In the Carboniferous, Arthropods scuttled about and infested the swamp forests. Mesothelae was one of these creatures and was the size of a tortoise, a relative of the true spiders, but not a true spider itself. Instead, Mesothelae was a Solifugid, more like a whip scorpion or camel spider - a form of arachnids of a primitive stock that go back to the Devonian Period as a whole. They are like a half way order between spiders and scorpions. The fangs of the Mesothelae are reminiscent of camel spiders, which are often mistaken for spiders by soldiers fighting in the 21st Century middle east. Instead they are Solifugids. Like the camel spider, the fangs are primarily used to crush their prey by sheer kinectic force - though whether Mesolthelae had poison like the camel spider in to make them even worse, it is of course impossible to say due to such things not being preserved.
Mesothelae was featured in Walking with Monsters, terrorizing Petrolacosaurus. It was later fried after lightning struck its burrow.
Interestingly, it was originally supposed to be named Megarachne, which at the time of production was believed to be a giant spider. Before the series aired it was discovered to be a sea scorpion instead, and the narration had to be changed. The producers decided to replace it with Mesothelae.