The earliest lichens

Lichens are not individual species, although they are given Linnaean names, but symbiotic associations of two or more species.  In the lichens the mutual relationship is between entirely different organisms: fungi with either algae or blue-green bacteria.  Although lichen form one of the plagues set to try geologists, their fossil record is extremely sparse.  Once again, Chinese lagerstätten in the Doushantuo Formation establish a first, in this case preserved in phosphorites (Yuan, X. et al. 2005.  Lichen-like symbiosis 600 million years ago.  Science, v. 308, p. 1017-1020).  The fossils show exquisite detail, sufficient to reveal both fungus-like hyphae and cells that resemble those of cyanobacteria.  They are from the late Neoproterozoic, Ediacaran period, when all manner of evolutionary developments were taking place.  One question that is unanswered is whether or not these fossils were marine or subaerial.  Modern lichens are intolerant of salt water.


Since the 1960s claims have been made for the oldest living organism being found in brine inclusions from salt deposits, and most have been dismissed as modern contaminants.  In 2000 that easy avoidance was ruled out by super-sterile culturing of the contents of a fluid inclusion in a Permian halite crystal from New Mexico (Vreeland, R.H. et al. 2000.  Isolation of a 250 million-year-old halotolerant bacterium from a primary salt crystal.  Nature, v. 407, p. 897-900).  The research produced a culture of a salt-tolerant bacterium that was dubbed Virgilbacillus.  However, the odd nature of the crystal could have formed much later than the deposition of the salt beds.  Confirming a Permian age for a fluid inclusion is not easy.  One approach is by comparing the composition and formation temperature of the bacterium-hosting fluid with that from other, more usual inclusions in the same deposit and from fluids that form when salt deposits are exposed to air (“weeps”), as might be included when salt deposits recrystallise long after their formation (Satterfield, C.L. et al. 2005.  New evidence for 250 Ma age of halotolerant bacterium from a Permian salt crystal.  Geology, v. 33, p. 265-268).  The study found that the inclusion fluids along with others from halite at the same level in the salt deposit have significantly different compositions from “weeps”.  The latter reflect the composition of the salts in the deposit which formed by precipitation of the less soluble components of seawater.  The inclusions have compositions more like sea water that has been concentrated by evaporation, albeit different from that of modern halite inclusions.  So it does indeed seem as if Virgilbacillus is a Permian creature.  Yet to emerge are DNA analyses that can be compared with modern salt-tolerant bacteria.


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