ANDERSON, P.S.*; WESTNEAT, M.W.; Univ. of Chicago; Field Museum of Natural History, Chicago: Feeding in Placoderms: A Biomechanical Model of Skull Kinetics in Devonian Fossil Fishes

Biomechanical models of feeding in fishes predict function from morphology and have recently been used to quantify ecological disparity. We apply this technique to the placoderms, a group of Devonian fossil fishes. Placoderms are particularly appropriate fossils for biomechanical modeling, as they are frequently preserved three-dimensionally. We used a third-order lever model to describe lower jaw closing mechanics in three species: Dunkleosteus terrelli, Gorgonichthys clarki, and Mylostoma variabile. We calculated the lengths of inlevers from the quadrate articulation to three possible adductor insertions and outlevers from the quadrate articulation to two main dental surfaces. Mechanical advantages (MA) are calculated as the inlever length over the outlever length. We compared MA among species, for different possible inlevers and outlevers within jaws. Our results indicate a wide range of MA values within jaws (depending on putative muscle geometry) and among groups. Average MA for each group: Dunkleosteus 0.34; Gorgonichthys 0.33(tip) 0.53(back denticles); Mylostoma 0.46. We hypothesize that the two groups with lower MA are piscivores while the group with higher MA ate benthic fauna. Note also that the denticles found further back on the Gorgonichthys jaw have higher MA than Mylostoma, indicating a range of force transfer abilities within the jaw. Physical and computer models of a full skull of Dunkleosteus reveal that this group employs a four-bar linkage in their jaws involving the pectoral girdle. Computer models of cranial kinesis of this species are presented as a tool to test hypotheses of feeding behavior. Lever and linkage mechanics can be used on fossil fishes to describe mechanical and ecological disparity among groups.