Trigonotarbids are an extinct order of arachnids that are among the earliest land arthropods known. They emerged during the late Silurian, about 410 million years ago, and died out in the early Permian, about 300 million years ago. Trigonotarbids thrived during the Devonian and Carboniferous periods, when the land went from being entirely barren to covered in thick forests and swamps. About 380 million years ago, true spiders evolved, which they lived alongside until they went extinct for unknown reasons. Trigonotarbids are not the ancestors of spiders, but in fact a separate offshoot of Arachnida.
Trigonotarbids obviously resemble spiders, to which they are closely related, though they aren't the same. Trigonotarbids are more primitive all around. Instead of having a thin waist, like all true spiders, the head and body of these animals were fused together into a large box-like structure. Unlike spiders, which have a smooth body, trigonotarbids had a segmented body, reminiscent of a lobster. They had lateral and ventral plates on their body for armor, like their closest living relatives, the hooded tickmites. Like most spiders, they were small, between 0.5 and a couple centimeters in length, had eight legs and were predators.
Trigonotarbids lacked spinnerets, which are distinctive to spiders and play a central role in their evolutionary success, though one recent fossil find may indicate microtubercules on the animal's back legs, which might be indicative of the ability to spin webs. The consensus for now is that they couldn't make webs, and instead were adapted to stalking prey on the ground. Trigonotarbids were blessed with numerous eyes, befitting of an apex predator in the new terrestrial environment. Instead of spiders, whose eyes are condensed onto a single tubercule, trigonotarbids had a central tubercule and two lateral tubercules. The central tubercule had two large lenses, while the lateral tubercules each had three large lenses and ten small ones. This adds up to a total of 28 eyes.
Paleontologists know a lot about trigonotarbids because they have been found in some of the best preserved fossils in the world, the Rhynie chert, which formed when volcanic materials quickly flooded a small ecosystem and fossilized everything in place. The resulting preservation is so perfect that the smallest features are visible, including well preserved mouthparts, setae (hairs), tiny eyes, and microscopic scales on the animal's body, which are diagnostic of the group in general. Trigonotarbids have been found within the structure of trees, which they presumably used as places to hide while waiting for prey to pass by.
Trigonotarbids are one of the few arthropod groups to go completely extinct. Other extinct arthropod groups include trilobites and eurypterids (sea scorpions).