Ammonoid habitats and habits in the Western Interior Seaway: a case study from the Upper Cretaceous Bearpaw Formation of southern Alberta, Canada

Multidisciplinary research on the habitats and life-habits of ammonoids of the Upper Cretaceous Bearpaw Formation is based on hydrostatics and hydrodynamic aspects of shell shapes, occurrence in lithofacies, evidence of predation, and stable-isotopic data. The large and well-streamlined, oxyconic Placenticeras, with large chitinous anaptychus, had the weakest shells known, and suffered mosasaur tooth perforations at minimal ambient pressure. Anomalously light δ18O signatures preserved in their aragonitic shells suggest that they grew in brachyhaline water. Juveniles and some adults of orthoconic Baculites had a vertical life orientation and primarily resided in the lower to middle levels of the water column, flourishing during oxygenation events. Some adults, however, may have changed to a mainly horizontal swimming habit in mid-water. Attitudinal change is indicated by relative shortening of the body-chamber during late ontogeny. Neutral buoyancy at maturity could have been achieved only with a partially liquid-filled phragmocone, implying the exceptional use of cameral liquid as an apical counterweight. Juvenile scaphitids were planktic or sluggishly nektic, as implied by sphaeroconic and discoconic immature whorls, respectively. The adults had different habits, as indicated by their strongly modified body-chambers, which gave high stability suited to vertical swimming. Adult Hoploscaphites and most Jeletzkytes, with open-hooked (scaphitoconic) body-chambers, were demersal and lived close to the sea floor during oxygenation events, whereas adult Rhaeboceras, with an elliptical, contracted body-chamber (elliptosphaerocone), lived in mid-water.