Remarkable Jurassic Fossil Reveals 165-Million-Year-Old Ammonite Muscles in 3D

3D Ammonite Model

3D model of an ammonite. Credit: Lesley Cherns et al.

  • Beautifully preserved fossil ammonite collected from 165-million-year-old Jurassic site in Gloucestershire, UK;
  • 3D reconstruction of combined neutron and X-ray images of fossil shows internal muscles never previously seen in ammonites;
  • Muscles used to retract body into the shell for protection;
  • Ammonites swam through jet propulsion, like modern octopus and squid.

An exceptionally preserved Jurassic fossil from Gloucestershire in the UK has given new insights into the inner workings of ammonites, according to a study published recently in the journal Geology.

Researchers created a detailed 3D computer reconstruction from combined neutron and X-ray images of the unique 165-million-year-old fossil ammonite, which allowed them to describe its internal muscles.

Ammonites are an iconic extinct group that thrived in ancient oceans hundreds of millions of years ago, when the dinosaurs ruled on land. They went extinct at the end of the Cretaceous period, around 66 million years ago.

Ammonites are among the most common fossils worldwide, but almost everything we know about them is based on their hard shells. The modern animal Nautilus has traditionally been used as a model for reconstructing the biology of ammonites, but the study suggests the two groups may not be as similar as previously thought. This is revealed by the muscles and organs within the ammonite body, visualized in three dimensions for the first time in the new fossil.

Paired muscles extending from the body mass are thought to have been used to retract the animal deep into the body chamber for protection. This would have been an important anti-predator adaptation in ammonites, which lacked other defensive features, such as an ink sac, seen in modern relatives like octopus, squid, and cuttlefish.

The arrangement of the muscles suggests ammonites swam by expelling water through the tube-like siphon, called a hyponome, next to the opening to the body chamber. This type of swimming is termed jet propulsion, and it is used by a wide range of living animals, including cephalopods (squid, octopus, cuttlefish, and Nautilus), the larger group to which ammonites belong.

Lead author Dr. Lesley Cherns, Honorary Research Fellow at Cardiff University, said, “Preservation of soft parts is exceptionally rare in ammonites, even in comparison to fossils of closely related animals like squid. We found evidence for muscles that are not present in Nautilus, which provided important new insights into the anatomy and functional morphology of ammonites.”

The fossil was discovered more than 20 years ago and has been studied on and off ever since, but only recent technological innovations allowed its internal structures to be visualized. The research team used a combination of cutting-edge techniques, including instruments at ISIS Neutron and Muon Source at Harwell and the Henry Moseley X-ray Imaging Facility at the University of Manchester, to digitally reconstruct the inside of a “virtual ammonite.”

Co-author Dr. Alan Spencer, Senior Strategic Teaching Fellow at Imperial College London and Scientific Associate at the Natural History Museum, said, “We combined high-resolution X-ray imaging with high-contrast neutron imaging to study the interior of the fossil in 3D without damaging it. This allowed us to visualize internal soft parts that had resisted all previous efforts to describe them. It’s a major breakthrough in ammonite palaeobiology.”

Dr. Imran Rahman, a co-author and Principal Researcher at the Natural History Museum, added, “Our study suggests that combining different imaging techniques can be crucial for investigating the soft tissues of three-dimensional fossils. This opens up a range of exciting possibilities for studying the internal structure of exceptionally preserved specimens. We will be busy!”

Co-author Dr. Russell Garwood, a senior lecturer at the University of Manchester and Scientific Associate at the Natural History Museum said, “It has taken over 20 years of patient work and testing of new non-destructive fossil scanning techniques, until we hit upon a combination that could be used for this rare specimen. This highlights both: the importance of our national museum collections which permanently hold and give access to these important specimens; and the pace of technological advances within paleontology over recent years.”

Dr. Genoveva Burca, neutron imaging and diffraction scientist at the ISIS Neutron and Muon spallation source and one of the co-authors said, “The outcome of this exciting project shows the advantages of a creative and interdisciplinary approach, the huge potential of neutron imaging applications and use of complementary non-destructive techniques which can a be a real game changer in many areas of scientific investigations including Palaeontology broadening its horizon and taking the research in this field to a whole new level.”

Dr. Neville Hollingworth, a co-author and Public Engagement Manager at the Science and Technology Facilities Council, discovered the fossil. He added, “When I found the fossil, I immediately knew it was something special. The shell split in two and the body of the fossil fell out revealing what looked like soft tissues. It is wonderful to finally know what these are though the use of state-of-the-art imaging techniques.”

Reference: “Correlative tomography of an exceptionally preserved Jurassic ammonite implies hyponome-propelled swimming” by Lesley Cherns, Alan R.T. Spencer, Imran A. Rahman, Russell J. Garwood, Christopher Reedman, Genoveva Burca, Martin J. Turner, Neville T.J. Hollingworth and Jason Hilton, 7 December 2021, Geology.
DOI: 10.1130/G49551.1

This work was carried out by a team of researchers from Cardiff University, Imperial College London, the Natural History Museum, University of Manchester, ISIS Neutron and Muon Source, the Science and Technology Facilities Council and the University of Birmingham.

The fossil is housed at the National Museum Wales.

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