Cosmopolitodus planus ( Agassiz, 1856 ) Yun, 2022

Yun, Chan-gyu, 2022, A Tooth Of The Extinct Lamnid Shark, Cosmopolitodus Planus Comb. Nov. (Chondrichthyes: Elasmobranchii) From The Miocene Of Pohang City, South Korea, Acta Palaeontologica Romaniae 18 (1), pp. 9-16 : 11-12

† Cosmopolitodus planus ( Agassiz, 1856) comb. nov.

Description: CNUNHM-F341 is a very well preserved fossilized lamnid shark tooth that is still embedded in the matrix ( Fig. 2 ). The exposed labial surface of the tooth is flat and nearly complete, except for some breakage at the base of the mesial crown edge ( Fig. 3 ). The tooth lacks lateral cusplets, but instead has a thick enameloid shoulder where the main cusp meets the root along the distal side (unfortunately this feature is not preserved on the mesial edge). The main cusp is wide and triangular; it measures 17 mm in height, and 15 mm in preserved mesiodistal width (if completely preserved, the estimated mesiodistal width of the main cusp was likely 18 mm). The upper two-thirds of the mesial crown edge is slightly convex, whereas the lower one-third is slightly concave. The distal edge of the crown is nearly straight apically, but strongly concave basally, thereby resulting an overall “hooked” morphology with the apex strongly inclining distally at a 28° angle measured between the midline of the tooth and the crown apex. Both cutting edges are smooth and devoid of any serrations. The preserved length of the mesial crown edge measures 21 mm, but was likely about 25 mm when complete. The distal crown edge length measures 14 mm. The root is large and has rounded mesial and distal root lobes. Both the mesial and the distal roots are extended slightly beyond the crown edges, and the root has a triangular interlobe area. The maximum width and height of the root are 20 mm and 5 mm, respectively.

Remarks: CNUNHM-F341 is assigned to Cosmopolitodus planus based on the presence of smooth cutting edges on the crown, nearly flat labial surface, a triangular and wide overall morphology of the crown with an apex that is strongly inclined distally, rounded root lobes, and the absence of lateral cusplets (e.g., Kuga, 1985; Karasawa, 1989; Nazarkin, 2013; Yun, 2020). Although some Isurus oxyrinchus ( Rafinesque, 1810) teeth bear resemblances to those of C.planus in having a relatively flat and distally inclined wide crown, CNUNHM-F341 differs from these by having an apicobasally thick root with rounded lobes (e.g., Kuga, 1985; Boessenecker, 2011; Nazkarin, 2013). CNUNHM-F341 is differentiated from the upper lateral teeth of Cosmopolitodus hastalis in being more robust and strongly inclined distally, even compared with the cases with similar profile (e.g., Kim et al., 2018; Yun, 2020). Lastly, the root on CNUNHM-F341 is massive and bears rounded mesial and distal root lobes, and this rounded form strongly differs from narrower and square root lobes on Cosmopolitodus hastalis (e.g., Kuga, 1985; Karasawa, 1989; Nazarkin, 2013). Thus, the referral of CNUNHM-F341 to Cosmopolitodus planus can be confidently justified.

The hooked nature of the main cusp of specimen CNUNHM-F341 suggests it was from the left palatoquadrate of the shark. Furthermore, the height and mesiodistal width of the main cusp, the degree of distal inclination, and the overall size of suggests it is either from an upper intermediate or posterior tooth file, as described upper anterior and larger upper lateral teeth of C. planus generally have a mesiodistally wider and taller main cusp (e.g., Nazarkin, 2013). The possibility also exists that the specimen may have been derived from a juvenile individual.

Figures

Fig. 2 Isolated tooth of Cosmopolitodus planus with unremoved matrix (CNUNHM-F341), probably from the Duho Formation (Middle Miocene), Pohang City of South Korea.

Fig. 3 Detail of CNUNHM-F341, a Cosmopolitodus planus tooth, in labial view.