Taphonomy of fossilized embryos

There are these fossilized embryos from the Ediacaran, approximately 570 million years ago, that have been uncovered in the Doushantuo formation in China. I’ve mentioned them before, and as you can see below, they are genuinely spectacular.

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Parapandorina raphospissa

But, you know, I work with comparable fresh embryos all the time, and I can tell you that they are incredibly fragile—it’s easy to damage them and watch them pop (that’s a 2.3MB Quicktime movie), and dead embryos die and decay with amazing speed, minutes to hours. Dead cells release enzymes that trigger a process called autolysis that digests the embryo from within, and any bacteria in the neighborhood—and there are always bacteria around—descend on the tasty corpse and can turn it into a puddle of goo in almost no time at all. It makes a fellow wonder how these fossils could have formed, and what kind of conditions protect the cells from complete destruction before they were mineralized. Another concern is what kinds of embryos are favored by whatever the process is—is there a bias in the preservation?

Now Raff et al. have done a study in experimental taphonomy, the study of the conditions and processes by which organisms are fossilized, and have come up with a couple of answers for me. Short version: the conditions for rapid preservation are fairly easy to generate, but there is a bias in which stages can be reliably preserved.

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gen•e•sis

Some fields of science are so wide open, such virgin swamps of unexplored territory, that it takes some radically divergent approaches to make any headway. There will always be opinionated, strong-minded investigators who charge in deeply and narrowly, committed to their pet theories, and there will also be others who consolidate information and try to synthesize the variety of approaches taken. There are dead ends and areas of solid progress, and there is much flailing about until the promising leads are discovered.

Origins of life research is such an unsettled frontier. I wouldn’t want to work there, but the uncertainty and the confusion and the various small victories and the romance of the work do make for a very good story. And now you can read that story in Robert Hazen’s Gen•e•sis: The Scientific Quest for Life’s Origins (amzn/b&n/abe/pwll).

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Modeling metazoan cell lineages

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A while back, I criticized this poorly implemented idea from Paul Nelson of the Discovery Institute, a thing that he claimed was a measure of organismal complexity called Ontogenetic Depth. I was not impressed. The short summary of my complaints:

  • Unworkable idea: There was no explanation about how we could implement and test the idea, and despite promises at the time, Nelson still hasn’t produced his methods.
  • False assertions and confusing examples: He claims that all changes in early lineages are destructive, for instance, which is false.
  • Bad metaphors: He uses a terribly flawed metaphor of a marching band to explain how development works; I’d say that it’s a better example of how development doesn’t occur.
  • No research: Which is really a major shortcoming for a research program, that no research is being done.

Recently, Nature published a paper by Azevedo et al. that superficially might resemble Nelson’s proposal, in that it attempts to quantify the complexity of developing organisms by looking at the pattern within their early lineages. The differences are instructive, though: this paper clearly explains their methodology, presents many of the limitations, and draws mostly reasonable conclusions from the work. It is an interesting paper and contains some good ideas, but has a few flaws of its own, I think. My main objections are that its limitations are even greater than the authors mention, and there are some conclusions that are driven by an adaptationist bias.

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Warring sexes

Both Twisty and Amanda seem a bit weirded out by this news that the fetus can be viewed as a kind of parasite. This story has been around long enough that a lot of us just take it for granted—I wrote about the example of preeclampsia a while back.

There are worse feminist-troubling theories out there, though. In particular, there is the idea of intersexual evolutionary conflict and male-induced harm. In species where there is some level of promiscuity, it can be to the male’s evolutionary advantage to compel his mate to a) invest more effort in his immediate progeny, b) increase her short-term reproduction rate, and c) suppress her ability to mate with other males. After all, his optimal strategy is to flit from female to female, copulate, and put her to work producing his offspring. The female’s preferred strategy, on the other hand, is to take her time, maximize her lifetime reproduction rate, and select the best genetic endowment for her children.

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This sets up a cycle of counter-adaptations in the population. If a male acquires a mutation that increases his fitness at the expense of his mate’s—for instance, if some component of his semen works on her brain to suppress her interest in remating—it will spread through the population due to its positive effect on male fitness, even though it reduces female fitness. Subsequently, a female who acquired a counter-adaptive resistance to the male’s hormonal sabotage would have an advantage, and that gene would spread through the population, reducing male fitness by making them less capable of controlling female reproduction. Then, of course, males could evolve some other sneaky way of maximizing their reproduction rate—vaginal plugs, secretions that make the mated female unattractive to other males, proteins that put her ovaries into overdrive to produce more eggs now at the expense of the female’s long term survival.

It all sounds improbable and dystopian, but all of these mechanisms and more have been observed in that exceptionally promiscuous species, Drosophila. Drosophila seminal fluid has the property of reducing the female’s interest in remating, increasing her rate of egg-laying, and is also mildly toxic. Artificial selection in the lab can produce females that are resistant to the effects, and males that produce more and more potent semen to overcome their resistance, to the point where the line of “super potent” males, when crossed to unselected females, kill their partners with their ejaculations. There is literally a battle of the sexes in these species.

To speak up in my defense, though, not all males are evil exploitive pigs. The logic of this pattern of sexual competitiveness vanishes as species exhibit greater and greater monogamy—if you have only one mate, it is to your advantage to take good care of him or her, because a loss diminishes your reproductive fitness.

Rice WR (2000) Dangerous Liaisons. Proc. Nat. Acad. Sci. USA 97(24):12953-12955.

Rice WR (1996) Sexually antagonistic male adaptation triggered by experimental arrest of female evolution. Nature 381(6579):189-90.

Niobrara

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What do you think of when someone mentions the word “Kansas”? Maybe what leaps to your mind is that it is a farming state that is flat as a pancake, or if you’ve been following current events, the recent kangaroo court/monkey trial, or perhaps it is the drab counterpart to marvelous Oz. It isn’t exactly first on the list of glamourous places. I admit that I tend to read different books than most people, so I have a somewhat skewed perspective on Kansas: the first thing I think of is a magic word.

Niobrara.

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Lamprey skeletons

Bone is a sophisticated substance, much more than just a rock-like mineral in an interesting shape. It’s a living tissue, invested with cells dedicated to continually remodeling the mineral matrix. That matrix is also an intricate material, threaded with fibers of a protein, type II collagen, that give it a much greater toughness—it’s like fiberglass, a relatively brittle substance given resilience and strength with tough threads woven within it. Bone is also significantly linked to cartilage, both in development and evolution, with earlier forms having a cartilaginous skeleton that is replaced by bone. In us vertebrates, cartilage also contains threads of collagen running through it.

These three elements—collagen, cartilage, and bone—present an interesting evolutionary puzzle. Collagen is common to the matrices of both vertebrate cartilage and bone, yet the most primitive fishes, the jawless lampreys and hagfish, have been reported to lack that particular form of collagen, suggesting that the collagen fibers are a derived innovation in chordate history. New work, though, has shown that there’s a mistaken assumption in there: lampreys do have type II collagen! This discovery clarifies our understanding of the evolution of the chordate skeleton.

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Jurassic beaver

Say hello to Castorocauda lutrasimilis, a primitive mammalioform from the middle Jurassic—164 million years ago. Despite its great age, it has evidence of fur and guard hairs still preserved in the fossil, and was rather large for its time. It’s estimated to have weighed about 500g (about a pound) and was over 400mm (over a foot) long in life, and as you can see from the reconstruction, shows signs of being aquatic. In size and lifestyle, it probably resembled the modern platypus.

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