What Was the Largest Arthropod Ever? A Predator Longer Than a Person

Eurypterids are an extinct group of arthropods known from the Ordovician through to the end of the Permian, related to horseshoe crabs and to arachnids rather than to crustaceans, and the popular name is misleading since they are not scorpions and most were not marine. The body carried a shield-like head bearing eyes, a segmented trunk and a tail ending in a spike or a paddle depending on the group. Appendages beneath the head served for walking, for grasping and in many forms as a pair of flattened swimming paddles. Sizes ranged from a few centimetres to well over two metres in the largest known, which are the largest arthropods that have ever existed and considerably exceed anything alive.

Several kinds of evidence constrain how they lived:

• •Body fossils, frequently preserved as moulted exoskeletons rather than dead animals
• •Limb structure indicating whether an animal walked, swam or both
• •Eye structure and position, which indicate whether a species hunted visually and in what light
• •Mouthparts and spines, which indicate what could be caught and processed
• •Trackways preserving walking gaits, including one showing an animal moving out of water
• •The rocks they occur in, which indicate marine, brackish or freshwater settings

Explaining animals this large among arthropods requires accounting for the constraints that limit modern ones. Support is less of a problem in water, where buoyancy carries much of the weight, and most of the largest forms were aquatic. Oxygen supply through the arthropod respiratory arrangement is a real limit on land and less so for gill-breathing aquatic forms, and elevated atmospheric oxygen during part of the relevant interval has been proposed as a contributing factor, which is better supported for the giant insects of the Carboniferous than for these animals. Moulting is the severe constraint, since an animal that must shed and regrow its entire exoskeleton is helpless and structurally vulnerable while the new one hardens, and the difficulty scales badly with size. Absence of large vertebrate competitors early in their history probably mattered most.

A peculiarity of the fossil record for this group is that most specimens are not dead animals, which affects everything interpreted from them. An arthropod grows by shedding its exoskeleton repeatedly, so a single individual leaves many moulted casts during its life and one body at the end, which means casts vastly outnumber corpses and are what is usually found. Moults can be recognised by the way the head shield is displaced or the front of the animal split open along lines where the old covering parted. That abundance is useful, since it means the record is richer than the population was, and it is misleading, since counting specimens overestimates numbers and since a moult preserves the shape of the covering without the internal anatomy. Growth series assembled from moults are how development in the group is reconstructed.

The decline was gradual rather than sudden and the causes are argued about. Fish diversified substantially through the Devonian, including armoured predators and later jawed groups, which introduced both competition and predation pressure on a scale these animals had not faced, and the peak of eurypterid diversity precedes that expansion. Later forms are increasingly found in brackish and freshwater deposits rather than fully marine ones, which is consistent with retreat from environments where the competition was greatest. The group persisted at reduced diversity into the Permian and disappeared at the end of it in the largest extinction known. No descendants survive, which makes reconstructing their biology entirely dependent on the fossils and on comparison with relatives that are not particularly close.

These were arthropods related to horseshoe crabs and arachnids rather than to scorpions, with the largest exceeding two and a half metres, which makes them the largest arthropods that have existed. Buoyancy, gill breathing and the absence of large vertebrate competitors all contributed to that size. They declined as fish diversified, retreated to fresher water and vanished at the end of the Permian.