By Will Dunham
WASHINGTON (Reuters) – Earth’s approximately 76,000 species of molluscs come in an impressive variety of forms, including clams, oysters, scallops, mussels, snails, slugs and even some that possess exceptional intelligence, such as octopuses, cuttlefish and squid. But its ancestral form and early evolutionary history have been difficult to decipher.
Fossils discovered in southern China of a curious little sea creature that lived during the Cambrian Period about 514 million years ago — essentially a spiny slug — are now providing some clarity about the early stages of the mollusc lineage.
The newly identified species, called Shishania aculeata, had a flat, oval-shaped body, averaging just over 3 cm long and 2 cm wide. Some of the 18 specimens described by researchers preserved soft body parts that rarely fossilize, providing an unusually detailed account of their anatomy.
The upper part of its body was densely covered in hollow, cone-shaped spines – similar to those on the durian fruit, native to Southeast Asia – for protection against predators. The spines are made of chitin, the same material as crab shells.
“On the bottom, we see a ring of tissue called a girdle that surrounds an organ called the foot. This is the same feature of the slimy, muscular sole that you see in slugs and snails. It would have used this to crawl along a muddy seabed, such like slugs and snails do today on land,” said paleontologist Luke Parry, from the University of Oxford, one of the leaders of the study published in the journal Science.
“It may have fed on algae and other organic matter in a shallow marine environment,” added paleontologist Xiaoya Ma, from Yunnan University in China and the University of Exeter in England, another of the study leaders.
The anatomical features of the lower part of its body demonstrated that it is one of the earliest known members of the mollusk lineage.
Molluscs are a diverse group of invertebrates, second in size in the animal kingdom only to arthropods, a group that includes insects, spiders, lobsters, crabs, centipedes, millipedes and others. Molluscs have soft bodies composed almost entirely of muscles, have a well-organized nervous system, and are usually protected by a shell. Those that do not have a shell, like squid, come from lineages that previously had it.
“So this new fossil records what molluscs looked like before they developed their shells,” Parry said. “This tells us that the first mollusks were covered in protective spines. We found evidence of the cellular mechanism by which Shishania secreted their protective spines by observing them with an electron microscope. We found that they contained tiny elongated channels less than a thousandth thick . one millimeter in diameter.”
The group of invertebrates that includes earthworms also has this type of secretion system.
The Shishania remains were found by the study’s lead author, Guangxu Zhang, when he was a doctoral student at Yunnan University, salvaging fossils from the earth excavated from a road construction project in Yunnan province.
“I saw under the magnifying glass I had with me that the fossils looked strange, spiny and completely different from any other fossils I had seen,” Zhang said.
Other fossils at the site included sponges and horseshoe crab-like trilobites that shared the marine kingdom of Shishania.
The great diversity among modern molluscs, in body shape and lifestyle, has made it difficult to elucidate their last common ancestor and first evolutionary steps. Its diversity evolved rapidly during an evolutionary event called the Cambrian Explosion, a critical moment in the history of life on Earth when a dizzying array of animals first arrived on the scene.
Parry said Shishania should be seen as “an evolutionary aunt or cousin” of today’s molluscs, maintaining a more primitive body plan than the last real common ancestor for all now-living members of the group.
“I think it’s amazing that we can trace animals that are smart enough to use tools like octopuses back to their humble, slug-like beginnings more than half a billion years ago,” Parry said.
(Reporting by Will Dunham, Editing by Rosalba O’Brien)
