Planohybodus sp.
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publication ID |
https://doi.org/ 10.26879/454 |
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persistent identifier |
https://treatment.plazi.org/id/8131E10D-1D3D-E876-CB61-FB2521CEFB1A |
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treatment provided by |
Felipe |
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scientific name |
Planohybodus sp. |
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Figure 6.1-5 View FIGURE 6
Type species. Planohybodus peterboroughensis Rees and Underwood, 2008 .
Referred material. IGM 9316, isolated imperfect collected in association with a pliosaurid vertebral centrum ( Figure 6.1 View FIGURE 6 ). IGM 9317, isolated fragment of predorsal fin spine ( Figure 6.2-5 View FIGURE 6 ). IGM 9316 comes from the shale strata between levels M and L1 and IGM 9317 from shale layer between L6 and L7 ( Figure 4 View FIGURE 4 ).
Occurrence. Yosobé, Tlaxiaco, Oaxaca. Kimmeridgian-Tithonian shales of the Sabinal formation.
Description. The specimen IGM 9316 is a single isolated tooth with the cusp tip broken and a large part of the root missing. The crown is about 8 mm and possibly 7 mm high. The root is remarkably shallow, just about 1 mm high. The crown includes a cusp and a pair of lateral cuplets on each side. The cusp is high, very broad at the base, slightly inclined lingually, and shows a pronounced labio-lingually compression (this can be seen in cross-section along the fracture of the tip). The cusplets form triangular structures well separated from each other, with broad bases and of decreasing size. In this series the outer cusplets are particularly small and the cusp could be five times higher than intermediate cusplets. The crown is largely smooth except for some small vertical and almost straight folds present only in the base of the cusp and cuplets. The cutting edge is continuous along the crown; it is weakly sinuous or serrated at the base and both sides of the main cusp. Although it is incomplete, the root was a flat rectangular and porous structure ( Figure 6.1 View FIGURE 6 ).
Specimen IGM 9317 is a fragment of the middle part of a predorsal spine ( Figure 6.2-5 View FIGURE 6 ). In lateral view, this fragment is trapezoidal-shaped, almost as long as high. In cross-section it is drop shaped with straight lateral edges, rounded anterior and posterior edges, and remarkably wider posteriorly (its maximum width 9 mm near posterior edge). The length of this fragment is 1.4 cm at the base and 1.2 cm at the top. The anterior two thirds of both lateral surfaces of the spine are ornamented with seven straight, upward projected, parallel, and evenly spaced ridges. The middle posterior edge bears two rows of denticles that are broken in the specimen. There is no groove between these denticles.
Remarks. Capetta (1987) included Hybodus Agassiz, 1837 , Pororhiza Casier, 1969 , and Priohybodus Erasmo, 1960 , in the family Hybodontidae ; however, the recent morphological and systematic review of Hybodus allowed the recognition of other genera, which are Egertonodus Maisey, 1987 , Planohybodus Rees and Underwood, 2008 , and Secarodus Rees and Underwood, 2008 . Maisey (1987, 1989) suggested that Hybodus and Egertonodus form the most derived and crown group in the family (Subfamily Hybodontinae ) because they develop “high-crowned multicuspid teeth.” Later, Rees and Underwood (2008) noted that Planohybodus , Secarodus , and Priohybodus also have high-crowned multicuspid teeth and may comprise a closely related group within Hybodontinae because their teeth are deeply labio-lingually compressed and have cutting borders with serration. The teeth of Priohybodus have well developed serrations that practically go along the entire cutting border while in Planohybodus and Secarodus the serrations are very weak and restricted to the main cusp base. In contrast, teeth of Secarodus and in Priohybodus have divergent cusplets, while in Planohybodus the cusplets are projected in a parallel or convergent manner in relation with the cusp.
The diagnosis of Planohybodus states that this genus differs from other hybodont sharks because the teeth have a high, wide-based, and strongly labio-lingually compressed cusp, its crown has a strongly developed cutting edge and is ornamented with short, simple folds ( Rees and Underwood, 2008, p. 126). Later, Bermúdez-Rocha (2009) also noted that this genus has a maximum of two or three pair of cusplets that are remarkably smaller than the cusp, as well as its cutting edge is sinuous or vaguely serrated at the cusp base. Even though it is incomplete, the IGM 9316 shows these characteristics supporting its inclusion in an undetermined species of Planohybodus .
Specimen IGM 9317 shares with the type species of Planohybodus , P. peterboroughensis (and probably with the related species), one of those two combined features of the predorsal spine, which were noted by Rees and Underwood (2008, p. 126, text-figure 2) as possible diagnostic characters that differentiate this genus from other hybodontid sharks. Both Planohybodus and IGM 9317 lack a groove between the rows of denticles on the posterior spine surface. On the other hand, in IGM 9317 the posterior third surface of the spine is smooth, whereas in Planohybodus the spine is almost entirely ornamented with parallel ridges (the ridges are absent only on a relatively small área on the rear of the upper half of the spine). Although this difference could represent a specific feature, it will require more and better-preserved fossils for better taxonomic identification of these hybodontid sharks from Yosobé.
Hybodontidae is a shark family that lived from the Triassic to the Maastrichtian and had a nearly worldwide distribution (Capetta, 1987). Despite a dubious report on the occurrence of Pennsylvanian and Permian Hybodontid remains from San Salvador Patlanoaya, Puebla, Mexico ( Derycke-Khatir et al., 2005), the present is the first concrete finding of Hybodontid sharks in Mexico. Moreover, the occurrence of Planohybodus in Yosobé is interesting because it complements the broken temporal and geographical distribution previously known of this genus. According to Pinheiro et al. (2013, p. 214), this hybodontid was present in the western Tethys Sea Domain in Europe from the Early Jurassic to the Early Cretaceous, it survived until the Late Cretaceous (Santonian and Campanian) in deposits of the Western Interior Seaway in southern USA and Canada. Although this shark inhabited the epicontinental seas of South America during the Early Cretaceous; it probably reached this last subcontinent during the Late Jurassic ( Cupello et al., 2011). Hence, the presence of Planohybodus in the Kimmerigian shales of Yosobé reveals its possible journey from the Tethys Sea of Europe southward through the Hispanic Corridor, where the marine Sabinal formation strata were deposited.
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