Progymnodon hilgendorfi Dames, 1883

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 : 182-183

Progymnodon hilgendorfi Dames, 1883

Fig. 68

Progymnodon hilgendorfi Dames, 1883: 148 , pl. 3, fig. 13.

Diodon (? Chilomycterus) hilgendorfi – Weiler 1929: 29 , pls 6–7, 9, 13–14.

? Diodon sp. – Thurmond & Jones 1981: 108, fig. 51.

Progymnodus cf. hilgendorfi – Suraru & Suraru 1987: 128 , fig. 1.

Chilomycterus hilgendorfi – Dica 2002: 40 , pl. 1, figs 1–2.

Material examined

UNITED STATES OF AMERICA – Alabama • 121 isolated teeth; Claiborne Group ; MSC 2396 , ALMNH PV1993.2.456 (57 specimens), ALMNH PVPV 2005.6 .438, MSC 37349.1 – 2 , MSC 37398.1 – 2 , MSC 37435, MSC 37440.1 – 44 , MSC 37443, MSC 37450, MSC 37556, MSC 37602, MSC 37895, MSC 38494.1 – 5 , MSC 38551.1 – 2 , MSC 38842 .

Description

Outer margin (the ‘beak’) composed of numerous stacked rows of very small (less than 1.0 mm) circular to wedge-shaped teeth. Premaxillary margin angular; dentary margin broadly convex. Triturating surface consisting of two stacks of subtriangular teeth; teeth separated from anterior margin by deep gap of bone. Individual teeth consist of thin enameloid crown, weakly convex; medial border (adjacent to tooth on other half of jaw plate) straight; anterior and posterior margins weakly convex, and the teeth have a sub-triangular outline in occlusal view.

Remarks

We identified premaxillary and dentary elements belonging to the Diodontidae based on the size and shape of the triturating teeth, the number of triturating teeth exposed at any one time through in vivo usage, and the nature of the anterior beak. Five fossil diodontid genera are considered valid, including Eodiodon from the middle and late Eocene, Progymnodon from the Paleocene to late Eocene, Oligodiodon from the Oligocene and Miocene, Kyrtogymnodon from the Pliocene, and Chilomycterus and Diodon from the Miocene to Recent (see Tavani 1955; Tyler 1980; Weems 1998; Schultz 2006; Ciobanu & Trif 2012). Tyler (1980) and Dica (2002) questioned the validity of Eodiodon , with both authors citing that the lone diagnostic characteristic of this genus, the lack of teeth at the biting edge (the “beak”), could be attributed to wear and/or preservation. Diodontid tooth plates have been shown to be highly variable in terms of the shape of isolated triturating teeth and the number of teeth concurrently exposed (see Weems 1998: fig. 6a). Nevertheless, our Claiborne samples were assigned to Progymnodon because the triturating teeth are separated from the beak by a bone gap, which is a diagnostic characteristic of this genus (see Ciobanu & Trif 2012). Furthermore, the tooth plates in our sample appear conspecific to those of Progymnodon hilgendorfi as described by Dames (1883). Although Dica (2002) synonymized P. hilgendorfi with Chilomycterus , citing the gap between the beak and the triturating teeth as being characteristic of the latter genus, Ciobanu & Trif (2012) rejected this synonymy, citing the Miocene origins of Chilomycterus .

Thurmond & Jones (1981: 108, fig. 51) reported and figured a specimen from the basal Gosport Sand at site ACl- 4 in Clarke County, AL that they assigned to “? Diodon sp.” Our reexamination of this specimen (ALMNH PV 2005.6.438) revealed that it instead belongs to P. hilgendorfi .

Stratigraphic and geographic range in Alabama

The specimens in our sample were collected from the basal Gosport Sand at sites ACl-4 and AMo-4, and the Gosport Sand at site ACh-21. Middle Bartonian, Zone NP17.

Figures

Fig. 68. Progymnodon hilgendorfi Dames, 1883, tooth plates. A–D. MSC 37450, basal Gosport Sand, upper tooth plate. A. Aboral view. B. Oral view. C. Posterior view. D. Profile view. E–H. MSC 37398.1, basal Gosport Sand, tooth plate. E. Aboral view. F. Oral view. G. Posterior view. H. Profile view. Scale bars: A–D= 1 cm; E–H= 5 mm. Anterior at top in oral and basal views.