BIOMECHANICS AND ECOMORPHOLOGY OF LATE DEVONIAN ARTHRODIRES FROM THE GOGO FORMATION OF WESTERN AUSTRALIA

Arthrodire placoderms show a great deal of morphological diversity by the Late Devonian. Much of this diversity is seen in the skulls and jaws, indicating a wide variety of possible feeding niches, including durophagous crushers and piscivorous hunters. Arthrodires are also diverse in habitat, with high numbers in freshwater deposits, reef settings and open basins. The high diversity and numerical dominance of placoderms amongst Devonian fish faunas makes the organization and function of their feeding systems important to understanding Devonian marine ecology. The Gogo Formation of Western Australia is a Frasnian reef deposit known for beautifully preserved placoderm skulls and jaws, with a rich fauna containing some 30 species along with other fossil fishes and invertebrates. The quality of preservation and high diversity make it a good setting to analyze morphological diversity and corresponding biomechanical variation among placoderms.

Functionally-relevant morphological and mechanical data were collected from 75 jaws (placoderms and associated lungfish) from the Gogo Formation. Functional characters include mechanical advantage (force transfer ability across the lower jaw lever), second moment of area (distribution of material along the cross section of the lower jaw), aspect ratio of the lower jaw and several dental characters such as cusp sharpness and attack angle. This database is used to create a multivariate eco-function space designed to capture the ecological diversity of the Gogo fish fauna. Function-space occupation and pairwise distances between arthrodire species were compared with both phylogeny and morphotype categories previously defined for arthrodires. Sensitivity analyses were performed on the matrix of measurements to determine the influence of character variation. Preliminary results indicate that the range of feeding strategies in Gogo placoderms was greater than estimated previously. Most Gogo placoderms have been considered durophagous; however, several taxa show mechanical advantages much lower than any modern durophagous fish. Such results lend strong support to the possibility of obtaining quantified hypotheses of ecological diversity within a phylogenetically remote, and long extinct basal gnathostome taxon.