Sphyrna guinoti, Adnet & Marivaux & Cappetta & Charruault & Essid & Jiquel & Ammar & Marandat & Marzougui & Merzeraud & Temani & Vianey-Liaud & Tabuce, 2020
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publication ID |
https://doi.org/ 10.26879/1085 |
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publication LSID |
lsid:zoobank.org:pub:B6B8E985-F1CF-4C10-BB00-602E5BF36C1C |
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persistent identifier |
https://treatment.plazi.org/id/779694E6-E3B1-4A5A-9C1A-4CB401DE852D |
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taxon LSID |
lsid:zoobank.org:act:779694E6-E3B1-4A5A-9C1A-4CB401DE852D |
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treatment provided by |
Felipe |
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scientific name |
Sphyrna guinoti |
| status |
sp. nov. |
Sphyrna guinoti nov. sp.
Figures 5 View FIGURE 5 , 6 View FIGURE 6 A-C zoobank.org/ 779694E6-E3B1-4A5A-9C1A-4CB401DE852D
?2011 Rhizoprionodon sp ; Underwood et al., p. 52-62, tab. 1, figs. 5O–P.
Etymology. In recognition of Guillaume Guinot for his great contribution in the study of extinct hammerhead shark-like taxa.
Type locality and stratum. KEB 1-118 ( Figure 5J View FIGURE 5 ) from Souar-Fortuna formations in Djebel el Kébar, Amamria hamlet near Soug-Jedid village (Sidi Bouzid Township), Tunisia: locality named KEB- 1 in Merzeraud et al. (2016) and previously dated (radiometric K-Ar datings) to middle Bartonian (38.7 ± 1.0 Ma to 40.7 ± 1.1 Ma, covering the NP17) in Marivaux et al. (2014a) .
Other material. Additional material, including figured KEB 1-109 to 1-117, 1-119 to 1-122 ( Figures 5 View FIGURE 5 A-F, 5H-K, 6A-C), consists of several hundred of upper and lower teeth from the KEB- 1 locality, Souar-Fortuna formations, Djebel el Kebar, Tunisia.
Diagnosis. Medium sphyrnid species only known by isolated teeth reaching 8 mm in width with a well-marked dignathic heterodonty which is reflected in bended blade-like cusps in upper teeth and high, erected cusps in lower teeth. Cutting edges of cusp and heels are totally smooth in both jaws. If upper teeth are reminiscent to those of all other upper teeth of fossil Sphyrna , lower teeth have a particularly well erected cusp and root is peculiarly massive medially with a deep and wide nutritive groove as observed in Recent S. tudes but the asymmetrical root lobes and heel distinct S. guinoti from this last.
Description
Upper teeth. The cusp is relatively thick and short in the mesio-distal long axis; the cutting edges lack serration and the mesial cutting edge is straight ( Figure 5 View FIGURE 5 A-B, E-G) to slightly concave ( Figure 5 View FIGURE 5 C- D). The apex of cusp never exceeds the distal extremity of heel in lingual and labial views, even in the more posterior teeth ( Figure 5G View FIGURE 5 ). The distal heel is well developed, well separated from cusp by a notch, rounded and entirely unserrated. The mesial heel is less individualized from cusp, its cutting edge being more or less aligned with that of cusp. Crown and root are not strongly labio-lingually compressed in profile ( Figure 5B View FIGURE 5 3 View FIGURE 3 ), the basal face of root making always an angle with the cusp. The crown-root boundary is reduced and limited to the fin enamel junction, which is relatively straight in labial view. The root is slightly broader than the crown, both in antero-lateral ( Figure 5 View FIGURE 5 A-C) and lateral teeth ( Figure 5E View FIGURE 5 ); root lobes are well aligned, horizontal and separated by a straight and deep nutritive groove in lingual view (e.g., Figure 5B View FIGURE 5 2 View FIGURE 2 , C 1 View FIGURE 1 , D 2 View FIGURE 2 ). Small but distinct protuberances are sometimes located in the medio-basal part of the lingual face of the root, and can be observed on labial view ( Figure 5A View FIGURE 5 1, F1 View FIGURE 1 ) or lingual view ( Figure 5A View FIGURE 5 2, F2 View FIGURE 2 ). These protuberances are in fact marked by the erecting of root lobe extremities, close to the median nutritive groove, but they can disappear totally on several upper teeth ( Figure 5B, C View FIGURE 5 ).
Lower teeth. Compared with upper teeth, the lower teeth, holotype included ( Figure 5J View FIGURE 5 ), are slightly narrower mesio-distally and thicker labiolingually. Contrary to the upper teeth, the cusp of lower teeth is strongly erected, as observable in the holotype ( Figure 5J View FIGURE 5 ). The cusp is relatively high (twice higher than the root) and its axis is quite vertical ( Figure 5H View FIGURE 5 ) in anterior teeth to slightly bended distally in more lateral teeth, like in the holotype ( Figure 5K View FIGURE 5 ). Its mesial cutting edge remains however very concave in lingual or labial views, and its distal cutting edge convex. Cutting edges of the cusp and heels are also unserrated. As for the upper teeth, the distal heel is well developed and distinct from the cusp, when the mesial heel is faintly marked. The root is relatively thick and the medio-lingual protuberance is particularly well marked in some teeth ( Figure 5 View FIGURE 5 J-K), with a large nutritive groove where a central foramina opens ( Figure 5J View FIGURE 5 2, K2 View FIGURE 2 ). Root lobes are often asymmetric, the mesial being shorter than the distal one contrary to what is observed in upper teeth.
Remarks
Nowadays, the representatives of this genus exhibit rather different dentitions and tooth morphologies, inasmuch as these living species are characterized by clutching-crushing type ( Sphyrna tiburo ) to cutting type with serrated teeth (e.g., living Sphyrna mokarran ). In addition, the dignathic heterodonty may be weak (e.g., S. tiburo ), moderate (e.g., Sphyrna lewini , S. mokarran , and Sphyrna zygaena ) to strong (e.g., Sphyrna media and Sphyrna tudes ), where lower teeth share straightened cusp with a mesial cutting edge very concave (e.g., like in some Rhizoprionodon ) when upper teeth are reminiscent to those of other large carcharhinid (e.g., Negaprion ). Sphyrna guinoti nov. sp. belongs to the last morphological group exhibiting strong heterodonty and thus an overall morphology of both dentitions very close to those observed in living and fossil species of the genus Rhizoprionodon . This explains that when the Paleogene material is scarce and restricted to a couple of teeth, confusion with Rhizoprionodon is possible. But in general, the teeth of Rhizoprionodon (see also Appendix 2C-E) have a narrower crown, with lower teeth having a straightened but short cusp with a mesial cutting edge more concave in labial view. This morphology is less usual on lower teeth of Sphyrna (see also Cappetta and Case, 2016) where the cusp tends to become higher than the width of crown, as observed in S. guinoti nov. sp.
Frequently reported from Neogene deposits, with occurrences of extant species (e.g., Sphyrna zygaena , including fossil Sphyrna laevissima (Cope, 1869) considered as synonym by Purdy et al., 2001) or of extinct Sphyrna arambourgi Cappetta, 1970 , the earliest appearance of the hammerhead sharks is quite discrete and sometimes dated to the Lutetian (Cappetta and Case, 2016, see also Ebersole et al. 2019 for discussion about affiliation). If the record of the genus in the Lutetian of Nigeria (White, 1926) seems to be erroneous (Cappetta, 2012), isolated Sphyrna teeth become more commonly recovered in Tethys and Atlantic deposits from the Late Eocene – Early Oligocene (Genault, 1993; Cicimurri and Knight, 2009; Adnet et al., 2010; 2011) with teeth often larger than for those of the present new species Sphyrna guinoti . Moreover, none of the fossil tooth set attributed to Sphyrna (including the unique fossil species S. arambourgi ) shows such a strong heterodonty. However and because of their morphological resemblances with teeth of Rhizoprionodon , the medium-sized teeth of Sphyrna are probably misidentified in the fossil record. A careful examination of other Middle-Late Eocene assemblages could reveal a more widespread occurrence of small and medium-sized species of Sphyrna , Sphyrna guinoti nov. sp. included. For Instance, Underwood et al. (2011) did not report the occurrence of Sphyrna in the Fayum area, but they mentioned two morphs attributed to Rhizoprionodon in their text, figuring the more robust one (Underwood et al., 2011, Figure 5O–P View FIGURE 5 from BQ), the morphology of which is reminiscent to that of the tooth of a hammerhead shark tooth.
At the present state of comparison, only the extant species Sphyrna media or Sphyrna tudes (see also Appendix 2A-B) have unserrated teeth that display the same heterodonty and morphology than Sphyrna guinoti nov. sp. However, lower teeth of S. media (Appendix 2A) seemly have finer and more elongated cusp (more than 66% of the tooth height) than in the S. guinoti nov. sp where the elongated cusp represents no more than 66% of tooth height. The upper teeth of S. media (Appendix 2A2) have also more gracile cusps with a more sigmoid mesial cutting edge in labial view. Distinction with S. tudes (Appendix 2B) is more tenuous. S. guinoti nov. sp. have upper teeth with similar erected cusp compared to that of S. tudes but the mesial heel is lesser individualized and shorter than the distal one; where both are quite equal and more similar in shape in S. tudes . This characteristic, added to the corresponding asymmetry of root lobes, was not observed on the teeth in the Recent S. tudes jaw.
Specimens repositories. Holotype and paratypes are deposited in the paleontological collections of the museum of the “ Office National des Mines ” of Tunis , 24 rue 8601, 2035 La Charguia, 1080 Tunis, Tunisia
Temporal range. Middle Bartonian ( Tunisia) and possibly to Priabonian ( Egypt).
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