The new images have allowed the team to analyse the muscles and organs of an ammonite for the very first time, throwing new light on how the cephalopod mollusc was able to swim through the oceans and defend itself from predators.
Previous investigations using solely X-ray techniques were unable to determine the structure of the soft tissues. By combining these results with neutron imaging on the IMAT instrument at ISIS, the researchers were able to study the fossil in unprecedented detail.
The team were the first UK users of the IMAT beamline and, by combining the high-contrast neutron imaging collected here with high-resolution X-ray imaging at the Henry Moseley X-ray Imaging Facility, they created a detailed 3D computer reconstruction of the muscles and organs inside the ammonite fossil based on the remnant soft tissues and muscle scars inside the shell. From this detailed model of muscle structure showing relative size and orientation, they were able to infer their functions.
The arrangement and relative strength of muscles reveal that ammonites swam by jet propulsion using the hyponome – a muscular tube-like funnel through which water is expelled – as found today in other cephalopods like modern squid and octopuses.
Paired muscles from the ammonite body enabled the ammonite to retract itself deep into its shell for protection, the images also revealed This would have been important since ammonites lacked defensive features such as the ink sac found in modern squid and cuttlefish, and the plate-like hood of Nautilus.
Publishing their findings today in Geology, the team say the findings add more insight and evidence that ammonites might be evolutionarily closer to coleoids, the sub-group of animals containing squid, octopuses and cuttlefish, than previously thought.
Up until now, because ammonite soft tissues are so rarely preserved, scientists have used the modern cephalopod Nautilus, which still swims in our oceans, as a body-plan for reconstructing ammonite biology.
In both groups, a buoyant coiled shell houses the soft body. However, the new study highlights that ammonites and Nautilus may not be as similar as previously thought.
Lead author Dr Lesley Cherns, Honorary Research Fellow from Cardiff University's School of Earth and Environmental Sciences, 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."
Ammonites, which became extinct around 66 million years ago, once thrived in the oceans while dinosaurs ruled the Earth. They are among the most common fossils worldwide and are excellent index fossils for dating rocks, linking the rock layer in which a particular species or genus is found to specific geological time periods.
However, almost everything we know about them so far is based on their hard shells, as these are more easily preserved over millennia than bodily tissues. Finding ammonites with the fossilised remains of soft parts is virtually unknown in the fossil record.
This amazing ammonite was originally found within Jurassic rocks exposed at a site in Gloucestershire, UK in 1998. Found by Neville Hollingworth, who works for the public engagement team at STFC, the specimen is now housed at the National Museum Wales in Cardiff. It is only through recent advances in world class imaging technology at ISIS that the secrets of this fossil have finally been unlocked.
Neville says, “I am thrilled to see that, at long last, this ammonite has provided important new evidence on these enigmatic creatures that swarmed in Jurassic seas. Breaking open the enclosing rock to reveal this incredible ammonite was a very lucky find indeed."
Study co-author Dr Alan Spencer, from Imperial's Department of Earth Science and Engineering and the Natural History Museum, said: “This ammonite is remarkably well preserved, which is very rare. New imaging techniques allowed us to visualise the internal soft parts of ammonites that have so far resisted all our previous efforts to describe them. This is 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 palaeontology 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.”
The work was carried out by a multi-institutional and interdisciplinary research team 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 full paper can be found online at DOI: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49551.1/610120/Correlative-tomography-of-an-exceptionally
