Corollasphaeridium : A cryptic record of Cambrian loricate protists

Cambrian palynomorphs include conventional acritarchs (organic-walled microfossils of unknown affinity, often interpretable as phytoplankton cysts) but also spine-shaped forms sometimes interpreted as animal body parts. Here, we examine new specimens of the problematic spine-shaped palynomorph Corollasphaeridium Martin (in Dean and Martin, 1982) and reassess its taxonomic composition and biological affinities. The type species C. wilcoxianum, known previously from Cambrian–Ordovician boundary intervals in western Canada and north China, is reported here from the Nolichucky Shale Formation of the USA and the Deadwood Formation of Canada and the USA, extending its biostratigraphic range back to the lower upper Cambrian (Furongian) or middle Cambrian (Miaolingian). The new specimens of C. wilcoxianum exhibit the diagnostic form of a trumpet-shaped conical body with 5–12 apical spines, a flaring and infolded aperture rim, and an ornamentation of branched ridges. In contrast, specimens from the lower Cambrian (Series 2, Stage 4) Forteau Formation of Canada are assigned to C. lissum sp. nov. In common with lower Cambrian species from Australia and Siberia, C. lissum lacks the ridged ornamentation of the type species, but shares a distinctive infolded aperture rim and is intermediate in shape, supporting reunification of all species under Corollasphaeridium. The morphology and size distribution of Corollasphaeridium are difficult to reconcile with a phytoplanktic cyst or animal identity, and instead support comparisons with the protective cases of protists such as amoebozoan tests and tintinnid ciliate loricae. Therefore, we interpret Corollasphaeridium as a loricate protist, albeit without clear synapomorphies with any extant group. The palaeoenvironmental and palaeogeographical distribution of Corollasphaeridium suggests localisation in nearshore habitats, possibly limited to warm-water environments (palaeo-tropics to subtropics). Our results further emphasise the hidden high-level taxonomic diversity among “acritarchs”.