Prehistoric chelicerates

Xiphosura (; , in reference to its sword-like telson) is an order of arthropods related to arachnids. They are more commonly known as horseshoe crabs (a name applied more specifically to the only extant family, Limulidae). They first appear in the fossil record in the Early Ordovician, around 480 million years ago. Currently, there are only four living species. Xiphosura contains one suborder, Xiphosurida, and several stem-genera.

Eurypterids, often informally called sea scorpions, are a group of extinct marine arthropods that form the order Eurypterida. The earliest known eurypterids date to the Tremadocian stage of the Ordovician period, 480 million years ago. The group is likely to have appeared first during the Late Cambrian period. With approximately 250 species, the Eurypterida is the most diverse Paleozoic chelicerate order. Following their appearance during the Ordovician, eurypterids became major components of marine faunas during the Silurian, from which the majority of eurypterid species have been described.

Limulus is a genus of horseshoe crab, with one extant species, the Atlantic horseshoe crab (Limulus polyphemus). One fossil species is currently assigned to the genus though several other species have been named, which have since been assigned to other genera.

Mollisonia is an extinct genus of Cambrian marine arthropod. Four species have been described from North America and China. Studies suggest it is a basal member of Chelicerata, a group which includes horseshoe crabs and arachnids.

Chasmataspidids, sometime referred to as chasmataspids, are a group of extinct chelicerate arthropods that form the order Chasmataspidida. Chasmataspidids are probably related to horseshoe crabs (Xiphosura) and/or sea scorpions (Eurypterida), while more recent studies suggest that they form a clade (Dekatriata) with Eurypterida and Arachnida. Chasmataspidids are known sporadically in the fossil record through to the mid-Devonian, with possible evidence suggesting that they were also present during the late Cambrian. Chasmataspidids are most easily recognised by having an opisthosoma divided in

left|thumb|Life restoration of Wisangocaris feeding on trilobites. Habeliida is an order of extinct arthropods that existed during the middle Cambrian. It is divided into two families, Habeliidae (monotypic, containing only Habelia), and Sanctacarididae (containing Sanctacaris, Utahcaris and Wisangocaris). They are thought to have been durophagous, with robust gnathobases (spined basal sections of limbs) used to shred hard-shelled organisms. Remains of trilobites have been found as stomach contents in Wisangocaris. Messorocaris has been suggested to be part of the order in some studies, but th