Isabelle Kruta

Ammonoid Buccal Mass and Jaw Apparatus

Current knowledge on the ammonoid buccal mass and jaw apparatus is synthesized based on in situ fossil records from 109 genera that are distributed in 30 superfamilies of 8 suborders of Devonian to Cretaceous age. As in those of modern cephalopods, the jaw apparatus of ammonoids consists of upper and lower elements. Comparative anatomical examination of the attachment scars of chitin-secreting cells (beccublasts) on the jaw lamellae allow us to reconstruct the buccal mass structure of ammonoids. The jaw apparatuses of ammonoids can be classified into the normal, anaptychus, aptychus, intermediate, and rhychaptychus types based mainly on the differences in overall morphology and lamellar composition in the lower jaws. The fairly large variation of the jaw morphology and the variety of food remains preserved in the digestive tract and buccal cavities in Mesozoic ammonoids may reflect diversity of feeding and diets ranging from predatory via scavenging to microphagous habits.

Adaptations to squid-style high-speed swimming in Jurassic belemnitids

Although the calcitic hard parts of belemnites (extinct Coleoidea) are very abundant fossils, their soft parts are hardly known and their mode of life is debated. New fossils of the Jurassic belemnitid Acanthoteuthis provided supplementary anatomical data on the fins, nuchal cartilage, collar complex, statoliths, hyponome and radula. These data yielded evidence of their pelagic habitat, their nektonic habit and high swimming velocities. The new morphological characters were included in a cladistic analysis, which confirms the position of the Belemnitida in the stem of Decabrachia (Decapodiformes).

Ammonoid Paleobiology: From macroevolution to paleogeography

This two-volume work is a testament to the abiding interest and human fascination with ammonites. We offer a new model to explain the morphogenesis of septa and the shell, we explore their habitats by the content of stable isotopes in their shells, we discuss the origin and later evolution of this important clade, and we deliver hypotheses on its demise. The Ammonoidea produced a great number of species that can be used in biostratigraphy and possibly, this is the macrofossil group

The Jaw Apparatus of the Late Cretaceous Ammonite Didymoceras

Abstract We report on well-preserved upper and lower jaws found inside the body chambers of two specimens of Didymoceras nebrascense (Meek and Hayden, 1856) from the Upper Cretaceous Pierre Shale of the USA. The finds are described and compared to existing material, and their possible functions are discussed.

Inquilinism of a Baculite by a Dynomenid Crab from the Upper Cretaceous of South Dakota

ABSTRACT We describe a small crab inside the phragmocone of a heteromorph cephalopod Baculites sp. smooth from the Gammon Ferruginous Member (lower Campanian) of the Pierre Shale in Butte County, South Dakota. The crab Ferricorda kimberlyae (Bishop, 1987) is well preserved with its carapace and pereiopods parallel to and between two septa of the phragmocone. Because of its superb preservation, the specimen is unlikely to have been washed into the phragmocone. The crab probably retreated into the phragmocone chamber to avoid predation or to molt and was subsequently buried by an influx of rapid sedimentation. This is the first instance of inquilinism by a crab in a heteromorph ammonite. Despite the rarity of such fossils, the occurrence of crabs inside ammonite shells was probably not uncommon on sea floors during the Mesozoic. Morphological details of the specimen reveal that Ferricorda is a dynomenid crab.