ECOLOGY OF THE AMMONITE SUPERFAMILY ACANTHOCERATOIDEA FROM THE CENOMANIAN-TURONIAN WESTERN INTERIOR SEAWAY USING WESTERMANN MORPHOSPACE

The Cenomanian-Turonian (C-T) interval of the Late Cretaceous was a time of extremes: sea level reached its Phanerozoic peak, temperatures were up to seven degrees warmer than today, and CO₂ levels were several times higher than today. This greenhouse interval led to one of the ten largest mass extinctions of the Phanerozoic. Species level extinction rates for ammonoid cephalopods, for example, may have reached 93% within the Western Interior Seaway (WIS) of North America. An important step in determining the nature of the extinction is to determine the ecology of the animals affected by it. Among ammonoid cephalopods, Superfamily Acanthoceratoidea was the most diverse and widespread group during the C-T. This group was therefore used as a proxy for ammonoids as a whole. Acanthoceratoids have traditionally been assigned to a demersal mode of life, but this interpretation has not been rigorously tested in a quantitative framework. Westermann morphospace, a recently proposed technique for assigning cephalopod shell forms to modes of life, can serve as this framework.

Shell measurements from 225 acanthoceratoid specimens from the Middle Cenomanian through the Upper Turonian, including shell diameter, whorl heights, whorl breadth, and umbilical diameter, were made. Shape ratios were calculated and normalized, then used to produce ternary diagrams representing Westermann morphospace. Acanthoceratidae, the most diverse of the acanthoceratoid families, shows much higher morphological disparity than other families, occupying much of the central region of the morphospace. Other families, all descendants of Acanthoceratidae, segregate into one or another peripheral region of the morphospace. For example, coilopoceratid specimens fall in the nektonic region while collignoniceratid specimens fall towards the demersal-planktonic region. The high morphological disparity of Acanthoceratidae is consistent with previous work showing the plasticity and variability in the group. The distributions of the descendant families support the claim that each family was derived from a different taxonomic, morphologic, and ecological precursor within Acanthoceratidae. Future work will investigate phylogenetic relationships within Acanthoceratoidea to better clarify these evolutionary transitions.