Plourdosteus is an extinctgenus of placodermarthrodire which was relatively widespread in Euramerica during the Givetian to Frasnian ages of the Devonian.^[1] It was a small placoderm, with P. canadensis specimen MNHM 2-177 measuring 37.5 cm (14.8 in) long.^[2]

The name Plourdosteus commemorates the Plourde family at Miguasha National Park.^[3]

Plourdosteus was previously assigned to the familyPlourdosteidae within the Coccosteomorphi. However, subsequent studies found the family Plourdosteidae to be polyphyletic and should be dismissed. Plourdosteus was then proposed to be a member of Panxiosteidae.^[4][5] In the 2010 Carr & Hlavin phylogenetic study, Panxiosteidae was recognized as the sister taxon to the family Dunkleosteidae, which together comprised the superfamilyDunkleosteoidea (one of the three major clades of Eubrachythoraci).^[4]

The phylogeny of Plourdosteus within Panxiosteidae is shown in the cladogram below from the 2013 Zhu & Zhu study:^[5]

อย่างไรก็ตาม การศึกษาของ Zhu et al. ในปี 2016 ที่ใช้ ชุดข้อมูล สัณฐานวิทยา ขนาดใหญ่กว่า พบว่าPanxiosteidaeอยู่นอกDunkleosteoideaแต่กลับอยู่ในCoccosteomorphiและCoccosteoideaเป็นกลุ่มพี่น้องของCoccosteidaeดังที่แสดงในแผนภูมิวิวัฒนาการด้านล่าง: ^{[ 6 ]}