Chuck Bednar for redOrbit.com – Your Universe Online
An astonishingly complete aquatic scorpion fossil was discovered in Ontario this week, and provides evidence that the ability to walk unsupported by water appeared far earlier in the fossil record than previously believed, according to a new study.
Janet Waddington and David Rudkin from the Royal Ontario Museum (ROM) and Jason Dunlop of the Museum für Naturkunde in Berlin, Germany explain that the fossils, which were collected from the 430-million-year-old Eramosa Formation Konservat-Lagerstätte in Ontario, demonstrate that this key prerequisite from living on land was already evolving by this time.
Writing in the journal Biology Letters, the authors explain that the species, which are the oldest known scorpions in North America, exhibits a leg morphology that includes a short tarsus, which is a common feature of more modern scorpions. Because of this adaptation, the creatures likely had the ability to support its weight on a subterminal foot-like appendage, they noted.
Because of the intriguing similarities between the fossilized scorpions and the modern types of the creatures, the authors claim that the creature’s short foot could have been placed flat on the ground. That feature, along with the sturdy attachment of the legs to its body, would have made it possible for the scorpions to support their own weight without the buoyancy of water.
Based on their analysis of the remains, as well as the presence of fossils belonging to other animals that lived only in the sea, Waddington, Rudkin and Dunlop reported that the scorpions were most likely primarily aquatic creatures that only ventured into extremely shallow waters on rare occasions.
The fact that the fossils occur on rock surfaces that show ripples are indicative of brief exposure to air, the museum said, and since the scorpions are preserved in a splayed posture suggest that they present empty molted exoskeletons rather than carcasses of an animal that died.
One explanation is that the scorpions used their leg structure to make brief forays into a temporarily exposed area in order to molt, where they would be safe from predators such as large eurypterids and cephalopods. Afterwards, they returned to deeper water, the museum said.
The fossil scorpions range in size from approximately 29 millimeters to 165 millimeters in length and are representative of several different age categories. All of the specimens originated from the Bruce Peninsula, and found their way to the ROM in several different ways.
For instance, museum officials said that one of the specimens had been found by a young fossil hunter in a quarry spoil, while some were discovered by quarry workers. Others had been found in quarried stone delivered to landscaping projects located far away, they added.
“This extraordinary find contributes to our understanding of how scorpions moved from the sea onto land,” said Waddington, a departmental associate in the ROM’s Department of Natural History. “It is the enthusiasm and generosity of amateur fossil collectors that allows us to study and publish these findings, which are vital to the ROM’s collections and research.”