Vertebral development in the Devonian Sarcopterygian fishEusthenopteron foordiand the polarity of vertebral evolution in non-amniote tetrapods

Abstract

Study of a growth series of twenty-seven specimens from the Upper Devonian of Escuminac Bay, Québec documents a complex pattern of vertebral development in the osteolepiform fish Eusthenopteron foordi. Ossification begins with elements associated with the caudal, anal, and second dorsal fins. Development of the haemal arches, caudal radials, and caudal neural arches continues anteriorly and posteriorly from near the level of the anterior margin of the caudal fin. Trunk neural arches ossify later than the caudal neural arches and as a separate sequence. Trunk intercentra most likely begin ossification posteriorly and continue forward after the ossification of haemal arches is complete. Comparisons of many different patterns of vertebral development within the modern actinopterygians demonstrates that the sequence of development in Eusthenopteron foordi is unique. The diverse patterns of vertebral development observed in fossil and modern fish presumably result from an interplay between the inherent anterior to posterior sequence of development controlled by the Hox genes, and varying selective forces imposed by the physical and biological environment in which the fish develop. Initiation of vertebral development in the caudal region of Eusthenopteron foordi can be attributed to selection for early function of the tail in propulsion. In contrast, vertebral development in Carboniferous amphibians typically proceeds from anterior to posterior. This may reflect development in the still water of ponds and lakes in contrast with the coastal environment inhabited by the hatchlings of Eusthenopteron foordi. The sequences of vertebral development seen in Carboniferous labyrinthodonts and lepospondyls are divergently derived from that observed in Eusthenopteron foordi.