ABSTRACT
The most important event in the evolution of the vertebrates was the origin of the vertebrates themselves. This event should be considered in the context of the place that the vertebrates occupy within the chordates, and, in turn, deuterostomes-the branch of the animal kingdom that includes echinoderms and hemichordates as well as the chordates. In this review I show how recent work has changed the conventional view of deuterostome phylogeny. Characters thought diagnostic of derived groups within the deuterostomes may have a wider distribution. For example, metameric segmentation, and the presence of pharyngeal slits, once seen as diagnostic respectively of somitic chordates (that is, cephalochordates+ vertebrates) and pharyngotremates (hemichordates+chordates) should both now be regarded as having been primitive features of deuterostomes. That is, the most recent common ancestor of all extant deuterostomes would have been a segmented animal with pharyngeal slits. This has particular implications for the history of echinoderms, conventionally regarded as primitively unsegmented and without pharyngeal slits. It should also prompt a reassessment of the ‘calcichordate’ theory, in which certain Palaeozoic fossils known as carpoids, conventionally regarded as echinoderms, are interpreted variously as stem-group deuterostomes, stem-group echinoderms, stem-group chordates, or stem-group representatives of chordate subphyla. I argue that to dismiss carpoids as echinoderms simply on the basis of general resemblance to extant (that is, crown-group) echinoderms is both illogical and, in the light of recent independent work, indefensible.
