A new(ish) paper in National Science Review evaluates the evidence for various interpretations of Ediacaran microfossils from the Weng’an biota in South China (Xiao et al. 2014. The Weng’an biota and the Ediacaran radiation of multicellular eukaryotes. Natl. Sci. Rev., 1:498–520.). I recommend checking it out; it’s open access, and there’s a lot of interesting stuff in there that I’m not going to address.
These fossils are undoubtedly multicellular, probably eukaryotic, and extremely enigmatic. Their age (582-600 million years) means they could have important implications for the evolution of multicellularity, and their exceptional preservation in great numbers creates the potential for reconstructing their life cycles in great detail. Some of the Weng’an fossils have been interpreted as volvocine algae, an interpretation that I find highly unlikely.
Some of the Weng’an fossils are thought to represent red algae, and this would not be terribly surprising, since red algae have been around for at least 1.2 billion years. Others, for example the tubular fossils, are more problematic, with interpretations as diverse as cyanobacteria, eukaryotic algae, crinoids, and cnidarians.
Among the most interesting of the Weng’an fossils, and the main focus of the Xiao et al. article, is Megasphaera. Megasphaera consists of one or more cells enclosed within a smooth or ornamented envelope. Although they were initially described as different genera, fossils in this group with different numbers of cells are now thought to represent different developmental stages of one or a few related species. As a result, Xiao et al. refer to all of them as Megasphaera. ‘Parapandorina-stage’ and ‘Megaclonophycus-stage’ refer to the genera in which these fossils were formerly classified.
Some Megasphaera fossils are reminiscent of volvocine algae, and like (most) volvocine algae, they are thought to have undergone palintomic division (also known as multiple fission). In palintomic species, cells grow to many times their initial size and then undergo several rounds of division with little or no growth between divisions, so that cells are essentially halved in volume with each subsequent round. This would explain why Megasphaera fossils with very different numbers of cells are nearly the same size.
The volvocine interpretation of Megasphaera is tempting: there’s the palintomy, plus just look at H and J in Figure 10 above! Would it really shock you if those were electron micrographs of Pandorina and Volvox, respectively?
Sadly, though, the problems with a volvocine interpretation outweigh the evidence for it. First, the fossils are much too old; colonial volvocine algae probably originated around 200 million years ago. Of course, my estimates could be wrong, but they’re four or so times older than previous estimates, and they’d need to be three times older still to be consistent with a volvocine Megasphaera. Second, the Weng’an deposits are marine, and all known volvocine algae live in freshwater*. Again, it’s not impossible that the volvocine algae started in the ocean, spread to freshwater, and the marine lineages subsequently went extinct, but it doesn’t seem likely. Finally, the Megaclonophycus stage (I-J in Fig. 10 above) is not a hollow spheroid like Volvox, but a solid mass of cells, and this doesn’t really work for the volvocine style of locomotion. Xiao et al. conclude that Megasphaera as a multicellular eukaryotic alga “cannot be excluded,” but that a volvocine affiliation is unlikely, and I agree with their assessment. There have been several reports of volvocine fossils, but none that I believe (and from talking with people at the Volvox and Chlamydomonas meetings, none that anyone else believes, either). Maybe someday I’ll get around to writing a post on that.
Another possible interpretation of Megasphaera is as an animal, animal relative, or animal embryo. The idea that any Ediacaran fossils may represent animals is an interpretation that cdesign proponentsists do not like, because it would weaken their argument that the origins of animal body plans during the Cambrian explosion was too fast to be explained by unguided evolution.
So what is Megasphaera? An early animal or ancestor of animals is possible; so is a multicellular (non-volvocine) alga. It’s also possible that it doesn’t fit into any extant group, but rather a lineage that has since gone extinct. The evidence for any particular group is of the ‘cannot be exluded’ type, and this isn’t a situation in which process of elimination works very well, since we don’t really know the size of the set we’re eliminating from. For now I think we have to admit that we just don’t know.
*Or damp soil, but not in saltwater.