Isogomphodon aikenensis, Cicimurri & Knight, 2019

Cicimurri, David J. & Knight, James L., 2022, Late Eocene (Priabonian) elasmobranchs from the Dry Branch Formation (Barnwell Group) of Aiken County, South Carolina, USA, PaleoBios 36, pp. 1-31 : 11-13

publication ID	https://doi.org/ 10.5070/P9361043964
publication LSID	lsid:zoobank.org:pub:3F95876E-933FF-48AF-9CF0-A840A333220B
persistent identifier	https://treatment.plazi.org/id/03E787A6-FE25-FF88-A98D-FD1DFA32F8DD
treatment provided by	Felipe
scientific name	Isogomphodon aikenensis
status	N. SP.

Treatment

ISOGOMPHODON AIKENENSIS CICIMURRI AND KNIGHT N. SP.

( FIG. 4E–P View Figure 4 )

Holotype —SC2013.38.110 ( Fig. 4E View Figure 4 ), upper anterior tooth.

Paratypes —SC2013.38.111 ( Fig. 4P View Figure 4 ), upper anterolateral tooth; SC2013.38.119 ( Fig. 4H, I View Figure 4 ), lower anterior tooth; SC2013.38.112 ( Fig. 4J, K View Figure 4 ), upper lateral tooth; SC2013.38.115 ( Fig. 4N, O View Figure 4 ), posterior tooth.

Referred specimens —SC2001.1.62, upper tooth; SC2001.1.63, lower tooth; SC2001.1.64, 97 teeth; SC2013.38.113, symphyseal tooth; SC2013.38.114, posterolateral tooth ( Fig. 4L, M View Figure 4 );SC2013.38.116, upper tooth; SC2013.38.117, 15 symphyseal teeth; SC2013.38.118, anterior tooth; SC2013.38.120, four anterior teeth; SC2013.38.121, six anterior teeth; SC2013.38.122, 52 teeth; SC2013.38.123.1 lower lateral tooth ( Fig. 4F, G View Figure 4 ), SC2013.38.123.2, 84 teeth; SC2013.38.124, 112 teeth; SC2013.38.125, 16 posterior teeth.

Occurrence — Type locality: South Aiken Site ( SAS) 33.504444, -84.742778, Aiken County, South Carolina; yellow, orange and red variegated sand, Upper Eocene (Priabonian) Dry Branch Formation , approximately two meters below the contact with the overlying Tobacco Road Formation GoogleMaps .

Etymology —The species name recognizes the city of Aiken and Aiken County, South Carolina, the only known area from which Eocene Isogomphodon have thus far been documented from North America.

Diagnosis —Nearly 400 specimens are referred to the new species, which differs from Recent Isogomphodon oxyrhynchus (Müller and Henle 1841) in that all teeth have smooth cutting edges extending from the apex to the very base of the crown. In contrast, the upper teeth of the extant species are weakly serrated and edges of lower anterior teeth are often limited to the upper half of the crown. In addition, the Eocene teeth have shorter cusps, many exhibit very convex lateral shoulders (especially more lateral positions), and the root lobes are more elongated and divergent ( Herman et al. 1991, Compagno et al. 2005).

The new species differs from the fossil species I. acuarius (Probst 1879) , I. lerichei ( Darteville and Casier 1943) , I. caunellensis ( Cappetta 1970) , and I. gracilis ( Jonet 1966) in being smaller in overall size. Additionally, the Dry Branch species has narrower upper anterior teeth and complete cutting edges on all teeth when compared to I. caunellensis , and the convexity of the lateral shoulders of anterior teeth appears to be more pronounced than on I. acuarius ( Cappetta 1970, Case 1980, Müller 1999). The transition from main cusp to lateral shoulders appears to be slightly more angular on teeth of I. lerichei ( Darteville and Casier 1943) .

Description —These 393 distinctive teeth generally measure less than 5 mm in total height, although some anterior teeth are up to 7 mm. All teeth have a tall and narrow cusp, and enameloid shoulders that extend nearly to the tips of the root lobes. The cutting edges of all teeth are smooth and extend to the tips of lateral shoulders. The holotype, SC2013.38.110, is an upper anterior tooth that is slightly asymmetrical ( Fig. 4E View Figure 4 ). The crown consists of a tall, narrow and erect cusp, with cutting edges that diverge slightly towards the crown base. Elongate lateral shoulders extend obliquely onto the root lobes. The root is bilobate, with rather short and sub-rectangular lobes that are separated by a narrow but deep U-shaped interlobe area. The mesial lobe is slightly smaller than the distal lobe, and a deep but narrow nutritive groove bisects the flat lingual root face. Upper anterolateral teeth ( Fig. 4P View Figure 4 ) are wider than those near the jaw symphysis. The cusp is narrow and flat as seen in more anterior positions, but slightly distally inclined. The lateral shoulders are more elongated and nearly horizontal. The sub-rectangular root lobes are lower, more elongated and divergent than on anterior teeth, and the interlobe area is wider but shallower.

Lower anterior teeth ( Fig. 4H, I View Figure 4 ) have a very tall and narrow cusp, with sub-parallel cutting edges. Lateral shoulders are short and nearly horizontal. The bilobate root is bisected by a deep, narrow nutritive groove, and the short lobes are widely separated by a U-shaped interlobe area. Lower lateral teeth ( Fig. 4F, G View Figure 4 ) are similar to those in the upper jaw, but the cusp is narrower, lateral shoulders are shorter, the transition from cusp to shoulder is more angular, and the mesial root lobe is shorter than the distal one. The root is low and bilobate, with rounded and diverging lobes that are separated by a wide and shallow U-shaped interlobe area. A narrow but deep nutritive groove is located on the lingual face.

Posterolateral and posterior teeth ( Fig. 4L–O View Figure 4 ) are small with a T-shaped outline. The cusp is triangular but sharply tapering apically, rather low compared to teeth in more anterior positions. The cusp is vertical to slightly distally inclined, flat in profile. Lateral shoulders are short to elongate, perpendicular to the cusp, and the transition from cusp to shoulder is more angular than seen on more anterior jaw positions. The lateral shoulders in these positions are very convex at their distal extremities, resulting in a cusp-like appearance in lingual view. The root is bilobate with short, rounded, highly diverging lobes, which are separated by a broad, shallow to deep U-shaped interlobe area. The lingual face bears a centrally located narrow but deep nutritive groove.

Remarks —A number of fossil species have been assigned to Aprionodon Poey, 1868, a junior synonym of Isogomphodon , including Ap. acuarias (Probst, 1879) , Ap. amekiensis White, 1926 , Ap. caunellensis Cappetta, 1970 , Ap. collata ( Eastman, 1904) , Ap. elongatus ( Leriche, 1910) , Ap. gibbesi (Woodward, 1889) , Ap. gracilis ( Jonet, 1966) , Ap. frequens ( Dames, 1883) , Ap. lerichei Darteville and Casier, 1943 , Ap. macrorhiza Jonet, 1966 , Ap. marÇaisi Arambourg, 1952, and a subspecies, Ap. lerichei var. minuta Jonet, 1966 . Although some of these species are at least in part based on specimens correctly assigned to the genus, most are appropriately referred to other genera or are perhaps nomina dubia.

For example, Carcharhinus collata Eastman, 1904 was referred to Aprionodon by Powlowska (1960), but illustrations of three teeth in Eastman (1904: pl. 22, figs. 3-5) show them to represent Carcharhinus and, possibly, Negaprion . Antunes and Jonet (1970) concluded that Jonet’s (1966) Ap. macrhorhiza represents symphyseal teeth of indeterminate Carcharhinidae . The teeth that White (1926) illustrated as Ap. amekiensis (pl. 8, figs. 11-26)largely appear to be Carcharhinus (with some possibly representing Negaprion ), and Arambourg’s (1952) Ap. marÇaisi morphology was identified as Carcharhinus marÇaisi by Noubhani and Cappetta (1997: 151). Teeth of the Ap. gibbesi (Woodward 1889) and Ap. elongatus ( Leriche 1910) species have weakly to coarsely serrated heels, particularly the uppers, and both species have been placed within Carcharhinus (Reinecke et al. 2001, 2005, Haye et al. 2008, Cicimurri and Knight 2009).

With regard to Jonet’s (1966) Ap. lerichei var. minuta subspecies, Cappetta (1970) considered the material to represent the lateral teeth of Isogomphodon acuarius , but he later (2006) treated the morphology as a valid species distinct from I. acuarius without providing supporting evidence. However, our examination of the teeth illustrated in Jonet’s (1966) plate II leads us to conclude that many of them, including the type specimen shown in figure 1, are not Isogomphodon . Specimens shown in figures 7, 11 and 13 in Jonet’s (1966) plate II appear to be Isogomphodon , but the specific attribution is in need of further evaluation.

Dames’ (1883) Carcharhinus (Aprionodon) frequens from the late Eocene of Egypt contains more than one species of shark. His description of the teeth (pp. 143, 144) and accompanying illustrations (pl. 3, figs. 7a–p) indicate that some specimens he examined (figs. 7b, f) are Isogomphodon . However, the frequens morphology is considered valid but has been attributed to Carcharhinus (i.e., Cappetta 1970) and, more recently, Negaprion (Underwood et al. 2011) . As is the case with Jonet’s (1966) gracilis morphology, the Isogomphodon teeth reported by Dames (1883) are in need of further study.

Cappetta (1970) believed that Jonet’s (1966) Ap. gracilis species was actually the anterior teeth of I. acuarius . However, Cappetta (2006) later treated the species as valid, but in doing so provided no comment supporting the conclusion. We concur with Cappetta (1970) that teeth illustrated by Jonet (1966: pl. 2, figs. 14–21) represent anterior teeth, and they are not dissimilar from those of I. acuarius . Regardless of whether Jonet’s (1966) species is valid or conspecific with I. acuarius , the teeth are both larger and have more elongated cusps than I. aikenensis n. sp. Case and Cappetta (1990) placed Cappetta’s (1970) Ap. caunellensis species within Carcharhinus , but later Cappetta (2006, 2012) supported identification as Isogomphodon caunellensis . If valid, I. caunellensis teeth are larger than I. aikenensis , have wider upper anterior teeth, and lower anterior teeth with incomplete cutting edges. Probst (1879) illustrated a specimen (pl. 1, figs. 76-77) of I. acuarius from his sample of approximately 50 teeth that is not dissimilar to anterior teeth of I. aikenensis . However, his description of the material (p. 140) is generic for the genus, but the maximum tooth size he reported (10 mm) is larger than that of I. aikenensis (7 mm).

Isogomphodon lerichei was based on a suite of Miocene teeth described by Darteville and Casier (1943). However, this taxon also appears to be a mixture of multiple species, with teeth of Isogomphodon and non- Isogomphodon species included in the illustrated suite of teeth provided by Darteville and Casier (1943: i.e., pl. 13, figs. 41, 42, 49, 50). This calls into question the validity of I. lerichei , but examination of the daggernose shark teeth shown by Darteville and Casier (1943: i.e., pl. 13, figs.37, 39, 43-46) show that they are much larger and the angle between the cusp and lateral shoulders may be sharper than I. aikenensis . Antunes et al. (1981) tentatively reported I. lerichei from the Miocene of Portugal, but the two referred teeth (pl. 2, figs. 13 and 17) are, in our opinion, more appropriately referred to Carcharhinus .

Isogomphodon oxryrhynchus is the only extant species of daggernose shark, and it is largely restricted to coastal waters of northeastern South America (Lessa and others 1999, Lessa et al. 2016). Teeth of fossil Isogomphodon , particularly I. acuarias , are predominantly known from Mio-Pliocene deposits of the Tethyan regions of Europe and Africa, but an increasing number of reports document the taxon from temporally equivalent strata in South America (i.e., Carrillo-Briceño et al. 2016). Some of these latter records, however, are based on inaccurate identifications of teeth belonging to other Carcharhinidae (i.e., Mora 1999). Two reports of I. acuarius from North Carolina in the eastern United States have been published ( Case 1980; Müller 1999). The strata yielding the fossils were considered to be of Miocene age, but the deposits are actually of Oligocene age ( Harris and Zullo 1991; Zullo et al. 1992; Denison et al. 1993). Considering the limited geographic distribution and habitat preference of extant I. oxyrhynchus , it may be prudent to re-evaluate Oligo-Miocene records of the genus. Discovery of I. aikenensis in the Dry Branch Formation unequivocally extends the temporal range of the genus back to the late Eocene, and it is the only Eocene record in North America.