Bacterial fossil doubts resolved

I raised a few questions about those 3.4 billion year old bacterial fossils, primarily that I was bugged by the large size and that they cited a discredited source to say that they were in the appropriate range of diameters for bacteria. Now my questions have been answered by Chris Nedin, and I’m satisfied. In particular, he shows data from 0.8 and 1.9 billion year old fossils in which the bacterial sizes are in the same range. It’s also a good review of the other evidence used to infer that they actually are bacterial microfossils.

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A questions about those ancient bacterial fossils…

Both Jerry Coyne and Larry Moran have good write-ups on the recent discovery of what are purportedly the oldest fossil cells, at 3.4 billion years old. I just have to add one little comment: a small, niggling doubt and something that bugs me about them. All the smart guys are impressed with this paper, but this one little thing gives me pause.

I’m a microscopist — I look at micrographs all the time, and one of the things I always mentally do is place the size of things in context. And I was looking at the micrographs of these fossils, and what jumped out at me is how large they are. They’re not impossibly large, they’re just well out of the range I expect for prokaryotes.

Most prokaryotes have diameters in the range of 1-10µm, while typical eukaryotes are about 10 times that size. There are exceptions: Thiomargarita gets up to 500µm across, so like I say, there’s nothing impossible about these cells, it’s just that the micrographs show lots of cells with 10-30µm diameters. And the authors come right out and report that:

The size range is also typical of such assemblages, with small spheres and ellipsoids 5–25 µm in diameter, rare examples (<10) of larger cellular envelopes up to 80 µm in diameter, and tubes 7–20 µm across (see ref. 24).

How odd. When I poke into the nervous system of an embryonic insect or fish, those are the sizes of cells I often see (well, except there aren’t many tubes of that size!). When I poke into a culture or embryo contaminated with bacteria, they’re much, much smaller. So maybe paleoarchaean bacteria tended to be larger? And they do cite a source for that size range of prokaryotes…

Then here’s a new problem: the source cited, ref. 24, is the Schopf paper, the older paper that claimed to have found ancient bacterial fossils, a claim that has since been discredited! Uh-oh. What they’re calling “typical of such assemblages” is a data set that’s widely considered artifactual now. Furthermore, that’s a simplified version of what Schopf said — he actually broke the sizes down into categories, and the range was more like 1-30 µm.

  • Very small solitary, paired or clustered rods (ca 0.75 µm broad, ca 1.5 µm long), inferred to be prokaryotic (bacterial) unicells: one unit (ca 2600 Myr old), one morphotype.
  • Small, solitary, paired or clustered coccoids (average diameter ca 3 µm, range ca 2–5 µm), inferred to be prokaryotic (bacterial, perhaps cyanobacterial) unicells: three units (range 3320–2600 Myr old), three morphotypes.
  • Large solitary or colonial coccoids (average diameter ca 13 µm, range ca 5–23 µm), inferred to be prokaryotic (bacterial, perhaps cyanobacterial) unicells: three units (range 3388–2560 Myr old), four morphotypes.
  • Narrow unbranched sinuous filaments (average diameter ca 1.25 µm, range ca 0.2–3 µm), with or without discernable septations, inferred to be prokaryotic (bacterial, perhaps cyanobacterial) cellular trichomes and/or trichome-encompassing sheaths: 10 units (range 3496–2560 Myr old), 17 morphotypes.
  • Broad unbranched septate filaments (average diameter ca 8 µm, range ca 2–19.5 µm), inferred to be prokaryotic (perhaps cyanobacterial) cellular trichomes: four units (range 3496–2723 Myr old), 10 morphotypes.
  • Broad unbranched tubular or partially flattened cylinders (average diameter ca 13 µm, range ca 3–28 µm), inferred to be prokaryotic (perhaps cyanobacterial) trichome-encompassing sheaths: five units (range 3496–2516 Myr old), five morphotypes (e.g. figures 3a–e and 4l).

So Schopf was reporting larger cells in his older samples, and now Wacey et al. are describing what look like very large cells to me in their 3.4 billion year old rocks. I’m not a microbiologist so I could be way off on this, but…isn’t this just a little bit strange? Maybe there are some micro people out there who can reassure me that this isn’t a surprising result.

Wacey D, Kilburn MR, Saudners M, Cliff J, and Brasier MD (2011) Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia. Nature Geoscience Published online Aug. 21, 20110 [doi:10.1038/ngeo1238]

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Magnificent momma

This is one beautiful plesiosaur, Polycotylus latippinus.

(Click for larger image)

(A) Photograph and (B) interpretive drawing of LACM 129639, as mounted. Adult elements are light brown, embryonic material is dark brown, and reconstructed bones are white. lc indicates left coracoid; lf, left femur; lh, left humerus; li, left ischium; lp, left pubis; rc, right coracoid; rf, right femur; rh, right humerus; ri, right ischium; and rp, right pubis.

The unique aspect of this specimen is that it’s the only pregnant plesiosaur found; the fore and hind limbs bracket a jumble of bones from a juvenile or embryonic Polycotylus. It’s thought to actually be a fetal plesiosaur, rather than an overstuffed cannibal plesiosaur, because 1) the smaller skeleton is still partially articulated, and it’s large enough that it is unlikely it could have been swallowed whole, 2) the two sets are of the same distinctive species, 3) the juvenile is incompletely ossified and doesn’t resemble a post-partum animal, 4) the bones aren’t chewed, etched by acids, or accompanied by gastroliths. I think we can now confidently say that plesiosaurs were viviparous, which is what everyone expected.

There are other surprising details. The fetus is huge relative to the parent, and there’s only one — so plesiosaurs had small brood sizes and invested heavily in their offspring.

(Click for larger image)

Reconstructions of female P. latippinus and newborn young. Gastralia were present in both animals but have been omitted for clarity.

The authors speculate beyond this a bit, but it’s all reasonable speculation. That degree of parental investment in fetal development makes it likely that there would have been extended maternal care after birth, and rather more tenuously, that they may also have lived in larger social groups. The authors suggest that their lifestyle may have resembled that of modern social marine mammals — picture a pod of dolphins, only long-necked and lizardy.

O’Keefe FR, Chiappe LM (2011) Viviparity and K-Selected Life History in a Mesozoic Marine Plesiosaur (Reptilia, Sauropterygia) Science 333 (6044): 870-873.

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