The last meal of a very young ichthyosaur – a prehistoric sea-dwelling reptile – has been discovered, incredibly well preserved, in a fossil from the Jurassic.

Fully grown ichthyosaurs could grow to more than 3 metres long. A new-born ichthyosaur, only 70cm long, was found to have snacked on an ancient species of squid just before it came to the end of its short life. Exactly how the creature died is not known.

"It is amazing to think we know what a creature that is nearly 200 million years old ate for its last meal. We found many tiny hook-like structures preserved between the ribs. These are from the arms of prehistoric squid," said study author Dean Lomax of the University of Manchester.

The researchers at the University of Manchester used a micro CT-scanner to create a three-dimensional model of the inner and outer structure of the fossil. The researchers used medical imaging software to see inside the ichthyosaur's tail, middle and head to create a full model, revealing the unexpected shape of a squid inside.

The discovery of the contents of the creature's stomach overturns established ideas about how baby ichthyosaurs fed.

"This is interesting because a study by other researchers on a different type of ichthyosaur, called Stenopterygius, which is from a geologically younger age, found that the small – and therefore young – examples of that species fed exclusively on fish. This shows a difference in prey-preference in new-born ichthyosaurs," said Lomax.

This particular specimen was rediscovered in the collections of the Lapworth Museum of Geology at the University of Birmingham. Sadly there are no records of the date of original discovery or location where the specimen was found. Even without this information, the find is rare.

"There are several small Ichthyosaurus specimens known, but most are incomplete or poorly preserved. This specimen is practically complete and is exceptional. It is the first newborn Ichthyosaurus communis to be found," said Lomax.

The fossil is now on display at the Lapworth Museum of Geology at the University of Birmingham. The discovery is described in a study in the journal Historical Biology.