The origins of the octopus are revealed in a half-billion-year-old new species

A discovery of a 500-million-year-old spiny slug has provided insights into the origins of animals like oysters and octopuses. Researchers, including those from the University of Oxford, have identified a new species of mollusc named Shishania aculeata. This fossil indicates that the earliest molluscs were flat, shell-less slugs with spiny armor for protection.

The fossils, some preserved upside down, revealed that the bottom of the creature was exposed, featuring a muscular foot similar to modern slugs, which it likely used to move across the seafloor. Shishania was initially nicknamed “the plastic bag” due to its appearance.

Associate Professor Luke Parry from Oxford’s Department of Earth Sciences highlighted the significance of this find, explaining that understanding the common ancestor of diverse animals like squid and oysters is challenging without fossil evidence. Shishania offers a glimpse into a crucial period in mollusc evolution, showing that early mollusc ancestors were armored spiny slugs before the evolution of the shells seen in contemporary snails and clams.

These well-preserved fossils were found in eastern Yunnan Province, China, dating back to the early Cambrian period, around 514 million years ago. The Shishania specimens, only a few centimeters long, were covered in small spiky cones made of chitin, the same material found in the shells of modern crabs, insects, and some mushrooms.

Unlike most molluscs, Shishania lacked a shell, indicating its place in a very early stage of mollusc evolution. Today, molluscs exhibit a wide variety of forms, including snails, clams, and even intelligent species like squids and octopuses. This diversity emerged rapidly, leaving few fossils to document the early stages of mollusc evolution.

Guangxu Zhang, the first author and a recent PhD graduate from Yunnan University, discovered the specimens. Initially, he thought the thumb-sized fossils were unremarkable but noticed under a magnifying glass that they were spiny and unique. Further analysis in the lab confirmed them as molluscs, despite their initial “plastic bag” appearance.

The spines of Shishania feature an internal canal system less than a hundredth of a millimeter in diameter. These cones were secreted at their base by cell protrusions that increase surface area, similar to how our intestines aid in food absorption. This secretion method resembles a natural 3D printer, enabling invertebrate animals to produce hard parts with diverse shapes and functions, from defense to movement facilitation.

Co-author Jakob Vinther from the University of Bristol noted that the common ancestor of modern molluscs had a single shell, and Shishania provides insight into an earlier stage of mollusc evolution before the development of a shell.