Sphyrna ” robustum, Cicimurri & Ebersole & Stringer & Starnes & Phillips, 2025

“ Sphyrna ” robustum sp. nov.

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Fig. 11 View Fig

Diagnosis

” Sphyrna ” robustum sp. nov. teeth differ from those of coeval ” Sphryna ” gracile sp. nov. (see above) by their greater stoutness, larger overall size, lower cusp height with respect to tooth size, and differing shapes of the mesial cutting edge. These features also serve to distinguish “ S.” robustum from the extinct S. integra and S. arambourgi (see Cappetta 1970; Reinecke et al. 2011). Although morphologically similar to “ S.” robustum , fossil S. laevissima teeth have a more biconvex cusp and the mesial cutting edge is conspicuously sinuous ( Purdy et al. 2001: fig. 60). Of presumed extant descendant species, the teeth of S. mokarran and S. zygaena are comparable to those of “ S.” robustum . However, the mesial cutting edges of the new Oligocene species are less convex and lack serrations.

Etymology

The species name alludes to the stout appearance of the teeth.

Material examined

Holotype

UNITED STATES OF AMERICA – Mississippi • upper right lateral tooth; Catahoula Formation ; SC 2013.28.171 ( Fig. 11Q–S View Fig ).

Paratypes

UNITED STATES OF AMERICA – Mississippi • lower right antero-lateral tooth; Catahoula Formation ; SC 2013.28.169 ( Fig. 11J–L View Fig ) • lower right lateral tooth; Catahoula Formation ; SC 2013.28.167 ( Fig. View Fig 11C–E).

Other material

UNITED STATES OF AMERICA – Mississippi • 82 isolated teeth; Catahoula Formation ; SC 2013.28.165 (22 teeth), SC 2013.28.166 ( Fig. 11A–B View Fig ), SC 2013.28.168 , SC 2013.28.170 ( Fig. 11F–G View Fig ), SC 2013.28.172 ( Fig. 11M–N View Fig ), SC 2013.28.173 to 28.176 , SC 2013.28.178 , SC 2013.28.179 , SC 2013.28.180 ( Fig. 11O–P View Fig ), SC 2013.28.181 ( Fig. 11H–I View Fig ), SC 2013.28.182 ( Fig. 11T–U View Fig ), SC 2013.28.183 (34 teeth), SC 2013.28.184 (10 teeth), SC 2013.28.915 ( Fig. 11V–W View Fig ), SC 2013.28.916 ( Fig. 11X–Y View Fig ), SC 2013.28.917 ( Fig. 11Z View Fig – AA).

Stratum typicum

Shelly, argillaceous sand of the Jones Branch fossil horizon, lower Catahoula Formation, Chattian Stage (horizon no longer accessible).

Locus typicus

Site MS.77.011, Jones Branch, tributary flowing into the Chickasawhay River, south of Waynesboro, Wayne County, Mississippi, USA.

Description

The teeth are mesio-distally wider than high (apico-basally) in all jaw positions available, with specimens measuring up to 12 mm in width but only a maximum of 8 mm in height. All teeth consist of a large cusp and a distal heel. The mesial cutting edge can be straight, weakly sinuous, or concave, but it is always smooth. On teeth with a sinuous cutting edge, the basal portion is concave, and the apical portion is convex. The mesial edge of other teeth is uniformly concave, and the cusp apex is more vertically directed. The distal cutting edge is much shorter than the mesial edge but is always smooth. This edge is most often straight but may be weakly convex, and its orientation may be vertical or distally inclined. The distal heel is low, elongated, and usually oblique, but may be horizontal. The cutting edge on the distal heel is smooth and varies in convexity. This heel is usually separated from the distal cutting edge by an indistinct notch. The root is bilobate, with the elongated lobes being widely divergent and their lateral ends rounded. The basal margin of the root varies from slightly concave to virtually straight. A thickened lingual root face is bisected by a deep nutritive groove.

Remarks

The teeth of “ Sphyrna ” robustum sp. nov. differ from those of superficially similar carcharhiniform genera within the Catahoula Formation, including Hemipristis , Galeorhinus , Physogaleus , and Galeocerdo , by the lack of serrations and/or denticulations on the mesial and distal cutting edges. Although the upper teeth of Carcharhinus elongatus have mesial and distal shoulders that may be weakly to moderately serrated, these heels are distinctly separated from the main cusp by a shallow notch (only a distal notch occurs in “ Sphyrna ” robustum ). With respect to Rhizoprionodon , the teeth of “ Sphyrna ” robustum achieve a much larger overall size and have a broader cusp.

“ Sphyrna ” robustum sp. nov. teeth differ from those of the coeval “ Sphyrna ” gracile sp. nov. by their greater stoutness, larger overall size, lower cusp height with respect to tooth size, and differing shapes of the mesial cutting edge. Most of these features also serve to distinguish the species from presumed extant relatives, including Sphyrna lewini ( SC 2001.7.1), S. media (see Gilbert 1967), and S. tudes (see Gilbert 1967). The teeth of “ Sphyrna ” robustum bear similarities to those in the jaws of extant Sphyrna mokarran ( SC 2000.120.2) and S. zygaena (MSC 42600) that we examined, but on the Oligocene specimens the mesial cutting edge of lower teeth is more concave than it is on upper teeth, and the cutting edges are completely smooth on all specimens.

The Catahoula Formation “ Sphyrna ” robustum sp. nov. teeth are comparable to the material that Cope (1867) originally named as Galeocerdo laevissimus from the Miocene of Maryland. Leriche (1942) later illustrated its morphology and placed it within Sphyrna (i.e., S. laevissima ). Purdy et al. (2001: fig. 60) figured all the teeth within Cope’s G. laevissimus type suite and synonymized the taxon with extant S. zygaena , citing similarities in gross morphology. Although Cope (1867) specifically stated that the cutting edges of the teeth he examined were smooth, Purdy et al. (2001) noted that tooth serrations on fossil specimens became more prominent from the Miocene to the Pliocene, indicating phyletic change within the taxon. However, Reinecke et al. (2011) later provided quantitative data separating the laevissima morphology from that of S. zygaena . In any case, the “ Sphyrna ” robustum sp. nov. teeth differ from both S. laevissima and S. zygaena by having narrower cusps and much less convex cutting edges that are completely smooth. Ebersole et al. (2024a) reported an isolated tooth derived from the Rupelian Red Bluff Clay in Alabama that they conservatively assigned to “ Sphyrna ” sp.

Our evaluation of the dentitions of Sphyrna mokarran ( SC 2000.120.2) and S. zygaena (MSC 42600) provides support for our conclusion that monognathic and dignathic heterodonty was developed in the Catahoula Formation taxon.Anterior teeth have a rather narrow (mesio-distal) crown and somewhat erect cusp ( Fig. 11A View Fig ), whereas teeth from more lateral positions are wider and have a more distally inclined cusp ( Fig. 11Q View Fig ). The degree of distal inclination appears to have increased towards the commissure, and at the same time overall tooth height decreased ( Fig. 11O View Fig ). Dignathic heterodonty is reflected in the narrowness of the tooth cusp, with upper teeth having a wider cusp ( Fig. 11X View Fig ) compared to lower teeth ( Fig. 11C View Fig ). The variations in tooth shape and development of monognathic and dignathic heterodonty would seem to provide a clear distinction between “ Sphyrna ” robustum sp. nov. and “ S.” gracile sp. nov. However, we also note that robust and gracile Sphyrna or Sphyrna -like tooth morphologies have consistently been documented together within Oligo-Miocene strata. For example, following Purdy et al. (2001), Cicimurri & Knight (2009) reported teeth of Sphyrna cf. media (small, narrow-cusped) and S. zygaena (large, broad-cusped) from the Chattian Chandler Bridge Formation of South Carolina. Later, Cicimurri et al. (2022) documented a small, narrow-cusped and a larger, broad-cusped Sphyrna -like morphology for teeth from the Ashley Formation (Rupelian) of South Carolina that they simply referred to Sphyrnidae gen. et sp. indet. based on the work of Lim et al. (2010) (see also Cappetta 1970; Purdy et al. 2001; Carrillo-Briceño et al. 2016, 2019). Although these morphologies have been treated as separate taxa (including herein), it could be interpreted that these morphologies actually represent other forms of heterodonty within a single taxon.

As part of our analysis of these teeth, we considered the possibility that the “ Sphyrna ” gracile sp. nov. (small, narrow-cusped) morphology represents ontogenetic and/or gynandric heterodonty within S. robustum sp. nov. With respect to ontogenetic heterodonty, studies have demonstrated dietary shifts in extant Sphyrna spp. from juvenile to adult growth stages (i.e., Gonzalez-Pestana et al. 2017). However, little has been said about the changes, if any, in tooth shape during that shift. Mello & Brito (2013) stated that ontogenetic heterodonty was “weak among sphyrnids” (p. 467). These authors examined the embryonic teeth of Sphyrna tiburo ( Linnaeus, 1758) , S. tudes ( Valenciennes, 1822) , and Eusphyra blochii ( Cuvier, 1816) and found that there is an ontogenetic change in the anterior tooth files of these taxa, but that lateral and posterior tooth files remain stable. Purdy et al. (2001) noted that the teeth of juvenile/young adult S. zygaena have smooth cutting edges, whereas serrations occur on the teeth of “large” individuals. This latter form of ontogenetic heterodonty is comparable to that documented in Rhizoprionodon terraenovae , where tooth shape remains relatively constant from birth to adulthood, but serrations develop on the teeth as the shark matures ( Ebersole et al. 2023).

We examined the jaws of a juvenile and an adult Sphyrna lewini to determine whether ontogenetic heterodonty occurs in this taxon. We found that tooth size in the upper files increases with age, but a more conspicuous change is the orientation of the main cusp. For example, the fourth upper anterior tooth of a juvenile S. lewini (MSC 50182) measures 6 mm in width and has a cusp that is 4 mm high and 3 mm wide, whereas the same adult tooth in MSC 42605 is 9 mm wide with a cusp that is 5 mm high and 4 mm wide. More telling, the distal cusp inclination of the juvenile tooth is 50° and on the adult tooth it is 63°. These same changes are true for the seventh upper tooth file, with the tooth width of the juvenile measuring 7.5 mm, cusp height 4 mm, cusp width 3.5 mm, and cusp inclination 44°. In contrast, the same adult tooth is 11 mm wide with a cusp measuring 5 mm high and 4 mm wide, and cusp inclination is 58°. Additionally, teeth in the first upper file of both the juvenile and adult dentitions are comparable (roughly symmetrical with a vertical cusp and well-differentiated lateral heels), but the juvenile second tooth is more similar to those of the succeeding files (distally inclined). In the adult dentition, the upper second tooth is comparable to the tooth in the first file. Our observations show that although there is an increase in tooth size from juvenile to adult growth stages, a more significant change is that the tooth cusps become much more upright into adulthood. Considering our observations of dentitions of juvenile and adult Sphyrna lewini , we believe that the morphological differences between “ Sphyrna ” gracile sp. nov. and “ S. ” robustum sp. nov. are too great for the morphologies to represent ontogenetic heterodonty.

Concerning gynandric heterodonty, Mello & Brito (2013) stated that “sexual heterodonty is hardly developed” in Sphyrnidae . Although differences in growth rates, age at maturity, and dietary preference between male and female hammerhead sharks have been documented ( Klimley 1987), to our knowledge there is no published detailed description of gynandric variation within a given species. We herein treat the two Catahoula Formation “ Sphyrna ” spp. morphologies as taxonomically distinct because our samples do not appear to exhibit any morphological overlap in overall size, main cusp width, cutting edge shape, or presence/absence of serrations.