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.

[Read more…]

Deep homologies in the pharyngeal arches

i-ccbc028bf567ec6e49f3b515a2c4c149-old_pharyngula.gif

PvM at the Panda’s Thumb has already written a bit about this issue in the article “Human Gland Probably Evolved From Gills”, but I’m not going to let the fact that I’m late to the party stop me from having fun with it. This is just such a darned pretty story that reveals how deeply vertebrate similarities run, using multiple lines of evidence.

[Read more…]

Cephalopod gnashers

i-ccbc028bf567ec6e49f3b515a2c4c149-old_pharyngula.gif

Cephalopods can inflict a nasty bite. On their underside, at the conjunction of their arms, they have a structure called the beak which does look rather like a bird’s beak, and which can close with enough force to crush shellfish. Many also dribble toxins into the wound that can cause pain, tissue necrosis, and paralysis. They aren’t the best animals to play with.

If you think about it, though, cephalopods don’t have a rigid internal skeleton. How do they get the leverage to move a pair of sharp-edged beaks relative to one another, and what the heck are they doing with a hard beak anyway? There’s a whole paper on the anatomy of just the buccal mass, the complex of beak, muscle, connective tissue, and ganglia that powers the cephalopod bite.

[Read more…]

Whose side are you on, Flatow?

I’ve been listening to Bethell vs. Mooney on Science Friday, and I’ve come to one conclusion: I really need to slap Ira Flatow. Repeatedly. And maybe kick him a few times, too.

He was playing right into Bethell’s hands. Bethell was rambling and vague, and he went on and on, and Flatow fed into it. Mooney had to interrupt several times and demand a chance to rebut (and good for him—he was on the attack, as he needed to be), and at least once Flatow stopped Mooney for a commercial and then asked Bethell to follow up afterwards.

Worse, Flatow wouldn’t allow any depth. They’d start getting into HIV and Bethell’s denial, and just as Mooney was getting into it, he’d say, “Now we need to talk about global warming!” Come on, FOCUS. The strengths of science come into play when we have a chance to dig deep and actually grapple with the issues; Bethell is a superficial flibbertigibbet who knows nothing, and this show gave him a forum for his usual unsupported pronouncements of doubt.

Grrr. Mooney was appropriately assertive, but it sounds like we need to go to new levels of aggression: next interview, bring duct tape and a clothesline. Shut the interviewer up, and charge right into the data. I can’t believe Flatow let Bethell get away with that crap.

Polar lobes and trefoil embryos in the Precambrian

i-b275e2beecc20e27c50d7f1200419c67-dentalium_polar_lobe.gif

i-82a3d61658c78fe68f81f85cd6236021-lobed_embryo_tease.jpg

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?

[Read more…]