The fossil collection of the Lower Saxony State Museum in Hannover, Germany, is full of hidden discoveries lately. A plesiosaur skeleton that had been gathering dust for years turned out to be a new species, and now on closer inspection, an Ichthyosaurus fossil in the museum's care has been found to not only be the largest individual specimen on record, but it was also pregnant.

Ichthyosaurs were roaming the ancient oceans as far back as 250 million years ago, and while they weren't technically dinosaurs, they did share the planet with them for hundreds of millions of years. These marine reptiles were the apex predators of their time, until eventually the plesiosaurs knocked them off the top spot.

This particular specimen was discovered along the UK's Somerset coast during the 1990s, but it hadn't been studied in detail until early this year. It was investigated by a pair of paleontologists, Sven Sachs of the Bielefeld Natural History Museum in Germany, and Dean Lomax of the University of Manchester, who identified the species and found a few interesting features hiding in the bones.

The skeleton belonged to an Ichthyosaurus somersetensis, and measuring between 3 and 3.5 m (9.8 and 11.5 ft) long, this particular one was the largest example of its species. The reason its size can't be pinned down more precisely is probably because the tail doesn't belong to the rest of the skeleton. It was added later, apparently to make the fossil more presentable to the public.

"It is often important to examine fossils with a very critical eye," says Sachs. "Sometimes, as in this instance, specimens aren't exactly what they appear to be. However, it was not 'put together' to represent a fake, but simply for a better display specimen. But, if 'fake' portions remain undetected then scientists can fall foul to this, which results in false information presented in the published record."

The other eye-catching feature of the fossil was the fact that the animal was pregnant at the time of its death. Preserved inside the abdomen is a 7-cm (2.8-in) section of backbone, as well as a tiny forefin, ribs and other bones. The baby's bones hadn't fully formed yet, indicating the embryo was still in the early stages of development.

"It amazes me that specimens such as this can still be 'rediscovered' in museum collections," says Lomax. "You don't necessarily have to go out in the field to make a new discovery. This specimen provides new insights into the size range of the species, but also records only the third example of an Ichthyosaurus known with an embryo. That's special."

The study was published in the journal Acta Palaeontologica Polonica.

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