What is "chasmataspidida"

Chasmataspidida (often referred to informally as chasmataspids) is an extinct group of rare chelicerate arthropods. Chasmataspids are probably related to horseshoe crabs ( Xiphosura) and/or sea scorpions ( Eurypterida). Indeed the first species to be discovered were thought to be unusual fossil horseshoe crabs, while later species were often based on specimens initially misidentified as eurypterids. There is some evidence that chasmataspids were present during the late Cambrian and the group is known sporadically in the fossil record through to the mid- Devonian. Chasmataspids are most easily recognised by having an abdomen divided into a short forepart (or mesosoma) and a longer hindpart (or metastoma) comprising nine segments. There is some debate about whether they form a natural (i.e. monophyletic) group.