Lower jaws of two Late Cretaceous pachydiscid ammonites from Hokkaido, Japan with discussion on taxonomic and paleoecological implications of pachydiscid jaw apparatus

Abstract

The lower jaws of two pachydiscid ammonites, Menuites japonicus (Matsumoto) and Eupachydiscus sp., are described based on two specimens from the Coniacian to Santonian (Upper Cretaceous) strata of the Haboro and Mikasa areas in Hokkaido, Japan. They are preserved in situ in the body chamber and exhibit features intermediate between the lower jaws of anaptychus and aptychus types, specifically, a broadly open outer lamella composed of inner “chitinous” and outer calcareous layers, a flat rostrum, and a median furrow along the hood ridge of the inner layer. These characteristics have also been documented in the lower jaws of other previously described pachydiscid species, suggesting they are diagnostic of the family. The unusually large, shovel-shaped lower jaws with flat rostral margins may have evolved in relation to a microphagous feeding strategy. Additionally, the ventral outline of the lower jaw roughly matches the cross-sectional outline of shell aperture, although the former is smaller than the latter. As previously supposed in some Jurassic ammonoids, Late Cretaceous pachydiscids were presumably able to withdraw their soft parts deep into the body chamber, thereby avoiding being eaten by predators. Under such circumstances, their widely open lower jaws could have an operculum-like role to protect the retracted soft body from predator’s attack.