Spiders and Scorpions

By | September 9, 2016

Spiders and scorpions belong to an ancient family of predatory animals called chelicerates (“kell-ISS er-ates”), all of which have special mouthparts that they use either as pincers or fangs. Modern chelicerates are small, but their earliest ancestors grew to gigantic sizes and were among the top predators of their time. The biggest of these prehistoric monsters were the sea scorpions.


Key features:

■ Segmented bodies and jointed limbs

■ Hard external skeleton (exoskeleton)

■ Pincerlike feeding claws or fangs

■ Four pairs of walking legs


The chelicerates appeared late in the Ordovician Period, about 445 millon years ago. Over 77,000 identified species exist today.

Pterygotus (terry-GOAT-us)


When: 400–380 million years ago (Late Silurian to Middle Devonian)

Fossil location: Europe, N. America

Habitat: Shallow seas

Length: Up to 7 ft 4 in (2.3 m)

Pterygotus was a sea scorpion that grew larger than a fully grown man. Using its colossal eyes, it scanned the water for prey such as fish and trilobites. Perhaps it hid half-buried in sand until victims wandered close by, before lashing its tail to produce a violent burst of speed and snatching up the animal in its claws.
Fossils have been found worldwide, and some experts think Pterygotus not only terrorized the seas but swam up rivers and into lakes as well.

Eurypterus (you-RIP-terruss)


When: 420 million years ago (Late Silurian)

Fossil location: United States

Habitat: Shallow seas

Length: Up to 4 in (10 cm)

This small sea scorpion was less well armed than the fearsome Pterygotus. It used prickly legs to pull tiny animals toward its fangs, which it then used to tear the victim to shreds. Eurypterus hunted on the muddy floors of shallow seas.

Mesolimulus (mee-zo-LIM-you-luss)


When: 162–145 million years ago (Late Jurassic)

Fossil location: Germany

Habitat: Shallow ocean waters

Length: Up to 3.1/2–3.1/2 in (8–9 cm), without tail

Mesolimulus is also called a horseshoe crab (though it’s more closely related to spiders and scorpions than crabs). It had a huge shell, small, widely spaced eyes, and a stiff tail with a sharp tip, like a spear. It lived on the seafloor, where it hunted worms and shellfish.

Spider (SPY-der)




Spider in amber


When: 400 million years ago (Late Silurian) to now

Fossil location: Worldwide

Habitat: All land

Size: Up to 12 in (30 cm) across

Although the soft and delicate bodies of spiders do not fossilize well, thousands of species have been found, many of them preserved in pebbles of amber—a clear, golden material formed from fossilized pine-tree resin. Spiders are specialized hunters that often use silk traps to capture prey before killing victims with a lethal injection of venom from their fangs. The oldest fossilized spider’s web is 100 million years old.

Proscorpius (pro-SCOR-pee-us)



When: 400–300 million years ago (Silurian – Carboniferous)

Fossil location: Worldwide

Habitat: Uncertain

Length: 1.1/2 in (4 cm)

The first scorpions lived in the sea rather than on land and breathed through gills. One of the oldest fossils is Proscorpius from the Late Silurian Period. This creature’s mouth was under its head like that of a horseshoe crab, rather than at the front like a modern scorpion’s.
It isn’t clear whether it lived on land or in water.



Limulus horse crab

Modern horseshoe crabs such as Limulus are almost exactly like their prehistoric cousins from the Jurassic Period. Limulus lives in shallow water off the eastern coast of North America. It swims upside down, as its ancient relative probably did.



The Black Widow spider

Animals with jointed legs and external skeletons (such as insects, spiders, and scorpions) are called arthropods. Sea scorpions were the largest arthropods that ever lived—giant versions of the ones we find today in the yard. Today, arthropods are small but in the distant past they grew to greater sizes, perhaps because the Earth’s air contained more oxygen, making it easier for arthropods to breathe and grow.


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