The Late Devonian Gogo Formation is noteworthy for its exceptional preservation of fossils in limestone nodules -- particularly vertebrates. Amazing examples of nearly three-dimensional fossil fishes have been collected, showing life-like articulation. These fossil fishes have been exceptionally influential in our understanding of early vertebrate evolution, since they reveal such exceptional details. Now, Gogo is revealing new, unexpected details: the oldest soft tissue preservation in jawed vertebrates.
A recently published paper by Trinajstic et al. in the journal Biology Letters presents the details of muscles, blood vessels and individual neurons in an extinct type of early jawed fish, the placoderms. Unfortunately, the figures are, for the most part, less than dazzling. Nevertheless, here are some examples for your edification.
a) Shows an individual muscle fiber; b) individual neuron connecting to a muscle fibre; c) capillaries (blood vessels); d) calcium phosphate crystals that make up the preserved tissues.
One of the important discoveries in this paper helps us understand how the placoderms are related to modern fishes. Over the decades, numerous hypotheses have been offered for how all the various groups of jawed vertebrates were related to each other, particularly how the fossils fit in. Fossils, of course, give us essential clues to how evolutionary transformations have taken place, but it is first important to know how they are related to each other and modern forms. Placoderms have been proposed as the sister group of sharks and their kin, of bony vertebrates, or as the most "primitive" of the jawed vertebrates.
What some of these partially articulated placoderms show is the morphology of the actual muscle blocks of the body axis.
These will add much to the debate on how placoderms may be related to modern lineages of jawed fishes. The authors of the paper note certain similarities to lamprey in these muscle blocks, suggesting that placoderms were the most primitive jawed vertebrates. However, I'm going to leave my discussion of it there and leave it to the reader to investigate this question more fully.
Update 19/02/2007: As somebody in the comments asked: how did these tissues get preserved. Yes, of course! These days, I'm so wrapped up in phylogenetic analsysi work of my own that I totally forgot about other interesting science! Yes, how are these soft tissues actually preserved.
Well, the important thing to point out is that they've been phosphatized, just like the Doushanto embryos. No these are not "fresh meat" as Karl in the comments says. So this is this really analogous to the preserved dinosaur soft tissue, either.
The authors of the paper rely on palaeoenvironmental information about the site to infer that the conditions were in fact anoxic at the immediate site of tissue preservation. In the absence of oxygen, the calcium precipitated in the local environment would've preferentially been calcium phosphate rather than calcium carbonate (limestone). The presence of microbes on the surfaces of the cells served to concentrate the calcium phosphate precipipation in the place of the tissues. Remember, bacterial cells are much, much smaller than differentiated animal cells and so an entire colony of bacteria encasing an animal cell can effectively create a facsimilie of the original thing! However, my competence of the geochemistry involved in this type of preservation is quite limited and if you're interested in knowing more, I suggest looking into the process of soft tissue phosphatization for yourself.
Trinajstic, K. et al. (in press) Exceptional preservation of nerve and muscle tissues in Late Devonian placoderm fish and their evolutionary implications. Biology Letters. link