Fossils have been found in many parts of the world in marine sediments dating from the Middle to Late Devonian.

The phylogeny of Eastmanosteus can be shown in the cladogram below:[2] Westralichthys uwagedensis Protitanichthys rockportensis Panxiosteus ocullus Janiosteus timanicus Plourdosteus canadensis Eastmanosteus calliaspis Xiangshuiosteus wui Eastmanosteus pustulosus Kiangyousteus yohii Golshanichthys asiatica Dunkleosteus amblyodoratus Dunkleosteus terrelli Dunkleosteus raveri Alternatively, the subsequent 2016 Zhu et al. study using a larger morphological dataset recovered Panxiosteidae well outside of Dunkleosteoidea, leaving the status of Dunkleosteidae as a clade grouping separate from Dunkleosteoidea in doubt, as shown in the cladogram below:[3] Eastmanosteus calliaspis Eastmanosteus pustulosus Kiangyousteus yohii Golshanichthys asiatica Westralichthys uwagedensis Dunkleosteus raveri Dunkleosteus terrelli Dunkleosteus amblyodoratus Heterosteus ingens Yinosteus major From the Frasnian Gogo Formation of northwestern Western Australia.

[2] A poorly known species from Russia, originally described from an isolated nuchal plate from the Frasnian of Timman with additional material from the Famennian of Lipetsk.

It was a large and widely distributed form, with fossil material from the Middle-Late Devonian of the USA (Wisconsin, Iowa, Michigan, New York State) and the Frasnian of Poland.

[1] An undescribed species based on relatively well preserved material from the Frasnian of Kerman, East Iran.