Tuesday, February 13, 2007

Placoderm soft tissue preservation

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


Alex said...

Aw, c'mon Martin! How can you leave the discussion there? When it comes to all the fish and swimmy things, I rely on people like you to do the comparing and contrasting so I don't have to learn any more than absolutely necessary. But you're right, the figures themselves are less than dazzling. Hope all is well in Sweden.

Karl said...

How was the soft tissue preserved? Wikipedia tells me about anoxic environments, but I can't imagine fossilization preserved individual neurons and muscle fibers in stone. But I can't imagine fresh meat lasting 300 million years either!

Karl said...

Hey, can I jog you into a brief explanation of how the tissue could be preserved by telling you it's for an argument with some creationists?

Martin Brazeau said...

Hi Karl,

I'm not sure whether you noticed or not, but I added a short update to the end of the post that gives a little explanation about the preservation. You should look into the process of phosphatization.


Noumenon said...

First I felt embarrassed because I prodded you when you'd actually written a whole section of post for me. Then I felt glad anyway because I would not have noticed it if I hadn't been so pushy. Now I feel embarrassed again for waiting until I did the homework on this to let you know I read it. But in the time it took me, this post has already become the number one Google hit for soft tissue phosphatization, so maybe it needed to be written for the Internet to see. Thanks a lot-- Karl.

Danny said...

Take a look at this fossil. I say it looks like a sword fish. It was found in southern Virginia, USA in the Triassic zone near the famous Solite Quarry. Let me know that you think. Danny Ricketts
Here's the online picture:

Martin Brazeau said...

Hi Danny,

That photo isn't a body fossil of anything, it seems. It looks superficially like a swordfish, but it appears to be sedimentary structures. It could be a trace fossil, like the traces of fins or toes as they hit the substrate.