fossils
pharyngula | June 16, 2006
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…
pharyngula | May 24, 2006I 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.
pharyngula | May 23, 2006
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…
pharyngula | May 11, 2006Crap. 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…
Bird digits
Bird teeth
Bird brains
Jurassic bird brains
Bird lungs
Oviraptor pelves (does that count?)
Cretaceous bird embryos
Four-winged birds
Waimanu
And Archaeopteryx, of course.
pharyngula | May 8, 2006The 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…
pharyngula | April 27, 2006This 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…
pharyngula | April 19, 2006It's a busy time for transitional fossil news—first they find a fishapod, and now we've got a Cretaceous snake with legs and a pelvis. One's in the process of gaining legs, the other is in the early stages of losing them.
Najash rionegrina was discovered in a terrestrial fossil deposit in Argentina, which is important in the ongoing debate about whether snakes evolved from marine or terrestrial ancestors. The specimen isn't entirely complete (but enough material is present to unambiguously identify it as a snake), consisting of a partial skull and a section of trunk. It has a sacrum! It has a…
pharyngula | April 13, 2006No pictures yet, but isn't this exciting? Half-billion year-old worm feces have been found in Sweden. Alert the tourism industry!
pharyngula | April 12, 2006Martin Brazeau is looking for volunteers to spend July in Atlantic Canada helping him split rocks, looking for Late Devonian to Early Carboniferous fossils. Let's see…I wonder if the family would mind if I abandoned them for a month? I don't have any important responsibilities, do I?
pharyngula | April 5, 2006Paleontologists have uncovered yet another specimen in the lineage leading to modern tetrapods, creating more gaps that will need to be filled. It's a Sisyphean job, working as an evolutionist.
This creature is called Tiktaalik roseae, and it was discovered in a project that was specifically launched to find a predicted intermediate form between a distinctly fish-like organism, Panderichthys, and the distinctly tetrapod-like organisms, Acanthostega and Ichthyostega. From the review article by Ahlberg and Clack, we get this summary of Tiktaalik's importance:
First, it demonstrates the…
pharyngula | April 3, 2006There 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.
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…
pharyngula | March 18, 2006
We haven't had enough fossil penguins here, so let me rectify that deficiency. Below the fold you'll find a reconstruction of Waimanu, a 61-62 million year old penguin that was discovered in New Zealand.
Oh, and Carl Zimmer has posted a photo of the bird with its skin and feathers on.
Reconstruction of Waimanu (composite of W. manneringi and W. tuatahi, based on original art
by Chris Gaskin ©Geology Museum, University of Otago). ca, caudal vertebrae; ce, cervical
vertebrae; cm, carpometacarpus; cr, coracoid; fb, fibula; fe, femur; fu, furcula; hu, humerus; sk,
skull, md, mandible; oc, os…
pharyngula | February 27, 2006Bone 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…
pharyngula | February 23, 2006Say 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.
Holotype of Castorocauda lutrasimilis [Jinzhou Museum of Paleontology (JZMP) 04-117]. (A) Photograph of the…
pharyngula | January 19, 2006
There's a lovely article in this week's Nature documenting a transitional stage in tetrapod evolution (you know, those forms the creationists like to say don't exist), and a) Nature provides a publicly accessible review of the finding, and b) the primary author is already a weblogger! Perhaps there will come a day when I'm obsolete and willl just have to turn my hand to blogging about what I had for lunch.
For an extra super-duper dose of delicious comeuppance, though, take a look at this thread on the Panda's Thumb. I wrote about Panderichthys, and a creationist ("Ghost of Paley") comes…
pharyngula | January 14, 2006Bredocaris admirabilis
Ooooh, there's a gorgeous gallery of Orsten fossils online. These are some very pretty SEMs of tiny Cambrian animals, preserved in a kind of rock called Orsten, or stinkstone (apparently, the high sulfur content of the rock makes it smell awful). What are Orsten fossils?
Orsten fossils in the strict sense are spectacular minute secondarily phosphatised (apatitic) fossils, among them many Crustacea of different evolutionary levels, but also other arthropods and nemathelminths. The largest fragments we have do not exceed two mm. Orsten-type fossils, on the other hand,…