Archaeus ("the ancient") is a prehistoric genus of jackfish from the Paleogene of Europe. The oldest species, A. oblongus is from the late/terminal Thanetian epoch of Paleocene Turkmenistan, and the last species, A. glarisianus and possibly A. brevis, are from the early to middle Rupelian, of the Oligocene lagerstatten of Canton Glarus, Switzerland.In life, Archaeus species would have resembled a small trevally, though, Archaeus had a proportionally smaller mouth, and larger eyes, and the tail was more fan-shaped, rather than crescent-shaped.