Rhinoptera, Cuvier, 1829

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 : 68-69

“ Rhinoptera ” sp.

Fig. 20

Material examined

UNITED STATES OF AMERICA – Mississippi • 110 isolated teeth; Catahoula Formation ; MMNS VP-12059 ( Fig. 20E–H ), MMNS VP-12060 ( Fig. 20I–L ), SC 2013.28.336 to 28.343 , SC 2013.28.344 ( Fig. 20A–D ), SC 2013.28.345 , SC 2013.28.346 , SC 2013.28.347 ( Fig. 20M–Q ), SC 2013.28.348 to 28.353 , SC 2013.28.354 (4 teeth), SC 2013.28.355 (3 teeth), SC 2013.28.356 (4 teeth), SC 2013.28.357 (10 teeth), SC 2013.28.358 (11 teeth), SC 2013.28.359 (4 teeth), SC 2013.28.360 (4 teeth), SC 2013.28.361 (12 teeth), SC 2013.28.362 (38 teeth).

Description

Teeth vary in width, and relatively unworn specimens exhibit a thick crown. In occlusal view, the crown is six-sided with sharp and centrally located lateral angles that are acute to roughly 90º. The overall shape is variable (straight, sinuous, or weakly convex). In profile view, the labial and lingual faces are vertical to slightly lingually inclined. The labial and lingual faces are generally heavily corrugated with vertical ridges, which are overprinted with finer vertical ridges ( Fig. 20J–K ). The ornamentation on the lingual face is usually less developed than on the labial face, and ornament appears to become obsolete apically. Some specimens show that lingual ornamentation can consist of short basal vertical ridges that transition apically to beaded ridges. The labial crown face overhangs the root, and the basal surface of the crown base bears a shallow transverse furrow. The lingual crown foot is marked by a very thick and rounded, shelf-like transverse ridge. The root is low with nearly vertical labial and lingual faces. In basal view, the root is subdivided into numerous thin, parallel lamellae by nutritive grooves. The lamellae are perpendicular or oblique to the tooth width ( Fig. 20H ). The lingual face of the root does not extend beyond the crown foot.

Lateral teeth vary in mesio-distal width but are generally six-sided and similar to the symphyseal teeth in all other aspects. The ultimate lateral tooth, the last tooth at the margin of the dentition, has a five-sided occlusal outline. The mesial side has a sharply angular margin, whereas the distal side is a straight edge that parallels the length of the dentition. The crown of lateral teeth is higher on the mesial side than on the distal side ( Fig. 20C, J ). Root lamellae are oblique to crown width ( Fig. 20D, L ).

Remarks

Monognathic and ontogenetic heterodonty are evident in our sample. Monognathic heterodonty is expressed as a transition from a very wide symphyseal file to lateral files that become progressively less wide (gradient heterodonty) towards the commissure. From the symphysis ( Fig. 20E–H ), the mesio-distal tooth width in succeeding lateral files progressively decreases from roughly three times as wide as long ( Fig. 20I–L ) to two times as wide as long ( Fig. 20M–Q ), to symmetrically hexagonal. The exact number of files of each morphology is unknown, as our sample does not include complete tooth plates. The margin of the dentition was formed of teeth with a pentagonal outline, where the mesial margin is angular and the distal margin straight. Dignathic heterodonty is difficult to discern based on the sample, but transversely convex specimens ( Fig. 20G ) may have comprised the upper dentition. Overall, these teeth are arched, but there is a medial region where the crown is worn flat. In contrast, relatively flat teeth with roughly uniformly worn crowns were likely part of the lower dentition. Ontogenetic heterodonty is identified based on the variation in tooth size within the sample available to us, which presumably reflects juvenile (i.e., smaller teeth) and adult specimens.

Cicimurri et al. (2022) reported “ Rhinoptera ” sp. teeth from the Rupelian Ashley Formation, and Cicimurri & Knight (2009) identified Rhinoptera cf. studeri ( Agassiz, 1843) from the Chattian Chandler Bridge Formation of South Carolina. These identifications were based on very limited and fragmentary specimens, and it is difficult to make accurate comparisons between that material and the Catahoula Formation sample. Müller (1999) identified R. aff. brasiliensis Müller, 1836 and R. aff. bonasus Mitchill, 1815 in his sample of teeth from the Oligo-Miocene Belgrade Formation of North Carolina. That material does appear to be similar to the Catahoula Formation sample in terms of gross morphology and crown ornamentation on the vertical faces. However, it is unlikely that the fossil specimens represent extant taxa, particularly considering that the genus Rhinoptera apparently did not diverge from Myliobatidae until the Miocene ( Naylor et al. 2012; Villalobos-Segura & Underwood 2020). Additionally, the variation we observed in the Catahoula Formation sample, which is also reflected in the sample reported by Müller (1999), is attributed herein to intraspecific variation (heterodonty) rather than the presence of multiple species. This interpretation is supported by the work of Hovestadt & Hovestadt-Euler (2013), who documented variation within dentitions of extant myliobatiform species. We follow other authors in placing the generic name Rhinoptera in quotation marks to acknowledge the dental similarities between the Oligocene taxon and extant Rhinoptera , and to address the temporal separation of the occurrences ( Ebersole et al. 2019; Cicimurri et al. 2022).

We place the Catahoula Formation taxon within Rhinopteridae following the conclusions of phylogenetic studies for extant Rhinoptera (i.e., Palacios-Barreto et al. 2023) and the very close similarity of the fossil teeth to those of extant members of this genus. However, this assignment is tentative if Rhinoptera / Rhinopteridae diverged from Myliobatidae during the Miocene. Ebersole et al. (2019) reported “ Rhinoptera ” sp. teeth from Ypresian, Lutetian, and Bartonian strata of Alabama that were all comparable to each other in terms of overall shape and crown ornamentation. Those authors therefore could not determine, based on tooth shape alone, whether one or more species were represented in their temporally wide-ranging sample. The Catahoula Formation “ Rhinoptera ” sp. specimens exhibit features comparable to those Eocene examples (i.e., vertical ridges sometimes transitioning to apical beaded ridges, wide lingual transverse ridge), and we therefore refrain from making a more specific determination without the aid of more complete (i.e., skeletal) material.

Figures

Fig. 20. “Rhinoptera” sp., teeth. A–D. SC2013.28.344, proximal lateral tooth. A. Occlusal view. B. Lingual view. C. Labial view. D. Basal view. E–H. MMNS VP-12059, upper symphyseal tooth. E. Occlusal view. F. Lingual view. G. Labial view. H. Basal view. I–L. MMNS VP-12060, proximal lateral tooth. I. Occlusal view. J. Lingual view. K. Labial view. L. Basal view. M–Q. SC2013.28.347, distal lateral tooth. M. Occlusal view. N. Labial view. O. Basal view. P. Lingual view. Q. Profile view. Scale bars = 1 cm.