Mophogenesis of the Exoskeleton: Classical and New Approaches to the Study of the Hard Cover of Early Vertebrates

Based on original research and published data, a brief review of approaches to the study of the morphogenesis of hard structures of the early vertebrate integument is provided. The first stages of understanding morphogenetic processes were attempts to recognize and describe a series of successive states of exoskeleton elements in the ontogeny and phylogeny of various early vertebrate groups. Classical approaches are characterized by the recognition of modules (lepidomoria, odontodes), the description of their combinations (e.g., odontocomplexes), and possible ways to construct models of hard covers in different species. A geometric approach is proposed, that develops previous approaches and explains the existence of complex reliefs in vertebrate exoskeletons. The circular model of integument formation describes existing reliefs and predicts the discovery of new ones.