Zenaspida Benneviaspidida Thyestiida The class Osteostraci (meaning "bony shells") is an extinct taxon of bony-armored jawless fish, termed "ostracoderms", that lived in what is now North America, Europe and Russia from the Middle Silurian to Late Devonian.

[3] Most osteostracans had a massive cephalothorac shield, but all Middle and Late Devonian species appear to have had a reduced, thinner, and often micromeric dermal skeleton.

The shield of bone covering the head formed a single piece, and so presumably did not grow during adult life.

This reveals the presence of complex sensory organs and the sides and upper surface of the head, which may have been used to sense vibrations.

[6] Below is a cladogram showing the phylogenetic relationships of osteostracans from Sansom (2009):[3] Hirella Aceraspis Ateleaspis Hemicyclaspis Cephalaspis Spangenhelmaspis Wladysagitta Parameteoraspis Balticaspis Trewinia Escuminaspis Levesquaspis Tegaspis Stensiopelta Diademaspis Zenaspis Scolenaspis Ukrainaspis Machairaspis Superciliaspis Pattenaspis Zychaspis Hildenaspis Mimetaspis Waengsjoeaspis Camptaspis Yvonaspis Ectinaspis Securiaspis "Benneviaspis" longicornis "Benneviaspis" anglica "Benneviaspis" lankesteri Benneviaspis Hoelaspis Severaspis "Boreaspis" ceratops "Boreaspis" intermedia Boreaspis Dicranaspis Spatulaspis Belonaspis Hapilaspis Tauraspis Ilemoraspis Procephalaspis Auchenaspis Thyestes Witaaspis Saaremaspis Tyriaspis Aestiaspis Dartmuthia Timanaspis Oeselaspis Tremataspis Dobraspis Sclerodus Tannuaspis Didymaspis Kiaeraspis Norselaspis Nectaspis Axinaspis Acrotomaspis Gustavaspis