Rhinoptera, Cuvier, 1829

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 : 135-138

Rhinoptera sp.

Fig. 50

Material examined

UNITED STATES OF AMERICA – Alabama • 1891 isolated teeth; Claiborne Group ; ANSP 23423 , ANSP 23424 , GSA-V705 (2 specimens), MSC 1493 (4 specimens), MSC 199 (12 specimens), MSC 2388 (18 specimens), MSC 2390 (84 specimens), MSC 33272, MSC 33342, MSC 33590, MSC 33844.3 , MSC 33844.6 , MSC 33957, MSC 35744 (7 specimens), MSC 35746 (3 specimens), MSC 35749 (7 specimens), MSC 37200 (9 specimens), MSC 37396, MSC 37429 (3 specimens), MSC 37465 (5 specimens), MSC 37485 (3 specimens), MSC 37488, MSC 37489, MSC 37534 (3 specimens), MSC 37595, MSC 37601, MSC 37625 (2 specimens), MSC 38403, MSC 38779, MSC 38791 (48 specimens), MSC 38794 (78 specimens), MSC 38800 (13 specimens), MSC 38803 (2 specimens), MSC 38808 (7 specimens), MSC 38812 (8 specimens), MSC 38815 (20 specimens), MSC 38816 (2 specimens), MSC 38816 (2 specimens), MSC 38817 (3 specimens), MSC 38825 (10 specimens), MSC 38827 (14 specimens), MSC 38840 (1164 specimens), MSC 38847 (4 specimens), MSC 38848 (72 specimens), MSC 38852 (3 specimens), MSC 38854 (4 specimens), MSC 38856 (67 specimens), MSC 38870 (9 specimens), MSC 38873 (12 specimens), MSC 38875 (3 specimens), MSC 38884 (6 specimens), MSC 38943 (7 specimens), MSC 38949 (14 specimens), MSC 565 , NJSM 24035 (3 specimens), SC 2012.47.18 (13 specimens), SC 2012.47.19 (29 specimens), SC 2012.47.20 (9 specimens), SC 2012.47.230 (12 specimens), SC 2012.47.231 (65 specimens), SC 2012.47.232, SC 2012.47.233, SC 2012.47.234, SC 2012.47.235 (3 specimens), SC 2012.47.236 (4 specimens) .

Description

Teeth vary in width, depending on jaw position. Median teeth the widest; lateral teeth become progressively narrower towards the commissure (roughly hexagonal in outline in more distal rows). All teeth six-sided, regardless of position; have sharp lateral angles of approximately 90-degrees. The occlusal surface flat to weakly convex; covered with smooth enameloid. Labial and lingual crown faces vertical and flat. Labial face slightly overhangs the root. Root smooth except for some basal crenulation; more often irregularly spaced vertical wrinkles occur, which are coarsest basally and may terminate before the crown apex. Lingual face always more strongly ornamented than the labial face; ornament varies from fine to coarse vertical wrinkles that bifurcate apically; fine wrinkling of continuous and discontinuous vertical ridges, or beaded texturing in longitudinal rows. Lingual crown foot marked by thick and rounded transverse ridge; ornamentation generally does not intersect with transverse ridge. Root polyaulocorhize, generally very low compared to crown thickness. Labial face usually vertical; root lamellae typically do not pass the lingual crown foot.

Remarks

Rhinoptera occurs in all of the formations within the Claiborne Group, and it was coeval with several taxa with similar crushing dentitions, particularly within the Lisbon Formation. In general, Rhinoptera teeth have vertical and straight labial and lingual faces, and the root is low with a vertical labial face and distal lobes that do not extend past the crown foot. In contrast, teeth of Aetomylaeus and Myliobatis have concave labial and convex lingual faces, the root his high, with an oblique labial root face and distal lobes that extend beyond the crown foot. Additionally, the lingual crown face and the basal transverse ridge form a 90-degree angle, whereas this juncture is less than 90-degrees on Myliobatis and Aetomylaeus .

Rhinoptera lacks a tuberculated enameloid covering and medial swelling as seen on the median teeth of Leidybatis . Teeth of Aetobatus and Pseudaetobatus have thinner crowns compared to Rhinoptera , and the root of the former two taxa have an oblique labial face and the distal lobes extend well beyond the crown foot.

The best sample of Claiborne Rhinoptera teeth was derived from the Gosport Sand and they exhibit a high degree of variation. Some of this variation represents monognathic heterodonty, with very wide median teeth being flanked by several rows of lateral teeth that become progressively narrower towards the jaw hinge. Dignathic heterodonty is likely also represented, but this is difficult to discern without the aid of an articulated dentition. Gynandric heterodonty is not known in Rhinoptera ( Herman et al. 2000) and ontogenetic heterodonty does not appear to have been developed, as the smallest teeth in our sample are simply smaller versions of the largest teeth we examined. The teeth we examined from the Tallahatta and Lisbon formations possess the same gross morphology as those from the Gosport Sand, and the types of wrinkling observed also overlaps with what we observed in the Gosport Sand sample. Thus, the evidence available to us at this time suggests that the Rhinoptera teeth within all three Claiborne Group formations are conspecific.

Eocene Rhinoptera teeth are often not speciated or referred to R. sherborni ( White 1926; also Arambourg 1952; Hovestadt & Hovestadt-Euler 2013). The gross morphology of the Claiborne Group specimens we examined is comparable to R. sherborni , but unfortunately, White (1926) did not comment on the labial or lingual crown faces with respect to the presence or absence of wrinkling on R. sherborni . Additionally, the specimens illustrated by White (1926: pl. 10) are all shown in occlusal view and we cannot directly compare them to the Claiborne Group teeth. Although teeth that Arambourg (1952: pl. 32, figs 15–24) identified as R. sherborni exhibit wrinkling on the vertical crown faces, the teeth are all worn, and the degree of variation is unknown. This, coupled with the high degree of variability observed in our sample, leads us to refrain from assigning the specimens in our sample to any particular species.

Stratigraphic and geographic range in Alabama

The specimens in our sample were collected from the Meridian Sand Member of the Tallahatta Formation and the lower Tallahatta Formation at site ADl-1, the contact of the Tallahatta and Lisbon formations at sites ACh-14, ACon-6, and ACov-11, the basal Lisbon Formation at site ACov-11, the “upper” Lisbon Formation at site ACh-8, the contact of the Lisbon Formation and Gosport Sand at site AMo-4, the basal Gosport Sand at site ACl-4, and the Gosport Sand at sites ACl-15 and ACh-21. Upper Ypresian to middle Bartonian, zones NP12 to NP17.

Figures

Fig. 50 (opposite page). Rhinoptera sp., teeth. A–E. SC2012.47.233, median tooth, basal Lisbon Formation. A. Oral view. B. Basal view. C. Labial view. D. Lingual view. E. Profile view. F–J. MSC 37389.2, median tooth, lower Tallahatta Formation. F. Oral view. G. Basal view. H. Labial view. I. Lingual view. J. Profile view. K–O. MSC 37389.1, median tooth, lower Tallahatta Formation. K. Oral view. L. Basal view. M. Labial view. N. Lingual view. O. Profile view. P–T. SC2012.47.235, lateral tooth, basal Lisbon Formation. P. Oral view. Q. Basal view. R. Labial view. S. Lingual view. T. Profile view. U–Y. MSC 2388.1, median tooth, Gosport Sand. U. Oral view. V. Basal view. W. Labial view. X. Lingual view. Y. Profile view. Z–DD. MSC 2388.2, median tooth, Gosport Sand. Z. Oral view. AA. Basal view. BB. Labial view. CC. Lingual view. DD. Profile view. EE–II. MSC 2388.3, median tooth, Gosport Sand. EE. Oral view. FF. Basal view. GG. Labial view. HH. Lingual view. II. Profile view. JJ–NN. MSC 38873.2, lateral tooth, Gosport Sand. JJ. Oral view. KK. Basal view. LL. Labial view. MM. Lingual view. NN. Profile view. OO–SS. MSC 38403, lateral tooth, basal Gosport Sand. OO. Oral view. PP. Basal view. QQ. Labial view. RR. Lingual view. SS. Profile view. TT–XX. MSC 37595.1, lateral tooth, basal Gosport Sand. TT. Oral view. UU. Basal view. VV. Labial view. WW. Lingual view. XX. Profile view. YY–AD. MSC 38794.5, lateral tooth, basal Gosport Sand. YY. Oral view. ZZ. Basal view. AB. Labial view. AC. Lingual view. AD. Profile view. AE–AI. MSC 38794.1, lateral tooth, basal Gosport Sand. AE. Oral view. AF. Basal view. AG. Labial view. AH. Lingual view. AI. Profile view. Labial at top in oral and profile views. Scale bars: A–E, U–SS, YY–AD = 1 cm; F–O, TT–XX, AE– AI =5 mm; P–T=2 mm.