What is the largest arachnid that has ever lived?

According to traditional phylogeny, the largest arachnid of all time is probably a scorpion, not a spider. However, recent work suggests that we should revisit the evolutionary relationships of chelicerae. In this case, the biggest arachnid that ever lived was probably much bigger than you.

Arachnids are a class of chelicerate arthropods. This is the group that includes, among others, spiders, scorpions, mites and harvestmen. However, the question still remains open.

Officially, chelicerae are indeed subdivided into smaller groups, with arachnids being one group, while horseshoe crabs (horse crabs, order Xiphosura) and sea scorpions (order Eurypterida) form two others. This is the traditionally accepted phylogeny. However, a recently published paper suggests that these last two groups may actually be nested within that of arachnids.

In detail, this new phylogeny, based on molecular analysis, points out that terrestrial chelicerae groups (spiders and scorpions) preceded marine groups (horse crabs and sea scorpions). However, despite the fact that this claim is supported by genetic evidence, it is at present inconsistent with the fossil record.

Largest living and extinct arachnid

According to the traditional view of the arachnid family tree (thus ignoring horseshoe crabs and sea scorpions), the largest living arachnid is a spider.

There are two contenders here. The Tarantula Goliath (Theraphosa blondi) is considered as the most massive. Together, the cephalothorax and the opisthosoma (the body without the legs) can measure up to twelve centimeters. Then add the legs, and you get a total wingspan about 28 cm for a weight of about 170 grams on the scale.

However, if size alone is to be relied upon, then the largest spider might be Heteropoda maxima, a species in the Sparassidae family. Endemic to the province of Khammouane in Laos, it has a wingspan spread legs about 30 cm. Despite its size, this spider was only discovered in 2001.

When we delve into the fossil record, however, we find that the largest arachnid ever discovered, still according to the accepted phylogeny, was probably not a spider, but a scorpion. Brontoscorpio anglicuswhich lived in the Silurian Devonian period (between 350 million and 450 million years ago), could indeed measure almost a meter long. It should, however, be emphasized that the only known example of this species has been described from a single free finger, that of its right pedipalp (one of the two blades of its right claw, visible below).

Brontoscorpio anglicus. Credit: Junnn11

Sea scorpions larger than humans

If we now consider sea scorpions and horseshoe crabs, then the largest living arachnid is no longer a spider, but rather a horseshoe crab. According to a 2017 study published in the Journal of Asia-Pacific Biodiversity, the largest living species, Tachypleus tridentatuscan indeed reach a length of nearly 80 cm.

Considering the entire fossil record now, the title would likely go to a member of the now extinct eurypterid sea scorpions. Indeed, fossils suggest that many of these ancient marine predators would rival humans in size. The largest species in this group was Jaekelopterus rhenaniae, a species discovered in 2007 whose claws were up to 46 cm long. From these claws, researchers estimated that its body measured about 2.5m long.

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