Odontogriphus omalus

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(click for larger image)

A new report in this week’s Nature clears up a mystery about an enigmatic fossil from the Cambrian. This small creature has been pegged as everything from a chordate to a polychaete, but a detailed analysis has determined that it has a key feature, a radula, that places it firmly in the molluscan lineage. It was a kind of small Cambrian slug that crawled over matted sheets of algae and bacteria, scraping away a meal.

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Minnesota and Texas have something in common

Lindsay makes a factual error: Minnesota does not have a state fossil. We had a bill introduced almost 20 years ago to make Castoroides ohioensis, a 6-foot long, 250 pound giant beaver, our state fossil…but some people objected to the fact that it’s named after Ohio, and I suspect there might have been some concern about the beaver jokes.

We do have a list of potential nominees. I’m rooting for Endoceras proteiforme, myself—a giant nautiloid would be perfect!

Old spiders

Two short articles in this week’s Science link the orb-weaving spiders back to a common ancestor in the Early Cretaceous, with both physical and molecular evidence. What we have is a 110-million-year-old piece of amber that preserves a piece of an orb web and some captured prey, and a new comparative study of spider silk proteins that ties together the two orb-weaving lineages, the Araneoidea and the Deinopoidea, and dates their last common ancestor to 136 million years ago.

Araneoids and Deinopoids build similar looking webs—a radial frame supporting a sticky spiral—but they differ in how they trap prey. Deinopoids spin dry fibers that they fluff into threads that adhere electrostatically to small insects; Araneoids secrete glue onto the the strand, which takes less work (no fluffing), and is much more strongly adhesive. The differences are enough to make one question whether there was a single origin of orb weavers, or whether the two groups independently stumbled on the same efficient form of architecture.

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Polar lobes and trefoil embryos in the Precambrian

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The diagram above shows the early cleavages of the embryo of the scaphopod mollusc, Dentalium. You may notice a few peculiarities: the first cleavage is asymmetric, producing a cell called AB and a larger sister cell, CD. Before the second division, CD makes a large bulge, called a polar lobe, and it almost looks like it’s a three-cell stage—this is called a trefoil embryo, and can look a bit like Mickey Mouse. The second division produces an A, a B, a C, and a D cell, and there’s that polar lobe, about as large as the regular cells, so that it now resembles a 5-cell embryo. What’s going on in these animals?

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I think I shall never clickOn a poem as lovely as Tiktaalik

I saw it first at Virge’s place, but Mike Snider is also on the blogroll and I would have gotten to it eventually…but hey, if you’re a fan of fossil tetrapods and poetry, here’s a treat: a Tiktaalik sonnet. You can also view some drafts of its construction, which is developmentally interesting, I think, and not quite as messy as chopping up embryos.

I think my title reveals why I’ll leave the poesy to the pros.

No genes were lost in the making of this whale

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I just learned (via John Lynch) about a paper on cetacean limbs that combines developmental biology and paleontology, and makes a lovely argument about the mechanisms behind the evolution of whale morphology. It is an analysis of the molecular determinants of limb formation in modern dolphins, coupled to a comparison of fossil whale limbs, and a reasonable inference about the pattern of change that was responsible for their evolution.

One important point I’d like to make is that even though what we see in the morphology is a pattern of loss—whale hindlimbs show a historical progression over tens of millions of years of steady loss, followed by a near-complete disappearance—the molecular story is very different. The main players in limb formation, the genes Sonic hedgehog (Shh), the Fgfs, and the transcription factor Hand2, are all still present and fully functional in these animals. What has happened, though, is that there have been novel changes to their regulation. Even loss of structures is a consequence of changes and additions to regulatory pathways.

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Beautiful birds?

Crap. Coturnix tagged me with this beautiful bird meme, and I am the wrong person to ask. I don’t get out much, preferring to sit in the lab or the library, so my favorite birds are all in pieces and dead. But OK, since he asked…

Stromatoveris

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The Cambrian vendobiont S. psygmoglena, gen.sp.nov., composite photo of part and counterpart to show both upper and lower surfaces.

From the pre-Cambrian and early Cambrian, we have a collection of enigmatic fossils: the small shellies appear to be bits and pieces of partially shelled animals; there are trace fossils, the tracks of small, soft-bodied wormlike animals; and there are the very peculiar Edicaran vendobionts, which look like fronds and fans and pleated or quilted sheets. In the Cambrian, of course, we find somewhat more familiar creatures—sure, they’re weird and different, but we can at least tentatively see them as precursors to the modern members of their respective phyla. It’s not surprising, though, that the farther back in time we go, the stranger animals appear, and the more difficult it is to place them in our phylogenies.

So here’s something cool and helpful—it looks like a vendobiont, but it’s been found in the Lower Cambrian fossil beds of Chengjiang. It’s also very well preserved, and has features that suggest affinities to the ctenophores.

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Naked anaspids

This strange fish is Euphanerops longaevus, which is one of two species of 370 million year old jawless fishes (the other is Endeiolepis aneri, and the paper suggests that they may actually represent differently preserved members of the same species). These are soft-bodied animals that are usually poorly preserved, and are of interest because they seem to have some properties in common with both the lampreys and the gnathostomes, or jawed fishes. Their exact position in the vertebrate family tree is problematic, and the experts go back and forth on it; sometimes they are grouped with the lampreys, sometimes as cousins more closely related to the gnathostomes.

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Euphanerops longaevus, preserved as an imprint. Scale bar, 10 mm.

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