What is your favorite transitional form?


Carl Buell's restoration of Aetiocetus weltoni. From Demere et al., 2008.

By now many of you have no doubt seen the abysmally bad story on evolution and creationism in yesterday's Telegraph. After referring to the reactions of fundamentalist Christians to the forthcoming Charles Darwin biopic Creation (based upon the book Annie's Box), the anonymous author of the piece presents the "Top 5" arguments for both evolution and creationism. The choices were baffling; it appeared that rather than do any actual research the writer extracted the selections from a bodily orifice that I will refrain from naming specifically.

The first point selected in favor of creationism immediately set my teeth on edge;

No evidence for evolution

There is no evidence that evolution has occurred because no transitional forms exist in fossils i.e. scientists cannot prove with fossils that fish evolved into amphibians or that amphibians evolved into reptiles, or that reptiles evolved into birds and mammals. Perhaps becuase of this a surprising number of contemporary scientists support the Creation theory.

I do not care if the author was trying to play "devil's advocate"; it is grossly irresponsible to perpetuate this myth. Creationists deny the existence of transitional forms because for them evolution is not a possibility. It is a circular sort of reasoning. Anything that questions their fundamentalist interpretation of religion is automatically wrong and therefore any species, living or fossil, exhibiting transitional features is automatically barred from providing evidence for evolution.

Now I could point out the flaws in the piece point by point, but I do not think it would be of much benefit. (And PZ already did it.) If you are reading this blog you probably already agree with me about how important and exciting evolutionary science is and I have no desire to waste your time. Instead I have decided to write up a little something about one of my favorite transitional forms, and I encourage you to do the same in the comments or on your own blog. Contrary to the ignorant statements of the Telegraph piece there are so many transitional forms in the fossil record that I could spend months writing about them and still not cover them all. Rather than try to cover many examples in brief I thought it would be more profitable to examine one particular case in detail.


The origin of whales has long been a controversial subject. I am not referring to the lame protestations of creationists, but to debates between scientists over how, when, why, and from what cetaceans evolved. Our present understanding (built upon an convergence of fossil, genetic, and developmental evidence) is a relatively new thing. During the latter half of the 19th century and much of the 20th century the details of whale evolution were frustratingly difficult to draw out.

One of the now-forgotten debates hinged upon whether living whales shared a common ancestor or not. All living cetaceans fall into one of two groups, the odontocetes ("toothed whales" like dolphins) or the mysticetes ("baleen whales" like the humpback whale), but the two groups seemed so dissimilar that some scientists doubted that they could have shared a common ancestor. The characteristics shared between the two groups of whales would hence be examples of evolutionary convergence in the extreme, with baleen whales having one ancestor and toothed whales having another.

Enter Aetiocetus. In 1966 D. Emlong of the University of Oregon described this 25-million-year old whale which exhibited a strange mix of features. Its skull was long, broad, and flat like that of a baleen whale yet it also had teeth. While initially identified as an "archaic whale" on the basis of its toothy grin, in 1968 the paleontologist Leigh Van Valen proposed an alternative view. Even if mysticetes had different early ancestors than odontocetes those ancestors still would have had teeth. Thus Van Valen deemed Aetiocetus to be a "baleen whale", and this was the correct assessment.

Even though we can lump living whales into the "toothed" and "baleen" categories this distinction ceases to be useful as we look back into the fossil record. As Van Valen pointed out baleen whales clearly evolved from ancestors with teeth, so by what features can we more reliably tell the difference between an odontocete and mysticete? One way is to look closely at the bones of the skull.

Those who know little about evolution often assume that new anatomical features, like the blowhole of a whale, invariably appear out of nowhere. This just isn't so. The blowhole of a whale is its nasal opening which was pushed back to the top of the skull by the elongation of other skull bones, primarily the maxilla (or the bone that makes up the upper jaw in mammals), during evolution. This is important in that the elongation of the maxilla did not happen in exactly the same way in toothed and baleen whales. In mysticetes the maxilla was elongated to the extent where it scoops downward and backward beneath the eye socket. This osteological quirk is not seen in odontocetes, and when we look at the maxilla in Aetiocetus it is immediately clear that it was a "baleen whale" with teeth.


The skull of Aetiocetus weltoni, and a close-up of holes that once contained blood vessels that would have nourished its baleen. From Demere et al., 2008.

Things only got stranger from there. When paleontologists looked at the underside of the upper jaws of a particular species of Aetiocetus, A. weltoni, they discovered that it possessed nutrient foramina just like modern baleen whales. Nutrient foramina are little holes in bones that once housed blood vessels, and in living mysticetes these vessels supply blood to the plates of hair-like baleen that hang down from the roofs of their mouths. Whales without baleen do not exhibit the pattern of nutrient foramina seen in living baleen whales and Aetiocetus. This led scientists to a startling hypothesis; Aetiocetus had both teeth and baleen!

Eventually, though, baleen whales lost their teeth entirely, a fact confirmed by the presence of "fossil genes" that still exist in living mysticetes. In a 2008 paper a team of scientists not only laid out the evidence that Aetiocetus weltoni had baleen, but they also showed that baleen whales possess two genes involved in tooth formation, AMBN and ENAM, which are slowly being mutated away. Sometime during mysticete evolution a mutation caused a stop codon to form in these genes, which (as the name suggests) acts a kind of genetic "STOP" sign that prevents the genes from being fully expressed. (A recent PLoS Genetics paper also discusses this.) Some baleen whales develop tooth buds that are resorbed during fetal development, as well, clearly showing that they still possess some vestiges of their toothed ancestry. Whenever the loss of teeth in mysticetes occurred, though, Aetiocetus shows that it happened after the evolution of baleen.

It is difficult for me to conceive how anyone truly interested in science can look at this kind of evidence and deny that evolution is a reality. The existence of prehistoric whales with both teeth and baleen and the fact that modern mysticetes still carry the (albeit degraded) genes for tooth formation only make sense when considered in an evolutionary context. Despite the cries of creationists that "Darwinism" is ready to crumble the truth is that right now evolutionary science is an extremely vigorous area of research. It is extremely exciting to see more interdisciplinary work that incorporates evidence from various biological fields to help us better understand how life has (and continues to) evolve. To deny this is to be willfully blind.

More like this

The Telegraph story was accurate (not having seen the "movie" this is, of course, an assumption on my part!). The article was just reporting on what the "movie" was claiming. I don't know why people are so freaked about that. None of what the creationists claim is any surprise to those of use who have dealt with them.

You could argue that the telegraph article could have done a better job of exposing the creationist claims for the baseless trash that they are, but others may argue that this is not the reporter's job - unless the reporter happens to be a science reporter, of course!

But arguing that a movie reviewer ought to be refuting creationist nonsense is going too far. And arguing about how news media should present reports is a different issue from what people are asserting about the Telegraph article, which seems to be the erroneous claim that the Telegraph itself is making creationist arguments.

Good discussion Brian. I've often thought that whale evolution was an excellent example of a transitional series, particularly as explained in Donald Prothero's excellent book. However I'm not fully aquainted with the details and would be interested in your thoughts on John Woodmorappe's article on whale evolution:


I'm naturally not inclined to believe anything he writes, but I'd still be interested in some more expert thoughts. He talks about evolutionary chimera's and nested hierarchies as well as the specifics of whale evolution so I thought it might also be a useful way of talking about some wider issues. YECs elsewhere have argued similar things to Woodmorappe; in an essay on Tiktaalik, Safrati argues that these species don't contain true transitional forms, they're just mosaics of fully formed structures.


Not being fully informed about these things, I'd like to better understand the basis of this mosaic not transitional argument. In additional to the claims about whale evolution that Woody makes. But if you'd rather ignore YECs I'll understand!

By anonymoose (not verified) on 11 Sep 2009 #permalink

Ian; I do not think that the article was not a review. It was a report pointing out the fundamentalist response to the film, but I was puzzled as to why the author felt compelled to present the "Top 5" arguments for both sides of the issue. I am not asking movie reviewers to go out of their way to refute creationist nonsense, but I also ask that they don't promulgate falsehoods either!

There was no reason to include the lists and I have no idea where they drew their "Top 5" arguments from. Perhaps it is not the job of a movie reviewer to discuss science, but if they are going to discuss science I would hope that they had some understanding of the subject being addressed. As I pointed out, the piece uncritically states that there is no fossil evidence for evolution, something that is absolutely false. I don't care if they're trying to be "fair"; I still think they are responsible for peddling nonsense.

Anon; Thank you for the links. I will respond in more detail later, but after skimming it Woodmorappe it appears that piece's is just a bunch of handwaving. It's the kind of intellectual/anti-intellectual chimera creationism is known for. He attempts to trade in the language of science but, in his definition, creatures like Pakicetus were not whales because they simple were not whale-like enough for him. His piece appears to be a good example of denying the existence of transitional forms because the author is already convinced that evolution cannot happen.

The completely ludicrous Top 5 lists in the article have absolutely nothing to do with the Creation film, which is emphatically not a debate of creationism vs. evolution but rather a biopic about Darwin's personal life. The Top 5 lists were not compiled from material in the film; they were most definitely compiled by the author of the article, who shows that he's completely ignorant not only of evolution but also of creationism. (Creationism's Point 2 about the Young Earth is directly contradicted by Creationism's Point 4 which posits the "ages" apology meant to make Genesis compatible with the scientific age of the earth.)

And the Top 5 in support of Evolution is just a fucking mess. The non-existence of Noah's Ark has nothing to do with evolution and is completely irrelevant to evolution. If somebody found Noah's Ark tomorrow, evolution would still be true. (Although archaeological evidence does contradict most of the Old Testament -- see The End of Biblical Studies by Hector Avalos.) And transitional fossils absolutely belong to evolution. Just off the top of my head, as merely a casually interested biology layman, I can name Sinosauropteryx, Pakicetus, and Tiktaalik as showing evidence of major transitions. There are millions of other species that show smaller-scale relatedness, because in a very literal sense, almost every extinct species is transitional.

When discussing transitional species, Tiktaalik is a favorite of mine because it helps illuminate the nature of scientific theories. It demonstrates how evolutionary theory can be successfully used to make predictions.

But my actual favorite transitional form? That would be the missus.

I had a creationist astronomy professor last semester--his lectures were absolutely surreal. He couldn't exactly teach creationism directly, so every one of his talks would be interspaced with 'we don't fully understand this, therefore the Earth is only 6000 years old' (or something to that effect).

What astounds me more is that this guy got his degree at MIT of all places.

(at one point, he tried to justify plate tectonics by pointing out that the bible says that he created the 'land' and not 'lands'--thus proving that the Earth started with Pangea! Nevermind the fact that they had to coalesce to form the continent first; or that apparently they so suddenly slowed their pace the moment we started looking...).

By Tor Bertin (not verified) on 11 Sep 2009 #permalink

I might have to pick Archaeopteryx as my favourite transitional form, because the combination of features marking it as transitional are so straightforward.

It has feathers, which makes you think it's a bird.

It has teeth, which makes you think it's not a bird.

There are no transitional forms! We Creationists ask the scientists to show us ONE example, they give us TWENTY, and we twist 'em and turn 'em to make 'em fit our whackaloon literalist beliefs! Therefore, NO TRANSITIONAL FOSSILS!

*heavy, heavy sarcasm*

My favorite transitional form is good ole Archaeopteryx lithographica. It happens to be the only transitional form I own a museum-quality cast of (it's the Berlin specimen), so I guess maybe I'm biased.

By M. O. Erickson (not verified) on 11 Sep 2009 #permalink

My favorite transitional group is the progymnosperms and the aptly, sort of, named Archaeopteris, the perfect intermediate between free-sporing plants and seed plants.

I'd have to say my dad is my favorite transitional form, even if he isn't a fossil yet.

Another vote for Archaeopteryx! Much of my reasoning is in line with Zen's: very straightforward, very apparent. Easy to use for explanations to the curious layperson.

Though my knowledge of Tyrannosaurid phylogeny is not as great as it should be, I know a few people who have made a pretty decent case that Depletosaurus may be a transitionary form to Tyrannosaurus, in which case I might list it as one of my favorites as well.

By Tor Bertin (not verified) on 11 Sep 2009 #permalink

If I may choose a blatantly obvious species... As a child, I was - as many children are - completely obsessed with dinosaurs. Unlike many children (those that I knew at least), I was also fascinated with events and species before the Triassic. One particular illustration from a very old (and unfortunately now long-lost) book simply entitled 'Dinosaurs' detailed a Late Devonian scene. Emerging from a pool was a catfish-like creature whose antecendents and descendents would crop up time and again throughout my life, from books, to blogs to my zoology degree. I'm speaking of course of the pre-Tiktaalik sarcopterygian, Eusthenopteron.

By Anthony Hallam (not verified) on 11 Sep 2009 #permalink

Australopithecus, and his various distant relatives.

Darwin predicted that early hominids would be found in Africa, and other biologists predicted late Cenozoic age.

Considering most everything is transitional, it's hard to pick just one. Since it's Friday, what comes to mind is that Carl Buell image of him hoisting a beer with an Australopithecus.

Recently, I discovered a love for Nautiloids. They're what I think of as the traditional fossil. This could be because my dad usually told me the most about the Cambrian era. When visiting the Smithsonian as a kid, I always remembered how exotic and strange the Earth would have seemed back then.

My favorite dinosaur as a kid was dimetrodon and I always liked the pedopenna.

My favorite transitional forms are humans. After that comes Basilosaurus, leading to cetaceans.

I forget the species names, but in a book by Gould & Eldredge they describe the case of two successive species of the trilobites Phacops that are distinguished by the different number of rows of ommatidia in the eyes. An intermediate-timed formation preserves Phacops specimens that show the incipient development of the extra ommatidia row.

This is just one of numerous cases in the invertebrate (and protist) fossil record where the amount of preserved specimens is large enough for us to be actually able to directly observe the evolution of one species into another over the course of a formation.

It's, indeed, a recurring thing. For most "Creationists," as we so aptly label them (By "We" I mean those who accept evolution as fact, regardless of religious beliefs or not.), their teachings tend to give them a narrow point of view.

Now, I choose not to take sides, so here are my thoughts:

1. It's truly one thing to have a religion and seek spiritual "nutrition," but another to have it lead to narrow-mindedness and a judgemental attitude which is nothing new.

2. I try to accept both sides of any argument because I attempt to value other's opinions, regardless of who you are and what you believe. My stance is "Evolutionist," yet I believe in Jesus as the Massiah, much like the "Christians" do, though, in this day and age, I refuse to call myself that. I don't share ANY beliefs other than that. Honestly, I figure that on such a sensitive subject for most people, I have succeeded in posting a neutral comment on one's blog without attacking them as I have unintentionally done on others. :P

Now, that you know my position and if I were to let my emotions get the better of me, I appreciate the chance to speak out and convey this before they get the better of me, and I repeat my mistakes on others'.

As for my favorite transitional species, it is a toss up between the newly discovered Jinfengopteryx, M. gui, Archaeopteryx, the Early Hominines. Although, I would venture to lean towards the theropods. :) Nice of you to touch up on it. :)

@Raptor Lewis,

I have a good friend of mine that's quite similar. She's Christian in religious beliefs, but loves geology. She believes in evolution, but doesn't mix in her religion to it. Her father and sister are the same way--they're scientists of a sort, as well as follow Christianity, but do not mix the two.

Easy - the precambrian Kimberella, the oldest bilaterian triphoblastic organism known (probably the earliest known mollusc too). I chose this fossil because of its age (an indication of animal diversification long before the Cambrian) along with the fact that fossils of this species show fine details of its internal and external anatomy along with hints as to how it may have moved and fed. These features are stunning for such an old fossil, and it is unusual for an Ediacaran fossil in that it may be an early member of a modern phylum.

By James Rieman DVM (not verified) on 12 Sep 2009 #permalink

Pezosiren portelli - four-limbed, semiaquatic, Sirenia
Not only is Pezosiren a nice transitional form for the other fully aquatic group of marine mammals, but they evolved at much the same time and in nearly the same places. The parallel evolution of transitional forms of the Cetacea and Sirenia is maybe even cooler than each of them are separately. Particularly in how and why they are found to have distributed in similar ways early on, but once they were fully aquatic they went totally different directions, thanks to the limitations of herbivory in marine systems.
So, Pezosiren has my vote!

> My favorite dinosaur as a kid was dimetrodon

*Dimetrodon* is a synapsid, not a dinosaur. BTW, speaking of synapsids, the fact that the morganocodontids have both the dentary-squamosal and articular-quadrate jaw joints makes a mockery of the creationist claim that "there are no transitional forms".


Since someone picked Tiktaalik, I'm going to go with Acanthostega. There is no denying that these creatures were transitional...my 7 year old son was fascinated by Tiktaalik. A fish with "arms"! He couldn't believe it.

I'd have to go with some of the other commentators and say my favorite is Archaeopteryx, despite some other great fossils like transitional amphibians and the very rich fossil record of whales. Part of Archaeopteryx's appeal is the fact that it's obvious position as a transitional form clearly makes creationists very nervous. (You'll recall the claim made by cranks that the original 1861 specimen was a forgery, a claim that has, of course, been thuroughly debunked.)

By Raymond Minton (not verified) on 12 Sep 2009 #permalink

wow. epic journalism fail.

Some fine research done by the telegraph:

WikiAnswers??? really??

I don't know whether to laugh or puke.

Whether the reporter is or is not a creationist, it's his duty to report FACTS. Like the FACT that there are thousands upon thousands of transitional fossils. And the FACT that scientists have understood the evolution of the eye for decades now, and that protoeyes are found throughout nature-- and they are all functional, from the complex eye of the eagle, to the barely-an-eye-at-all of planaria. half an eye IS better than no eye.



Two reasons why creationists believe what we believe
about the reproduction of creatures such as whales
and land animals (according to their kinds) are:
1.) Biblical
2.) Scientific.


1.) Biblical:

Genesis 1
"21 So God created the great creatures of the sea
and every living and moving thing
with which the water teems,
according to their kinds,
and every winged bird
according to its kind.
25 God made the wild animals
according to their kinds,
the livestock
according to their kinds,
and all the creatures that move along the ground
according to their kinds.
And God saw that it was good."

Genesis 1

2.) Scientific:

"The specific complexity of genetic information in the genome does not increase spontaneously. Therefore, there is no natural process whereby reptiles can turn into birds, (or) land mammals into whales, ..."

What does the Catholic Church Teach about Origins?
What Does Cutting-Edge Science Teach about Origins?

Joseph; Thanks for the comment, but your arguments do not make much sense to me. We're specifically talking about creatures that break any artificial barriers ("kinds") that creationists want to set up. There is no evidence that there are self-contained, immutable "kinds" of creatures set up by God. If there were then creationists would have to explain the mechanism by which creatures were prevented from evolving, something they have not (and cannot) do. "Created kinds" are just artificial categories made by creationists based upon a denial of evolution. As your comment shows, this rejection of science is made first on Biblical grounds and the argument is circular. "Evolution doesn't happen because my interpretation of scripture says everything was made in a certain way."

Your scientific argument does not hold any water, either. It denies the genetic fact of mutation and simply ignores mountains of interdisciplinary evidence that supports the evolution of birds from dinosaurs, whales from land-dwelling artiodactyls, &c. Your argument only shows that you are rejecting science because of your particular religious affiliation, and in turn you're trying to make science submit to faith. Science is not so easily subjugated.

Hard to choose a favorite, but I really like Yinlong, that very basal ceratopsid from China. It shows a pebbled squamosal (pachycephalosaur trait) and a rostral bone (ceratopsid trait) and it shows that transitions don't just happen between big groups like terrestrial artiodactyls and marine whales--it happens in smaller groups too.

Brian, does the top picture of A weltoni show small baleen? My computer screen is too small and dark to see it. I expected the baleen might have been a bit longer, like a human mustache, as I think the archaeic whales' upper lip vibrissae (still seen on patches of the face of the right whale) was the original source of the mysticete baleen, and I can't imagine why it would have grown very short. Are there any mysticetes with very short baleen? Aside from that speculation... life is transitional, forms are ephemeral yet dependent on balanced structural rigidity and tension, so symmetry dominates. I've never really bought into the Gaia hypothesis, but the Earth is in fine form, and is certainly in transition. Without it, where would we be?

I think I heard the funniest explanation of evolution and transitional during a recent visit to the AMNH's Extreme Mammals exhibit. A young boy was explaining to his even younger brother how reptiles eventually evolved into mammal-like reptiles and then to mammals. He basically said, "You see, one day a reptile had a retarded baby, and it had a lot of hair like a mammals." Obviously, it's not scientifically accurate, but it does remind us that what we associated as "deformed" babies are often an expression of significant genetic mutations similar to those that lead to evolution (although presumably the "retarded babies" would die out from natural selection).

Could it be that the otter is our modern day pakicetus? A land-mammal living more and more of its life in water could start a whole new cycle! Who knows what species of sea-mammal could be traced back to the otter in 25 million years time.

By Eddie Janssen (not verified) on 13 Sep 2009 #permalink

As someone who works with sirenians, I'll say my favorite transitional fossil is Pezosiren portelli. A seacow with hindlimbs that could still hold the animal weight out of water as well as pachyosteosclerotic ribs, an adaptation for life in the water, how amazing is that!!

My favorite transitional form is Indohyus.

But Archaeopteryx is a close second. It will always be a classic.

By Stevo Darkly (not verified) on 13 Sep 2009 #permalink

Christopher Taylor @11:

I forget the species names, but in a book by Gould & Eldredge they describe the case of two successive species of the trilobites Phacops that are distinguished by the different number of rows of ommatidia in the eyes. An intermediate-timed formation preserves Phacops specimens that show the incipient development of the extra ommatidia row.

Do you know which book (or where else I could find info about it)? That sounds like a seriously awesome transition.


Synapsids and their morphing jaws have been favourites of mine for a while, but maybe that's just because I know so little about fossils.

BTW, can anyone explain to me how a double jaw joint actually works? I have a hard time imagining how a jaw could rotate around two hinges. *is confused*

One of the standard PRO-evolution talking points has been the existence of functionless vestigial organs: hip-bone rudiments in whales being a standard example. To which Creationists have objected that they aren't REALLY functionless: the hip-bones of whales function as anchors for muscles connected to the reproductive organs.

O.k., give them that one. (Really we should think through the "functionless" part of the argument, but that's too complicated to do in the middle of a debate.) Molecular studies have turned up TONS of "vestigial" segments of genetic code, like the degraded copies of the genes for tooth enamel in baleen whales that Brian mentioned. Even if you wanted to argue that they might have SOME function ("spacers" separating other genes on the chromosome?), they don't function as codes for proteins the way "active" genes do, so their resemblance in sequence to active genes is something better explained by evolution than by any Creationist special pleading. ... In other words: if it was a race, the Evolutionists would be winning: discovering new "vestigial" evidence faster than the Creationists can explain the old ones away!
And my favorite transitional form TODAY is Raranimus: a recently described (latest issue of Acta Palaeontologica Polonica) Synapsid (= protomammal) that helps fill the gap between Spenacodont-grade Synapsids (like Dimetrodon) and Therapsid-grade ones. Creationists point to ever so many gaps, and of course when you fill one gap they can point to two new ones, one on each side. But the gaps keep getting narrower!

By Allen Hazen (not verified) on 13 Sep 2009 #permalink

re: "Synapsids and their morphing jaws have been favourites of mine for a while, but maybe that's just because I know so little about fossils.
BTW, can anyone explain to me how a double jaw joint actually works? I have a hard time imagining how a jaw could rotate around two hinges. *is confused*"

----> Since I'm an amateur and not a professional paleontologist, I get to choose the organisms I'm inter5ested in on emotional grounds, and as a "mammal chauvinist" I also like the Synapsids best. As to your question about the two joints: on hinge is "outboard" of the other, so the axes of rotation aren't all that far apart. And (jut your jaw so your lower front teeth are in front of the uppers, then relax so they come bac behind the uppers) there is enough play in the system that the small amount of misfit isn't a problem.

By Allen Hazen (not verified) on 13 Sep 2009 #permalink

Another vote for Tiktaalik, for the same reason as Sean Craven @ #6. Although a fish with legs and a neck is pretty cool, here is the best and most important part: Shubin and colleagues didn't just stumble on it. Because they applied everything they knew about evolutionary biology, they had an excellent idea of where to look for it.

By Not that Louis (not verified) on 14 Sep 2009 #permalink

I quite enjoy hearing about modern speciation, like the chapter in Richard Dawkins' The Ancestor's Tale about the ring species of salamander. Amazing.

For a fossil though, both Tiktaalik for the way it was discovered (and its cool name) and the delicacy of the Archaeopteryx fossils. Beautiful specimens.


That flexibility was one thing I had in mind, but I wasn't sure that shifting one axis around and making it work in various positions is quite the same thing as trying to use two axes at the same time.

As to your question about the two joints: on hinge is "outboard" of the other, so the axes of rotation aren't all that far apart.

I don't think I've ever seen a picture showing the two joints in anything other than a side view, so a HUGE thank you for pointing that out. It makes a lot more sense now. (of course, I HAVE seen top/bottom views of therapsid skulls, so maybe I should have figured it out for myself...)

Now if only I could interpret the Digimorph Thrinaxodon... Why don't people colour and annotate those things for us anatomy noobs? :P

Allen @40, you've fallen for a creationist straw man: the hip bones of whales are vestigial because they no longer serve their original purpose. That they may have been co-opted for a second purpose is irrelevant. They are still vestiges of limbs for walking and swimming that the whales have lost.

My own personal favourite transitional is Miacis. I'm fascinated by the development of Carnivora anyway, but it also makes a wonderful teaching tool to haul out whenever creationists start saying stupid things like "you never see a dog give birth to a cat!" No, that's like asking to see your contemporary cousin give birth to you. But here, look, you have the same grandparent, and you each carry some traits -- not necessarily the same ones! -- from that grandparent, and you each have a few new traits that Grandmamma didn't have....

Besides, Miacids are just cool.

Allen Hazen @ 40: There are more than simply vestigial hip bones. Numerous whales have been caught which had vestigial thigh bones -- and one of the reasons why these can be classed as truly functionless vestigials is just how highly variable these are in development and presence, even in individuals of the same species. When you can have anything from nothing at all to a three-inch formless blob of bone to a distinctly femur-shaped multi-foot thing with occasional bits of tibia, and none of these appear to have any effect on the whale's development or health or general success, and these almost never have any real connection to muscle or other parts of the whales' anatomy, then it is a bit difficult to argue that they still have any function.

By Luna_the_cat (not verified) on 15 Sep 2009 #permalink

Hey Brian,

I'm a few days late on this *but* - on a recent visit to UC Berkeley, I had a chance to see the type skull and jaws of Aetiocetus weltoni in person. Actually, that's a bit of a lie; I saw this fossil in 2007 when it was on loan to Tom Demere; I admit I was a bit skeptical when I saw his talk at SVP in 2006. Then, he showed me the palatal foramina on the actual thing, and let's just say now I'm a believer (this was a little under a year before their 2008 paper came out, which is pretty cool!)

At UC Berkeley, I was able to look at it again, and take lots of photographs of it (although my crappy point and shoot couldn't get good pictures of the palate). Anyway, point is, sometime I'll post some larger photos of the specimen on my blog, now that all the research has been finished and the specimen returned.

Bottom line - the Aetiocetus weltoni type skull is BEAUTIFUL, and the aetiocetids are a very elegant example of a transitional fossil. I agree; definitely one of my favorites.

thank you very good I don't think I've ever seen a picture showing the two joints in anything other than a side view, so a HUGE thank you for pointing that out. It makes a lot more sense now. (of course, I HAVE seen top/bottom views of therapsid skulls, so maybe I should have figured it out for myself...)

Well, playing devil's advocate, but scientifically speaking, it's incorrect to speak of "intermediate".

First because even if just every character's displayed by a particular fossil seem to be intermediate, that doesn't mean that the animal is the ancestor of a currently living species : it might be only closely related to the hypothetical ancestor...

I won't write a cladistic lesson... But what we see is probable intermediate characters bore by animals. Are they ancestors of other animals? We sometimes tough so, but we can't prove it, so the animal itself can't be called "intermediate".

I won't look down on a fossil as an intermediate, every animal, past and present haves its own particularity and that's fantastic : I wont reduce them as "intermediate" as you wont say to your parents they're only intermediates between you and you grand-parents :P .

Yet, I fear it's quite impossible to put some good science into a creationist head. But I'm more frightened to see that some scientist go down to creationists level and answer them inaccurately in term of real science, speaking of "indubitable intermediates" "living fossils" or such things.

Anyway, thanks for writing about this wonderful animal.

My two pence.