Rhizoprionodon undetermined

Rhizoprionodon sp. Fig. 8A–D Material examined UNITED STATES OF AMERICA – Mississippi • 36 isolated teeth; Catahoula Formation; MMNS VP-8396 (6 teeth), MMNS VP-8737, SC 2013.28.145 (Fig. 8A–B), SC 2013.28.146, SC 2013.28.147, SC 2013.28.148 (Fig. 8C–D), SC 2013.28.149, SC 2013.28.150, SC 2013.28.151 (7 teeth), SC 2013.28.152 (9 teeth), SC 2013.28.153 (7 teeth). Description Teeth are small and generally measure approximately 5 mm in width (mesio-distal). The crown consists predominantly of a main cusp and a much smaller distal heel. The cusp varies in height, degree of distal inclination, and width (compare Fig. 8A to C). The labial crown face is rather flat, but the lingual face is convex, and enameloid is smooth. The mesial cutting edge is elongated, smooth, and may be straight, weakly convex, or concave to varying degrees. The distal cutting edge is smooth and approximately one-half as long as the mesial edge, and it may be straight or convex (compare Fig. 8B to D). The distal heel is low and varies in length, and it may be uniformly convex or somewhat cuspidate. The distal heel is separated from the distal cutting edge by a distinct notch (Fig. 8A). The root is bilobate with short but widely diverging lobes being separated by a prominent lingual nutritive groove (Fig. 8B). Remarks These teeth are of small size and most of them are imperfectly preserved, which inhibits our ability to accurately identify them taxonomically. However, we believe that the sample demonstrates monognathic, dignathic, and gynandric heterodonty within the taxon based on the morphological criteria recently presented by Ebersole et al. (2023). With respect to monognathic heterodonty, the teeth increase in width but decrease in height from the symphysis to the commissure. Dignathic heterodonty is apparent in cusp width, with that of upper teeth being more than twice as large as that of lower teeth (compare Fig. 8B to D). Anterior teeth of mature, breeding season males have a much more concave mesial cutting edge compared to non-breeding male teeth and female teeth (gynandry). The Catahoula Formation specimens can be differentiated from sphyrnid teeth in our sample (see below) based on their smaller size, shorter main cusp, and shorter and more convex distal heel. However, several authors have commented on the difficulty in differentiating isolated teeth of extant Rhizoprionodon from those of taxa within Sphyrnidae (i.e., Purdy et al. 2001; Ward & Bonavia 2001), as there is morphological dental overlap between some of the species within these genera (Ebersole et al. 2023). The Catahoula teeth in our sample are morphologically similar to those of two named fossil species, including the Eocene Rhizoprionodon ganntourensis (Arambourg, 1952) and the Miocene R. ficheuri (Joleaud, 1912). The former species has been documented in lower-to-middle Eocene deposits in the Gulf Coastal Plain of Alabama and Mississippi (Ebersole et al. 2019, 2023). The Catahoula Formation sample is not large enough to determine morphological similarity to R. ganntourensis and, in any case, the R. ganntourensis morphology has not been documented from any fossil deposits that are stratigraphically younger than the middle Eocene. Additionally, there are a number of taxonomic issues surrounding R. ficheuri from the type locality that warrant further evaluation of this species (see Ebersole et al. 2023 for a discussion of this taxon). We refrain from assigning the Catahoula Formation teeth to any particular species of Rhizoprionodon due to our small sample size.