MALLATT, Jon M; CHEN , Jun-Yuan: The Cambrian fossil Haikouella and the origin of the craniates

Haikouella lanceolatum is a fossil deuterstome that is known from many well-preserved specimens from the Early Cambrian of China (Nature 402:518). It has been interpreted as craniate-like or else as resembling the basal deuterostome, but here we analyze its phylogenetic position in detail, based on over thirty structural characters evident in the Haikouellafossils. Our cladistic analysis generated the following tree: (protostomes, hemichordates, (tunicates, (cephalochordates, (Haikouella,(craniates))))), with the clade of “Haikouella + craniates” supported by bootstrap values varying from 76% to 96%, depending on how the analysis was structured. Thus, Haikouella is concluded to be the closest known sister group of craniates. It is compared to hypothetical precraniates that have been predicted by recent hypotheses (such as the “new-head” hypothesis of Gans and Northcutt and the “new-mouth” hypothesis of Mallatt) and it fits these predictions closely. Thus, it should help solve the longstanding problem of the origin of the vertebrates.