Catalog of Late Pennsylvanian ichthyoliths, addendum

Abstract

ABSTRACT A review of the evidence suggests that ancestral placoderms were not armored, but that dermal bones were acquired, independently of Osteichthyes, probably in the Late Silurian. It is believed that these ancestors were covered with denticles or tesserae, some of which were retained in a few Devonian orders. Primitive forms lacked the placoderm dermal cranial bone pattern that is recognizable in most orders; they also lacked a trunk shield which was derived in more advanced orders from the ancestral shoulder girdle. The evolutionary significance of some dermal bones is discussed: in the skull, the centrals, postpineal, nuchal, “premedian,” postnasals, and paranuchals; in the trunk shield, the anterior ventro-laterals, intero-laterals, antero-ventrals, anterior medio-ventrals, spinals, and posterior ventro-laterals.