Beginning approximately 250 million years ago, during the Mesozoic era, carnivorous marine reptiles known as ichthyosaurs dominated the shallow seas surrounding the supercontinent of Pangea.
Thanks to their abundance and the excellent bone-preserving properties of ocean sediments, fossils from dozens of ichthyosaur species, ranging in size from 0.3 to 21 meters (1 to 69 feet) in length, have been discovered across the world since the order was first described by paleontologists.
Yet a newly analyzed specimen, previously gathering dust in a private collection after it was unearthed in Yorkshire in 2010, has provided a University of Manchester researcher duo with fresh insights into one particular aspect of ichthyosaur life – parenthood.
The partial skeleton is from a female ichthyosaur of unknown total size that was carrying six to eight embryos at the time of her death. According to the team’s paper, published in the Proceedings of the Yorkshire Geological Society, the remains are the first example of a pregnant ichthyosaur found in the fossil-rich Jurassic layer of Yorkshire rock. Radiocarbon dating of the layer suggests the fossil is about 180 million years old.
Like plesiosaurs, the other order of air-breathing marine reptiles that existed alongside dinosaurs, ichthyosaurs evolved from a four-legged terrestrial ancestor that went back into the sea. Over time, as the animals spent more and more time in the water, the legs morphed into flippers, thus resulting in the fish-like appearance (ichthyosaur is Greek for “fish lizard”) of fully aquatic ichthyosaurs.
Yet unlike other reptile lineages, ichthyosaurs were likely warm-blooded and gave birth to live young, as this fossil demonstrates. Due to its imperfect preservation, authors Mike Boyd and Dean Lomax were unable to determine the exact number of embryos that the fossil’s internal contents represent, though they are fairly confident there are eight.
"We also considered the possibility that the tiny remains could be stomach contents, although it seemed highly unlikely that an ichthyosaur would swallow six to eight aborted embryos or newborn ichthyosaurs at one time,” said Boyd in a statement. “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 seem in Early Jurassic ichthyosaurs, such as the remains of squid-like belemnites."
Pregnant ichthyosaur specimens from a total of eight different species have been found previously. Embryo-bearing Stenopterygius are particularly common; more than 100 examples have been extracted from dig sites in Germany, showing a range of one to 11 embryos. Boyd and Lomax speculate that the new partial specimen could also be a Stenopterygius because it dates to the same period.
New species or not, the intriguing fossil is now on display to the public at the newly opened Yorkshire's Jurassic World exhibit at the Yorkshire Museum.