Conchorhynchus sp.

Tajika, Amane, Landman, Neil H., Koyasu, Hiromichi, Broussard, Amy, Anderson, Lian, Ikuno, Kenji & Ikuno, Kenji, 2023, New discovery of rhyncholites and conchorhynchs (cephalopod jaw elements) from the Upper Cretaceous Mount Laurel Formation of Delaware, American Museum Novitates 2023 (3998), pp. 1-20 : 9-11

Conchorhynchus sp.

Figures 6 , 7

MATERIAL: Twelve specimens ( AMNH 137086, AMNH 137087, AMNH 137095–137103, AMNH 137123 from the Mount Laurel Formation of Delaware. All specimens are somewhat poorly preserved.

DESCRIPTION: The overall shape is unclear due to the poor preservation. The maximum preserved length and width range from 4.2 to 8.7 mm and 4.0 to 8.2 mm, respectively. The shaft angle ranges from 68° to 90°. The hood is not visible, but the anterior edge seems slightly folded (anterior fold). There are broad and shallow grooves between the anterior edge and shaft on the ventral side. The ventral ridge is present, which widens posteriorly. The ventral ridge is often smooth, but some specimens such as AMNH 137101 and AMNH 137102 exhibit lateral grooves. The dorsal side is sometimes covered by sediments or eroded, which masks the denticle/rib pattern.

DISCUSSION: According to Košťák et al. (2010), Mesozoic and Cenozoic conchorhynchs are morphologically conservative. They also mention that Rhyncolites simplex and Conchorhynchus cretaceous represent the upper and lower jaws of a single nautilid species. Our specimens resemble the overall morphology of specimens of C. cretaceous illustrated by Fritsch (1910) and Košťák et al. (2010). However, our specimens do not possess the V-shaped groove on the ventral edge that is visible in their specimens. Some other conchorhynchs reported from the Cretaceous such as C. limburgicus and C. similis also resemble our specimens. Nevertheless, the preservation (erosion and sediment covering the dorsal side) masks fine morphological details and, therefore, hampers the precise assignment of our species.

Superfamily Nautilaceae de Blainville, 1825

Family Nautilidae de Blainville, 1825

Genus Eutrephoceras Hyatt, 1894

Figures

FIGURE 6. Conchorhynchus sp. from the Mount Laurel Formation, Delaware. A–C, AMNH 137086. D, H, I, AMNH 137095. E–G, AMNH 137123. J, K, L, AMNH 137087. M, P, Q, AMNH 137096, N, O, R, AMNH 137099. S, T, U, AMNH 137098. Scale bar = 2 mm.

FIGURE 7. Conchorhynchus sp. from the Mount Laurel Formation, Delaware. A–C, AMNH 137097. D–F, AMNH 137101. G, K, O, AMNH 137102. H, I, J, AMNH 13710. L, M, N, AMNH 137103. Scale bar = 2 mm.