Tuesday, January 20, 2015

Ancient Scorpions may have Lived in Water but Walked on Land

Paleontologists have hypothesized that scorpions evolved on the seafloor, but some recently discovered fossils have scientists reconsidering exactly how it happened. Eramoscorpius brucensis, possibly the second oldest known scorpion at 430 million to 433 million years old, appears to have lived primarily in water and occasionally spent time on land, putting scorpions on dry ground much earlier than previously thought.

There’s a clear evolutionary advantage to spending time on land – scorpions molt, and they’re extremely vulnerable during the process; leaving the sea would allow them to avoid predators until their new exoskeleton formed. However, walking on dry ground requires having land-legs.

Before E. brucensis, ancient scorpions had leg structures similar to those of crabs (horseshoe crabs are one of the closest living relatives of ancient scorpions), and, like crabs today, moved around on the tips of their foot segment. They would’ve needed water to help support their weight. E. brucensis had a leg structure similar to what scorpions have today; meaning their legs could’ve supported their body on land. So why not just live on dry ground? Their digestive systems lacked coxapophyses, which helps modern scorpions consume prey on land, suggesting E. brucensis still needed to eat in the water.

Further evidence of this dual existence comes from the quarries in Ontario where most of the fossils are from. They’re filled with fossils of sea creatures, suggesting E. brucensis mostly lived in the water as well. However, the rocks surrounding the scorpions have ripples that imply brief exposure to air. Also, the position of the fossil scorpions suggests empty molted exoskeletons rather than a carcass, and it’s much easier for exoskeletons to fossilize on land than in the sea.
If E. brucensis did indeed live in the water and walk on land, then scorpions developed their first adaption for living out of the sea much earlier than previously thought. The evolution of scorpions is still under debate and more research is needed.

Photo Credit: David Rudkin, Royal Ontario Museum

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