REINTERPRETATION OF YUNNANOZOON AS THE EARLIEST KNOWN HEMICHORDATE

THE Chengjiang fossil Lagerstatte is one of the earliest and most impo rtant palaeontological sites from the Phanerozoic era(1,2) about 530 m illion years ago(3). It yields extremely abundant and remarkably prese rved soft-bodied fossils and shells with soft parts of various kinds, including bradoriids(4-6), trilobites(7,8), crustaceans(9), brachiopod s, worms, sponges, algae and many unknown forms(10-13). One of these f ossils is Yunnanozoon(14), which we reinterpret here as the earliest k nown hemichordate. Possessing half of the characteristic chordate feat ures and providing an anatomical link between invertebrates and chorda tes(15), Hemichordata is a minor but important phylum in evolutionary biology. Hemichordates comprise two main groups: the enteropneusts, or 'acorn worms', and the pterobranchs. Apart from the presumable inclus ion of graptolites in pterobranchs(16-19), there are very few hemichor date fossils(2,17,20). Although Yunnanozoon is superficially similar t o the chordates(21), its typical tripartite body plan is broadly consi stent with that of living balanoglossid hemichordates (enteropneusts).