This spiky fossil shows what early mollusks looked like

An animal that could be mistaken for a spiky fruit is giving scientists a peek into what mollusks looked like around 500 million years ago.

Fossils of an ancient invertebrate dubbed Shishania aculeata show that the animal was a sluglike creature covered in prickly armor, researchers report in the Aug. 2 Science. The find bolsters evidence suggesting that early mollusks lacked shells and were covered in spikes made of chitin, a fibrous material found in present-day crab and other mollusk shells (SN: 10/13/22). 

Today’s mollusks are an incredibly diverse group of animals, says paleobiologist Xiaoya Ma of Yunnan University in China. With living species as different as clams and octopuses, it’s tough to find common traits that indicate what the group’s earliest ancestors looked like. But “fossils can often provide unique and direct evidence” for how early mollusks appeared, Ma says.

The fossils, which were uncovered in China, date to around 510 million years ago following an early Cambrian period when there was a rapid burst of evolution for mollusk ancestors (SN: 6/11/94). Ma and colleagues examined a total of 18 specimens, ranging in size roughly from 1 to 6 centimeters long. Each specimen was “not always beautiful,” Ma says. Soft tissues like those in S. aculeata’s body don’t fossilize well. “But they preserved or compressed from different angles … [which] helps us put a jigsaw [puzzle] together to reconstruct the animal.”

S. aculeata’s base is flat, with a singular foot. This mollusk characteristic helps the animals scooch across the ground or dig into soft sediments. What’s more, the hollow chitin cones that make the organism resemble a durian fruit on the outside are filled with narrow canals that are “spectacular and extremely rare,” Ma says. These canals are similar to those found in the exoskeletons of extinct and living worms and brachiopods, suggesting a common origin.