Ontogeny, phenotypic variation and phylogenetic implications of arthrodires from the Gogo Formation, Western Australia

Compagopiscis croucheri and Gogopiscis gracilis (Gardiner and Miles, 1994) were collected in high numbers and in different size classes from the Upper Devonian Gogo Formation, Western Australia. This materia I has provided the basis for an investigation of ontogentic and phenotypic variation of characters and their phylogenetic implications. We used the Principal Component Analysis to examine if variation between specimens assigned to C. croucheri and G. gracilis could be attributed to random variation. In our analysis we found that only the first two principal components showed any significant ability to discriminate between the specimens assigned to C. croucheri and G. gracilis. The first principal component is a measure of overall size of the specimen, so that variation in this principal component can be explained in terms of maturity and growth within a single species. Not all the anatomical variation observed in this study call be attributed to ontogeny and it is considered that the two-class structure defined by the second principal component is consistent with individual variation. It is, therefore, concluded that Compagopiscis and Gogopiscis represented part of the continuous variation exhibited by the whole population rather than two distinct taxa and therefore Compagopiscis and Gogopiscis are synonomized to a single germs, Compagopiscis containing a single species, Compagopiscis croucheri.