L. B. Halstead and the heterostracan controversy

This brief review is aimed at showing the impact of L. B. Halstead's works on heterostracan fishes during the past twenty-five years. His ideas on the internal structure of these early vertebrates, the histology of their dermal skeleton, their habitat and their phylogenetic position have always raised stimulating controversies which have led to the present day knowledge of these fossils. Halstead's interpretation of the snout and mouth of the heterostracans and its bearing on their relationships to the jawed vertebrates, are discussed in the light of new data on the anatomy of Ordovician vertebrates.