Database: Prehistoric Life
Most of the sandstones in AlUla were formed by sediments of enormous rivers 500 million years ago. There was no life on land, or in the rivers at this time, so we don’t find any fossils. But towards the north of AlUla the sandstones were formed from sediments in shallow seas around 465 million years ago. And here, in the rocks, there is evidence of past life.
There are dozens of areas where we can find clues left by a huge number of species of marine animals. Footprints, burrows, and even feeding traces are incredibly well preserved.
Something spectacular in these rocks is the oldest fossil discovered in AlUla, which is an extinct species of horseshoe crab. This unique fossil has only been found here and nowhere else in the world. It is the largest horseshoe crab of the time, at least ten times larger than other species of the period.
It is also unusual because it is well preserved and upside down. From the fossils and the features in the rocks, we think that these horseshoe crabs were gathering in very large numbers, perhaps to mate, and large storms washed them onto the beach. Many were turned upside down and their bodies sunk into the sediment. Fortunately, most were apparently able to lift their body and tail and flip themselves. But sadly, some were never able to flip the right way round again. After they died, the body and tail usually snapped off, and their upside down heads were buried by sediment, and exposed 465 million years later.