Trigonotarbids were an extinct order of arachnids that lived from the late Silurian to the early Permian period. They were closely related to spiders but had a fused head and body, segmented bodies, and lacked spinnerets. They were predators and had up to 28 eyes. Trigonotarbids are known from well-preserved fossils found in the Rhynie flint. They went extinct for unknown reasons and are one of the few arthropod groups to do so.
The trigonotarbids are an extinct order of arachnids that are among the earliest known terrestrial arthropods. They emerged during the late Silurian, about 410 million years ago, and became extinct in the early Permian, about 300 million years ago. Trigonotarbids thrived during the Devonian and Carboniferous periods, when the land went from being completely barren to covered in dense forests and swamps. About 380 million years ago, true spiders evolved, to which they lived until they became extinct for unknown reasons. Trigonotarbids are not the ancestors of spiders, but actually a separate offshoot of Arachnida.
Trigonotarbids obviously resemble spiders, to which they are closely related, although they are not 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 in a large box-like structure. Unlike spiders, which have smooth bodies, trigonotarbids had a segmented body, resembling a lobster. They had lateral and ventral plates on their body for armor, like their closest living relatives, the hooded mites. Like most spiders, they were small, between 0.5 and a couple of centimeters long, had eight legs, and were predators.
Trigonotarbids lacked spinnerets, which are distinctive of spiders and play a central role in their evolutionary success, although a recent fossil find may indicate microtubercles on the animal’s hind legs, which may be indicative of the ability to spin webs. The consensus for now is that they couldn’t make webs, and instead were adapted to stalk prey on the ground. Trigonotarbids were blessed with numerous eyes, worthy of an apex predator in the new terrestrial environment. Instead of spiders, whose eyes are concentrated on a single tubercle, trigonotarbids had one central tubercle and two lateral tubercles. The central tubercle had two large lenses, while the lateral tubercles each had three large and ten small lenses. This adds up to a total of 28 eyes.
Paleontologists know a lot about trigonotarbids because they’ve been found in some of the best-preserved fossils in the world, the Rhynie flint, which formed when volcanic materials quickly flooded a small ecosystem and fossilized everything in its place. The resulting preservation is so perfect that the smallest features are visible, including well-preserved mouthparts, bristles (hairs), tiny eyes, and microscopic scales on the animal’s body, which are diagnostic of the group at large. Trigonotarbids have been found within the structure of trees, which they presumably used as hiding places while waiting for prey to pass.
Trigonotarbids are one of the few arthropod groups to go extinct completely. Other extinct arthropod groups include trilobites and eurypterids (sea scorpions).
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