Brachycarcharias twiggsensis ( Case, 1981 )

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 : 43-45

Brachycarcharias twiggsensis ( Case, 1981)

Fig. 14

Lamna twiggsensis Case, 1981: 58–59 , pl. 3, figs 4–8, text fig. 3.

Cretolamna twiggsensis – Case & Cappetta 1990: 9–10 , pl. 3, figs 40–55.

Brachycarcharias cf. twiggsensis – Underwood et al. 2011: 52 .

Tethylamna twiggsensis – Cappetta & Case 2016: 51 .

Material examined

UNITED STATES OF AMERICA – Alabama • 21 isolated teeth; Claiborne Group ; ALMNH PV1989.4.160.2 (2 specimens), ALMNH PV1989.4.32.4, ALMNH PV1989.4.34.3, ALMNH PV1989.4.47.2 (2 specimens), ALMNH PV1989.4.6.1.3 (3 specimens), ALMNH PV1989.4.63.1.2, ALMNH PV1989.4.94.2, MSC 2372.15 , MSC 2375.4 , MSC 37510.1 , MSC 37510.3 , MSC 37561.2 , MSC 37561.17 , MSC 37561.28 , MSC 37561.41 , MSC 37561.42 , MSC 37645 .

Description

Sample includes anterior, anterolateral, and lateroposterior teeth.Anterior teeth with tall, erect, somewhat narrow, symmetrically triangular main cusp. Sharp, sinuous cutting edges extend from cusp base to apex; apex sharply pointed. Labial face nearly flat, lingual face very convex; enameloid smooth. In profile, apex of main cusp curves labially. Single pair of tall lateral cusplets present. Root bilobate with elongated, slightly diverging lobes separated by U-shaped interlobe area. Lingual nutritive groove on medial boss. Anterolateral teeth with slight distal inclination to main cusp; cusp labiolingually thinner than on anterior teeth. In profile, labial curvature of main cusp apex more pronounced than on anterior teeth. Two pairs of diverging lateral cusplets present; first pair large, tall, broadly triangular; second pair diminutive. Root bilobate with more divergent lobes; lobes range from pointed to rounded. Interlobe area shallow, V-shaped to U-shaped. Shallow nutritive groove on triangular lingual root protuberance. One or more large nutritive foramina occur within nutritive groove.

Remarks

The taxonomic history of the twiggsensis morphology has been tumultuous, having been assigned to Lamna ( Case 1981) , Cretalamna ( Case & Cappetta 1990) , Brachycarcharias ( Underwood et al. 2011) , and more recently Tethylamna ( Cappetta & Case 2016). As part of their generic diagnosis for Tethylamna, Cappetta & Case (2016) referred the Priabonian “ Lamna ” twiggsensis Case, 1981 to this genus. However, the lateral cusplets on the teeth of “ L. ” twiggsensis differ significantly from those of the type species of Tethylamna, T. dunni Cappetta & Case, 2016 , suggesting the species does not belong to the latter genus. For instance, the anterior teeth of T. dunni often have two pairs of lateral cusplets, the larger of which is very narrow and often medially curved. In contrast, there is generally only a single pair of erect cusplets on “ L. ” twiggsensis anterior teeth. Additionally, although both T. dunni and “ L. ” twiggsensis lateral teeth have two pairs of cusplets, those of T. dunni are most often both mesially directed, whereas they are diverging on the twiggsensis morphology. Also, the first cusplet on the distal crown shoulder of T. dunni lateral teeth is conspicuously wider than the corresponding cusplet on the mesial side. In contrast, the equivalent cusplets on “ L. ” twiggsensis lateral teeth are roughly of equal size. Underwood et al. (2011) placed the twiggsensis morphology within Brachycarcharias because the degree of ontogenetic heterodonty they observed within their middle-to-late Eocene sample from Egypt suggested a close affinity to this genus, as opposed to Cretalamna or Lamna . Because the characteristics noted above are more consistent with our observations of Brachycarcharias teeth in Alabama, we follow Underwood et al. (2011) in assigning the twiggsensis morphology to this genus.

The B. twiggsensis teeth in our sample were separated from those of Brachycarcharias atlasi by being more robust and larger in overall size, by having a wider crown base, and by lacking coarse lingual ornamentation on the main cusp and lateral cusplets. Similarly, the teeth of B. twiggsensis can be differentiated from those of Brachycarcharias lerichei by their larger overall size, broader crown, and consistent development of a second pair of lateral cusplets. In contrast, teeth of B. lerichei only occasionally exhibit a second lateral cusplet (generally on one side of the tooth, and poorly separated from the larger, first cusplet). There are certain smaller teeth in the Gosport Sand sample that resemble specimens of B. lerichei (i.e., Fig 14 V–X), but based on larger samples of B. twiggsensis teeth from the Clinchfield Formation of Georgia and Tupelo Bay Formation of South Carolina, we regard these teeth as belonging to juvenile individuals of B. twiggsensis . In these samples, smaller and more gracile teeth could be identified as B. lerichei , but the lateral teeth have a conspicuous second or third lateral cusplet like their larger and more robust B. twiggsensis counterparts.

Anterior teeth of Striatolamia differ from those of B. twiggsensis in their taller but much narrower dimensions, diminutive lateral cusplets, and presence of coarse lingual longitudinal ridges. Lateral teeth of Striatolamia differ in having a main cusp with blunted apex and fine lingual ornamentation, the lateral cusplets are rounded with the distal cusplet being much larger in size than the mesial one. In contrast, cusplets of B. twiggsensis lateral teeth are sharply pointed and of roughly equal size. Anterior and lateral teeth of Jaekelotodus have much smaller lateral cusplets than those of B. twiggsensis , and the lateral teeth of Jaekelotodus are very conspicuously distally hooked and with a much more sharply apically tapering main cusp.

Stratigraphic and geographic range in Alabama

The specimens in our sample were collected from the basal Gosport Sand at site ACl-4, and the Gosport Sand at sites ACh-21 and ACl-15. Middle Bartonian, Zone NP17.

Figures

Fig. 14. Brachycarcharias twiggsensis (Case, 1981), teeth. A–C. MSC 2372.4, anterior tooth, Gosport Sand. A. Labial view. B. Lingual view. C. Mesial view. D–F. MSC 2372.15, anterolateral tooth, Gosport Sand. D. Labial view. E. Lingual view. F. Mesial view. G–I. ALMNH PV1989.4.47.1, anterior tooth, Gosport Sand. G. Labial view. H. Lingual view. I. Mesial view. J–L. ALMNH PV1989.4.16.1, anterior tooth, Gosport Sand. J. Labial view. K. Lingual view. L. Distal view. M–O. ALMNH PV1989.4.47.2, lateral tooth, Gosport Sand. M. Labial view. N. Lingual view. O. Mesial view. P–R. ALMNH PV1989.4.63.1, lateral tooth, Gosport Sand. P. Labial view. Q. Lingual view. R. Mesial view. S–U. ALMNH PV1989.4.94, lateral tooth, Gosport Sand. S. Labial view. T. Lingual view. U. Mesial view. V–X. MSC 37561.2, lateroposterior tooth, Gosport Sand. V. Labial view. W. Lingual view. X. Mesial view. Scale bars = 1 cm.