Ancient toothed whales had baleen

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"It is little wonder, then, why this great creature failed to continue its lineage: with no ears, it could not sustain a top hat upon its head, and thereby expired from lack of common dignity." A cartoon featuring Koch's "Hydrarchos," cobbled together from several Basilosaurus skeletons. [Update: I've been told that this cartoon is from Married to the Sea, whereas I had assumed that it was so old that it was public domain (I just had it sitting around in my pictures file and I don't remember where it came from). I'll err on the side of caution: Married to the Sea.]


ResearchBlogging.orgYesterday was the celebration of the 199th anniversary of Charles Darwin's birth, and so it is perhaps most fitting to start of this post about whales with a famous passage from the 1st edition of On the Origin of Species by Natural Selection (coincidentally published 149 years ago as of this year) that caused Darwin some amount of consternation;

In North America the black bear was seen by Hearne swimming for hours with widely open mouth, thus catching, like a whale, insects in the water. Even in so extreme a case as this, if the supply of insects were constant, and if better adapted competitors did not already exist in the country, I can see no difficulty in a race of bears being rendered, by natural selection, more and more aquatic in their structure and habits, with larger and larger mouths, till a creature was produced as monstrous as a whale.

Darwin's hypothesis is not entirely unreasonable (see Stephen Jay Gould's essay "Hooking Leviathan by it's Past," 1997), but he was widely criticized for such speculation during a time when science was becoming more systematic and "armchair theorizers" were looked down upon. Curiously, though, Richard Owen made some interesting comments about the paragraph. As related by Darwin in a letter to Charles Lyell, Darwin had a rather strange meeting with the "bitter & sneering" Owen;

Lastly I thanked him for Bear & Whale criticism, & said I had struck it out. -- "Oh have you, well I was more struck with this than any other passage; you little know of the remarkable & essential relationship between bears & whales". -

I am to send him the reference, & by Jove I believe he thinks a sort of Bear was the grandpapa of Whales!

What Darwin was suggesting was that natural selection could act upon the bear change its characteristics given enough time & selection pressure, but he was still ridiculed for the passage as some interpreted the illustration as meaning that bears were the actual ancestors of whales. Darwin stuck to his guns privately even if he did remove the passage, though, as shown in an 1860 letter to W.H. Harvey;

The Bear case has been well laughed at, & disingenuously distorted by some into my saying that a bear could be converted into a whale; as it offended persons I struck it out in 2d. Edition; but I still maintain that there is no especial difficulty in a Bear's mouth being enlarged to any degree useful to its changing habits,--no more difficulty than man has found in increasing the crop of the pigeon, by continued selection, until it is literally as big as whole rest of body. If this had not been known, how absurd it would have appeared to say that the crop of a bird might be increased till it became like a balloon.

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During Darwin's time some fossil whales were already known, the most famous being "Zeuglodon"* (known to us as Basilosaurus), and given the dentition of the ancient whale there seemed to be a good connection between an terrestrial, carnivorous whale ancestor and the later aquatic, predatory mammal. Darwin knew of "Zeuglodon," but he wasn't sure if it truly represented an intermediate form between an ancient, terrestrial carnivore and modern whales. In October of 1871 Darwin wrote to his ally T.H. Huxley for advice on the issue before mentioning the connection in the 6th edition of On the Origin of Species by Natural Selection, and Huxley soon replied that it was very probably that "Zeuglodon" represented a transitional form between the Carnivora and the Cetacea. Ultimately, Darwin included a passing mention of "Zeuglodon" in his book and rested the somewhat tenuous case on the assertions of Huxley;

The cetaceans or whales are widely different from all other mammals, but the tertiary Zeuglodon and Squalodon, which have been placed by some naturalists in an order by themselves, are considered by Professor Huxley to be undoubtedly cetaceans, "and to constitute connecting links with the aquatic carnivora."

*By 1845 enough material from Basilosaurus had been discovered and compiled to allow Albert Koch to tour the country with his fantastic "Hydrarchos" skeleton. This monstrosity was composed of remains of five different skeletons (most being Basilosaurus, but other remains were mixed in). The sideshow-like attraction was ultimately destroyed in the Great Chicago Fire of 1871.

I wonder what Darwin would have made, then, of the recent discoveries of carnivorous artiodactyls that are representative of the ancestors of the toothed (odontoceti) and baleen (mysticeti) whales alive today. Much of the attention has been on the earliest members of the whale lineage (creatures like Pakicetus, Ambulocetus, Kutchicetus, etc.,**), but one of the most vexing problems of whale evolution has involved a change that occurred more recently in geologic time. Baleen is a derived feature, something that is either present in extant whales or is absent, but it has been difficult to determine when & how baleen evolved. Fossils of ancient, toothed whales that are related to modern baleen whales are known (being that the ancestors of modern baleen whales would have teeth for much of their evolutionary history), but the evolution of baleen has been enigmatic. A new study out in Systematic Biology by Demere et al., however, appears to have given us a window to a time when some whales had both teeth and baleen, making them very curious creatures, indeed.

**There has been some suggestion that Indohyus may best represent the form of archaeocetes before Pakicetus, the close ancestors of Pakicetus therefore being herbivorous. There are several problems with this hypothesis; 1) Indohyus is younger than Pakicetus, 2) that the cladogram presented in the recent Nature paper places Indohyus and archaeocetes as closely-related sister groups outside the rest of the Artiodactyla, distant from hippos (the closest living relatives of whales), 3) analysis on the teeth did not allow for much resolution of the diet of Indohyus, the authors only being able to hypothesize that it was different from the diet of Pakicetus and there is a good chance Indohyus was an omnivore, and 4) there are reports of a forthcoming paper in Cladistics that may or may not place mesonychids closer to cetaceans. Indohyus was certainly closely related to cetaceans compared to most other artiodactyls, but just how significant Indohyus is and what it can say about whale evolution is still in dispute and awaits further resolution.

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


As reported in the new paper, toothed baleen whales have been known from the Oligocene (about 24 to 34 million years ago for the specimens mentioned in the paper), and while some have speculated that these whales might have had baleen in addition to teeth there hasn't been any anatomical trait that could support this hypothesis until now. Indeed, the problem has been that baleen itself rarely is preserved during fossilization, but there are osteological proxies that can give researchers clues as to the presence of baleen.

Around the edge of the upper jaw in living whales, there are little holes with furrows running off them to the anterior (towards the front of the mouth) and buccal (towards the "cheek", starting about 3/4 of the way back on the skull) directions, and these features denote the presence of baleen. (The holes are not anchors for baleen, but rather foramina that once housed blood vessels that nourished the baleen.) Surprisingly, the extinct toothed whale Aetiocetus weltoni also possesses these features (see illustration above), and so there is now concrete evidence of a toothed whale that also had baleen. Further analysis of Aetiocetus cotylalveus and Chonecetus goedertorum also revealed similar foramina, but they were not nearly as well preserved as the A. weltoni skull pictured. Likewise, an undescribed species of the genus Morawanocetus might have similar features, but details of whether it did or did not have not yet been published.

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


Eventually baleen whales lost their teeth entirely, but genetic vestiges remain. Genetic analysis by the researchers revealed that genes associated with the development of tooth enamel (AMBN and ENAM) contained premature stop codons. This makes them "pseudogenes" that are undergoing decay in extant baleen whale lineages and corroborate the evidence from the fossil record that the ancestors of modern baleen whales once possessed teeth! Indeed, it is practically certain that Aetiocetus weltoni had baleen, but the question remains as to what sort of baleen arrangement it had.

The researchers state in their paper that A. weltoni may have only had bundles of tubules along the inside of the tooth row, perhaps filling large gaps between the teeth. The attachment of large swaths of baleen to plates may have occurred later, so it should not be inferred that A. weltoni had baleen just like modern whales, only on a smaller scale due to a lack of evidence. The precise purpose of such structures is also somewhat hazy, although it likely served to filter prey/keep prey from escaping the jaws and likely triggered a transition to filter feeding (if such a behavioral transition was not already under way). This still leaves the question of why this trait arose in the first place to be acted upon, so for now it's probably best to abstain from panglossian posturing over what the feature evolved "for."

The present research does refute the idea that baleen whales underwent an implausible saltation, though, a stepwise pattern being apparent rather than toothed whales suddenly obtaining racks of baleen hanging from plates virtually overnight. It is still possible that the ancient baleen whales did undergo so major, macroevolutionary changes, although the process was more drawn out and involved intermediate stages rather than all at once. This would have involved the transition from toothed whale (primitive) to toothed whale with some baleen (primitive dominant/intermediate) to exclusive baleen feeding (derived dominant). The teeth might have shown reduction as baleen become dominant and filter-feeding the primary prey-capture method, but the ultimate loss of teeth in the adult stage may have been a macroevolutionary change involving stop codons (I say adult stage because baleen whales briefly have teeth during development but then reabsorb them).

The Demere et al. paper is a very important one, and it will certainly change the way we think about whale evolution. Much like some of the recent announcements involving osteological clues as to the presence of advanced feathers in theropod dinosaurs, the findings of the team now offer paleontologists markers to look for in specimens to determine whether ancient whales had baleen or not. Likewise, the findings suggest a mosaic of macroevolutionary events between which natural selection shaped the form of the baleen-bearing whales. This refutes the hypothesis of a massive saltation giving rise to baleen whales, but also giving new importance to the study of development and changes brought about by the creation of stop codons along genes. Indeed, this is the best kind of science; research that not only explains something important about the natural world, but also opens up innumerable new questions about the unity and diversity of life on earth.

[Hat-tip to Kevin Z for the paper]

Deméré, T., McGowen, M., Berta, A., Gatesy, J. (2008). Morphological and Molecular Evidence for a Stepwise Evolutionary Transition from Teeth to Baleen in Mysticete Whales. Systematic Biology, 57(1), 15-37. DOI: 10.1080/10635150701884632

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Zimmer has a summary of the latest discoveries in the evolution of the baleen whales. It's beautiful stuff, with the lineage showing their origin from toothed whales, through a phase where they had both teeth and baleen, to their current condition lacking teeth and having only baleen.

Definitely an interesting paper, thanks for blogging about it. I'm certainly developing an appreciation for Carl Buell (I think you might have spelled his last name wrong) - his amazing work seems to be everywhere these days.

Thanks for the compliment and correction, Justin. Buell definitely is everywhere these days, although my favorite as of late has been his cover for Evolution of the Artiodactyls (which I still owe everyone a review of).

Thanks for the link, Brian. I have an 80 gig'er (I've filled a whopping four) but my wife wants a Nano. Maybe we'll get lucky by the time her birthday rolls around next month!

Impressive!

However...

There has been some suggestion that Indohyus may best represent the form of archaeocetes before Pakicetus, the close ancestors of Pakicetus therefore being herbivorous. There are several problems with this hypothesis; 1) Indohyus is younger than Pakicetus,

So what? The idea is that Raoellidae and Cetacea are sister-groups, and that the former are morphologically intermediate between the latter and more conventional artiodactyls.

2) that the cladogram presented in the recent Nature paper places Indohyus and archaeocetes as closely-related sister groups outside the rest of the Artiodactyla, distant from hippos (the closest living relatives of whales),

That's indeed suspicious. The way to fix this is to make a bigger analysis. Most notably, the analysis of the Indohyus paper lacks all anthracotheres except the hippos themselves.

3) analysis on the teeth did not allow for much resolution of the diet of Indohyus, the authors only being able to hypothesize that it was different from the diet of Pakicetus and there is a good chance Indohyus was an omnivore,

Didn't the paper say so? But the point is that it was more herbivorous than any whale, and that stands.

and 4) there are reports of a forthcoming paper in Cladistics that may or may not place mesonychids closer to cetaceans.

If it was in press when Indohyus was discovered, I. is obviously not included in its data matrix. When the paper is out, someone should put I. in and look what happens.

Besides, it's a bit strange -- are the mesonychians supposed to have lost the extra ankle joint, or what?

By David Marjanović (not verified) on 14 Feb 2008 #permalink

The evolutionary history of whales gained much more personal interest for me in the last years, especially as so many new striking discoveries emereged. The evolution of baleen whales is especially interesting. Perhaps you have not already read it, but in Carl Zimmer´s "At the Water´s Edge" (illustrated with ink-illustrations by Carl Buell)there is a lot of highly interesting information about this topic. Especially the discoveries of Lawrences Barnes, who actually found a lot of archaic fossils of baleen whales in museum archives over the world, where nobody knew for years what they were, are fantatic. There is still a lot to discover about this topic, but actually there is also comparably much known. A highly interesting fact is that there is a small recent dolphin (can´t remember its name, it stand somewhere in ATWE), which has corneaus spiky strutures at its gum behind the teeth, which it uses like extra-teeth when it catches fish. It turned out that this structures are homologous. It surely does not mean that this is a late survivor of the baleen wahle ancestors, but that baleens did not came from nothing and could have developed in the mouth of a piscivorous whale with a certain purose.
I heard about some new amazing discoveries about the evolution of baleen whales, and how their early ancestors lived, but I was said to keep it under my hat. I hope it will be published soon.
If we´re already at the genealogical tree of the baleen whales, the strange offshoot Janjucetus hunderi should not be forgotten. There is a great paper by Erich Fitzgerald, which deals also with the baleen-topic. Some months ago I scupted in cooperation with Erich Fitzgerald and Brian Choo (which made the first life-reconstruction of Janjucetus which can be seen in countless articles)a small model of Janjucetus. It was a lot of work to sculpt the head, and it turned at the end out, that it became a bit too big, and the neck probably a little bit too short, and looks therefore more like a juvenile specimen, but it was really interesting to see this extinct creature finally not only as a twodimensional painting or as a fossilized skull, but more like a real plastic animal. Pictures can be seen here: http://bestiarium.kryptozoologie.net/artikel/janjucetus-hunderi-der-sch…
If I find time, I will made a new model, with more "adult" proportions.

A highly interesting fact is that there is a small recent dolphin (can´t remember its name, it stand somewhere in ATWE),

(Translation from the German: it's written somewhere in the ATWE, whatever that is... :-} )

which has corneaus spiky strutures at its gum behind the teeth, which it uses like extra-teeth when it catches fish. It turned out that this structures are homologous.

:-o

By David Marjanović (not verified) on 14 Feb 2008 #permalink

Sorry that I shock you again and again with my horrible english...
I just looked again for this special whale, it is Dall´s propoise whose minuscule teeth are completely surounded by a horny set of gums that is uses to grip squid and fish, and those gums are on a microscopic scale almost identical to true baleens.
Really fascinating. It seems really much more probable that the first baleens evolved as extra-teeth to hold prey, and not to filter very small animals out of the water. In fact animals like Aetiocetus or Archaeomysticetus probably still ate quite big animals, and swallowed them whole.

Ah, thanks.

By David Marjanović (not verified) on 15 Feb 2008 #permalink

"It is little wonder, then, why this great creature failed to continue its lineage: with no ears, it could not sustain a top hat upon its head, and thereby expired from lack of common dignity." A cartoon featuring Koch's "Hydrarchos," cobbled together from several Basilosaurus skeletons. [Update: I've been told that this cartoon is from Married to the Sea, whereas I had assumed that it was so old that it was public domain (I just had it sitting around in my pictures file and I don't remember where it came from).

A quick google-images for "Hydrarchos" brings up this page:

http://www.alabamaheritage.com/vault/monsters.htm

Which has the image, and cites it as being from:

(Engraving from The Iconographic Encyclopedia of Science, Literature, & Art, edited by Spencer Fullerton Baird, 1851.)

While the amusing caption about dignity and top hats might well be from "Married to the Sea", it seems that the image itself is Koch's Hydrarchos, as you say.

By Owlmirror (not verified) on 15 Feb 2008 #permalink

Isn't "macroevolution" a dirty word used by creationists?

I'm surprised that no-one has mentioned Carl's blog http://olduvaigeorge.com yet.

You all owe it to yourselves to go there and see his work. For some reason, he has trouble keeping it updated regularly. I can easily see why. Still, there is more than enough there for any lover of paleontology.

You'll find Ambulocetus natans, Pakicetus and many others.

Why isn't he in your blogroll Laelaps?

By JohnnieCanuck, FCD (not verified) on 16 Feb 2008 #permalink

Ahmad; "Macroevolution," "saltation," and other terms aren't ones that I like to use, but I can't think of a better way to describe the creation of a stop codon that stops the expression of genes that would cause baleen whales to lose their teeth. The way creationists use the term is completely wrong, but in actual science there are macroevolutionary events where a big change can happen in one generation due to mutation.

Johnnie; I probably should link to Carl's blog, but it's not in my blogroll because he doesn't regularly update it. I try to populate my list with people who actively write and update their blogs, and Carl's doesn't fit into that category as much as I love his work.

Hi, I am very much a lay person who is not even well read, but sometimes I am startled into wonderment, and I like it! Do you see the formation of keratin on the upper jaw a renewed expression of ancient heritage of pharyngeal slit filter feeding?

Sorry. ...the upper jaw TO BE a renewed expression...

Bill;

That's a great question, but I would have to say "no." "Slit-filter feeding" seems to have arose at least partly due to exaptation; organisms already had their mouths open, passing water over the gills, so anything that caught that food would open up that evolutionary pathway. What is happening with the whales is more akin to a chance change having an advantage leading to a shift in feeding behavior or a chance mutation allowing a behavior that was already present to be more efficient. With slit-filter feeding respiration and feeding are linked, but in the whales it is not being that they don't breathe via gills. That was a great question, though, and I can't say I had thought of it before.

Laelaps, I really appreciate your response! I think I am somehow trying to avoid how truly providential evolution is, in a way that makes me uncomfortable to recognize. The implications that keratin would randomly manifest itself throughout the bodies of mutated individuals unsuccessfully until it hit the right spot is vastly mind-boggling. I am not ready to fully accept what you are saying. I'll have to study my one book some more! It just seems too coincidental that that pharyngeal filter feeding is so fundamental to chordates. I am thinking of the jaw of the whale as an evolved pharyngeal slit, back when fishes first evolved jaws, and became dissociated from respiration. What a fascinating matter! Perhaps baleen is an new thing on the first slit, but I wonder. I couldn't grok the articles I found on fish filter feeding and food separating mechanisms of the pharynx and the palate in carp for example. Organisms have been at the business of separating food from seawater for so long that baleen almost has to be seen as a riff on a pre-existing pattern. I just can't credit it as original but rather a variation!

Thanks again!

Oh I forgot about horns. Baleen is practically an ingrown horn! You've probably read this.

PS All my "insights" are specious, but they provide me with the entertainment of shuffle about in my murky bath of ignorance. As for truth, what would I'd do with that? I do have a feeling that my wrongs have a rightness to them if only as evidence that I am alive and thinking!

I'm enjoying reading about Dall's porpoises now at the suggestion of your commenter.

Homo erectus (diver) and Homo sapiens have everted lips, whale ancestors inverted the lips.

See any lips on whales? Nope. All cetaceans derive from ancestors that inverted the lips and vibrissae.

You see lips on seals, walruses, sirenia, all of which still have external vibrissae, and none have baleen.

Baleen may not be cornified gums, but rather lip hair vibrissae, still retaining the large vessels like other vibrissae.

What mystifies me is that baleen whales have very long baleen hair and humans have very long scalp hair.

Did whale ancestors during their semi-aquatic stage develop long head hair for better hydrodynamics around the neck area, then develop sub-cutaneous fat (blubber), then lose the long hair but retain it in the inverted mustache?

IIRC whales have those vessels both in the mouth and on the snout, whether or not hair protrudes, as part of the very sensitive nerves associated with the follicles.

The jaw is not a gill slit, it's the jaw. Filter feeding in basal chordates and in whales is not homologous. Baleen is hair growing in the wrong place -- and that place is the palate, not the pharynx. Don't confuse the palate of carps with their pharynx roof.

By David Marjanović (not verified) on 14 May 2008 #permalink

What can you say about the similarity of teeth of Mesonychia and early whales? Is it a result of convergence, or a common origin?

By Pavel I. Volkov (not verified) on 14 May 2008 #permalink

> Is it a result of convergence, or a common origin?

Good question. On the one hand, mesonychians and artiodactyls - including whales - are sister groups, and closely related. It is therefore not unreasonable to suggest common origin.

On the other hand, *Nanocuris* had mesonychian-like teeth, and it probably wasn't even a placental. This is clearly convergence. If those form of teeth evolved at least two times by convergence, why shouldn't it evolve three times? Perhaps the question should be why not more predatory mammals evolved mesonychian-like teeth. Most other terrestrial predatory tetrapods have ziphodont teeth, and mesonychians (and entelodonts) showed a tendency toward this kind of dentition, too. There seems to be strong evolutionary pressure in favour of ziphodont dentition in large predators. It's strange that most mammalian predators - mesonychians, entelodonts and *Nanocuris* are exceptions - stuck with a basal mammalian set of teeth plus carnassials.

Thank you, johannes! Are you a scientist? I want to translate your message for my review about whales and creationists. How to introduce you to readers? Or post here a link to the source of this info.

By Pavel I. Volkov (not verified) on 15 May 2008 #permalink

> Are you a scientist?

No, I'm a mere enthusiast, so quoting me as an authority probably wouldn't impress anyone.

For a good overview of tooth morphology in mesonychians, cetaceans and severeal other related groups see here:

> www.springerlink.com/index/F5U0754554297600.pdf

sadly the article isn't free.

The question why there aren't more mammals with ziphodont dentition has been discussed in Darren's comment section, but if anybody ever did formal research on this, it has escaped my knowledge.

Does anybody know a reference to article explaining the convergent similarity between teeth of Mesonychia and early cetaceans?

By Pavel I. Volkov (not verified) on 19 May 2008 #permalink