Rhizoprionodon ganntourensis ( Arambourg, 1952 )

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 : 78-80

Rhizoprionodon ganntourensis ( Arambourg, 1952)

Fig. 28

Scoliodon ganntourensis Arambourg, 1952: 164 , pl. 24, figs 49–63; text-fig. 33.

Rhizoprionodon sp. – Thurmond & Jones 1981: 68, fig. 33.

Rhizoprionodon cf. ganntourensis – Cappetta & Traverse 1988: 361 .

Rhizoprionodon sp. – Clayton et al. 2013: fig. 2i.

Sphyrna sp. – Cappetta & Case 2016: 61, pl. 10, figs 2–4.

Material examined

UNITED STATES OF AMERICA – Alabama • 156 isolated teeth; Claiborne Group ; ALMNH PV1993.2.426 (24 specimens), ALMNH PV1993.2.442 (17 specimens), ALMNH PVPV 2005.6 .435, MMNS VP-8212, MSC 188.324 , MSC 2173.1 – 7 , MSC 2379.1 – 5 , MSC 35753.1 – 7 , MSC 36178, MSC 37098, MSC 37099, MSC 37264.1 – 5 , MSC 37265, MSC 37355, MSC 37648.1 – 3 , MSC 37674.1 – 4 , MSC 37904.1 – 3 , MSC 37905.1 – 3 , MSC 37906.1 – 2 , MSC 37907.1 – 2 , MSC 38199.1 – 2 , MSC 38486.1 – 6 , MSC 38504.1 – 10 , MSC 38544.1 – 11 , SC 2012.47.160, SC 2012.47.168 (5 specimens), SC 2012.47.172, SC 2012.47.52, SC 2012.47.53, SC 2012.47.54 (9 specimens), SC 2012.47.55 (12 specimens), WSU 5017 , WSU 5037 (6 specimens) .

Description

Teeth small, most not exceeding 0.75 cm in mesiodistal width. Crown wide, with broad-based and distally inclined triangular cusp; apex of cusp not extending past the distal edge of the crown. Mesial cutting edge elongate, continuous, smooth. Distal cutting edge very short, oblique, straight to weakly convex, smooth. Conspicuous distal heel forms acute angle with cusp, separated by distal notch. Cutting edge of distal heel convex to angular, smooth. Labial crown face is flat; lingual face is convex; both crown faces smooth. Root shallow with rounded root lobes. Root lobes extend mesially and distally beyond the base of the root. Tooth base is flat to slightly concave in mesial and distal views. A deep nutritive groove present on lingual root face that often forms a basal notch.

Remarks

Our Rhizoprionodon sample exhibits monognathic, dignathic and gynandric heterodonty. Monognathic heterodonty is expressed as a more erect crown on upper and lower anterior teeth when compared to lateral teeth, and lateral teeth are wider than tall with a more inclined main cusp. With regard to dignathic heterodonty, the cusp of upper teeth is taller and more broadly triangular than on lower teeth. Also, lower anterolateral and lateral teeth have a more elongated mesial edge. Male teeth have a mesiodistally thinner crown than those on females, and the distal edge of the crown is often less convex.

Thurmond & Jones (1981: 68, fig. 33) first reported Eocene representatives of Rhizoprionodon in North America when they referred teeth to this taxon that were derived from the basal Gosport Sand at site ACl- 4 in Clarke County, AL. The presence of this genus within the Claiborne Group of Alabama was confirmed as part of this study by comparing the teeth in our sample to the dentitions of extant Loxodon (one species), Rhizoprionodon (eight species), and Scoliodon (one species) housed at MSC and SC and as illustrated by Springer (1964). Of these taxa, the teeth in our sample most closely resemble those of extant species of Rhizoprionodon with respect to the shape of the mesial and distal edges of the crown, the mesiodistal thickness of the crown base, overall crown height, and the extent of distal inclination.

Examination of the dentitions of Recent female Rhizoprionodon acutus (Rüppell, 1837) and Rhizoprionodon terraenovae (Richardson, 1836) specimens in the SC collection revealed differences between the distal heels between the two species. The finely serrated distal heel of R. acutus teeth are broadly convex, but a single small denticle often provides some angularity, most evident on upper anterior teeth. Teeth in the R. terraenovae dentition exhibit a similarly serrated distal heel, but a distinctively large denticle results in a more angular appearance than R. acutus , particularly on the upper teeth. We did not observe any significant variation within the Claiborne sample that could indicate multiple species, and we refer all of the material to one taxon. Our Claiborne Group teeth are similar in age and overall morphology to the type specimens of Rhizoprionodon ganntourensis as described and figured by Arambourg (1952: pl. 26, figs 49–63), and both samples have teeth with a distal heel that varies in shape from rounded to angular. Due to these similarities, the Claiborne material is assigned to R. ganntourensis , currently the only recognized Eocene species within the genus (see Cappetta 2012).

The dentition of extant Scoliodon laticaudus Müller & Henle, 1838 exhibits gynandric heterodonty, with tooth crowns on female lateral and posterolateral teeth being much more distally inclined than those in the male dentitions (which instead have a thin crown with an upturned apex and an elongated mesial shoulder). Only two species of extant Rhizoprionodon possess gynandric heterodonty ( Springer 1964), with male anterolateral teeth having a thinner cusp base than on female teeth. A similar pattern can be observed within our Rhizoprionodon sample. Male anterolateral teeth of Rhizoprionodon are similar to those of Scoliodon (see Cappetta 2012: 300, fig. 283), but can be differentiated by their lack of a lingually twisted cusp apex.

Cappetta & Case (2016) did not identify Rhizoprionodon in their sample of teeth from site ACov- 11 in Covington County, AL. However, the authors figured three specimens (pl. 10, figs 2–4) they referred to Sphyrna sp. that, in our opinion, belong to Rhizoprionodon .

Stratigraphic and geographic range in Alabama

The specimens in our sample were collected from the lower Tallahatta Formation at site ADl-1, the Tallahatta Formation at site AMo-8, the contact of the Tallahatta and Lisbon Formations and the basal Lisbon Formation at site ACov-11, the basal Gosport Sand at site ACl-4, and the Gosport Sand and site ACh-21. Upper Ypresian to middle Bartonian, zones NP14 to NP17.

Figures

Fig. 28. Rhizoprionodon ganntourensis (Arambourg, 1952), teeth. A–C. MSC 2173.4, lower lateroposterior tooth, basal Gosport Sand. A. Labial view. B. Lingual view. C. Mesial view. D–F. MSC 37674.3, lower anterior tooth, basal Lisbon Formation. D. Labial view. E. Lingual view. F. Mesial view. G–I. MSC 2173.5, lower lateral tooth, basal Gosport Sand. G. Labial view. H. Lingual view. I. Mesial view. J–L. MSC 37907.1, lower lateral tooth, lower Tallahatta Formation. J. Labial view. K. Lingual view. L. Mesial view. M–O. MSC 2173.6, upper anterolateral tooth, basal Gosport Sand. M. Labial view. N. Lingual view. O. Mesial view. P–R. MSC 38504.7, lower lateroposterior tooth, basal Gosport Sand. P. Labial view. Q. Lingual view. R. Mesial view. S–U. MSC 35753.3, upper anterolateral tooth, lower Tallahatta Formation. S. Labial view. T. Lingual view. U. Mesial view. V–X. MSC 37648.2, lower lateral tooth, basal Gosport Sand. V. Labial view. W. Lingual view. X. Mesial view. Y–AA. MSC 37098, lower lateral tooth, Tallahatta/Lisbon formation contact zone, courtesy of James Lowery. Y. Labial view. Z. Lingual view. AA. Mesial view. BB–DD. MSC 37674.1, lower lateroposterior tooth, basal Lisbon Formation. BB. Labial view. CC. Lingual view. DD. Mesial view. EE–GG. MSC 37648.1, lower lateral tooth, basal Gosport Sand. EE. Labial view. FF. Lingual view. GG. Mesial view. HH–JJ. MSC 37674.4, lower lateral tooth, basal Lisbon Formation. HH. Labial view. II. Lingual view. JJ. Mesial view. Scale bars = 2.5 mm.