Articulated halkieriids from the Lower Cambrian of north Greenland

Abstract

METAZOAN skeletons appear abruptly in the fossil record near the base of the Cambrian (∼540MyrBP (before present))1,2. In the initial stages of diversification, familiar skeletal remains such as sponge spicules, brachiopod valves and echinoderm ossicles are outnumbered by a remarkable array of skeletal types, many of millimetric size3–9. Particularly conspicuous in these early assemblages are various sclerites, some of which are inferred to have formed cataphract coverings of slug-like metazoans. Recognition of articula tory facets and fused sclerites contribute to partial scieritome reconstructions8–11, but discovery of articulated material is a vital ingredient in understanding the palaeobiology of these extinct groups. Here we report complete halkieriid scleritomes from the Lower Cambrian of north Greenland. This finding broadly confirms the predictions of an earlier hypothetical reconstruction11, but reveals entirely unexpected anterior and posterior shells. It supports also the proposed phylogenetic link between the halkieriids and Wiwoxia corrugata, a metazoan with a cataphract scieritome best known from the Middle Cambrian Burgess Shale.