Explosion of Cambrian fossil molluscs unveiled in Australia

A treasure trove of 500-million-year-old fossils found in the Australian state of Queensland has been studied.

The findings, published in the journal PeerJ, sheds new light on the complex morphology of Plectronoceratids, an important group of molluscs from the late Cambrian period (539–485 million years ago).

The Cambrian saw an evolutionary avalanche known as the “Cambrian explosion” in which all the basic animal body plans – like the shelled arthropod ancestors of today’s insects and first vertebrates which led to humans – emerged. It also saw the Plectronoceratida – Earth’s first shelled cephalopods.

Today, cephalopods include octopuses and squid, and shelled cephalopods like the nautilus and cuttlefish.

Plectronoceratida is believed to be the ancestor of all subsequent cephalopod groups. They are characterised by cone-shaped shells which were either straight or slightly curved. Though very little is known about the tip or opening of the shells due to sparse and broken fossils.

While these creatures were usually tiny, their ancestors would become massive.

Endoceras, straight-shelled cephalopods which lived about 485–419 million years ago, are the longest known extinct cephalopods measuring up to 6 metres or more. Parapuzosia is an extinct type of ammonite (similar to today’s nautilus) which lived during the ‘Age of the Dinosaurs’ and could grow to a diameter of about 2 metres.

The largest cephalopod is the modern giant squid. The biggest of these mysterious deep-sea creatures measured approximately 13 metres.

More than 200 exquisitely preserved Plectronoceratid specimens are included in the latest analysis. This surpasses the total of all other Plectronoceratids studied previously.

The fossils were found at Black Mountain, 1,400 km northwest of the Queensland state capital Brisbane. They were unearthed in the 1970s and ‘80s by the late Australian palaeontologist Mary Wade.

This new analysis focused on the three-dimensional structure of the molluscs’ siphuncle – the strand of tissue which passes along the length of a cephalopod’s shell. The team examined the tiny ancient structures using 3D reconstructions including micro-CT scanning.

Cambrian shell cephalopod fossils on black background
All specimens whitened with NH4Cl. Credit: PeerJ (2024). DOI: 10.7717/peerj.17003.

A new species Sinoeremoceras marywadeae, named in honor of Mary Wade, has unusual features which add to the cephalopod evolutionary tree.

First author Dr Alexander Pohle, from the Ruhr University Bochum in Germany, pays tribute to Mary Wade.

“Would it not be for her, these faunas would still largely be unknown,” Pohle says.

The study deepens our understanding of Earth’s ancient oceans which were the birthplace of life as we know it today.

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