An analysis of the fossilized remains of an ichthyosaur – an aquatic reptile of the Jurassic era – has revealed the presence of between six and eight small embryos packed inside their ribs. It is the earliest evidence of embryos of ichthyosaurs that have been found in the British Isles.
The fossil, embedded within a small rock that broke in half, was discovered eight years ago near Whitby in North Yorkshire. Its owner, Martin Rigby, had the feeling that some of the small fragments of bones contained in the fossil could be traces of unborn embryos, so he caught the attention of paleontologists at the University of Manchester Mike Boyd and Dean Lomax, who took a closer look.
Their research, published this week in the Proceedings of the Yorkshire Geological Society confirmed the presence of six embryos and the probable remains of two others, each approximately four inches long. The size of the mother could not be firmly established due to the poor preservation of her remains, but the ichthyosaurs grew to more than 13 feet (4 meters) in length. It is the first time that paleontologists have discovered the fossilized remains of a pregnant ichthyosaur (pronounced "ick-thee-oh-pain") in Yorkshire, and it is the first appearance of ichthyosaur embryos in the United Kingdom, dating from about 180 years ago. millions of years.
The ichthyosaurs were a group of carnivorous aquatic reptiles that fed on other reptiles, fish and marine invertebrates, such as the squid-like belemnites. These creatures did not lay eggs, but gave birth to babies, that is, they were "viviparous." This incredible adaptation meant that the ichthyosaurs did not have to leave the sea and maintain terrestrial physical attributes, such as legs and feet (unlike modern sea turtles, for example, that have to lay eggs on beaches). This meant that ichthyosaurs could live, reproduce and evolve exclusively in aquatic environments, allowing them to acquire distinctive dolphin characteristics 150 million years before the dolphins appeared on the scene (this is a great example of convergent evolution, where similar features appear in unrelated species).
Unlike dolphins, however, ichthyosaurs did not provide milk for their young. And as this new fossil testifies, the ichthyosaurs gave birth to large batches of young, while the dolphins tend to give birth to unique calves; It has been found that fossils of ichthyosaurs contain from one to 11 embryos. The multiple births among these Jurassic reptiles suggest that the conditions were difficult for the offspring, who probably had to fend for themselves in what was undoubtedly an intensely dangerous environment; the high volume of offspring meant that at least some could survive to sexual maturity.
Ichthyosaur fossils are abundant in the United Kingdom and Europe, but before this example, only five were discovered in Britain with embryos, and none with this amount of unborn offspring. During the analysis, Boyd and Lomax had to make sure that the embryos were not the result of cannibalism, and that the specimen was in fact pregnant when she died.
"We … considered the possibility that the small remnants might contain stomach contents, although it seemed very unlikely that an ichthyosaur could swallow six to eight aborted embryos or newborn ichthyosaurs at the same time," Boyd said in a statement. . "And this does not seem to have been the case, because embryos do not show erosion by stomach acids, and embryos are not associated with any stomach contents commonly seen in lower Jurassic ichthyosaurs, such as the remains of squid-like belemnites. "
As a final note, the first ichthyosaur fossil was discovered in 1846, and notably, it contained an embryo within its pelvic region. For years, paleontologists believed that this and later discoveries were a sign of cannibalism, but they finally realized that these Jurassic reptiles were actually viviparous. Moving from an egg-laying creature to one that gave birth to a living young man was a monumental evolutionary leap, but as Jeff Goldblum said in Jurassic Park "Life finds a way".
[Proceedings of the Yorkshire Geological Society]