Pycnogonids have almost no fossil record, though three genera have been found in the Devonian in the Hunsruck Slate in Germany. Despite the lack of fossil evidence, scientists have deduced from morphological and embryonic studies that sea spiders are an old lineage of animals.
Pycnogonids are an unusual group that has been difficult to place relative to other arthropod groups. Pycnogonids are thought to represent an early divergence from the evolutionary line leading to other Chelicerates. Both pycnogonids and chelicerates have claws on the first appendages and a tubercle with simple eyes, and both lack antennae. Pycnogonids, however, differ by possessing features such as the proboscis, reduced abdomen, and ovigers.
About eight families and 1,000 species of sea spiders are known.
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