In my post on Bangiomorpha, I said
…Bangiomorpha was probably a red alga. This conclusion seems to be accepted by most everyone in the field. In fact, I don’t know of any dissenters, and that kind of consensus is rare for fossils this old.
I guess I didn’t look hard enough, because reader not the FTB Stewart commented
Cavalier-Smith dissents (dissented?) from the consensus
Sure enough, in a 2002 paper Thomas Cavalier-Smith, who is a serious bigshot in eukaryotic systematics, said
Earlier large Precambrian fossils, notably the mid to late Proterozoic ‘bangiophyte red algae’ (Butterfield et al., 1990; Butterfield, 2000) and the 2.1-Gy-old early Proterozoic Grypania (Han & Runnegar, 1992), are more likely
cyanobacteria; an origin for red algae or any other macroscopic eukaryotic algae much earlier than animals is entirely implausible given the large number of molecular trees that indicate that they are of approximately equal age (e.g. Moreira et al., 2000; Baldauf et al., 2000; Cavalier-Smith, 2002). If one tries to integrate the sequence trees and the fossil record in the most parsimonious way, the most reasonable estimate for the origin of plastids is only about 570 My ago (Cavalier-Smith, 2002). Given that concatenated protein trees (Moreira et al., 2000) indicate that red algae probably diverged from green plants after glaucophytes, it is highly improbable that red algae originated much before 500 My ago. Accepting the accuracy of these identifications of 2.1- or 1.2-Gy-old fossils and thereby implicitly postulating 1600 My of cryptic eukaryotic evolution in which macroorganisms never became diverse would be evolutionarily untenable. I interpret the 1.2-Gy-old Bangiomorpha (Butterfield, 2000), which has the best cell preservation, as a slightly more complex than usual, Oscillatoria-like cyanobacterium. Although it has a few features of cell arrangement that make it resemble the red alga Bangia still more, I think they are almost certainly convergent and well within the capacity of a filamentous cyanobacterium to evolve. The so-called different spore sizes on separate plants might have nothing to do with the analogous situation in Bangia. They might simply be two different cyanobacterial species. There are no features of these fossils that require them to be eukaryotic. If Bangiomorpha was eukaryotic, we should be totally at a loss to explain why eukaryotes failed to diversify and leave millions of unambiguously eukaryotic fossils earlier than 850 My ago. [emphasis added]
Nor has his view changed, apparently:
There is no convincing evidence that these early acritarchs are even crown eukaryotes – that assumption seems refuted by dated multigene analyses (Eme et al., 2014); their estimates for the age of red algae (thus a maximum age for chromists) range hugely from 636 to 1206 My depending on assumptions, the most realistic favouring the youngest dates, favouring the view that Bangiomorpha is probably not a red alga (Cavalier-Smith, 2006a)…Though error margins are wide, these results imply that crown eukaryotes are no older than ∼1.2 Gy and support the unpopular but carefully argued view that many palaeontologists have been too ready to accept doubtful evidence for substantially earlier origins of eukaryotes, plants, and chromists (Cavalier-Smith, 2002a; Cavalier-Smith, 2006a; Cavalier-Smith, 2010b). [emphasis added]
Only papers by Cavalier-Smith himself are cited in support of Bangiomorpha not being a red alga, suggesting that he may be a lone dissenter. Nevertheless, I was clearly too quick to call consensus on this one.
Butterfield NJ (2000) Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes. Paleobiology, 26, 386–404.
Cavalier-Smith T (2002) The neomuran orign of archaebacteria, the negibacterial of the universal tree and bacterial megaclassification. Int J Syst Evol Microbiol 52:7–76.
Cavalier-Smith, T., E. E. Chao, and R. Lewis. (2015) Multiple origins of Heliozoa from flagellate ancestors: New cryptist subphylum Corbihelia, superclass Corbistoma, and monophyly of Haptista, Cryptista, Hacrobia and Chromista. Molecular Phylogenetics and Evolution 93:331–362.