Abstract

Deuterostome phylogeny is now fairly secure, with a clear-cut division between the Ambulacraria (echinoderms and hemichordates) and Chordata (vertebrates, cephalochordates (aka amphioxus) and urochordates (aka tunicates)). Collectively they show an impressive disparity, such as the pentaradial echinoderms and encephalized mammals. Identifying what the ancestral forms looked like calls upon a diverse and sometimes problematic fossil record which oscillates between conviction and lunacy. Some of the groups are unfamiliar to the wider world, not least the vetulicolians, yunnanozoans and vetulicystids. Others, such as the iconic Pikaia, are chordates but show a series of puzzling features that are open to several interpretations. More secure ground may be reached, perhaps, with fish such as Metaspriggina whose pharyngeal area suggests some cherished assumptions as to the utility of the lamprey as a model system are now ripe for reassessment. And then there are the conodonts …..