Researchers, including scientists from the University of Oxford, have made a significant discovery that sheds light on the origins of various animals, such as oysters and octopuses. A new species of mollusc, named Shishania aculeata, has been unearthed, providing valuable insights into the early evolution of molluscs over half a billion years ago.

The fossils of Shishania, found in well-preserved specimens from eastern Yunnan Province in southern China, date back approximately 514 million years to the early Cambrian period. These fossils reveal that the earliest molluscs, devoid of a backbone, were flat, shell-less slugs adorned with a protective spiny armor.

Remarkably, some of the fossils were preserved upside down, exposing the naked underside of the creature, which featured a muscular foot similar to that of a slug. This suggests that Shishania would have used its foot to creep across the seafloor. Initially referred to as "the plastic bag" due to its unique appearance, the discovery of Shishania provides a rare glimpse into a period in mollusc evolution for which only a few fossils exist.

Associate professor Luke Parry, corresponding author and expert from the department of earth sciences at Oxford, emphasizes the significance of understanding the common ancestor of diverse animals like squids and oysters. Parry explains that this evolutionary puzzle cannot be solved solely by studying present-day species. However, Shishania offers unprecedented insights, suggesting that the earliest mollusc ancestors were spiny slugs armored for protection, predating the evolution of modern shell structures found in snails and clams.

The Shishania species, which measures only a few centimeters in length, is covered in small spiked cones made from chitin, a material also found in the shells of crabs, insects, and certain mushrooms. Unlike most molluscs, Shishania lacked a shell covering its body, indicating that it represents an early stage in the animal's evolution. This finding highlights the rapid diversification of molluscs, with the group evolving into various forms including snails, clams, squids, and octopuses.

Guangxu Zhang, the first author of the study and a recent PhD graduate from Yunnan University in China, recalls their initial impression of the fossils as strange and spiny. Zhang affectionately named them 'the plastic bag' due to their resemblance to a rotting piece of plastic. However, upon closer examination, it became evident that they were indeed fossilized molluscs.

The spines of Shishania display internal canals, each less than a hundredth of a millimeter in diameter. This indicates that the cones were secreted by tiny cell protrusions at their base, similar to how cells in our intestines aid in food absorption. Researchers describe this secretion process as resembling a natural 3D printer, enabling invertebrate animals to create hard parts with various shapes and functions, from defense mechanisms to facilitating movement.

Co-author Jakob Vinther from the University of Bristol underscores the significance of Shishania in understanding the common ancestor of all present-day molluscs, which would have had a single shell. The discovery provides crucial insights into a very early stage in mollusc evolution preceding the development of shells.

The findings of this groundbreaking research have been published in the reputable scientific journal Science, further contributing to our understanding of the fascinating ancient history and evolution of molluscs.