Вадим Дудченко
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Using correlative neutron and X-ray tomography, paleontologists have examined an exceptionally preserved specimen of Sigaloceras enodatum — a species of ammonite that lived some 165 million years ago (Middle Jurassic epoch) — from Claydon Pike pit, Fairford, Gloucestershire, UK.

Backlit shell of Sigaloceras enodatum with visible organs. Image credit: Cherns et al., doi: 10.1130/G49551.1.

Ammonites are an iconic extinct group that once thrived in oceans as dinosaurs ruled the Earth.

They became extinct at the end of the Cretaceous period, around 66 million years ago

They are among the most common fossils worldwide, but 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. Thus, ammonite fossils with preserved muscles and organs are extremely rare.

The modern animal Nautilus has traditionally been used as a model for reconstructing the biology of ammonites, but a new study led by Cardiff University researcher Lesley Cherns suggests the two groups may not be as similar as previously thought.

In the study, Dr. Cherns and colleagues studied the 5-cm-wide specimen of Sigaloceras enodatum which was found within an exposed Jurassic sediment at a Gloucestershire site in 1998.

They looked at the fossil’s remaining soft tissues and scarring where muscles once attached to the inside of its shell.

By combining high-resolution X-ray and high-contrast neutron imaging, they created a detailed 3D computer reconstruction of the structure, size and orientation of its muscles and organs.

From this detailed model, they were able to infer the functions of the muscles and organs.

“Preservation of soft parts is exceptionally rare in ammonites, even in comparison to fossils of closely related animals like squid,” Dr. Cherns said.

“We found evidence for muscles that are not present in Nautilus, which provided important new insights into the anatomy and functional morphology of ammonites.”

“We combined high-resolution X-ray imaging with high-contrast neutron imaging to study the interior of the fossil in 3D without damaging it,” said Dr. Alan Spencer, a researcher at Imperial College London and the Natural History Museum, London.

“This allowed us to visualize internal soft parts that had resisted all previous efforts to describe them. It’s a major breakthrough in ammonite paleobiology.”

“Our study suggests that combining different imaging techniques can be crucial for investigating the soft tissues of 3D fossils,” added Dr. Imran Rahman, principal researcher at the Natural History Museum, London.

“This opens up a range of exciting possibilities for studying the internal structure of exceptionally preserved specimens. We will be busy!”

The results appear in the journal Geology.

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Lesley Cherns et al. Correlative tomography of an exceptionally preserved Jurassic ammonite implies hyponome-propelled swimming. Geology, published online December 7, 2021; doi: 10.1130/G49551.1

 



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