Aglaspidida

Aglaspida, more correctly termed Aglaspidida, is an extinct taxon of small, superficially horseshoe crab-like arthropods that were once regarded as basal or ancestral horseshoe crabs. However, anatomical comparisons demonstrate that the aglaspidids cannot be accommodated within the chelicerates. Now, though, they are regarded as a distinct group, possibly being close to the trilobites. An older hypothesis suggesting that they link trilobites with Chelicerata has fallen out of favour.
Aglaspid fossils are found throughout the world, including North America (upper Mississippi valley, Missouri and Utah), Europe, and Australia.