Upper Cretaceous record of non-belemnitid coleoid jaws from Hokkaido, Japan, and its evolutionary implications

Abstract

Two lower jaws and one upper jaw of coleoid cephalopods recovered from the Santonian to lower Campanian (Upper Cretaceous) strata in Hokkaido, Japan were studied taxonomically. A comparison of the jaws of modern and fossil coleoids suggests that the two lower jaws are conspecific with the co-occurring non-belemnitid coleoid Longibelus matsumotoi represented by longiconic phragmocones. We designated them as L. matsumotoi (?) with reservations. The similarity between the lower jaws of L. matsumotoi (?) and the extant ram's horn squid Spirula spirula (order Spirulida) supports the evolutionary scenario from Longibelus to Decapodiformes, including spirulid Groenlandibelidae, which ranged from the late Campanian to the late Maastrichtian. The other incomplete but very large upper jaw lacking the posterior part of the inner lamella was identified as Oegopsida, family, genus, and species indeterminate, with a long rostrum terminated at a sharply pointed tip in the outer lamella, as observed in the upper jaws of some extant species of this order. The extremely large oegopsid jaws described in this study and in our other papers from the Santonian–Campanian of Hokkaido provide a reliable fossil record to infer the evolutionary history of this order.