Organic-walled microfossils from the lower Cambrian of North Greenland: a reappraisal of diversity

Abstract

The early Cambrian Buen Formation (North Greenland) hosts an exceptionally rich fossil biota that has contributed significantly to our knowledge of early metazoans, yet the fossil remains of primary producers from this deposit have received less attention. Here we examine the palynological component of the Buen Formation, with a focus on acritarchs and filamentous microfossils. Our analysis revealed the presence of 49 form taxa, 15 of which are described for the first time in the Buen Formation. These include large elements of presumably benthic origin, together with cyst-like acritarchs. Comasphaeridium longispinosum Vidal 1993 is renamed Comasphaeridium? brillesensis nom. nov., and Comasphaeridium densispinosum Vidal 1993 is reassigned to a new genus, Pearisphaeridium, becoming Pearisphaeridium densispinosum comb. nov. The diagnoses of Pearisphaeridium densispinosum (Vidal 1993) comb. nov. and Skiagia pura Moczydłowska 1988 are emended. Further, careful analysis of disparity in the recovered assemblage has revealed the presence of numerous transitional morphologies among the recorded acritarch form taxa. Though some of these transitional forms likely represent biologically meaningful entities (e.g. life cycle stages, ecophenotypes), others appear to have been artificially generated by taphonomic processes. Accounting for taphonomic factors and other sources of morphological variation has curtailed diversity down to 30 acritarch morphotypes, ten of which represent distinct abundance peaks broadly corresponding to acritarch genera. This analysis illustrates how population-based studies of early Cambrian acritarchs can help to discern the different factors that impinge on acritarch morphology, detect instances of taxonomic inflation, and refine our measures of diversity at the base of early Palaeozoic food webs.

Colonial green algae in the Cambrian plankton

“…By implication, comparable fossils might have gone unnoticed in other acritarch assemblages. Among the many reported clusters of Cambrian acritarchs, most lack any indication of ordered arrangement or in vivo cell attachments [ 23 , 24 ]. However, among specimens assigned to Synsphaeridium from the Buen Formation (Cambrian Stage 3–4 of Greenland) are examples with closely packed planar arrangements including a rounded plate with a central cell and seven surrounding, to more Tapetisphaerites -like forms, and a Kahfia -like ring with additional cells in the lumen (pl.…”

“…Even more pronounced is growth in Hydrodictyon, where zoospores of under 10 µm grow to mature cells of several millimetres or even centimetres in length [49]. The wide size range of Forteau cells compared to many Cambrian acritarch genera [24] is thus consistent with hydrodictyacean-type growth.…”

“…Some former acritarchs have been reclassified as the phycoma stages of green algal prasinophytes [ 21 ], and a case has been made for interpreting certain Cambrian (and Ediacaran) acritarchs as the life-history stages of unicellular green algae [ 9 , 17 , 22 ]. Recently, contrasting interpretations of colonial lifestyles have been mooted for Cambrian acritarchs preserved in monotypic clusters [ 23 ], although non-biotic means of cluster formation cannot be ruled out [ 23 , 24 ].…”