Exceptionally preserved ammonite shows its inner soft tissue in 3D

Ammonites are a type of cephalopod that thrived in the oceans during the Mesozoic Era. They became extinct at the same time as non-avian dinosaurs about 66 million years ago.

Scientists have learned a lot about these marine animals through their fossils - their hard shells are a common find on beaches and in rocks. However, very little is known about the soft body which rarely preserves well.

Dr Imran Rahman, a principal researcher at the Museum, says, 'Ammonites are an iconic extinct group of marine animals renowned for their rich fossil record that stretches back hundreds of millions of years. Their hard shells made of calcium carbonate preserve well, however, important details of the living animal were largely still a mystery.

'In almost all cases, it's only the hard shell - not the soft parts - preserved in fossil ammonites. On the rare occasions where soft parts have been found, they are mostly flattened.'

But 23 years ago an extraordinary fossil ammonite was discovered. It has given researchers a rare insight into how the soft tissue of these invertebrates functioned.  

Neville Hollingworth, an avid-fossil hunter, discovered the exceptionally preserved ammonite in an open gravel pit in Gloucestershire in 1998. Neville was experienced enough to recognise it was extraordinary and allowed it to be studied in detail.

Since then, the ancient fossil has been analysed using a variety of techniques. Scientists learned the fossil contained copious amounts of soft tissue - more than any other ammonite found - and in 3D form. This made the ammonite unique.

Imran says, 'This is the first specimen where we have substantial three-dimensional preservation of internal organs and muscles, which is why it's so fascinating.'

Because of this, scientists only examined the exterior as they were unwilling to break the fossil open.

'The specimen has been known for more than two decades and the research team has been searching for ways of looking inside the fossil without damaging it,' continues Imran.

'Taking photos of the exterior has provided some information because the shell is partly translucent. This is how we were able to identify that there was something special inside, but that only gave us a partial picture.'

While CT scanning has been performed, this did not provide enough information to describe the inner features. The soft body and the shell are both preserved rich in calcium carbonate, making it difficult to distinguish between the two in these scans.

Ammonite tissue examined by neutron imaging

Recently, a group of researchers led by Lesley Cherns from Cardiff University teamed up with scientists at the Museum and other institutions to analyse the fossil again.

This time, the difference was the state-of-the-art technology available. In addition to CT scans, the team also carried out neutron scanning, which can be used to differentiate various materials with similar compositions.

Using this approach, the team were able to analyse the internal muscles within the shell for the first time. This also allowed them to digitally reconstruct the muscles and organs in 3D. Both processes gave them a better understanding of how the sea creature moved and protected itself.

'Modern technology has allowed us to fully describe those soft parts for the first time ever,' says Imran. 'It provided three-dimensional information on the internal structures and, based on that, we were able to reconstruct this wonderfully detailed 3D computer model of the specimen, which allowed us to describe its anatomy.'

Dr Genoveva Burca, neutron imaging and diffraction scientist at the ISIS Neutron and Muon spallation source and one of the co-authors says, ‘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.’

Neutron scans glean new information from ancient ammonite

The scans confirmed ammonites moved using their hyponome - a muscular, funnel-like opening which expelled water - allowing the creature to push itself backwards through the ocean. A similar swimming mechanism is used by many modern cephalopods, such as squids and octopuses. 

The scans also showed ammonites had dorsal muscles, which run along the back of the animal, and worked in pairs. This allowed the animal to retract deep into its shell for protection. This is particularly important as they lacked defensive mechanisms found in their modern relatives, such as ink sacs found in cuttlefish.

Scientists have often compared ammonites to the living Nautilus, mostly due to the similarity in their shells. However, the study shows the extinct sea creatures may have more similarities with coleoids, the sub-group of animals containing squid, octopuses, and cuttlefish.

'The arrangement of the muscles in the ammonite is different to Nautilus, which indicates the two groups swam in different ways,' explains Imran. 'We’ve got direct evidence of muscles that had previously been hypothesised.'

In Nautilus, the retractor muscles and hyponome work together to enable jet propulsion, but in ammonites they worked independently. Scientists think this trait evolved early in the ammonoid-coleoid lineage.

'Although it's taken a couple of decades to get there, we now have suitable techniques available to study fossils like this one without causing any damage,' says Imran. 'It shows there's always a balance between the immediacy of studying a specimen and what might be possible in future, which is hard to predict but worked out in this case.'

This is the first extensive 3D preservation of ammonite soft tissues in the body chamber documented through CT scanning. 

The specimen is housed at the National Museum Wales in Cardiff and the study has been published in Geology, an online science journal. The authors of the study are Lesley Cherns, Alan Spencer, Imran Rahman, Russell Garwood, Christopher Reedman, Genoveva Burca, Martin Turner, Neville Hollingworth and Jason Hilton.