[3] Fossils of Erratus have been found in the Chengjiang Lagerstätte of China,[4] dating to around 520 million years ago.

[2] The relationship between the components of the biramous limb of many euarthropod groups, which consists of an exopod and an endopod (appendages), and the lopopods (stubby legs) and flaps of lobopodians and radiodonts has long been controversial.

Fu et al. 2022 describes the species as having "unique trunk appendages formed of lateral anomalocaridid-type flaps and ventral subconical endopods".

The arthropod had a bivalved carapace that covered the top, which possesses an anterior spine but rounded posteriorly.

Ventral head structures between the eyes and body flaps are yet to be discovered, it is also uncertain if the trunk-end possesses a telson.

[2] Megadictyon Kerygmachela Pambdelurion Opabinia Anomalocarididae Hurdiidae Erratus Isoxys Surusicaris Nereocaris Pectocarididae Odaraia Canadaspis Perspicaris Pectocarididae Fuxianhuiida Fortiforceps Yohoia Leanchoilia Olenoides A phylogenetic analysis conducted by Fu et al. 2022 found that Erratus was a basal arthropod, branched before isoxyids (Isoxys and Surusicaris) and other euarthropods (hymenocarines, fuxianhuiids, megacheirans, trilobites and so on), but more derived than other stem-arthropods like gilled lobopodians (Pambdelurion and Kerygmachela), Opabinia and radiodonts (Anomalocaris, Hurdia and so on).

[2] Erratus was a member of the Chengjiang Biota, which dates to the Cambrian period, 520 million years ago.