[5] It has been placed as a basal member of the clade Vicissicaudata within Artiopoda, a group of arthropods containing trilobites and their relatives.

[3][5] The semicircular head bore an elongate pair of antennae, sometimes exceeding over 80 segments in E. brocki.

The trunk is made up of 10 (in E. brutoni) or 11 in (E. brocki) segments/tergites, which have well developed pleurae, which are followed by a cylindrical segment to which "caudal flaps" attached, followed by the tailspine.

Today, both Sidneyia and Emeraldella are placed as part of the clade Vicissicaudata within Artiopoda, which includes trilobites and other arthropods with similar bodyforms.

However, Sidneyia and Emeraldella are usually not recovered as each others closest relatives within Vicissicaudata, rendering "Xenopoda" invalid.