Bhat et al.—Late Triassic freshwater sharks from India
Morphotype V.—This morphotype is based on twenty-four isolated teeth comprising seventeen newly collected specimens (Supplemental Data 1) and the specimens originally assigned to Lissodus duffini (VPL/JU/TF/143–VPL/JU/TF/149, Supple- mental Data 2). Morphotype V is characterized by robust teeth with a rounded principal cusp (Fig. 7.1, 7.2) and one incipient lateral cusplet on each side. The teeth have distinct triangular outlines in occlusal and basal views where MDL/LLW (=2.3) and BL/BW (=3.1) are low in comparison to the other mor- photypes (Table 2). However, their crown proportions are similar to those seen in morphotypes III and IV (MDL/LLW = MDL/ABH, Table 2). The crown has an enlarged and robust labial peg that extends beyond the crown-root junction (Fig. 7.1), whereas the lingual peg is absent (Fig. 7.2). The horizontal ridge is not visible in labial view, but it is prominent, wide, and crenulated in lingual view. The occlusal crest is broad and almost flat (Fig. 7.3). The base of the crown is distinctly triangular and has a central depression (Fig. 7.4).
Discussion.—The low-crowned teeth and presence of anaula- corhize root vascularization suggest that Pristrisodus n. gen., conforms to the hybodontiform tooth morphology (Ginter et al., 2010). Pristrisodus n. gen., fits the familial diagnosis of the Lonchidiidae, which includes small teeth with mesiodistal expansion, well-developed labial protuberances, and presence of irregularly arranged foramina on the root (Rees and Under- wood, 2002). The family Lonchidiidae, established for the genus Lonchidion (Herman, 1977), was synonymized with Polyacrodontidae (Cappetta, 1987), although recent works show that the former is valid (Rees and Underwood, 2002; Cappetta, 2012). The family comprises eleven genera char- acterized by the shape of the crown in labial and occlusal views, height of the principal cusp, number and nature of lateral cus- plets, ornamentation, and nature of the root (Wang et al., 2009; Klug et al., 2010; Cappetta, 2012). Of these, Dabasacanthus and Gansuselache are essentially Paleozoic forms (Ginter et al., 2010). Both forms bear similarity with Lissodus, although these possess several distinguishing features. Dabasacanthus is represented by an articulated juvenile with very small teeth, which have prominent labial pegs but lack lateral cusplets and ornamentations (Ginter et al., 2010). On the other hand, teeth of Gansuselache have a high crown-root junction labially, widely separated lateral cusplets from the principal cusps, and a strongly developed, flared occlusal crest (Wang et al., 2009; Ginter et al., 2010). The tooth morphology of five Mesozoic lonchidiid genera is distinctly different from the teeth of Pristrisodus n. gen., and includes Bahariyodon (Fig. 8.1, 8.2), Hylaeobatis (Fig. 8.3–8.5), Isanodus (Fig. 8.6, 8.7), Vectiselachos (Fig. 8.8–8.10), and Diplolonchidion (Fig. 8.11, 8.12). In contrast to Pristrisodus n. gen., Bahariyodon is char- acterized by a high cuspwith wide base, prominent cristae, and a lingual crown face that is produced into a distinct shelf (Cappetta, 2012). The shapes of teeth of Hylaeobatis are rectangular (Fig. 8.5), whereas Isanodus teeth are deeper than long and have a stout crown with rounded apex (Fig. 8.6, 8.7; Cuny et al., 2006). The teeth of Vectiselachos, conversely, are small, have a well- demarcated principal cusp, no lateral cusplets, and a root that is much thinner than the crown (Fig. 8.8–8.10). Diplolonchidion differs from other lonchidiid genera, including Pristrisodus
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n. gen., in possessing two distinct and robust labial pegs slightly offset mesiodistally from the central crown (Fig. 8.11, 8.12). Diplolonchidion was included in Polyacrodontidae by Heckert and Lucas (2006), and Lissodus was placed in an unknown family by Rees (2008), although Cappetta (2012) placed them in the family Lonchidiidae with a scope for future revision of the family. The teeth of Pristrisodus n. gen., show superficial
similarities to the genera Lissodus (Fig. 8.13–8.15), Lonchidion (Fig. 8.16–8.18), Parvodus (Fig. 8.19, 8.20), and Jiaodontus (Fig. 8.21–8.24) based on symmetry and shape and size of the principal cusp and lateral cusplets. However, the lateral teeth of Pristrisodus n. gen., (Figs. 3–7, 8.21) are mesiodistally at least three times more elongated than those of Lissodus africanus (Broom, 1909; Duffin, 1985), and the lateral cusplets are well developed in contrast to the weakly developed ones or their near absence in all species of Lissodus (Duffin, 1985; Rees and Underwood, 2002; Duncan, 2004; Fischer, 2008; Prasad et al., 2008), such as L. cassangensis (Teixeira, 1956). In addition, the crown of Lissodus possesses weakly developed folds (Under- wood and Rees, 2002), in contrast to Pristrisodus n. gen., Lonchidion (Fig. 8.16–8.18) differs from Pristrisodus n. gen., (Fig. 8.25) in having a weakly developed principal cusp, multiple cusplets, a prominent and narrow labial peg, and a broad root. The root of Pristrisodus n. gen., has a single row of foramina proximal to the crown-root junction, and distally, the foramina are randomly oriented (Fig. 8.25). Such a pattern contrasts with that of Hybodus parvidens (Patterson, 1966), where the foramina are irregularly distributed throughout the root without any specialized pattern. The lateral teeth of Parvodus (Fig. 8.19, 8.20; Rees and Underwood, 2002) are different from those of Pristrisodus n. gen., in having a very high crown profile and strong vertical folds. The two known species of Jiaodontus (J. montaltissimus, [Fig. 8.21, 8.22] and J. vedenemus [Fig. 8.23, 8.24]) are distinctly different from Pristrisodus tikiensis in having teeth with a high coronal profile, protruding and bulging labial peg, strong ornamentation in the form of prominent ridges on their labial and lingual faces, and a strongly convex labial face (Klug et al., 2010). Hence, Pristrisodus constitutes a new genus belonging to the family Lonchidiidae, which is distinct from all other valid lonchidiid genera.
Quantitative assessments
Results.—To overcome discrepancy arising due to positional differences, only lateral teeth of the genera examined are con- sidered (Supplemental Data 2). In the case of Pristrisodus tikiensis, the anterior and anteriorly placed teeth are omitted from the analyses, whereas those teeth that are definitely lateral in position are used in the analyses. This includes twenty-two lateral teeth, which are placed posteriorly (morphotypes III–IV) with respect to morphotypes I–II (Supplemental Data 1). The examined teeth of Lissodus duffini were identified as lateral in position by Prasad et al. (2008), although these are found to be morphologically similar to morphotype V (anterior) teeth of Pristrisodus tikiensis. PCA was applied to the variance-covariance matrix of the
four variables, ABH, MDL, LLW, and PCH, which characterize the crown elongation, width, thickness, and height of the
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