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A NEWLY discovered group of prehistoric animals could be the forebears of all
vertebrates, including fish, reptiles and humans.

The ancient creatures look like outsize tadpoles and date from the Cambrian
explosion around 550 million years ago. Fossils from the Chengjiang region of
southern China reveal that the animals, placed in a new phylum called
Vetulicolia, were around 6 centimetres long and had a swollen sac-like head with
a huge mouth, joined directly to a muscular, segmented tail that they used for
swimming.

鈥淲e鈥檙e confident they are the ancestors of the group that includes
vertebrates,鈥� says palaeontologist Simon Conway Morris of Cambridge University.
Vertebrates belong to a group of animals called deuterostomes. This group
includes echinoderms like starfish, and tunicates鈥攕trange jelly-like
creatures such as sea squirts. Although vertebrates and starfish appear
unrelated, they share several key features, like gill slits. In people, remnants
of these form the inner ear.

But the identity of the earliest deuterostomes has been a mystery. 鈥淭he
difficulty is that deuterostomes all look terribly different from one another,鈥�
says Conway Morris, who examined the Chinese fossils. 鈥淭rying to imagine a
common ancestor is practically impossible.鈥�

Then in 1987, researchers found a strange fossil, which they named
Vetulicola. It was thought to be a 鈥渧ery weird arthropod鈥� because of its
segmented tail. But it didn鈥檛 have any limbs, which is unheard of for
arthropods. Now Degan Shu from Northwest University in Xian and his team have
discovered two similar fossils, which they have named Xidazoon and Didazoon.

They each had five pairs of gill slits鈥攁 feature inherited by all
deuterostomes. The team soon realised that the strange animals were all related,
and are probably the ancestors of all living deuterostomes. 鈥淭hese were the
鈥榤issing link鈥�,鈥� says Conway Morris.