A review of placoderm scales, and their significance in placoderm phylogeny

An historical review of the literature relating to placoderm scales preserv ed in association with articulated dermal plates, or as isolated units in m icrovertebrate assemblages, is followed by a discussion of their relevance in phylogenetic analyses of the Placodermi. The dentinous tissue forming th e tubercles of Early Devonian acanthothoracid scales and dermal bone is sim ilar to that of the dermal bone ornament of some osteostracans, and denticl es of the vertebrate Skiichthys from the Ordovician Harding Sandstone. This similarity supports the proposition that the gnathostomes are the sister-g roup of the Osteostraci, with the Placodermi branching earliest within the gnathostomes, and the Acanthothoraci branching earliest within the Placoder mi. The meso-semidentine in acanthothoracid tubercles, rather than semident ine (sensu stricto), is most likely to be synapomorphic for the Placodermi.