What is "synziphosurine"

Synziphosurines are a paraphyletic group of xiphosurid ancestors (within the class Xiphosura) from the Silurian to the Carboniferous periods. The earliest accepted synziphosurines are Venustulus waukeshaensis, found in Early Silurian (Late Llandovery) deposits in Wisconsin, and Bembicosoma pomphicus, from the same period, known from deposits in Scotland.