Erratus

legs.^[2] It also supported the theory that the gills of aquatic arthropods probably evolved into the wings and lungs of terrestrial arthropods later in the Paleozoic.^[3]

deposit feeding or filter feeding

anterior spine, as well as seven anterior body fragments.^[2]

endopod (appendages), and the lopopods (stubby legs) and flaps of lobopodians and radiodonts has long been controversial. Erratus appears to show one of the earliest steps in the evolution of the endopod, with an unsclerotized endopod fused to a flap that is attached to the body wall, rather than being a branch of the appendage like an exopod. Fu et al. 2022 describes the species as having "unique trunk appendages formed of lateral anomalocaridid-type flaps and ventral subconical endopods". The fossils indicate that the species represent "an intermediate stage of biramous limb evolution".^[2]

lenses, and the stalks appear to be as long as the eyes. On the bottom of the arthropod were 11 pairs of wide body flaps with gill-like wrinklings. The flaps decreased in size towards the bottom of the arthropod. As in gilled lobopodians and radiodonts, the flaps were connected to the body wall. The anterior 7 pairs of body flaps each possesses a medially-arranged endopod (inner branch). Each endopod was a simple, 7-segmented leg, lacking any evidences of endites (internal projections) and claws. The posterior 4 pairs of body flaps lacking endopods, but the wrinklings are more prominent. 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]

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]

Chengjiang Biota, which dates to the Cambrian period, 520 million years ago.^[2] During this time the area was a tropical region with sea level changes and tectonic activity. Most of the fauna were primarily benthic, and were probably buried via turbidity currents. Brachiopods, ctenophores, phoronids, lobopodians, worms, primitive chordates, and other arthropods are represented in the biota.^[7][8] The site is known for its incredible preservation of fossils similar to that of the younger Burgess Shale.^[9]