PRASINOPHYTE WORLD: BIODIVERSITY OF ORGANIC-WALLED MICROFOSSILS FROM THE TONIAN VISINGSÖ GROUP, SWEDEN

Primary diversification of the eukaryotic single-celled, organic-walled microorganisms reached its peak in the early Neoproterozoic. The Tonian Visingsö Group in southern Sweden has yielded exceptionally preserved palynomorphs, including the fossils of characteristic prasinophyte morphologies.

Microfossils were recovered from a drill core through the Visingsö Group, which is also exposed on the Vising Island and along cost of Lake Vättern. Time interval for the deposition of the Visingsö Group is c. 740-800 Ma, constrained by the biostratigraphic correlation with isotopically dated successions and the last appearance of the vase-shaped microfossils (VSM). The studied fossiliferous upper formation contains well laminated, dark grey, micaceous shales. Microfossils were extracted by a standard palynological method of acid maceration and filtering. They were studied with light and scanning electron microscopy, and compared to extant protists for the purpose of reconstructing their phylogenetic relationship to modern groups, as well as to other early eukaryotic microorganisms.

Diversity rises through the Visingsö succession from spheroidal to ornamented morphotypes. The distinct taxa in the Visingsö assemblage are Pterospermopsimorpha, Simia, Tasmanites, Cerebrosphaera, Vandalosphaeridium, Trachysphaeridium, Chuaria, Kildinella, multiple leiosphaerids and unnamed acanthomorphs, in addition to the colonial Synsphaeridium.

Certain microfossils share morphological characters with extant prasinophyte algae such as Pterosperma and Pachysphaera. Likewise, the presence of an envelope around a central body in some taxa is indicative of a phycoma – a resilient reproductive structure, produced by present-day prasinophytes. Prasinophycean algae have a long evolutionary history, appearing in the Mesoproterozoic and diversifying in the Tonian. Even though the group is paraphyletic, the prasinophytes represented in the fossil record (order Pyramimonadales that includes Pterosperma-like and tasmanitid taxa) is monophyletic and well resolved. Visingsö microfossils with well-preserved morphological characters provide a detailed insight into the early evolution of this ancient algal group.