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Some of the first complex animals lived in a shallow sea near the mouth of a river delta about 518 million years ago. The changeable environment, which was prone to violent floods, may have helped drive the evolution of early animals, including some of the first worms and animals with hard shells.

The Cambrian period, from 541 to 485 million years ago, saw a great flowering of animal life, dubbed the Cambrian explosion. A number of major animal groups originated at this time, including many that still exist today.

Some of the best Cambrian fossils come from the Chengjiang biota, a set of fossil-bearing rocks in China that are around 518 million years old. It was first recognised in 1984 by , now at Yunnan University in Kunming, China, and has been studied ever since.

Most of this research has focused on the animals themselves, says at the University of Exeter in Penryn, UK. 鈥淲e have very limited understanding in terms of why the animals are there,鈥� she says.

Ma and her colleagues, including Hou, have now analysed a core taken from the rocks that include the Chengjiang layer. By studying the exact kinds of sediments that make up the rocks, they say they have identified the environment in which the Chengjiang organisms lived.

They concluded that it was a shallow sea, close to the mouth of a delta. This was unexpected, because the animals are beautifully preserved, which often indicates that they lived in the deep sea and were buried in oxygen-free sediments where they couldn鈥檛 decay.

However, Ma says the presence of the delta explains the preservation. Deltas are prone to violent floods, which sweep sediment out to sea 鈥� so, from time to time, the Chengjiang organisms would have found themselves sliding away in a washed-out delta. The delta sediments would have buried them, preserving them exquisitely.

The evidence from the core is sound, but it would be good to have multiple cores to confirm that all the Chengjiang organisms come from the same environment, says at the Nanjing Institute of Geology and Palaeontology in China.

Living near a delta would have had advantages, says Ma. The delta water would have been rich in nutrients and minerals, and the shallow sea offered plenty of oxygen. But it also came with stressors, like the floods and changing levels of salt in the water.

The shifting environment may have helped drive the rapid evolution in the Cambrian, says Ma. 鈥淚f it鈥檚 a very stable environment, what animals do is already sufficient in that environment, hence there is no force to move them,鈥� she says. In contrast, unstable environments encourage evolutionary change over the generations.

Zhao is sceptical of this. 鈥淭he sedimentary environment of the Chengjiang biota does not tell us much of an evolutionary story,鈥� he says, because other Cambrian fossils show animals were also living in deeper regions.

In a separate study, , Zhao and his colleagues have re-examined a Chengjiang fossil called . It was a soft-bodied animal, roughly cone-shaped and up to 7.1 centimetres long.

The researchers tried to figure out what animal group C. striata belonged to. They knew it was a lophotrochozoan, but that doesn鈥檛 narrow it down much because the group includes like earthworms, molluscs like snails and octopuses, and many others.

They concluded that C. striata has traits of several lophotrochozoan groups, suggesting it resembled a shared common ancestor. It had a coiled set of tentacles, like those of horseshoe worms, but it also had an outer mantle that was undivided, like that of a shelled animal called a brachiopod. The implication is that C. striata was close to the common ancestor of horseshoe worms and brachiopods.

That is possible, but 聽because they are all so different, says Ma.