Discovery of new 430 million-year-old fossils challenges our understanding of ancient molluscs

A University of Leicester palaeontologist is part of a team of researchers who have unearthed two new fossils, which they have named Punk and Emo, revealing that ancient molluscs were more complex and adaptable than previously known.

Molluscs are one of life’s most diverse animal groups; they include snails, cephalopods, mussels and their relatives. Analysis of the rare 430 million-year-old fossils is challenging long-held views on the early origins of the group.

The fossils, dating from the Silurian period of geological time, were retrieved from deposits in Herefordshire that accumulated at a time when most of southern Britian was covered by sea.  They shed light on the molluscs’ complex evolutionary history and how they moved.

The discovery challenges the longstanding view that early molluscs from the group known as Aculifera – which include chitons and worm-like molluscs – were basic and primitive.

Instead, the ‘rebellious’ fossils – which have been named Punk ferox and Emo vorticaudum – show that these early molluscs possessed some unique features and were, in fact, quite complex and displayed a wider variety of forms and movement strategies than researchers knew existed in this group of early molluscs.  Researchers drew these conclusions by recreating the fossils in 3D using advanced imaging techniques, including X-ray scanning. 

The study, published in Nature, was led by Dr Mark Sutton, from the Department of Earth Science and Engineering at Imperial College London, working with collaborators at the University of Leicester, the University of Oxford, the Senckenberg Research Institute and Natural History Museum Frankfurt, and Yale University. 

Emeritus Professor David Siveter from the University of Leicester School of Geography, Geology & the Environment said: “These 430-million-year-old fossils demonstrate the crucial importance of so-called ‘exceptional preservation’ (in which soft-parts of the animal are fossilised) to the interpretation of the evolution of life, in this case shedding new light on the early history of one of the fundamental branches of the evolutionary tree of a major group of life on Earth, the molluscs.”

Dr Sutton said: “Molluscs are one of the largest and most diverse animal groups on Earth. However, early Aculiferan molluscs are much less well-known than some of their relatives. We have limited information about this group, and for a very long time, we assumed they were rather basic, simple and primitive. 

“Retrieving fossils that are so exceptionally well preserved and reveal details of the soft tissues is extremely rare. We have been able to create ‘virtual fossils’ – 3D digital models – providing us with a gold mine of information and helping us understand that the branch of molluscan evolution containing Emo and Punk was much more evolutionarily rich and diverse than we thought; as much as other mollusc groups.”

The team used two different methods to get a clear picture of the fossils both inside and out. First, they used X-ray scanning to get a detailed look at the internal structures without causing any external damage. They then carefully ground the fossils down in very thin layers, taking photos at each step to create a 3D image of the external features.

Researchers found both fossils had smooth undersides, suggesting that they lived on the sea floor, and they both possessed some unique features and unconventional movement strategies. 

The Emo fossil is preserved in a folded posture, suggesting that it moved like an inchworm, using its spines to grip and push forward. Meanwhile, how Punk was able to move remains unclear to researchers, but they found it had a ridge-like foot, unlike any existing mollusc today.

“The names Punk and Emo were actually our initial pet names for these ancient molluscs, inspired by some of their unique features and individuality. Punk in particular, with its spiky appearance, clearly resembles a rebellious punk rocker – and we thought Emo complemented it well,” said Dr Sutton.

While Punk resembles worm-like molluscs with long spines, it also has a broad foot and gills like chitons. Emo, similarly worm-like with a long body and spines, also features shells and a compressed body similar to chitons.

This mix of features helps researchers better understand the mollusc evolutionary tree – pointing to a story that involves more complexity and diversity than previously thought.