Morphometric analysis ofSkiagia-plexus acritarchs from the early Cambrian of North Greenland: toward a meaningful evaluation of phenotypic plasticity

Abstract

The Cambrian evolutionary radiations are marked by spectacular biotic turnovers and the establishment of increasingly tiered food chains. At the base of these food chains are primary producers, which in the Cambrian fossil record are chiefly represented among organic-walled microfossils. The majority of these microfossil remains have traditionally been attributed to an informal category of incertae sedis called “acritarchs,” based entirely on form taxonomy. Acritarch form taxa have been intensely used for biostratigraphy and in large-scale studies of phytoplankton diversity. However, both prospects have been challenged by cases of taxonomic inconsistencies and oversplitting arising from the large phenotypic plasticity seen among these microfossils. The acritarch form genus Skiagia stands as an ideal case study to explore these taxonomic challenges, because it encompasses a number of form species widely used in lower Cambrian biostratigraphy. Moreover, subtle morphological differences among Skiagia species were suggested to underlie key evolutionary innovations toward complex reproductive strategies. Here we apply a multivariate morphometric approach to investigate the morphological variation of Skiagia-plexus acritarchs using an assemblage sourced from the Buen Formation (Cambrian Series 2, Stages 3–4) of North Greenland. Our analysis showed that the species-level classification of Skiagia discretizes a continuous spectrum of morphologies. While these findings bring important taxonomic and biostratigraphic hurdles to light, the unequal frequency distribution of life cycle stages among Skiagia species suggests that certain elements of phytoplankton paleobiology are nonetheless captured by Skiagia form taxonomy. These results demonstrate the value of using morphometric tools to explore acritarch phenotypic plasticity and its potential ontogenetic and paleoecological drivers in Cambrian ecosystems.

Colonial green algae in the Cambrian plankton

“…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.…”

“…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.…”

“…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].…”

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

“…The issue of continuous variation among established acritarch taxa is increasingly being acknowledged (e.g. Stricanne and Servais 2002; Servais et al 2004; Kroeck et al 2021; Wallet et al 2022). Nevertheless, how best to accommodate such continua of forms into rigid classification schemes relying on discrete morphological categories has yet to be resolved.…”

“…In this context, Baltisphaeridium was dismantled into Skiagia (Downie 1982), Comasphaeridium (Downie 1982), Globosphaeridium (Moczydłowska 1991) and Lophosphaeridium species (Moczydłowska 1991); while Micrhystridium was split into Asteridium, Ammonidium , Heliosphaeridium and Comasphaeridium species (Lister 1970; Moczydłowska 1991; Sargeant and Vavrdová 1997). Although the splitting of ‘waste-basket’ taxa has substantially refined acritarch taxonomy and biostratigraphy, it has brought about a noticeable degree of confusion (Stricanne and Servais 2002; Servais et al 2004; Mullins and Servais 2008) when applied in practice for two main reasons: (1) the partial overlaps existing between many acritarch species make identification equivocal, and conceal the presence of transitional forms (Wallet et al 2022; Figure 18); and (2) the submicron-scale diagnostic features used for distinguishing between certain acritarch taxa require a combination of high optical resolution and exquisite state of preservation to be reliably detected. While the former likely obscures our understanding of phytoplankton palaeobiology by artificially splitting ontogenetic or ecophenotypic sequences into numerous form-taxa, the latter implies that imperfectly preserved acritarchs cannot be identified with certainty.…”