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Yorkshire’s first embryo-bearing ichthyosaur was pregnant with octuplets

A specimen described today in Proceedings of the Yorkshire Geological Society is the star attraction in the Yorkshire Museum’s new exhibition, Yorkshire’s Jurassic World, which opened on March 24.

Pregnant ichthyosaur (c) Nobumichi Tamura

The fossil, part of the skeleton of a 180 million-year old ichthyosaur, contains the remains of between six and eight tiny embryos between its ribs. Whilst ichthyosaur fossils are relatively common in the UK, only five have ever been found in Britain containing embryos. The new find contains more embryos than any previous examples, and is the first of its kind recorded from Yorkshire.

Dating back to the Toarcian Stage of the Jurassic, the specimen was collected around 2010 near Whitby, North Yorkshire. A small limestone boulder, it has been cut in half and polished, exposing several large adult ribs and several strings of vertebrae, along with various indeterminate tiny bones. It has since been acquired by the Yorkshire Museum from the collection of fossil collector Martin Rigby, and studied by palaeontologists Mike Boyd and Dean Lomax from the University of Manchester.

Close-ups of Block A, YORYM: 2016.316a, showing ichthyosaur embryos. Scale bars equal 10mm. (From Boyd, M. J. and Lomax, D. R. 2018. The youngest occurrence of ichthyosaur embryos in the UK: A new specimen from the Early Jurassic (Toarcian) of Yorkshire. Proceedings of the Yorkshire Geological Society)

Viviparity – the development of an embryo inside the body of the parent, as opposed to laying eggs – was first reported in ichthyosaurs in 1846 by Joseph Chaning Pearce (1811-1847). A surgeon and avid fossil collector, Pearce noted the presence of an embryo within the pelvic region of a specimen of Ichthyosaurus from the Early Jurassic (probably Hettangian) Blue Lias Formation of Somerset, and published a paper on his find. Others argued that such specimens represented cannibalism among ichthyosaurs – a debate which continued until the 1990s.

‘We also considered the possibility that the tiny remains could be stomach contents’ Mike said, ‘although it seemed highly unlikely that an ichthyosaur would swallow six to eight aborted embryos or new born ichthyosaurs at one time. And this does not seem to have been the case, because the embryos display no erosion from stomach acids. Moreover, the embryos are not associated with any stomach contents commonly seen in Early Jurassic ichthyosaurs, such as the remains of squid-like belemnites.’

Studied specimen: YORYM: 2016.316. A cut and polished boulder containing between six and eight ichthyosaur embryos, collected from Saltwick Bay, near Whitby, Yorkshire. (From Boyd, M. J. and Lomax, D. R. 2018. The youngest occurrence of ichthyosaur embryos in the UK: A new specimen from the Early Jurassic (Toarcian) of Yorkshire. Proceedings of the Yorkshire Geological Society)

Ichthyosaurs were aquatic reptiles that dominated the Jurassic seas. They were carnivores, feeding on other reptiles, fish and marine invertebrates such as belemnites. Of those documented specimens which have been discovered carrying embryos, by far the most commonly found species is Stenopterygius, over a hundred of which have been found in Holzmaden and surrounding areas in Germany, with between one to eleven embryos.

‘The German sites are approximately the same age as the new specimen from Whitby’ said Dean, ‘and it is possible that the new specimen is also Stenopterygius, but no identifiable features are preserved in the adult or embryos. Nonetheless, this is an important find.’

‘This is an incredible find’ said Sarah King, curator of natural science at the Yorkshire Museum. ‘The research by Dean and Mike has helped us confirm it is the first example of fossilised ichthyosaur embryos to be found in Yorkshire. Its display in Yorkshire’s Jurassic World incorporates the latest digital technology to reveal the embryos and to explain the significance of the discovery.’

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