Half-billion-year-old sea squirt could push back origins of vertebrates

An exquisitely preserved 500-million-year-old fossilised sea squirt offers new insight into the evolution of tunicates, the sister group to vertebrates and a key to unravelling our own evolutionary origins. The fossil resembles living tunicates that have two life stages: free-swimming larvae and adults that live rooted to the sea floor. This suggests that a crucial evolutionary moment — when sessile tunicates diverged from free-floating ones — happened 50 million years earlier than currently estimated on the basis of molecular clocks.