Some problems of the origin of the vertebrata

Abstract

A.O. Kowalevsky was the first to examine in 1865–1867 the groups related to ancestors of vertebrates. They were represented by lancelet (Branchiostoma) and tunicates (Ascidia). As Grobben (1908) divided metazoans which are more advanced than coelenterates into protostomes and deuterostomes, searching for remote relatives of vertebrates was performed among deuterostomes. For a long time, enteropneusts were considered to be probable ancestors of vertebrates and chordates as a whole. Subsequently, this concept was replaced by the hypothesis that chordates evolved from aberrant deuterostomes with a calcitic skeleton, which were named Stylophora or Calcichordata (Jeffries, 1986). Based on the data on the homeobox genes, Malakhov (2006) proposed that chordates could have acquired characters of deuterostomes independently of echinoderms. A separate position is occupied by the theory of Sepp (1959), who proposed that vertebrates appeared as a result of “duplication” of marine annelids. The most primitive living vertebrates, Cyclostomata, have a hypophysis which is enclosed in an unusually long canal under the brain and opens in an aperture just anterior to the brain. Cyclostomata, along with Paleozoic armored fishlike forms, compose the most primitive vertebrate group, Agnatha.