Horseshoe crabs, the blue-blooded burrowers in the genus Xiphosura, first appeared in the fossil record about 450 million years ago. Around that same time, arachnids crawled into existence.

Looking at the genomic sequencing of 53 species of arachnids, horseshoe crabs and sea spiders, Ballesteros' team found overwhelming data putting Xiphosura within the anarchid family.

Fossil Xiphosura are known from all geological systems from the Cambrian onward. A distinct trend of development, particularly from the Silurian to Recent time, is demonstrated. The early xiphosuran ...

A new genus and species of synziphosurine (Chelicerata) is described from the Silurian (Wenlock) Scotch Grove Formation Konservat-Lagerstätte in Clinton County, Iowa. Camanchia grovensis n. gen. n. sp ...