Like its body appendages, these were biramous—their inner branch was spiny, segmented, flexible and leg-like, while the outer portion had a large surface area and resembled a flap.

[1] The organism probably swam, wafting its outer flap-limbs in waves along its body to gain propulsion, in the manner of Anomalocaris.

[1] Alalcomenaeus was initially thought to fall in the stem group to the crustacea, and was placed in a clade with Leanchoilia, Actaeus and Yohoia.

[1] More up to date analyses place it along with Leanchoilia and Yohoia as well as Sanctacaris, Habelia, Sarotrocercus and Sidneyia, somewhere within the arachnomorpha.

[4] Alalcomenaeus is very rare in the Walcott quarry of the Burgess Shale, so was described on the basis of half a dozen specimens.