Archive for the 'Fossils'

If you ever get a chance, spend some time looking at fish muscles in a microscope. Larval zebrafish are perfect; they’re transparent and you can trace all the fibers, so you can see everything. The body musculature of fish is most elegantly organized into repeating blocks of muscle along the length of the animal, each segment having a chevron (“V”) or “W” shape. Here’s a pretty stained photo of a 30 hour old zebrafish to show what I mean; it’s a little weird because this one is from an animal with experimentally messed up gene expression, all that red and green stuff, but look at the lovely blue muscle fibers stretching across the length of each segment. Lateral view of a 30 hour old zebrafish, embryonic myotome, displaying ectopic eng2a:eGFP (green) fibers in response to Smad7 (marked in red) expressing clones. Slow muscle fibers are marked by monoclonal antibody F59 (blue). But wait—why the chevron shape? Also, the reason I told you to look in a microscope sometime is that a 2-D image can’t illustrate the lovely intricacy of the muscles; they don’t go straight across, but twist in a partial spiral in 3-dimensions. Trust me, it’s beautiful to see. And it’s also nearly universal in chordates — even Amphioxus has this arrangement. And there’s a good biomechanical reason for this arrangment. Read more

A while back, I told you all about this small piece of the biochemistry of the fly eye — the pathways that make the brown and red pigments that color the eye. I left it with a question: if even my abbreviated summary revealed considerable complexity, how could this pathway evolve? Changing anything produces a failure or change in the result. Before I answer, let’s make the problem even harder, because I love a challenge (although actually, I’m cheating — it’s going to turn out that complexity is not a barrier, but an opportunity). The pigment pathways above are far downstream: they operate in the differentiated compound eye of the fly. Long before that, there are a set of genes that have to be activated first to trigger formation of the head and eye in the larva. And this is that pathway: Regulatory scheme on the top of the eye developmental pathway. Twin of eyeless (toy), eyeless (ey), and possibly eyegone (eyg), three Pax genes, are master control genes on the top of the hierarchy. Sine oculis (so), eyes absent (eya), dachshund (dac), and optix are second-order transcription factors regulated by the master control genes. Note that the pathway is not linear, but rather a network interconnected by feedback loops. At the top of the hierarchy are two genes in Drosophila, eyeless (ey) and twin of eyeless (toy). Remember, genes are named for their mutant effect, so the normal function of eyeless is to initiate eye development. These genes switch on sine oculis and eyes absent (notice the effort to find synonyms to describe genes that cause missing eyes when broken) that activate each other and feed back on eyeless to generate a robust response. Another gene, dachshund (this one named for another part of its phenotype: it makes flies with very short legs) also feeds back on eyeless. This circuit has multiple outputs: so, dac, optix and eyg. All of these have effects further downstream, in that catch-all category labeled “eye...
Read more

I’ve written about the spectacular phospatized embryos of the Doushantuo formation before. It’s a collection of exceptionally well preserved small multicellular organisms, so well preserved that we can even look at cellular organelles. And they’re pre-Cambrian, as much as 630 million years old. They’ve been interpreted as fossilized embryos for which we have no known adult forms. They certainly look like embryos, but one thing has always bothered me — they all look like blastula-stage embryos at various points in their early divisions, and the absence of later stages was peculiar: how did gastrulae and neurulae and other stages avoid getting preserved? One explanation was that we weren’t seeing metazoan fossils at all — they were colonies of large bacteria. That’s disappointing if you have an animal bias, but still cool — as I pointed out then, it just highlights the fact that the transition from single-celled to multi-celled life isn’t that remarkable. Now we have another alternative explanation that seems even better to me: they aren’t animals, and they aren’t bacteria, they’re protists. Some of the Doushantuo specimens are rather peanut-shaped, and others are vermiform, odd for an animal embryo, but entirely compatible with the idea that these are encysted stages of propagating protists. Here are some of these oddly shaped Doushantuo specimens. Tianzhushania from the Ediacaran Doushantuo Formation, Datang Quarry, Weng’an, Guizhou Province, China. (A) Regular and (B to J) irregular forms, the latter interpreted to be in the germinating stage: MESIG 10022 Read more
Read more

Look with your puny camera eyes! Some new specimens of Anomalocaris, the spectacular Cambrian predator, have been discovered in South Australia. These fossils exhibit well-preserved eyes, allowing us to see that the bulbous stalked balls on their heads were actually fairly typical compound eyes, like those of modern insects. Anomalocaris eyes from the Emu Bay Shale. a–d, Eye pair, SAM P45920a, level 10.4 m. a, b, Overview and camera lucida drawing. Scale bars, 5 mm. Grey fill in b represents visual surface, the proximal part in the upper eye extrapolated from the lower eye. c, Detail of ommatidial lenses located by horizontal white box in a. Scale bar, 1 mm. d, More complete eye, showing transition between visual surface and eye stalk (white arrows). Scale bar, 2 mm. e, Detail of ommatidial lenses in counterpart SAM P45920b. Scale bar, 0.3 mm. es, eye stalk; I.c., Isoxys communis; us, undetermined structure; vs, visual surface. Tilted white box in a represents area analysed using SEM-EDS. The cool part of this discovery: the investigators were able to count the density of lenses and estimate how many were present in the intact eye. The number is 16,000 ommatidia in each eye, which is more than a little impressive: to put it in context, Drosophila has about 800. The emphasis on high-resolution vision suggests that Anomalocaris was diurnal predator in shallow water. Oh, and just in case you’re one of those strange beings who isn’t instantly familiar with what the anomalocarids looked like, here’s a video to remind you. Paterson JR, García-Bellido DC, Lee MS, Brock GA, Jago JB, Edgecombe GD (2011) Acute vision in the giant Cambrian predator Anomalocaris and the origin of compound eyes. Nature 480(7376):237-40. (Also on Sb)

A few weeks ago, PLoS One published a paper on the observation of preserved chitin in 34 million year old cuttlebones. Now the Institute for Creation Research has twisted the science to support their belief that the earth is less than ten thousand years old. It was all so predictable. It’s a game they play, the same game they played with the soft tissue preserved in T. rex bones. Here’s how it works. Compare the two approaches, science vs. creationism. The creationists basically insert one falsehood, generate a ludicrous conflict, and choose the dumbest of the two alternatives. The Scientific Approach ↓ ↓ find traces of organic material in ancient fossils ↓ Cool! We have evidence of ancient biochemistry! ↓ Science! The Creationist Approach ↓ declare it impossible for organic material to be ancient ↓ steal other people’s discovery of organic material in ancient fossils ↓ Cool! Declare that organic material must not be ancient, because of step 1, which we invented ↓ Throw out geology, chemistry, and physics because they say the material is old ↓ Profit! Souls for my Lord Arioch! You see, the scientists are aware of the fact that organic materials degrade over time, but recognize that we don’t always know the rate of decay under all possible conditions. When we find stuff that hasn’t rotted away or been fully replaced by minerals, we’re happy because we’ve got new information about ancient organisms, and we may also be able to figure out what mechanisms promoted the preservation of the material. The creationists start with dogma — in this case, a false statement. They declare Chitin is a biological material found in the cuttlebones, or internal shells, of cuttlefish. It has a maximum shelf life of thousands of years… and Because of observed bacterial and biochemical degradation rates, researchers...
Read more