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

Cicimurri, David J., Ebersole, Jun A., Stringer, Gary L., Starnes, James E. & Phillips, George E., 2025, Late Oligocene fishes (Chondrichthyes and Osteichthyes) from the Catahoula Formation in Wayne County, Mississippi, USA, European Journal of Taxonomy 984 (1), pp. 1-131 : 38-41

“ Sphyrna ” gracile sp. nov.

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Fig. 10

Diagnosis

Mesio-distally wide teeth consisting of a large main cusp and a distal heel. The main cusp is broadly triangular and distally inclined to varying degrees. The mesial cutting edge is straight to weakly convex on the main cusp, but it extends to the end of the mesial root lobe generally through a sloping transition at the base of the cusp. The distal cutting edge is shorter and straight to weakly convex. The distal heel is elongated, low, straight to weakly convex, and differentiated from the distal cutting edge by a shallow notch. All cutting edges are smooth. The root is bilobate with short, sub-rectangular lobes that are highly diverging. The basal margin is straight to weakly concave. The lingual root face is thick, and there is a distinctive medially located nutritive groove. These teeth differ from fossil species reported in the literature, like those of the Miocene Sphyrna arambourgi Cappetta, 1970 , by having a wider main cusp and weakly sinuous (as opposed to straight) mesial cutting edge. Additionally, “ S. ” gracile sp. nov. teeth can be separated from those of both S. arambourgi and S. integra ( Probst, 1878) by having an elongated and straight to weakly convex distal heel (as opposed to being rather short and occasionally cuspidate in the latter taxa). Furthermore, the lower teeth of the former taxon have an angular mesial cutting edge, whereas this edge is curved in the latter taxa. “ Sphyrna ” gracile teeth differ from those of the Miocene S. laevissima ( Cope, 1867) by being less robust and by being smaller in mesio-distal width (up to 6 mm for “ S. ” gracile vs 1 cm for S. laevissima ).

Etymology

The species name alludes to the small size and delicate appearance of the teeth.

Material examined

Holotype

UNITED STATES OF AMERICA – Mississippi • upper right lateral tooth; Catahoula Formation ; SC 2013.28.158 ( Fig. 10O–Q ).

Paratypes

UNITED STATES OF AMERICA – Mississippi • lower left anterior tooth; Catahoula Formation ; SC 2013.28.155 ( Fig. 10A–C ) • lower left lateral tooth; Catahoula Formation ; SC 2013.28.162 ( Fig. 10L–N ) .

Other material

UNITED STATES OFAMERICA– Mississippi • 28isolated teeth; Catahoula Formation ; SC 2013.28.154 , SC 2013.28.156 ( Fig. 10J–K ), SC 2013.28.157 ( Fig. 10R–S ), SC 2013.28.158 to 28.160 , SC 2013.28.161 ( Fig.10T–U ), SC 2013.28.162 , SC 2013.28.163 (7 teeth), SC 2013.28.164 (10 teeth), SC 2013.28.912 ( Fig. 10F–G ), SC 2013.28.913 ( Fig. 10D–E ), SC 2013.28.914 ( Fig. 10H–I ).

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

These small teeth measure up to 6 mm in mesio-distal width and slightly over 5 mm in overall height (apico-basal). The crown consists of a conspicuous cusp and a distal heel. The mesial cutting edge is sharp, smooth, and weakly to strongly concave. The mesial cutting edge exhibits a basal heel that may be short or elongated, poorly or conspicuously differentiated from the cusp, and oblique to nearly perpendicular to the cusp. The distal cutting edge is smooth and sharp, straight to convex, may be nearly vertical to moderately distally inclined, and is shorter than the mesial edge. The mesial and distal cutting edges intersect apically to form the cusp, which itself is rather narrow but sharply pointed. An elongated distal heel is very low, weakly convex to angular, and the edge is smooth. A conspicuous notch is located at the junction of the heel and the distal cutting edge, and the apex of the heel is located just distal to the notch. The bilobate root has elongated, widely diverging, sub-rectangular lobes with rounded ends. The interlobe area is low and broadly U-shaped or may be absent (straight basal margin). The lingual root face is bisected by a short but deep nutritive groove.

Remarks

Although the teeth described above share morphological features that occur on teeth of extant Sphyrnidae , there are differences among the various taxa (see below). Assigning the fossils to the extant genus Sphyrna is problematic based on molecular divergence work by Lim et al. (2010), which indicates that the genera Eusphyra and Sphyrna did not diverge from their most recent common ancestor until the Early-to-Middle Miocene, four to seven million years after deposition of the Catahoula Formation fossil bed. For the purposes of this report, we follow Ebersole et al. (2024a) in placing the generic name Sphyrna within quotations, acknowledging dental similarities between the Oligocene and extant species, and taking into account divergence estimates that may result in future placement of the fossil species in a new genus.

To aid our evaluation of the Catahoula Formation sphyrnid sample, we examined the jaws of several extant Sphyrna species, including S. lewini ( Griffith & Smith, 1834) ( SC 2001.7.1), S. mokarran ( Rüppell, 1837) ( SC 2000.120.2), S. tiburo ( Linnaeus, 1758) ( SC 96.77.3), and S. zygaena ( Linnaeus, 1758) (MSC 42600). Additionally, we utilized the illustrated dentitions of S. media Springer, 1940 and S. tudes ( Valenciennes, 1822) provided by Gilbert (1967: figs 14 and 19, respectively). “ Sphyrna ” gracile sp. nov. teeth are much smaller in overall size and less stout compared to the teeth of presumed extant relatives Sphyrna mokarran and S. zygaena , which are also serrated to varying degrees. The upper teeth of “ Sphyrna ” gracile differ from those of S. lewini by having a less elongated mesial crown foot and a less convex medial portion of the mesial cutting edge. Additionally, the mesial cutting edge on the lower teeth of the former taxon has an angular appearance, whereas this edge on the lower teeth of the latter taxon appears strongly curved. The upper teeth of extant S. media have more convex distal cutting edges, a more medially convex portion of the mesial edge, and a shorter distal heel compared to “ Sphyrna ” gracile . Furthermore, the lower teeth of the former taxon have a narrower, taller and strongly curved cusp compared to the lower teeth of the latter taxon. and the distal heel of the former taxon is comparatively shorter than that of the latter taxon. The upper teeth of “ Sphyrna ” gracile have a somewhat wider and more distally inclined main cusp compared to the upper teeth of extant S. tudes . The lower teeth of the latter taxon are also narrower and more erect compared to those of the former taxon.

Numerous Neogene fossil species have been assigned to Sphyrna , but our evaluation of the published illustrations of the type or referred specimens leads us to conclude that most of them do not belong to Sphyrnidae , let alone Sphyrna . For example, teeth identified as S. magna Cope, 1867 , S. americana Leriche, 1942 , and S. lata Agassiz, 1843 , among many others, are more appropriately identified as Carcharhinus . Other examples include S. gilmorei Leriche, 1942 , which has been placed in Negaprion (i.e., Ebersole et al. 2019), and S. tortillis White, 1926 should be identified as Physogaleus .

Of the remaining Neogene species, Sphyrna arambourgi Cappetta, 1970 , S. integra ( Probst, 1878) , and S. laevissima ( Cope, 1867) appear to be correctly identified as sphyrnids. The Lower Miocene S. integra is based on one complete tooth and one partial tooth (see Pollerspöck & Unger 2023: pl. 11 figs 3–4), but Cappetta (1970: pl. 19 figs 1–18) utilized a larger suite of Middle Miocene specimens to diagnose S. arambourgi . Although the complete S. integra specimen shown by Pollerspock & Unger (2023: pl. 11 fig. 3) appears to be a lower tooth with a clear separation of a mesial heel compared to the contiguous convex mesial edge on S. arambourgi teeth, the latter taxon was synonymized with S. integra ( Barthelt et al. 1991) . The teeth of “ Sphyrna ” gracile sp. nov. are like those of S. arambourgi and S. integra , as illustrated by Cappetta (1970: pl. 19 figs 1–18) and Reinecke et al. (2011: pls 81–85), but there are differences between the Catahoula material and the two European taxa. For one, the main cusp of S. arambourgi is somewhat narrower than that of “ Sphyrna ” gracile , particularly on lower teeth. Additionally, the mesial cutting edge on the upper teeth of S. arambourgi is straighter than that on the Catahoula Formation teeth, which are weakly sinuous. Furthermore, the distal heel in S. arambourgi and S. integra is short and sometimes weakly cuspidate, whereas in “ Sphyrna ” gracile sp. nov. the distal heel is elongated and straight to weakly convex. Additionally, the mesial cutting edge of “ Sphyrna ” gracile lower teeth has an angular appearance, whereas this edge is curved on comparable teeth of S. arambourgi / S. integra (albeit sharply curved on the latter). Purdy et al. (2001) indicated that the S. arambourgi morphology was similar to Mio-Pliocene teeth they referred to S. media , but they did not specifically synonymize the former with the latter.

Unfortunately, Cope (1867) did not include illustrations of teeth when he named the Galeocerdo laevissimus morphology, but Leriche (1942) later assigned the morphology to Sphyrna . Purdy et al. (2001: fig. 60) figured Cope’s G. laevissimus type suite ( Cope 1867), which shows that these teeth are much larger in overall size (greater than 1 cm in mesio-distal width) and much more robust compared to those of “ Sphyrna ” gracile sp. nov. Additionally, the very wide main cusp of the former species has very convex mesial and distal cutting edges. The S. laevissima morphology is discussed further below.

Cicimurri & Knight (2009) reported a similar small and gracile hammerhead-like tooth morphology from the Chandler Bridge Formation (Chattian) of South Carolina, which they assigned to Sphyrna cf. media (following the observations of Purdy et al. 2001). Cicimurri et al. (2022) later identified comparable teeth from the Ashley Formation (Rupelian) of South Carolina simply as Sphyrnidae gen. et sp. indet. Examination of specimens from the Ashley (accession SC 2007.36) and Chandler Bridge (accession SC 2005.2) formations indicate that the teeth are conspecific with “ Sphyrna ” gracile sp. nov. Ebersole et al. (2024a) later described an isolated tooth as “ Sphyrna ” sp. that was derived from the Rupelian Red Bluff Clay in Alabama. This tooth has a shorter and wider main cusp and more convex mesial edge than those of “ Sphyrna ” gracile .

Based on our evaluation of extant Sphyrna spp. dentitions, monognathic and dignathic heterodonty are evident in our “ Sphyrna ” gracile sp. nov. sample. Teeth from anterior files are rather narrow and have a more vertically directed cusp apex ( Fig. 10A–B ). In contrast, lateral teeth are wider and have more distally inclined cusps ( Fig. 10D–E ). Additionally, in progressively more distal tooth files, cusp inclination increases but overall cusp height decreases towards the commissure (compare Fig. 10J, H, D ). Dignathic heterodonty is reflected in cusp width and the nature of the mesial cutting edge. Upper teeth generally have a wider main cusp with a convex medial portion of the mesial cutting edge compared to lower teeth (i.e., Fig. 10R vs A). In addition, the elongated mesial cutting edge of upper teeth may only be slightly concave basally ( Fig. 10O, L ), but on lower teeth the basal one-half of the mesial edge is clearly distinguished as an elongated, roughly horizontal heel ( Fig. 10G, U ).

The teeth of “ Sphyrna ” gracile 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 upper teeth of Carcharhinus elongatus can be identified by the shallow notch on the mesial and distal sides of the crown, only a distal notch occurs on the teeth of “ Sphyrna ” gracile . Lower teeth of “ Sphyrna ” gracile have a conspicuously elongated mesial heel and root lobe, whereas the mesial and distal heels of C. elongatus lower teeth are roughly equal in length.

Figures

Fig. 10. “Sphyrna” gracile sp. nov., teeth. A–C. SC2013.28.155 (paratype), lower left anterior tooth. A. Labial view. B. Lingual view. C. Mesial view. D–E. SC2013.28.913, lower right lateral tooth. D. Labial view.E. Lingual view.F–G. SC2013.28.912, lower left lateral tooth.F. Labial view.G. Lingual view. H–I. SC2013.28.914, lower left lateral tooth.H. Labial view.I. Lingual view.J–K. SC2013.28.156, upper left lateral tooth. J. Labial view. K. Lingual view. L–N. SC2013.28.162 (paratype), lower left lateral tooth. L. Labial view. M. Lingual view. N. Mesial view. O–Q. SC2013.28.158 (holotype), upper right lateral tooth. O. Labial view. P. Lingual view. Q. Mesial view. R–S. SC2013.28.157, upper left anterior tooth.R. Labial view. S. Lingual view. T–U. SC2013.28.161, upper right lateral tooth. T. Labial view. U. Lingual view. Scale bars = 3 mm.