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The comparison of certain genera representative of the ichthyofauna of the Escuminac Formation (Quebec, Canada) with those coming from contemporaneous deposits (Upper Devonian, Frasnian) located in the Baltic countries (U.S.S.R.), Western Australia, Nevada (U.S.A.), and Germany provides new evidence against a lacustrine origin of the Escuminac Formation. It is suggested that the Escuminac Formation was deposited in a coastal marine environment. This hypothesis is supported by results of a geochemical study.

The study of Plourdosteus canadensis (Brachythoraci, Placodermi) from the Escuminac Formation (Frasnian) led us to erect a new family: the Plourdosteidae. Firstly, we discuss all the characteristics that allowed Ørvig to differentiate the genus Plourdosteus from Coccosteus. It is proposed to consider Plourdosteus mironovi as a synonym of Plourdosteus trautscholdi, found in the Baltic region of the USSR. The characteristics that support the monophyly of the Plourdosteidae are listed and are represented on a phylogenetic diagram. In the conclusion, the Dunkleosteidea sup. fam. nov., the Plourdosteidae fam. nov., and the type species Plourdosteus canadensis are defined. The different species of Plourdosteus are also listed.