Pachyscyllium sp.

Pachyscyllium sp.

Fig. 9K–P View Fig

Material examined

UNITED STATES OF AMERICA – Mississippi • 19 isolated teeth; Catahoula Formation ; MMNS VP-8746 (6 teeth), MMNS VP-8796 ( Fig. 9O–P View Fig ), SC 2013.28.126 ( Fig. 9K–L View Fig ), SC 2013.28.127 ( Fig. 9M– N View Fig ), SC 2013.28.128 to 28.131 , SC 2013.28.132 (2 teeth), SC 2013.28.133 (4 teeth).

Description

All but one of the specimens are broken, but these are morphologically comparable to the complete tooth, SC 2013.28.126 ( Fig. 9K–L View Fig ). This specimen measures 2.5 mm in crown width and just under 3 mm in total height. The crown consists of a main cusp that is slightly distally inclined, and a single pair of large lateral cusplets. The main cusp is tall, triangular, rather narrow, sharply pointed, and its labial and lingual faces are convex. The lateral cusplets are broad, short, pointed, located very low on the crown, and well separated from the main cusp ( Fig. 9K View Fig ). Smooth cutting edges extend along the main cusp and lateral cusplets. In apical view, the lateral cusplets appear to be located anterior to the labial face of the main cusp. The labial crown foot is thickened, concave, and conspicuously overhangs the root. The crown enameloid lacks ornamentation. The root is low (in labial view) and bilobate with lobes extending laterally just beyond the crown margin. The root lobes are very widely diverging and separated by a broad and shallow U-shaped interlobe area. The lingual attachment surface is flat and rather thin on the lobes, but a large medial boss is bisected by a thin, deep nutritive groove ( Fig. 9L View Fig ).

Remarks

As noted above, the additional incomplete teeth in the sample are morphologically similar to the complete tooth represented by SC 2013.28.126. However, we did note some slight differences amongst the teeth, namely the width, robustness, and inclination of the main cusp, as well as the height of the lateral cusplets. Several specimens have a vertical main cusp and symmetrical crowns ( Fig. 9M–N View Fig ), whereas those like SC 2013.28.126 have a slightly distally inclined cusp ( Fig. 9K, O View Fig ). We believe that these differences represent monognathic heterodonty within the same taxon, where anterior teeth are symmetrical and lateral teeth have distally inclined main cusps. Variations in tooth robustness (i.e., some specimens are more gracile than others) could reflect dignathic, ontogenetic, or even gynandric heterodonty, the latter of which has been documented in other scyliorhinid sharks ( Soares & de Carvalho 2019). Cicimurri et al. (2022) reported an incomplete Pachyscyllium sp. specimen from the Rupelian (NP24) Ashley Formation of South Carolina, but that tooth exhibits short longitudinal ridges that are not present on these Catahoula Formation specimens. Ebersole et al. (2021) reported a single Pachyscyllium sp. tooth from the Rupelian (NP23) Glendon Limestone Member of the Byram Formation of Alabama, and Ebersole & Cicimurri (in press) also reported a similar Pachyscyllium sp. tooth from the Rupelian Rosefield Formation in Louisiana. Although these teeth are comparable to those described above, additional material from these lithostratigraphic units are needed to ascertain whether the morphologies are conspecific.

The gross morphology of the Catahoula Formation teeth is comparable to that of various Oligo-Miocene Pachyscyllium species that have been described, but there are some apparent differences. The Catahoula Formation Pachyscyllium sp. teeth have smooth enameloid, whereas those of P. distans possess distinct vertical ridges on the lower portion of the labial face and often on the lingual face of the lateral cusplets. The Catahoula Formation specimens appear to differ from those of P. dachiardii ( Lawley, 1876) by having consistently shorter lateral cusplets, and from P. albigensis Reinecke et al., 2005 by their larger overall size ( Reinecke et al. 2005; Reinecke & Radwański 2015). Although there is some morphological overlap with P. braaschi Reinecke et al., 2005 , the few Catahoula Formation specimens available to us appear to have shorter and less divergent lateral cusplets ( Reinecke et al. 2005, 2014; Haye et al. 2008).