Chasmataspidida

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 into a wide forepart (preabdomen) and a narrow hind part (postabdomen) each comprising 4 and 9 segments respectively. There is some debate about whether they form a natural (i.e. monophyletic) group.

== Distribution == 200px|thumb|Geographic distribution of chasmataspidids. Chasmataspidids survived at least since Ordovician to mid-Devonian in age. As of 2019, most chasmataspidids (with a total of 9 species) are known from the Devonian strata, while the preceding Silurian and Ordovician period each have 3 and 2 species being described. Diploaspis is the only genus of chasmataspidids that unambiguously comprises species from different periods (D. casteri and D. muelleri from Devonian and D. praecursor from silurian). There was also a trace fossil composed of resting imprints with Chasmataspis-like outline discovered from late-Cambrian stratum in Texas, which might suggest an earlier occurrence of chasmataspidids. In addition, likely fossils from the Jiangshanian have been recorded from the Kimiltei site. However, affinities of those Cambrian records are questioned in 2023 study, identifying imprint from Texas (named Minterichnus shieldi) as made by phyllocarid, and suggesting Kimiltei fossils are also phyllocarids, while original authors argues.