“A sea scorpion’s strike: new evidence of extreme lateral flexibility in the opisthosoma of eurypterids”

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W. Scott Persons IV and John Acorn

Researchers report prehistoric sea-scorpions had slashing tail spines

Flexing fossil reveals sea scorpion’s strike

A sea scorpion attacking an early vertebrate.
(Illustrated by Nathan Rogers; figure 2 in Persons & Acorn 2017)

Four hundred thirty million years ago, long before the evolution of barracudas or sharks, the scariest predators lurking in the primordial seas were eurypterids—better known as “sea scorpions”. A group of arthropods related to modern terrestrial scorpions and horseshoe crabs, the eurypterids included some species that grew to over three meters long, had pinching claws like a modern spider crab, and could even crawl out of the water and hunt on land. Now, University of Alberta researchers Scott Persons and John Acorn have added a new weapon to the arsenals of these ancient sea creatures: a slashing tail spine! Sparked by a fossil of the eurypterid Slimonia acuminata, which preserves a serrated-spine-tipped tail fully articulated and curved strongly to one side, Persons and Acorn have made the biomechanical case for eurypterids dispatching their prey with sidelong tail strikes. Eurypterids had a flattened tail and body form. Unlike lobsters and shrimps, which can flip their broad tails up and down to help them swim, eurypterid tails were vertically inflexible but horizontally highly mobile. Their flattened weaponized tails could be aggressively slashed sideways, while meeting a minimum of hydraulic resistance and without propelling themselves away from an intended target. Among the likely prey of Slimonia acuminata and other eurypterids were our ancient early vertebrate ancestors. Read the Article

The new specimen of Slimonia acuminata.
(Figure 1 in Persons & Acorn 2017)