Aleosteus is an extinct monospecific genus of arthrodire placoderm fish of the Early Devonian period. The type species Aleosteus eganensis was described in 2000, and was found in the Late Emsian strate of the Sevy Dolomite Formation, in the Egan Range of east-central Nevada, USA. Almost complete fossils belong to juvenile and adult specimens and show a short and broad skull, posteriorly concave.[1]
The generic name Aleosteus comes from the Ancient Greco-Roman game "alea", a game of chance involving dice, as an allusion to Nevada's reputation for gambling. The species name eganensis is based on the type locality in the Egan Mountain Range in Nevada.[1]
Aleosteus is one of the more basal members of the order Arthrodira, closely related to Simblaspis, as shown in the cladogram below: