The complete Shishania aculeata fossil resembles a durian fruit (left); its spines (right) are made of an organic compound called chitin.Credit: G Zhang/L Parry
From much-maligned garden slugs and snails to tool-using octopuses and bivalves such as clams and oysters, molluscs are among the most diverse animal groups on the planet. But their origins are a mystery.
A newly found mollusc fossil — which resembles a durian fruit sliced in half — offers clues as to what the earliest species looked like. “They were this sort of weird, spiny slug,” says Luke Parry, a palaeontologist at the University of Oxford, UK, who is part of the team that described the roughly 510-million-year-old fossils in Science on 1 August1.
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Like many other animal groups, molluscs exploded in diversity during the Cambrian period, 539 million to 485 million years ago, and nearly all the groups found on Earth now emerged during this time.
But this rapid pace of change makes it difficult to determine the characteristics of early molluscs, says Parry. “Just from looking at a modern clam and a modern octopus, it’s sort of hard to envision what their common ancestor might have looked like.”
The durian-fruit-like mollusc fossils, a couple of centimetres in diameter, were discovered on a road-building site in Kunming, China. Study co-author Guangxu Zhang, a palaeontologist at Yunnan University in Kunming who was then doing his PhD, thought the first specimen he found resembled a rotting plastic bag. “It wasn’t immediately important or striking,” says his adviser, Xiaoya Ma, a palaeontologist at the University of Exeter in Penryn, UK, who is also a co-author of the study.
An artist’s reconstruction of the Shishania aculeata mollusc.Credit: Reconstruction by M. Cawthorne
But further, better-preserved specimens revealed the creature’s soft underside, including features found in modern molluscs, such as a foot. The mollusc’s topside was covered in hollow spines made of chitin — an organic compound that also forms insect exoskeletons. The researchers named the species Shishania aculeata, after accomplished Yunnan province geologist Shishan Zhang, who is now 87.
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Hollow chitinous spines in early molluscs such as Shishania probably gave rise to calcium carbonate ‘spicules’ found in modern molluscs called chitons. The spines also seem to share an origin with chitinous bristles covering segmented worms in a distantly related invertebrate group called annelids.
“This is really another piece in the jigsaw puzzle,” says palaeobiologist Mark Sutton at Imperial College London. “It helps nail down our ideas about molluscan evolution and we’re finally getting a coherent story.”
The spines, which might also have been sense organs, probably helped Shishania and other early molluscs to avoid predators as they crept along Cambrian sea floor, says Parry. “The fact that we have any of these fossils is pretty amazing.”
