The scales of Tulerpeton
were made of compact bone lacking enamel, dentine and isopedine layers found in more basal sarcopterygians (Mondéjar-Fernandez et al. 2014). This was the first step in a process that ultimately also removed bone from scales leaving only keratin scales arising from the epidermis.
Several times later
bone reappeared in the dermis producing osteoderms.
Mondéjar-Fernandez J, Clément G and Sanchez S 2014. New insights into the scales of the Devonian tetrapods Tulerpeton curtum Lebedeve, 1984. Journal of Vertebrate Paleontology 34:1454-1459.