Meeting Abstract
Xiphactinus audax was a large, predatory ichthyodectid fish that lived during the Late Cretaceous, from approximately 120 – 66 million years ago. It inhabited the Western Interior Seaway (WIS) – an epicontinental sea that extended from the Arctic Ocean to the Gulf of Mexico – where this top predator reached lengths of 6.0 m. Megalops atlanticus is an extant megalopid fish that inhabits both Atlantic coasts and the Gulf of Mexico. While not as imposing, M. atlanticus is nonetheless a hefty predator, reaching lengths of 3.0 m. Despite their temporal and phylogenetic distance, both species have superficial morphological similarities – notably their body shape and strongly supraterminal mouths. These two species are often noted as examples of convergent evolution, with M. atlanticus cited as a modern analog for X. audax. This study sought to quantify differences in the shape of the mandibular elements of these two fish species using both 2D and 3D landmark-based geometric morphometrics (GM) in order to assess claims of convergence. Landmark-based GM compares shape by statistically analyzing landmarks placed on homologous points between objects. A number of statistical procedures were carried out comparing shape differences between both complete skulls and individual elements. Results indicate considerable difference between the shape of both complete skulls and individual elements, suggesting differences in the feeding biomechanics between X. audax and M. atlanticus. These results also suggest a lack of convergence on a functional phenotype, indicating a re-assessment of what constitutes convergence and how it is detected – particularly in the fossil record – may be necessary.
