KAMMERER, Christian F.*; WESTNEAT, Mark W.; GRANDE, Lance; University of Chicago; Field Museum of Natural History; Field Museum of Natural History: A Comparative and Developmental Analysis of Jaw Mechanics in Gars

Gars (Lepisosteidae) are a group of predatory basal actinopterygians with an elongate snout and a rapid feeding strike. Gars have traditionally been considered a morphologically conservative group whose habits have remained fairly static since their appearance in the Cretaceous. Feeding mechanisms in gars are less complex than in teleosts and Amia, involving a minimum of cranial kinesis. This study examines the jaw mechanics of gars using a lever model of the mandible that allows for muscle simulation of lever action. Mandibular mechanics of extant and fossil gar species were calculated using morphometric data from 228 specimens. Lever function was also calculated for a developmental series of the extant species Lepisosteus osseus and Atractosteus spatula. We found surprisingly high mechanical diversity within the group both between species and during development. Mechanical advantages (MA) for closing of gar mandibles were found to range from 0.05 to 0.20. The lower extreme represents the lowest recorded jaw closing MA in fishes and corresponds to very high transmission of motion. This mechanical diversity was found to correspond to trophic diversity among extant gars, and suggests hypotheses for the feeding habits of fossil gars. Developmental data reveal a distinctive curve for MA during ontogeny, with a large decrease in MA between larvae and juveniles followed by a steady increase during adult growth. This curve reflects a change in prey type from invertebrates taken by young gar giving way to increasing fish predation during growth.