Chasmataspidids, sometime referred to as chasmataspids,[2][3][4] are a group of extinct chelicerate arthropods that form the order Chasmataspidida.

[6][7][8][9] Chasmataspidids are known sporadically in the fossil record through to the mid-Devonian,[10] with possible evidence suggesting that they were also present during the late Cambrian.

[2] 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.

[12][13] 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).

[14] There was also a trace fossil composed of resting imprints with Chasmataspis-like outline discovered from late-Cambrian stratum, which might suggest an earlier occurrence of chasmataspidids.

[11] Chasmataspidids are readily distinguished from other chelicerates by the subdivision of the 13 opisthosomal segments into a widened, 4-segmented preabdomen and a slender, 9-segmented postabdomen.

[15][11] the tergite (dorsal exoskeleton) of the first opisthosomal or preabdominal segment is retained as a narrow element known as 'microtergite',[15] which is not observed in eurypterids.

[2] The next species to be discovered were Diploaspis casteri and Heteroaspis novojilovi; both described by the Norwegian palaeontologist Leif Størmer from the early Devonian of Alken an der Mosel in Germany in 1972.

[17] A third species, Diploaspis praecursor (Late Silurian, Bertie Group, New York State), was described by Lamsdell and Briggs in 2017.

Forfarella mitchelli from the early Devonian of the Forfar region in the Midland Valley of Scotland was described by Jason Dunlop and colleagues in 1999; although the fossil had actually been recognised as a chasmataspidid and provisionally labelled as such some years previously by Charles Waterston.

[3] The stratigraphically youngest chasmataspidid is Achanarraspis reedi, described by Lyall Anderson and colleagues in 2000, from the mid-Devonian Achanarras quarry in Caithness, Scotland, a site rich in fish fossils.

[18] Well preserved chasmataspidids were recovered from the early Devonian of October Revolution Island, part of the Severnaya Zemlya group in the Russian Arctic.

Described by Erik Tetlie and Simon Braddy in 2003, it was placed in Diploaspididae, but interpreted as being somewhat more intermediate in form between the Chasmataspis and Diploaspidid body plans.

[15] Hoplitaspis hiawathai is the second known species of Ordovician chasmataspidid, discovered from the Big Hill Lagerstätte of Michigan in United States, described by James C. Lamsdell and co-authors in 2019.