Rhinoptera undetermined

Cicimurri, David J. & Knight, James L., 2019, Late Eocene (Priabonian) elasmobranchs from the Dry Branch Formation (Barnwell Group) of Aiken County, South Carolina, USA, PaleoBios 36, pp. 1-31 : 19-21

RHINOPTERA SP.

( FIG. 7K–R )

Referred specimens —SC96.97.54, medial tooth; SC96.97.55, 8 lateral teeth; SC96.97.56, 38 tooth fragments; SC 2001.1.11, 233 partial medial teeth; SC 2001.1.12, lateral tooth; SC 2001.1.13, lateral tooth; SC 2001.1.14, lateral tooth; SC 2001.1.15, lateral tooth; SC 2001.1.16, medial tooth; SC 2001.1.17, 19 medial teeth; SC2013.38.69, medial tooth; SC2013.38.70, lateral tooth; SC2013.38.71, lateral tooth; SC013.38.72, lateral tooth; SC2013.38.73, lateral tooth; SC2013.38.74, four medial teeth; SC2013.38.75, three medial or proximal lateral teeth; SC2013.38.76, ten lateral teeth; SC2013.38.77, eight distal lateral teeth; SC2013.38.78, distal-most lateral tooth; SC2013.38.79, 100 incomplete teeth.

Description — Rhinoptera is represented in our material by 435 teeth. Unworn teeth have a crown measuring up to 1 cm in thickness, but in cases of extreme wear crowns are only 2 mm thick. The labial and lingual faces of all teeth are vertical and flat, and ornamented with fine vertical wrinkles which grade apically into a granular texture on the lingual face. A large rounded transverse ridge is located at the lingual crown/root junction ( Fig. 7L, P, R ). The labial crown face overhangs the root, and there is a shallow groove on the ventral surface of the crown just anterior to the root ( Fig. 7K ). Median teeth (i.e., Fig. 7O–Q ) measure up to 35 mm wide and 8 mm long (4.4:1 ratio). We observed, as did White (1926), that median teeth are evenly worn and may be arched ( Fig. 7P ). The largest teeth that we consider to have been from the first lateral row measure between 23 mm and 25 mm in width and 5 mm to 9 mm in length. These teeth are easy to identify because the crown is obviously thicker on the mesial side than on the distal side ( Fig. 7K, L, R ), and root lobes are often oblique to the long axis of the tooth ( Fig. 7N ). Other specimens that we believe to be from the outermost row of lateral teeth are symmetrical or only slightly wider than long (1:0.7 ratio). These teeth are distinguished by having an angular mesial margin that articulates with the remainder of the dentition, but the distal margin is rounded. At this time we cannot ascertain if intermediate lateral positions were part of the dentition.

Remarks —Unworn median teeth we refer to Rhinoptera have a square cross section, whereas those of Pseudaetobatus are rectangular. The labial and lingual crown faces of Rhinoptera sp. are vertical and bear fine vertical wrinkling, whereas the labial face of Aetomylaeus is concave with extensive pitting, and the lingual face is convex with granular ornamentation. In addition, the root lobes of Rhinoptera do not extend lingually past the crown base as they do on Pseudaetobatus and Aetomylaeus . The lingual crown/root ridge on Rhinoptera is thick and rounded as opposed to thin and sharp on Aetomylaeus .

The Dry Branch Rhinoptera is easily distinguished from Paleocene Rhinop. prisca Woodward, 1907 and Rhinop. raeburni White, 1934 in that the crown is not nearly as thick, labial and lingual faces are nearly vertical and straight, and ornamentation is reduced to fine wrinkles or scattered nodes. Occlusal ornamentation appears to be absent, and nutritive grooves are more numerous and more closely spaced. Although of similar morphology, Dry Branch Rhinoptera teeth are larger than Rhinop. sherborni White, 1926 from the African Eocene, with the crown of the largest complete tooth in our sample measuring 3.6 cm in width as opposed to only 1.7 cm (see White 1926: pl. 10, figs. 16–26; also Arambourg 1952). Even considering specimens originally identified as Myliobatis by White (1926: pl. 10, fig. 5–7) but later considered by Arambourg (1952) to be Rhinop. sherborni medial teeth, the largest specimen measures only 2.5 cm.

Bearing these differences in mind, assigning a species name to the Dry Branch Rhinoptera is somewhat problematical. The available material compares very well to Leidy’s (1855) Zygobates dubius from the “Ashley Phosphate beds” region of South Carolina (Leidy did not illustrate teeth until 1877; see pl. 31, figs. 21–33). Leidy (1877:248) mentioned the possibility that his Z. dubius was conspecific with Trygon carolinensis named by Emmons (1858) from the Eocene of North Carolina. However, Emmons’ illustrated teeth (p. 243, figs. 91, 92) appear to be undulating rather than straight or weakly arched. Cappetta (2006) syonymized Z. dubius with Mio-Pliocene Rhinoptera studeri Agassiz, 1843 , but unfortunately the stratigraphic position and age of Leidy’s fossils are unknown. Due to a lack of stratigraphic control and more limited knowledge of Cenozoic stratigraphy in Leidy’s time, material purportedly from the “Ashley Phosphate beds” of South Carolina could range in age from the Oligocene through Pleistocene epochs (Sanders 2002).

Leriche (1927) illustrated a partial Rhinoptera dentition (p. 44, fig. 6) that may shed some light on the issue. That specimen, from the Swiss Miocene, was identified as Rhinop. studeri and it clearly shows that a median tooth row and at least three lateral tooth rows were present. The first lateral tooth of the Swiss specimen is only 86% of the width of the median tooth, but the second median tooth is only 29% of the width of the medial tooth. We can only confirm two lateral tooth rows in the dentition of the Dry Branch taxon (first and distal-most lateral positions). Additionally, the first lateral tooth is 63% of the width of the median tooth. The Swiss Rhinop. studeri dentition appears to have been disjunct with respect to tooth size reduction towards the commissure, but size reduction within the Dry Branch appears to have been more gradual and similar to Recent Rhinop. brasiliensis ( Bigelow and Schroeder 1953). Leidy’s Z. dubius appears to be identical to the Dry Branch taxon, and his fossils could be older than Miocene. Unfortunately, we cannot corroborate these hypotheses, and at this time we identify the Dry Branch taxon only to the generic level. Interestingly, Darteville and Casier (1959) illustrated two teeth from the Eocene of western Africa (pl. 34, fig. 2a–c. figs. 7, 8), identified as Myliobatis sp. , that are very similar to the Dry Branch Rhinoptera .

Figures

Figure 7. Myliobatidae teeth from the Dry Branch Formation (Priabonian) of Aiken County, South Carolina. A–E. Aetomylaeus sp. Incomplete lower median tooth in labial (A), lingual (B), and occlusal (C) and views, SC2001.1.2. Lateral tooth in occlusal (D) and profile (E) views, SC2001.1.29. F–I. Pseudaetobatus undulatus. Upper median tooth in occlusal (F) view, SC2013.38.86. Lower median tooth in occlusal (G) view, SC2013.38.84. Upper right distal-most lateral tooth in occlusal (H) and labial (I) views, SC2013.38.91. Lateral tooth in occlusal (J) view, SC2013.38.96.2. K–R. Rhinoptera sp. Lateral tooth in labial (K), lingual (L), occlusal (M), and basal (N) views, SC2001.1.12. Median tooth in occlusal (O), lingual (P), and basal (Q) views, SC2001.1.16. Lateral tooth in lingual (R) view, SC2013.38.70. Labial at top in C, D, F–H, J, M–O, Q; at right in E. Scale bars=1 cm in A–C, F–I, K–R; 5 mm in D, E, J.