Ischyodus townsendi ( Buckland, 1835 )

Otero, Rodrigo A., Bravo, Constanza Figueroa, Soto-Huenchumán, Paula, Fernández-Collemann, Sara, Valenzuela-Toro, Ana M. & Gutstein, Carolina S., 2021, First record of chimaeroid fish Ischyodus from the Upper Jurassic of southwestern Gondwana, Acta Palaeontologica Polonica 66 (3), pp. 623-630 : 625-626

Ischyodus townsendi ( Buckland, 1835)

Fig. 3A .

Material.— MHNC.35.444, two articulated mandibular plates from Mina Del Fierro , Andes Cordillera , Región del Maule, Chile; Baños del Flaco Formation , Tithonian (Upper Jurassic) .

Description.—Both plates are visible in basal view ( Fig. 3A ). The mandibular plates remain articulated in nearly anatomical position. The right mandibular plate is the most damaged by diagenesis or transport, with the basal surface lost and a few tritors broken, being exposed in different sections. This preservation allows us to observe the tritor distribution. Its posterior occlusal surface is preserved as a thin bone layer attached to the matrix, which is why this was not removed. The preserved surface indicates a sudden mechanical break-up, judging by the fresh bone exposed. This is consistent with damage derived from rock blasting, discarding erosive agents. On the broken surface (basal view) it can be recognized a larger tritor pad placed medially, and therefore, being identified as the median tritor which covers two-thirds of the plate width. A cylindrical tritor, identified as the posterior outer tritor (pot), flanks labially to the median tritor (mt). Over the labial margin of each plate there is a prominence. The broken right mandibular plate shows the anterior outer tritor (aot) near to this prominence. The left mandibular shows two marked ridges r) over the basal surface, which reflect the internal position of the symphyseal tritor (syt) of laminated pleromin and the median tritor, respectively. On its basal margin, the symphyseal tritor is broken, leaving a cavity. The median tritor is visible, it is also partially broken, but prominent. Flanking the latter, there is a posterior exposure of the posterior outer tritor. The beak (bk) is preserved in both plates, being prominent and slightly recurved laterally. Its anteriormost tip is worn. Measurements of both specimens are provided on Table 1.

Remarks.—MHNC.35.444 can be referred to the genus Ischyodus based on the possession of mandibular plates with a large central median tritor flanked by a short anterior outer tritor, an elongate posterior outer tritor, and the existence of a symphyseal tritor of laminated pleromin at the mesial tip of the tooth plate ( Stahl 1999; Popov and Machalski 2014). The occurrence of two small tritors exposed at the prominence on the wavy labial margin of the mandibular plate has been considered as another diagnostic feature of the genus ( Stahl 1999: 131); however, this feature is not present in the holotype of Ischyodus townsendi , which only has a single tritor (anterior outer) in this position. Nevertheless, the absence of a second tritor is related to the wearing suffered in larger/old mandibular plates, being instead a consequence of the ontogeny rather than being a diagnostic value ( Ward and Grande 1991; Popov and Machalski 2014).

Stratigraphic and geographic.—Baños del Flaco Formation, Tithonian (Upper Jurassic). Mina Del Fierro, Andes Cordillera, Región del Maule, Chile.

Figures

Fig. 3. Chimaeroid fishes from Mina Del Fierro, Andes Cordillera, Región del Maule, Chile; Baños del Flaco Formation, Tithonian (Upper Jurassic). A. Ischyodus townsendi (Buckland, 1835) (MHNC.35.444); two articulated mandibular plates in basal view. B. Ischyodus sp. (MHNC.35.445); ventral view of the bony contour of a right palatine attached to the matrix, preserving the occlusal surface of the tritors.Abbreviations: ait, anterior inner tritor; aot, anterior outer tritor; bk, beak; lm, left mandibular; mt, median tritor; out, outer tritor; pit, posterior inner tritor; plm, prominence of the labial margin; pot, posterior outer tritor; r, ridges; rm, right mandibular; syt, symphyseal tritor. Photographs (A1, B1), schematic illustrations (A2, B2).