BIOMECHANICS AND EVOLUTION OF THE FEEDING STRUCTURES IN LATE DEVONIAN PLACODERMS

Arthrodire placoderms are a Middle Paleozoic group of morphologically diverse basal gnathostomes. The diversity seen in their skulls and jaws, indicates a wide variety of feeding niches from freshwater durophagous bottom dwellers to marine piscivorous cruisers. Current phylogenies place the Placodermi as sister-group to all other jawed vertebrates. The organization and function of placoderm jaw systems is central to understanding early gnathostome evolution. The goal of this study is to understand the relationship of arthrodire skull morphology to feeding ability. The Gogo Formation of Western Australia is a Frasnian fossil reef deposit known for beautifully preserved placoderm skulls; 30 species have been described along with other fossil fishes and invertebrates. The quality of preservation and high diversity make it a good setting to analyze biomechanical diversity.

Functionally relevant morphological and mechanical variables were measured on 9 arthrodire skulls from the Gogo Formation as well as 5 skulls of Dunkleosteus terreli from the Cleveland Shale. All skulls were analyzed using Placodermodel 2.0, a dynamic computer model based on lever and linkage mechanics that calculates skull kinetics and mechanical feeding metrics in fossil vertebrates. I use the model to compare the ten arthrodire species to each other and to modern groups. Results reveal a range of kinematic transfer coefficients (KT) and mechanical advantage values that change during the feeding cycle. Arthrodire KT values (3.2-3.4) are generally higher then a subset of modern labrid oral linkages (0.45-1.5), but near the high end of the range of modern hyoid linkages (0.07-4.7), indicating that arthrodires utilized suction as part of their feeding mechanism. The high range of mechanical advantage measurements (0.1-0.68) indicates considerable diversity in feeding strategy. Recent phylogenies imply that different placoderm taxa attain high KT values in a variety of ways and that the linkage system modeled here evolved multiple times within placoderms. Such results show how quantified hypotheses of ecological diversity can be obtained even in a phylogenetically remote and long extinct basal gnathostome taxon.