Heterodontus, de Blainville, 1816

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 : 15-17

Heterodontus sp.

Fig. 5

Heterodontus sp. cf. H. woodwardi – White 1956: 128 . — Thurmond & Jones 1981: 42, fig. 9.

Material examined

UNITED STATES OF AMERICA – Alabama • 7 isolated teeth; Claiborne Group; MSC 2394, MSC 37449, MSC 37320, MSC 35769.1–2, SC 2012.47.33, SC 2012.47.156.

Description

Only lateral teeth were identified in our sample. Lateral teeth are apicobasally compressed and mesiodistally elongated. Teeth range from oval to rectangular to slightly sigmoidal in occlusal outline. Crown is dome-shaped and ranges from non-cuspidate to weakly cuspidate in profile view. Teeth have a medial transverse ridge that extends mesiodistally across the tooth, but not to the lateral margins. The ridge bisects the oral surface into labial and lingual crown faces of unequal size. Crown enameloid ranges from completely smooth to heavily ornamented, with ornamentation consisting of a combination of reticulated, folded, and smooth surfaces. Reticulations and folds often intersect the transverse ridge, but do not reach the crown foot. Transverse furrow at base of lingual face serves as articulation surface for succeeding tooth. Root is thin with flat attachment surface, slightly smaller in area than the crown.

Remarks

Our sample of Claiborne specimens includes seven Heterodontus lateral teeth, five of which preserve a complete crown. These teeth include both cuspidate ( Fig. 5E, G, I ) and non-cuspidate forms ( Fig. 5A, C ). Of the cuspidate teeth, the central cusp is located either in the center of the crown ( Fig. 5G, I ) or is offset laterally ( Fig. 5E ), indicating they are from different tooth positions. On the non-cuspidate teeth, the crown is more dome-shaped in profile view than those with cusps. On all the teeth, the crown ornamentation differs on the lingual and labial surfaces. In general, the lingual crown surface is more ornamented than the labial surface, often consisting of bifurcating ridges or folds that intersect with the transverse ridge but do not reach the edges of the crown. The labial surface on at least one tooth (MSC 37449; Fig. 5I –J ) has a reticulated ornamentation, but the ornamentation is nondescript or absent on the other teeth in our sample, likely due to wear.

The teeth of Heterodontus in our sample were directly compared to the jaws of two modern members of the genus, Heterodontus portusjacksoni (Meyer, 1793) and Heterodontus zebra (Gray, 1831) . Our observations of these Recent specimens showed that the lateral teeth on the jaws of H. portusjacksoni are non-cuspidate, whereas several lateral files on the jaw of H. zebra have teeth with cusps. On the jaws of H. zebra , the teeth within the first few lateral rows are more cuspidate than those in the remaining lateral files, having the appearance of a low-crowned crushing tooth with a cusp. Within the dentition of H. zebra , the teeth were observed to become progressively more cuspidate in the younger replacement rows, and the transverse ridges in both species become more robust in these rows. This suggests that the formation of cuspidate teeth and expression of the transverse ridge might be related to ontogeny, and that both features increase in size/robustness as an individual aged. With respect to general morphology and ornamentation, the upper lateral teeth of both species appear to be identical to the lower laterals, making it difficult to distinguish upper and lower files. Furthermore, the teeth of both species appear more worn in the more labial, functional rows, possibly suggesting a longer period of tooth retention in Heterodontus than in other shark taxa.

Our observations of dentitions of extant Heterodontus suggest that the presence or absence of cuspidate lateral teeth might be a taxonomically useful characteristic, but only when presented with a complete dentition. When isolated teeth are involved, this characteristic is not taxonomically useful because non-cuspidate teeth may simply be from a more laterally positioned file or from a younger individual. Although both cuspidate and non-cuspidate teeth are present in our sample, due to their extremely small size we cannot rule out the possibility that they all belong to the same taxon, and the variation present reflects monognathic and/or ontogenetic heterodonty. Furthermore, the crown ornamentation varies across our suite of teeth and several specimens are ablated, which does not allow us to determine with confidence which species they represent. The combined seven specimens in our sample demonstrates that Heterodontus occurs in all three Claiborne Group formations, but at this time we cannot determine if they are conspecific or represent two or more distinct species.

Stratigraphic and geographic range in Alabama

Specimens of Heterodontus sp. were collected from the lower Tallahatta Formation at site ADl-1, the basal Lisbon Formation at site ACov-11, the basal Gosport Sand at site ACl-4, and the Gosport Sand at site ACh-21. Upper Ypresian to middle Bartonian, zones NP12 to NP17.

Figures

Fig. 5. Heterodontus sp. lateral teeth. A–B. MSC 35769.2, lower Tallahatta Formation. A. Lingual view. B. Oral view. C–D. MSC 35769.1, lower Tallahatta Formation. C. Lingual view. D. Oral view. E–F. SC2012.47.33, basal Lisbon Formation. E. Lingual view. F. Oral view. G–H. MSC 37320, basal Gosport Sand. G. Lingual view. H. Oral view. I–J. MSC 37449, basal Gosport Sand. I. Labial view. J. Oral view. Labial surface at top in oral views. Scale bars = 2 mm.