Development of the scales in Lepisosteus as a model for scale formation in fossil fishes

The living holostean fish Lepisosteus and the closely related Atractosteus offer a unique opportunity to study the development and homology of the ganoine-covered scales typical of early groups of actinopterygian fishes. Unlike the living chondrostean Polypterus, the scales of lepisosteids lack dentine and thus the role of the dentine, epidermis and mesodermal mesenchyme can be compared. Formation of the ‘preganoine’ is shown for the first time in Lepisosteus. Initial results show that in Lepisosteus ganoine is formed on the inner surface of a cell layer, apparently mesenchymal in origin, proliferating deep to the epidermal basement membrane. If this is correct ganoine in Lepisosteus is mesodermal and its formation has changed significantly from the pattern in the closely related fossil Semionotidae where dentine is present and is presumably involved in ganoine formation. The canals of Williamson in early fossil fishes were formed by migrating cells that played a major role in bone morphogenesis. The Lepisosteidae offer a unique opportunity to study these cells in a living vertebrate.