On a Monday morning when I am feeling every little nuance of the fact that it's a Monday morning, it seems appropriate to discuss a section of organismal diversity whose study seems pretty severely crippled before it has even begun. I speak of the study of fossil bacteria, and the subject of today's Taxon of the Week post is the Proterozoic fossil taxon Myxococcoides.
Myxococcoides is a small (1-35 µm) spherical to ellipsoidal fossil without distinctive ornamentation or other visible features found either singly or in loose colonies without an enclosing sheath or other distinct colony shape. It is an oft-repeated, but perhaps little appreciated, fact that bacteria were around and about long before a few of them considered getting together and making a eukaryote. I mean really long before. The earliest evidence of bacteria in the fossil record dates back nearly four billion years, while the earliest unequivocal evidence for eukaryotes is only about 850 million years old* (Cavalier-Smith, 2002). In other words, fully three-quarters of the history of life on this planet is represented only by prokaryotes. Only members of a species with severely anthropocentric delusions of grandeur would imagine that biodiversity did nothing in all that time except twiddle its thumbs and wait for the nucleus to develop, but there are some serious hurdles to understanding what was happening for the first three billion years of life.
*It will probably come as no surprise that the earliest date for eukaryotes is rather debatable - Cavalier-Smith (2002) gives a brief, if somewhat partisan, review. The 850 Mya date represents the earliest appearance of protist fossils of eukaryote cell size and complex cell morphology that implies the existence of a well-developed microfilament skeleton to hold it all in place. Certain fossils dating back as far as 1200 Mya or even 2100 Mya have been identified as eukaryotes, such as the putative "red alga" Bangiomorpha. However, these taxa have fairly simple cell morphologies and their identification as eukaryotes rather than prokaryotes rests on relatively few characters such as cell size. As argued by Hofmann (1976), supposed 'nuclei' in fossil cells may represent degradational artefacts where cytoplasm has become detached from the surrounding cell wall. While prokaryote cells are generally much smaller than eukaryote cells, bacteria can occassionally reach considerable sizes - the largest known bacterium, the sulphur-oxidizing Thiomargarita namibiensis, has cells almost a millimetre in diameter, a size that, as pointed out by Schütt (2003), is more than twice that of the smallest known spiders, which is a great piece of information to bring up at parties (technically, some actinobacteria such as Streptomyces are arguably even larger, but have a fungus-like filamentous hyphal morphology). It is therefore a perilous activity to label Proterozoic fossils as eukaryotes on the basis of size alone, especially as it is not unlikely that bacteria may have occupied a number of niches prior to the appearance of eukaryotes from which they were later excluded.
Lacking as they do the well-developed eukaryote cytoskeleton, the morphology of most prokaryotes is decidedly simple, with the majority of taxa conforming to the basic rod or sphere. For instance, Thermoproteus and Mycobacterium are both rod-shaped prokaryotes with colonies formed through snapping division that may be morphologically almost indistinguishable despite one being a archaebacterium and the other a Gram-positive eubacterium. Instead, bacterial taxa are generally distinguished by features of their genetics, biochemistry and physiology - all features that, of course, are generally completely unavailable when studying fossilised remains. As a result, taxa based on fossilised bacteria are doomed to be form taxa or morphotaxa - labels to indicate a particular morphology without necessarily indicating the actual relationships of the fossils involved. To complicate matters further, a single living morphology may potentially give rise to multiple fossil 'taxa' due to the level of degradation prior to preservation, as shown in the figure below from Hofmann (1976) of various stages of degradation from a Myxococcoides-like morphology.
Needless to say, the relationships of forms such as Myxococcoides to modern taxa is difficult if not impossible to establish. Most Precambrian fossil bacteria have been found in association with stromatolites and interpreted as cyanobacteria. They have then been assigned to modern orders on the basis of colony morphology, so forms without defined colony structures such as Myxococcoides have been assigned to the Chroococcales. However, phylogenetic analysis of recent taxa has shown that the Chroococcales (not surprisingly seeing as it was defined solely on negative characters) is a strongly paraphyletic assemblage from which filamentous forms have arisen polyphyletically (Litvaitis, 2002).
So why, some of you may be asking yourselves at this point, should we study fossil bacteria at all? Well, the simple fact is that, murky as it is, the bacterial fossil record remains our main window into three billion years of evolution. Some distinctive probable cyanobacterial groups, such as the family Aphralysiaceae (Vachard et al., 2001), have been identified solely from fossils, while others, such as the stromatolite-forming Entophysalidaceae, held far more ecological significance in the past than presently. If, as alluded to above, forms such as Grypania and Bangiomorpha represent prokaryotes convergent on eukaryotes that were later replaced by actual eukaryotes, then such diversity would have remained unknown except through the fossil record. Three billion years is a long time to miss out on.
Cavalier-Smith, T. 2002. The neomuran origin of archaebacteria, the negibacterial root of the universal tree and bacterial megaclassification. International Journal of Systematic and Evolutionary Microbiology 52: 7-76.
Hofmann, H. J. 1976. Precambrian microflora, Belcher Islands, Canada: significance and systematics. Journal of Paleontology 50 (6): 1040-1073.
Litvaitis, M. K. 2002. A molecular test of cyanobacterial phylogeny: Inferences from constraint analyses. Hydrobiologia 468: 135-145.
Schütt, K. 2003. Phylogeny of Symphytognathidae s.l. (Araneae, Araneoidea). Zoologica Scripta 32 (2): 129-151.
Vachard, D., M. Hauser, R. Martini, L. Zaninetti, A. Matter & T. Peters. 2001. New algae and problematica of algal affinity from the Permian of the Aseelah Unit of the Batain Plain (East Oman). Geobios 34 (4): 375-404.