Bizarre 500-million-year-old sea creatures called vetulicolians are relatives of vertebrates.
Palaeontologists have struggled to identify the relationship between living animals and these extinct organisms, because of their odd combination of features such as gill slits and a segmented abdomen. A team led by Diego García-Bellido at the University of Adelaide and John Paterson at the University of New England in Armidale, both in Australia, analysed a fossil vetulicolian from a South Australian island.
The fossil, a new species named Nesonektris aldridgei, shows the outline of a notochord — a rod-like structure that develops into the backbone in vertebrates.
Although N. aldridgei is distantly related to vertebrates, its closest relatives are tunicates — invertebrates that swim or attach themselves to underwater rocks. It was probably a free-swimming filter-feeder, say the authors.