On the Phylum Vetulicolia

Abstract

The origins and radiations of metazoans, bilaterians, deuterostomes and vertebrates have been recognized as the four most important milestone events in animal history and, conversely four of the toughest and puzzling questions in evolutionary biology. Among them, the first two occurred in the deep time of the Precambrian and have left little information in the fossil records. Fortunately, intensive investigation into the Early Cambrian Chengjiang faunas in the past two decades have revealed numerous significant data on the main episode of the Cambrian Explosion, particularly information on the various earliest-known deuterostomes, leading to a much better understanding of the last two events. The “first fish”Haikouichthys andMyllokunmingia have been proved the oldest-known vertebrates, and to be among the most primitive ones, representing a key transitional group from acraniates to craniates. Indeed, “Paleontological work of Shu and collaborators is revising our understanding of the early evolution of chordates”. Moreover, the discovery of the Phylum Vetulicolia may also throw new light on the origin of deuterostomes. This extinct group with simple gill slits might represent one of the roots in the deuterostome lineage, as the evidence of molecular biology and developmental biology has predicted. Here the first discovery of tiny vetulicolians from the Chengjiang Lagerstatte is reported. The evolutionary trend from the giant to the small might represent a special adaptation in early animal history. Since all vetulicolians share similar characters in their anterior section (pharynx), and on the basis of differences in their posterior section, a new classification system is proposed, with a new class, the Heteromorphida erected. As both the “root” group vetulicolians and the “top” group vertebrates in the early deuterostome lineage, as well as those members in between, such as vetulocystids, yunnanozoans, cephalochordates and urochordates, have been recovered, a relatively complete early deuterostome phylogeny, which is compatible with phylogenies of extant deuterostomes, is nearly mature.