Digging deeper: why we need more Proterozoic algal fossils and how to get them

Abstract

Known Proterozoic algal fossils raise compelling questions about the origin and diversification of cyanobacteria and eukaryotic algae, and their ecological influence in deep time. This Perspectives article describes particular examples of persistent evolutionary and biogeochemical issues whose resolution would be aided by additional algal fossil evidence from Proterozoic deposits, which have been the subjects of recent intensive study. New Proterozoic geosciences literature relevant to the early diversification of algae is surveyed. Previously underappreciated algal traits that might improve taxonomic attributions of fossil remains are highlighted. Processes that phycologists could use to improve detection of algal fossils are recommended. Potential geological sources of new Proterozoic fossils are suggested.

A one-billion-year-old multicellular chlorophyte

“…However, the morphological simplicity of these taxa means that diagnostic chlorophyte features are few and subject to evolutionary convergence 54 , 55 . The interpretation of Proterocladus as a chlorophyte and specifically as a siphonocladalean has also been questioned 10 , 11 , 18 , largely on the basis of its morphological simplicity 54 , 56 . Compared with previously described specimens of Proterocladus 17 , the new material reported here offers additional phenotypic features—including a differentiated holdfast, akinetes, siphonocladous organization, and distinct branching pattern—that strengthen a morphological comparison and suggest a phylogenetic affinity with siphonocladalean chlorophytes ( Supplementary Table 1 ).…”

“… 15 that questions its rhodophyte or even archaeplastid interpretation), the ~1.05 Ga Bangiomorpha 7 , and ~0.6 Ga florideophytes from the Doushantuo Formation 16 . Similarly, Proterozoic chlorophytes are represented by only one plausible genus, the ~0.72 Ma Proterocladus 17 , which is preserved as fragments and thus its phylogenetic interpretation has been questioned due to the scanty morphological information 10 , 11 , 18 . The poor record of Proterozoic archaeplastid fossils means large uncertainties in their stratigraphic ranges 19 , hence limiting their values as fossil calibrations in molecular clock studies.…”

Neoproterozoic origin and multiple transitions to macroscopic growth in green seaweeds

“…Similarly, the assignment of the middle Neoproterozoic filamentous fossil Proterocladus (ca. 750 Mya) to the Cladophorales (27,28) is questioned (3,29). Reliable chlorophytan fossils include resistant outer walls of prasinophyte cysts known as phycomata in Ediacaran and Paleozoic deposits (30,31) and fossils of siphonous seaweeds (Bryopsidales, Dasycladales) from the Cambro-Ordovician onward (32)(33)(34)(35)(36).…”

Phylotranscriptomic insights into a Mesoproterozoic–Neoproterozoic origin and early radiation of green seaweeds (Ulvophyceae)

“…Recently, a new species, P. antiquus, has been described from one-billion-year-old deposits 28 , prompting further interest in the phylogenetic and molecular clock analyses of the Ulvophyceae. Based on the apparent coenocytic thallus architecture and its resemblance with extant Cladophora and Cladophoropsis species, Proterocladus has been attributed to the Cladophorales, although its taxonomic placement remains contentious 18,29 . Thus, it is appropriate to more thoroughly evaluate Proterocladus in a phylogenetic context to better constrain the evolution of ulvophyceans.…”