Phytoplankton dynamics from the Cambrian Explosion to the onset of the Great Ordovician Biodiversification Event: A review of Cambrian acritarch diversity

Abstract

Most early Palaeozoic acritarchs are thought to represent a part of the marine phytoplankton and so constituted a significant element at the base of the marine trophic chain during the 'Cambrian Explosion' and the subsequent 'Great Ordovician Biodiversification Event.' Cambrian acritarch occurrences have been recorded in a great number of studies. In this paper, published data on Cambrian acritarchs are assembled in order to reconstruct taxonomic diversity trends that can be compared with the biodiversity of marine invertebrates. We compile a database and calculate various diversity indices at global and regional (i.e. Gondwana or Baltica) scales. The stratigraphic bins applied are at the level of the ten Cambrian stages, or of fourteen commonly used biozones in a somewhat higher resolved scheme. Our results show marked differences between palaeogeographical regions. They also indicate limitations of the data and a potential sampling bias, as the taxonomic diversity indices of species are significantly correlated with the number of studies per stratigraphic bin. The total and normalized diversities of genera are not affected in the same way. The normalized genus diversity curves show a slow but irregular rise over the course of the Cambrian. These also are the least biased. A radiation of species and to a lesser extent of genera in the 'lower' Cambrian Series 2 appears to mirror the 'Cambrian Explosion' of metazoans. This radiation, not evident on Gondwana, is followed by a prominent low in species diversity in the upper Series 3 and lower Furongian. Highest diversities are reached globally, and on both Baltica and Gondwana, in the uppermost Cambrian Stage 10, more precisely in the Peltura trilobite Zone, preceding a substantial phase of acritarch species extinction below and at the Cambrian/Ordovician boundary. Nearly all the genera present in Stage 10 survived into the Ordovician. The forms that emerged during the Cambrian therefore became the foundation for the more rapid radiation of acritarchs during the 'Great Ordovician Biodiversification Event'.

The Great Ordovician Biodiversification Event (GOBE): definition, concept and duration

“…At a larger scale, phytoplankton diversity increased continuously from the early Cambrian to the Middle-Late Ordovician (Fig. 3), mirroring to some extent the biodiversity of marine invertebrates, at least during the Cambrian (Nowak et al 2015), and possibly also during the Ordovician (Hints et al 2010).…”

“…), mirroring to some extent the biodiversity of marine invertebrates, at least during the Cambrian (Nowak et al . ), and possibly also during the Ordovician (Hints et al . ).…”

“…The diversity of the phytoplankton, considered a key part of the base of the marine trophic chain, 'exploded' in the late Cambrian (Nowak et al 2015), followed by a slow but continuous increase of phytoplankton diversity during the Early and Middle Ordovician (Servais et al 2004(Servais et al , 2008. At a larger scale, phytoplankton diversity increased continuously from the early Cambrian to the Middle-Late Ordovician (Fig.…”

Possible patterns of marine primary productivity during the Great Ordovician Biodiversification Event

“…Nowak et al . () effectively observed a dramatic increase in diversity of the acritarchs, that is the organic‐walled fraction of the phytoplankton, in the late Cambrian – Early Ordovician interval. But do the higher diversities of phytoplanktonic organisms also indicate an increased abundance of phytoplankton and an increased biomass?…”

“…Based on Martin et al (2008), Servais et al (2016) further considered that the increasing presence of planktonic organisms in the late Cambrian -Early Ordovician must coincide with increasing nutrient supply, increased primary productivity and expanded biomass production that resulted during the initiation of the GOBE with a higher diversity and increased abundance of plankton-feeding groups during the Ordovician. Nowak et al (2015) effectively observed a dramatic increase in diversity of the acritarchs, that is the organic-walled fraction of the phytoplankton, in the late Cambrian -Early Ordovician interval. But do the higher diversities of phytoplanktonic organisms also indicate an increased abundance of phytoplankton and an increased biomass?…”