Pterygotid eurypterids (sea scorpions), giant aquatic arthropods with big claws, were all regarded as apex predators but some scientists then suggested that certain species had weak claws, so weren’t predatory. New research now reveals that their claws were more robust and were just used to catch prey; other appendages chewed it up. Fossils found with pterygotid eurypterids also indicate that some species specialized on lightly-armored crustaceans and fishes, but most species specialized on heavily-armored fishes.
Sea scorpion (eurypterid) size through time. The pterygotid eurypterid Jaekelopterus (background), the largest ever arthropod, and the hibbertopterid eurypterid Cyrtoctenus (foreground) in comparison to the size of a (average British male) human alongside silhouettes of some of their eurypterid relatives. Image credit: Simon Powell.
Sea scorpions (eurypterids) were ancient aquatic creepy-crawlies (arthropods) that lived 467 to 253 million years ago, long before the dinosaurs.
They include the ‘pterygotids’ (428 to 391 million years ago), with large fearsome claws, that grew up to 2.5 m long, the biggest bugs that ever lived.
An extinct millipede called Arthropleura was claimed to be even larger, but if 12 to 14 preserved body segments are 76 cm long, an animal with 32 segments was (76/12 x 32) just over 2 m long (excluding the head).
Pterygotid eurypterids were all assumed to have been fierce apex predators — the T. rex of their day.
Then, some scientists suggested that the claws of a pterygotid called Acutiramus could only catch and slice weak, soft-bodied prey, and their eyesight wasn’t sharp enough to be a predator. It was relegated from the top tier of predators and even labeled a ‘pussycat.’
A new study now suggests that the claws of Acutiramus were much more robust; the suggestion that they would snap was based on incorrect assumptions.
The apparent lack of an ‘elbow joint’ isn’t a problem either; this was at the base of the claw. It also only used its claws just to catch prey; the more powerful mouthparts at the base of their legs would kill and chew it up.
Their poor eyesight also isn’t a problem; their prey were large and some non-predatory insects (e.g. bees and butterflies) have similar eye metrics to arthropods that were considered predators.
Computer modeling and experiments with a robotic swimming eurypterid also indicates that pterygotids were slower swimmers than had been assumed; they were so large that their relatively small swimming paddles didn’t provide enough thrust, so their flat tail (telson) functioned as both a rudder and helped for propulsion.
Analysis of the types of fossils found alongside pterygotids also suggests that Acutiramus specialized on lightly-armored crustaceans (called phyllocarids) and pteraspid fishes, Erettopterus on thelodont fishes and Pterygotus and Jaekelopterus on more heavily-armored placoderm fishes.
Predation traces (claw marks) and fossil feces (coprolites) confirm that some eurypterids ate armored fishes, trilobites and even other eurypterids.
The suggestion that eurypterids influenced early vertebrate (fish) evolution, in a predator — prey arms race, was generally dismissed by previous studies but this new research suggests that pterygotids and other eurypterids probably did have some influence on early vertebrate evolution.
The evolutionary relationships of pterygotids has changed too; their shape, vision, fossil associations, ecology and stratigraphic record all indicates that Acutiramus was more basal to Jaekelopterus and Pterygotus.
This means that the largest ever arthropod (Jaekelopterus rhenaniae) can now be estimated at nearly 2.6 m long, 10 cm longer than previous estimates. The biggest bugs just got a bit bigger!
This paper appears in the Bulletin of Geosciences.
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S.J. Braddy. 2023. Pterygotid eurypterid palaeoecology: praedichnia and palaeocommunities. Bulletin of Geosciences 98 (4); doi: 10.3140/bull.geosci.1891