooth crowns of Aetobatus lack the thickened, tuberculated enameloid seen on the occlusal surface of Leidybatis teeth, and the ornament on the vertical crown faces is only weakly developed. A reticulated pattern of beaded ridges and pitting on the vertical crown faces, angular lateral margins, and the sharp and narrow lingual transverse ridge distinguish Aetomylaeus from Aetobatus . Myliobatis teeth are more similar to Aetomylaeus than Aetobatus . Although the teeth of both Aetobatus and Rhinoptera (Rhinopterinae) have a crown with similar ornament and cross section, the labial root face of Aetobatus is oblique and the lingual root margin extends well past the crown, whereas the labial root face on Rhinoptera teeth is low and vertical, and the lingual margin generally does not extend past the lingual crown foot.

Eocene specimens of Aetobatus are often assigned to A. irregularis . However, we could not directly compare the Lisbon material to the type specimens and therefore cannot confidently assign our incomplete specimens to A. irregularis . Additionally, Hovestadt & Hovestadt-Euler (2013) demonstrated that dentitions of extant myliobatin taxa exhibit varying degrees of intraspecific variation, and it is difficult to ascertain if the generally fragmentary, globally distributed remains typically identified as A. irregularis actually represent a single species.

Stratigraphic and geographic range in Alabama

The specimens in our sample were collected from the contact of the Tallahatta and Lisbon formations at site ACh-14, the basal Lisbon Formation at site ACov-11, the contact of the Lisbon Formation and Gosport Sand at site AMo-4, and the Gosport Sand at site ACh-21. Lower Lutetian to middle Bartonian, zones NP14 to NP17.

Figures

Fig. 44. Aetobatus sp., teeth. A–E. MSC 37150, lower median tooth, Tallahatta/Lisbon formation contact zone, courtesy of Bruce Relihan. A. Oral view. B. Basal view. C. Labial view. D. Profile view. E. Lingual view. F–K. MSC 37330.1, median tooth, basal Lisbon Formation. F. Oral view. G. Basal view. H. Lingual view. I. Labial view. J. Distal view. K. Mesial view. L–Q. MSC 37330.2, median tooth, basal Lisbon Formation. L. Basal view. M. Oral view. N. Labial view. O. Lingual view. P. Profile view. Q. Distal view. R–V. SC2012.47.23, lower median tooth, basal Lisbon Formation. R. Oral view. S. Basal view. T. Lingual view. U. Labial view. V. Profile view. W–AA. SC2012.47.24, lower median tooth, basal Lisbon Formation. W. Oral view. X. Basal view. Y. Labial view. Z. Lingual view. AA. Profile view. Scale bars = 1 cm.