When described the genus Archimyrmex was known from a solitary fossil preserved as an impression in fine shale of the Green River formation in Colorado.

[1] One side of the A. rostratus holotype is currently deposited in the University of Colorado paleontology collections while the counterpart is in the US National Museum, and an additional eleven fossils were known as of 2002.

[2] The holotype was first studied by palaeoentomologist Theodore Dru Alison Cockerell of the University of Colorado and his 1923 type description of the new genus and species was published in The Entomologist.

Perfilieva, who moved the genus again, this time to the primitive Subfamily Myrmeciinae based on the close similarity between Archimyrmex and Prionomyrmex.

[2][4][5] In 1957, an additional species of fossil ant was described from the Middle Eocene Ventana Formation in the Rio Pichileufu region of Argentina by M.J. Viana and J.A.

Recovered from an outcrop of the Ventana Formation in the Rio Limay area, Rossi de Garcia distinguished the new species from "A."

A fourth species, A. wedmannae was described by Dlussky in 2012 based on fossils found in the Middle Eocene Messel Pit site of Germany.

However, Cesare Baroni Urbani of the University of Basel, Switzerland classified the genus as incertae sedis within the family Formicidae.

[7] Baroni Urbani justifies his decision by stating the characters of the ant are similar to those of Cariridris, an insect from the Lower Cretaceous that was once placed in Myrmeciinae, but is now incertae sedis within the subclade Aculeata.

[7] A 2012 report describing new myrmecine fossils accepted the classification of Archibald and colleagues while disregarding Baroni Urbani's comments.

The petiole structure in combination with little to no constriction between abdominal segments III and IV is unique to the genus and separates it from the other Myrmeciinae genera.

[5] The holotype, recovered from the Ventana Formation of the Neuquén Basin in Argentina is currently preserved at the Bernardino Rivadavia Natural Sciences Museum palaeoentomological collection.

69617 for the part and counterpart; females of the type species A. rostratus have a body length estimated to be between 13.2–15.8 mm (0.52–0.62 in), with a small gaster that is shorter than the mesosoma and sporting a short thick sting.

The preserved specimens from the Green River Formation are brown and the upper area of the head and gaster is black, which is similar to Myrmecia nigriceps.

The width of the scutum is greater than its length, the legs are noticeably elongated and thin, and the podeum (a narrow stalk that connects the mesosoma with the abdomen) is slightly convex.

[5] The lectotype female has an estimated body length of 3 cm (1.2 in) with an mesosoma that is described as "relatively massive" (being only 1.7 to 1.9 times as long as tall) and an oval shaped gaster.