Largest Ichthyosaurus Fossil Ever Discovered Contains An Unexpected Gift

An unstudied ichthyosaur fossil dating back to the Jurassic period is now the largest on record -- a remarkably well-preserved specimen that also contains the remnants of a developing fetus.

Artistic impression of Ichthyosaurus with embryo. (Image: Joschua Knuppe)

The female ichthyosaur, who was pregnant when she died some 200 million years ago, measured around 3.3m long, according to new research published earlier this week in Acta Palaeontologica Polonica. Found on the UK's Somerset coast back in the mid-1990s, the fossil languished in a museum until it was "re-discovered" by palaeontologist Sven Sachs of the Bielefeld Natural History Museum. Sachs recruited University of Manchester palaeontologist Dean Lomax to assist with a detailed analysis.

"It amazes me that specimens such as this [the biggest] can still be 'rediscovered' in museum collections," said Lomax in a statement. "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."

Image: Lomax & Sachs, 2017.

These dolphin-like marine reptiles went extinct 90 million years ago after an exceptionally long and successful run. Technically speaking, these animals are not dinosaurs, having emerged long before the reign of their terrible lizard cousins. One of the more interesting aspects of Ichthyosaurs is that, like mammals, they gave birth to live young. This represented a momentous evolutionary leap for these sea creatures, liberating them from having to lay eggs. A modern example of this evolutionary "limitation" can be seen in penguins, who are forced to live a dual existence, procreating on land and sustaining themselves in the water. Through evolution, the Ichthyosaurs stumbled on a biological solution.

Embryo of Ichthyosaurus somersetensis. (Image: Lomax & Sachs, 2017)

As noted in the new analysis, the fossil of the tiny Ichthyosaurus embryo -- only the third to ever be discovered -- is incomplete; it contains a portion of backbone, a forefin, ribs, and a jumble of other tiny bones. Its connection of vertebrae measures less than 7cm long. The foetus's bones hadn't yet ossified, meaning it was still in the midst of developing when the mother died. Analysis of the adult fossil confirmed it as belonging to the species Ichthyosaurus somersetensis, which has only recently been added to the scientific literature.

The analysis also revealed something quite curious about the fossil: It contained the tail and other body parts from a entirely different specimen. This was done by palaeontologists years ago, say the researchers, to make the fossil look presentable.

"It is often important to examine fossils with a very critical eye," said 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."

Ichthyosaurs are among the common fossil reptiles found in the UK, with thousands of specimens known. But this one -- overlooked for decades -- now represents the largest specimen of these fascinating sea creatures on record.

[Acta Palaeontologica Polonica]

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