Galeocerdo eaglesomei White, 1955

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 : 99-103

Galeocerdo eaglesomei White, 1955

Fig. 35

Galeus latidens Agassiz, 1843 : pl. 26, figs 22–23.

Galeocerdo eaglesomei White, 1955: 320 , text fig. 1.

Galeocerdo latidens – White 1926: 26 , pl. 6, figs 1–21. — Pedroni 1844: 283. — Stromer 1905b: 175, pl. 14, figs 10–15.

Galeocerdo alabamensis – Thurmond & Jones 1981: 63 , fig. 28, right.

Galeocerdo eaglesomei – Maisch et al. 2014 : fig. 3, 9–14.

Material examined

UNITED STATES OF AMERICA – Alabama • 38 isolated teeth; Claiborne Group ; ALMNH PV1989.4.1.1 (11 specimens), ALMNH PV1989.4.151.1a (3 specimens), ALMNH PV1989.4.219a (2 specimens), ALMNH PV1989.4.90, ALMNH PV2000.1.43.4 (8 specimens), ALMNH PV2000.1.44.3, ANSP 23412 , ANSP 23418 , ANSP 23419 , GSA-V695, MMNS VP-7496 (2 specimens), MSC 2378.1 , MSC 36904, MSC 37619.1 – 2 , MSC 39039, NJSM 24023 .

Description

Teeth broadly triangular, with distally inclined, narrow triangular cusp. Anterior teeth with elongate, sinuous mesial cutting edge; cutting edge becomes more uniformly convex towards commissure. Lower two-thirds to three-quarters of mesial edge coarsely serrated; remainder of edge with much smaller serrations; serrations simple in form. Distal edge short, weakly convex, finely serrated nearly to apex. Anterior teeth with distal heel not well differentiated from distal cutting edge; heel becoming more conspicuous towards commissure. Distal heel elongate, concave, very coarsely serrated; serrations simple, decreasing in size basally. Labial crown face flat; lingual face convex; enameloid smooth. Root bilobate; lobes short, rounded, highly diverging; narrow to broadly U-shaped interlobe area. Lingual root face high, flat, bisected by shallow nutritive groove.

Remarks

Five species of Galeocerdo have been reported from Paleogene deposits in Alabama, including G. aduncus (Agassiz, 1843) , G. alabamensis Leriche, 1942 , G. clarkensis White, 1956 ; G. eaglesomei White, 1955 ; and G. latidens (Agassiz, 1843) . Both G. aduncus and G. latidens were first reported from the “Tertiary of Alabama” by Tuomey (1858); however, these specimens were never illustrated and were destroyed by fire near the end of the Civil War in 1865 (see Ebersole & Dean 2013). Hence, the identity of his material cannot be corroborated. Woodward (1889) assigned 18 teeth, purportedly from Alabama but housed within various NHMUK collections, to G. aduncus . White (1956) reexamined Woodward’s (1889) specimens and referred several to a new species, Galeocerdo clarkensis , and determined that the remaining teeth were indeed correctly identified as G. aduncus , but they instead originated from Malta. A few years prior, Leriche (1942) named Galeocerdo alabamensis based on a single tooth derived from Priabonian deposits in Choctaw County, AL. Both G. eaglesomei and G. latidens have since been reported from various Claiborne Group deposits in the state. Westgate (2001), for example, reported 13 G. latidens specimens from the Gosport Sand at site ACl-4 (TMM 43412.2) in Clarke County. Feldmann & Portell (2007) later reported the occurrence of G. latidens from the contact of the Tallahatta and Lisbon formations at site ACov- 11 in Covington County. However, because these authors never figured their specimens we cannot confirm or refute their identifications. Clayton et al. (2013) and Cappetta & Case (2016) also reported G. latidens from site ACov-11, but examination of actual and figured specimens leads us to conclude that they all to belong to Physogaleus . Thurmond & Jones (1981: fig. 28, left) figured a tooth from Monroe County, AL that they referred to Galeocerdo alabamensis , but our reexamination of this tooth (ALMNH PV 2005.6.448) revealed that it instead belongs to G. eaglesomei . Maisch et al. (2014: fig. 3, 9–14) described and figured two G. eaglesomei teeth from the contact of the Tallahatta and Lisbon formations in Choctaw County, and our analysis confirmed their identification.

As part of this study, teeth of Paleogene Galeocerdo species that have been reported from Alabama were compared to those within several Recent jaw sets of Galeocerdo cuvier Péron & Leseur, 1822 , the only extant member of the genus. The purpose of this analysis was to gain a better understanding of the types and degrees of heterodonty (monognathic, dignathic, ontogenetic) occurring within the jaws of Galeocerdo . This analysis allowed us to better define the teeth of this genus, test the validity of published differential characteristics for the various reported species, and ultimately help determine whether these species are valid, nominal, or should belong to a different genus. To test for differences that could be attributed to ontogeny, the fossil Galeocerdo teeth in our sample were directly compared to those within three sizes of G. cuvier jaws. Measured in terms of greatest internal mesiodistal width, the three jaw sizes examined were as follows, small =11.0 cm; medium = 23.5 cm; and large 40.6 cm. A critical examination and comparison of these three G. cuvier jaws provided the following key characteristics that, in turn, helped diagnose the teeth belonging to this genus:

1. Within a jaw, the number of distal heel cusplets decreases the more laterally a tooth is positioned.

2. Among the various jaws, the number of distal heel cusplets on the teeth increases as the shark gets older (presumably because the teeth get larger).

3. Among the various jaws, the tooth serrations become more compound and complex as the shark gets older.

4. Within a jaw, teeth in the anterior and anterolateral files tend to have a mesial edge that is slightly angular. Teeth in the lateral and posterior files have a smoothly concave mesial edge.

5. Within a jaw, teeth in anterior and anterolateral files have a distal edge that is more convex than those in the lateral and posterior positions.

6. Within the jaw, anterior teeth are taller than wide; lateral teeth are wider than tall.

7. Within the jaw, the cusps on the upper teeth are slightly more erect than those in the equivalent position of the lower jaw. This characteristic can only be observed in jaw sets, not on isolated teeth.

8. Within the jaw, the teeth that are about to shed (the oldest teeth) are smaller than the replacement teeth. Furthermore, replacement teeth generally have an additional distal cusplet. This phenomenon was most evident on the smallest jaw, indicating rapid tooth-size increase in juveniles.

9. Within the jaw, serrations on the teeth increase in size apically to the most convex portion of the mesial edge, at which point the serrations decrease in size. Serrations extend nearly to the cusp apex on both the mesial and distal edges.

10. Within the jaw, the serrations on the mesial edge of anterior teeth are larger than those on the distal edge. In the lateral positions, the teeth are more evenly serrated.

11. Among the various jaws, the size difference between the mesial and distal serrations on the anterior teeth was observed on each specimen, suggesting this phenomenon is not related to ontogeny.

Although discernable monognathic heterodonty exists within the dentition of G. cuvier , the teeth of this species, regardless of size/age or position, have the following defining characteristics: all have a distally inclined cusp; a conspicuous notch where the distal edge meets the distal heel; a convex mesial edge; serrations that are largest on the medial part of the mesial edge, but much finer serrations on the upper half, extending nearly to the apex; very large serrated cusplets on the distal heel but fine serrations on the distal edge, which extend nearly to the apex.

Galeocerdo eaglesomei exhibits monognathic heterodonty in that the anterior teeth are more erect, whereas lateral teeth have a lower crown with a more distally inclined cusp. Additionally, the distal heel on anterior teeth is not well differentiated from the distal cutting edge but is seen as a transition from very fine serrations on the distal edge to coarse cusplets on the heel. Lateral teeth have a more conspicuous heel, forming more of a notch with the distal cutting edge. Anterior teeth have a more sinuous mesial cutting edge, whereas lateral teeth are more uniformly convex, although basally weakly concave. Upper and lower teeth were difficult to differentiate with certainty, but lower teeth at times have a slight labial bend in profile view.

With regard to Galeocerdo eaglesomei and Galeocerdo latidens , the latter species was originally erected by Agassiz (1843) based on teeth from an unknown locality and horizon. White (1926) later referred 39 teeth that were derived from Eocene deposits in Nigeria to G. latidens . Dartevelle & Casier (1943), however, expressed their opinion that the G. latidens teeth figured by White (1926: pl. 6), as well those figured by Stromer (1905b: 175, pl. 14, figs 10–15), differed from the specimens reported by Agassiz (1843: pl. 26, figs 22–23) and possibly represented a new taxon. In agreement with these assessments, White (1955) subsequently assigned the teeth he figured in 1926 to a new species, Galeocerdo eaglesomei . White (1955) also referred the teeth figured by Stromer (1905b) to this new taxon and designated a specimen from his 1926 publication (pl. 6: 2) as the holotype. Although we are not certain, White (1926: pl. 6) may have originally referred his specimens to G. latidens because the lateral teeth (figs 7–10, 17–21) were wider than tall and appeared conspecific with the type specimen illustrated by Agassiz (1843: pl. 26, figs 22–23), which is a tooth from a lateral or posterior position. It appears that when Dartevelle & Casier (1943) pointed out that several other of White’s (1926) teeth did not match Agassiz’s (1843) type specimen, White (1955) made the decision to assign his 1926 (pl. 6) teeth, as well as those in Stromer (1905b: pl. 14), to G. eaglesomei . However, Stromer’s (1905b: pl.14) specimens are of interest because he combined teeth with the G. eaglesomei morphology with several lateral teeth (pl. 14, figs 11, 13, 15) that appear to be conspecific with Agassiz’s (1843) G. latidens holotype. In fact, one of the teeth figured by Stromer (1905b: pl. 14, fig. 15) appears nearly identical to Agassiz’s (1843) type specimen, possibly being from the same tooth position (but from the opposite side of the jaw). As part of White’s (1955: 320) type description for G. eaglesomei , he described how the teeth could be differentiated from those of G. latidens by their “greater relative height and shorter base, while the anterior margin is much more convex and posterior emargination less marked owing to the length of the denticles, which also reach nearer to the tip.” These characteristics, however, fall within the range of monognathic heterodonty as observed on the Recent jaw sets of G. cuvier . Therefore, it is our belief that White (1955) failed to recognize the degree of heterodonty within Recent Galeocerdo , leading him to erect a new species, G. eaglesomei , for what were instead the anterior teeth of G. latidens .

Despite our contention that the G. eaglesomei and G. latidens morphologies of White (1926, 1955) and Stromer (1905b) are conspecific, the designation of the former as a junior synonym of G. latidens is problematic. As explained by Agassiz (1843), his figured holotype was collected from an unknown locality and horizon. Because this likely renders G. latidens as a nomen dubium, we suggest the usage of G. eaglesomei for teeth with this morphology, as it is a name that is available and based on specimens from a known locality and horizon.

It is also our conclusion that many teeth previously referred to “ G. latidens ” (i.e., Thurmond & Jones 1981: fig. 28, right; Van den Eeckhaut & De Schutter 2009: pl. 20, figs 8–11; Clayton et al. 2013: fig. 3J; Cappetta & Case 2016: pl. 8, figs 12–16) are misidentified, and instead are lateral teeth belonging to Physogaleus alabamensis comb. nov. ( Leriche 1942). Teeth of Galeocerdo can be differentiated from P. alabamensis comb. nov. by the presence of serrated mesial and distal cutting edges, with serrations extending nearly to the cusp apex. Our examination of Recent G. cuvier jaws indicates that, these fine serrations are always present (unless taphonomically lost) regardless of tooth size and position, and their presence is therefore not related to heterodonty (monognathic or ontogenetic). On the lateral teeth of both P. alabamensis comb. nov. and P. secundus , cutting edges are smooth and mesial denticles never reach the main cusp apex.

Although Galeocerdo has traditionally been placed within the Carcharhinidae (see Compagno 2005; Cappetta 2012; Nelson et al. 2016), recent mitochondrial DNA studies have revealed Galeocerdo cuvier to be an outgroup from this otherwise monophyletic family (see López et al. 2006; Naylor et al. 2012). Furthermore, the dentitions of Recent G. cuvier specimens exhibit little dignathic heterodonty, with isolated upper and lower teeth being difficult to distinguish. The presence of strong dignathic heterodonty is a defining characteristic for nearly all members of the Carcharhinidae , as the overall morphology of upper teeth is generally substantially different from that of the lower teeth (see Compagno 1984; Voigt & Weber 2011). Because of this we follow Herman et al. (2010) in placing Galeocerdo , and all recognized fossil species, within the monogeneric family Galeocerdidae.

Stratigraphic and geographic range in Alabama

The specimens in our sample were collected from the contact of the Tallahatta and Lisbon formations at site ACh-14 and ACon-6, the “upper” Lisbon Formation at site ACl-3, the contact of the Lisbon Formation and Gosport Sand at site AMo-4, the basal Gosport Sand at site ACl-4, and the Gosport Sand at site ACh-21. Lower Lutetian to middle Bartonian, zones NP14 to NP17.

Figures

Fig. 35. Galeocerdo eaglesomei White, 1955, teeth. A–E. MMNS VP–7496.1, anterolateral tooth, “upper” Lisbon Formation. A. Labial view. B. Close–up of distal notch. C. Lingual view. D. Close–up of simple mesial serrations. E. Mesial view. F–J. MMNS VP–7496.2, lateral tooth, “upper” Lisbon Formation. F. Labial view. G. Close–up of distal notch. H. Lingual view. I. Close–up of simple mesial serrations. J. Mesial view. K–O. MSC 37619.1, lateral tooth, “upper” Lisbon Formation, reversed for comparison, courtesy of James Lowery. K. Labial view. L. Close–up of distal notch. M. Lingual view. N. Close–up of simple mesial serrations. O. Mesial view. Scale bars = 5 mm.