I’ll have to show my spiders this reconstruction of their great-great-many-times-great grandmama. She’s adorable.

Megachelicerax cousteaui

It’s all in the chelicerae.

In a study published in Nature, Research Scientist Rudy Lerosey-Aubril and Associate Professor Javier Ortega-Hernández, Curator of Invertebrate Paleontology in the Museum of Comparative Zoology, describe Megachelicerax cousteaui, a 500-million-year-old sea predator discovered in Utah’s West Desert. It is the oldest known chelicerate, the arthropod group that includes spiders, scorpions, horseshoe crabs, and sea spiders. The discovery pushes the evolutionary history of chelicerates back by 20 million years.

Hey, I’ve visited the West Desert of Utah — but all I found were fossilized trilobites. Lots of trilobites. Megachelicerax food. But I never found a predator like this one.

The key feature identifying it as a chelicerate are those big clawed appendages on the front of its face, which is also a prominent feature of modern spiders, much modified. The appendage is now a crushing/chewing anvil, and the thumb of the claw has evolved into a wicked needle.

Its most extraordinary feature, however, is its unmistakable chelicera — the pincer-like feeding appendages that define the subphylum Chelicerata and distinguish spiders from insects. While insects possess sensory antenna as their foremost appendages, chelicerates have grasping, often venomous tools. Despite a rich Cambrian fossil record, no unambiguous chelicera-bearing arthropod from that time had ever been found – until now.

The thing about modern spiders is that they’re mostly defenseless — no sting, no pointy legs, no claws — except for that one extraordinarily useful weapon, the chelicerae. Well, they also have silk, making them a kind of retiarius of the arthropod world. You’re probably not afraid of their webs, but you worry about their big chompers and the venom they carry.

I don’t think the retiarii would have poisoned their tridents, though.