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Whale evolution

Whale evolution has always fascinated me, ever since I learned that their evolutionary path took them from sea to land and then back to the sea. Here’s an animation showing how the land-to-sea-again transition may have occurred.


Scientists have now learned about a new aspect of this transition in the baleen found in the jaws of whales.

If you observe a feeding fin, blue or humpback whale, you are likely to catch a glimpse of the bristles of baleen that fill its gaping jaws. Baleen is unlike any other feeding structure on the planet. Made of keratin, the same material as hair and finger nails, baleen hangs from the roof of a whale’s mouth and is used to filter small fish and crustaceans from large gulps of water. As water rushes into the mouth of a whale, the small creatures easily pass through the comb-like bristles, but once a whale’s mouth is full of water, it flushes the water back out, trapping the tiny creatures in the wall of baleen to be swallowed whole.

Scientists don’t know how or when baleen evolved, but the recent discovery of an ancient whale fossil—roughly 30 to 33 million years old—hidden in the collections of the Smithsonian National Museum of Natural History may hold a critical clue.

Scientists have speculated that baleen may have come to replace teeth and the fossil they studied seems to have had both, making it a transitional fossil.

With such a unique method for feeding, it is only logical to ask why this whale would evolve in such a way. What could be the advantage to losing an important feeding tool like teeth?`

First, teeth are expensive. It takes a lot of energy an d resources to build teeth with strong enamel. Slurping food is much more economical when soft food is readily available. Second, this whale lived at a time in Earth’s history when the environment was rapidly changing. As Antarctica broke away from South America at the end of the Eocene, the ocean’s currents were disrupted. Now, with Antarctica alone, a massive current encircles the continent—a change that had massive implications for both atmospheric and ocean temperatures around the globe. While it remains unclear as to how this might specifically have impacted whales, what is clear is that the change in Earth’s climate was a spark that ignited dramatic evolutionary change.

Fascinating stuff.

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Comments

  1. Owlmirror says

    @Mano in the OP:

    Scientists have speculated that baleen may have come to replace teeth and the fossil they studied seems to have had both

    Did the article change since you posted about it? Because it currently reads that Maiabalaena had neither teeth nor baleen.

    Whales today have elaborate networks of blood vessels in the rooves of their mouths that nourish their baleen or teeth with nutrients. And so, their jaw bones are riddled with microscopic holes. That wasn’t the case for this fossil.
     
    Instead, the Maiabalaena skull had indicators of a fleshy gum line, a strong tongue, and muscular cheeks built for vacuuming its meals of squid and fish. With these strong muscles it could shape its mouth to function like a straw—a feat observed in more recent toothed whales like the narwhal and the extinct odobenocetops.

    You did remind me that I had read about transitional whales that had both teeth and baleen, at The Loom.

    On the Path Towards Leviathan. Whales mentioned: Janjucetus, Aetiocetus. Also excellent cetacean paleoart by Carl Buell

  2. Christian Stock says

    Mano,
    It seems that there were two lines of early mysticete evolution. The one that was ancestral to modern baleen whales evolved: teeth > teeth + baleen > just baleen. The other line never evolved baleen. This Maiabalaena fossil is younger than some of the teeth + baleen genera, and therefore is likely not ancestral and belongs to the extinct lineage that never evolved baleen.

  3. Mano Singham says

    Owl mirror @#1 and Christian Stock @#2,

    It looks like I read too quickly and misunderstood the article. Thanks for the corrections!

  4. Owlmirror says

    Actually, this is very confusing. And what Christian Stock says contradicts the original paper, although I originally thought the same thing; that Maiabalaena was an offshoot lineage.

    But the claim in the original paper is that all baleen whales derive from whales that had lost teeth, like Maiabalaena. There was never a whale with both teeth and baleen (the authors of this paper say); the actual transition was teeth → toothlessness → baleen.

    Tooth Loss Precedes the Origin of Baleen in Whales (open access)

    Key sentences from the abstract:

      • Maiabalaena nesbittae is 33 million year old fossil baleen whale from Oregon
      • Maiabalaena has neither teeth, nor baleen
      • Early whales lost teeth entirely before the evolutionary origin of baleen
      • Despite no teeth or baleen, these whales were effective suction feeders
    [. . . ]

    Previous hypotheses for the origin of baleen [4, 5] are inconsistent with the morphology and phylogenetic position of Maiabalaena. The absence of both teeth and baleen in Maiabalaena is consistent with recent evidence that the evolutionary loss of teeth and origin of baleen are decoupled evolutionary transformations, each with a separate morphological and genetic basis [2, 6].

    (bolding mine, of course)

    They are opposing the claims from the paper from 2008 (Morphological and Molecular Evidence for a Stepwise Evolutionary Transition from Teeth to Baleen in Mysticete Whales (also open access)), which Carl Zimmer wrote about in the article I linked to above. The key paragraph in that article is:

    The evidence the scientists offer for early baleen comes from fossils of a whale called Aetiocetus. Baleen doesn’t fossilize easily, but it leaves evidence of its existence on whale skulls, which do. In all living whales, baleen tissue is supplied with blood by vessels that fit snugly into troughs cut into the palate. Toothed whales don’t have these troughs (known as nutrient foramina), nor do fossils of primitive whales. Janjucetus, an early baleen whale, doesn’t have the troughs either. Yet Aetiocetus–a baleen whale that still had teeth–had the troughs. So it seems that they had both teeth and baleen: a classic transitional form.

    It really bothers me that the Smithsonian article leaves that important point out, to the point where the exact opposite of what the paper was saying can be concluded from it. And the paper itself leads with that point and emphasizes it repeatedly! It’s really bad science writing to not point out that the authors of the work in question are making a new and controversial claim. (And as long as I’m griping, it’s badly spell-checked as well; “rooves” is an incorrect pluralization of “roof”)

    There may be challenges to this conclusion, too. Will the authors of the 2008 paper just say “Oh, of course, your logic and reasoning are impeccable. We were wrong!”? Or will they try and defend their own conclusions?

  5. Owlmirror says

    Another open-access paper about the tooth-baleen transition, The Origin of Filter Feeding in Whales, from July of 2017, summarizes the controversy while siding with the teeth+baleen model.

    The origins of baleen are controversial: one hypothesis suggests that teeth were lost during a suction-feeding stage of mysticete evolution and that baleen evolved thereafter [2, 3, 4], whereas another suggests that baleen evolved before teeth were lost [5]. Here we report a new species of toothed mysticete, Coronodon havensteini, from the Oligocene of South Carolina that is transitional between raptorial archaeocete whales and modern mysticetes. Although the morphology and wear on its anterior teeth indicate that it captured large prey, its broad, imbricated, multi-cusped lower molars frame narrow slots that were likely used for filter feeding. Coronodon havensteini is a basal, if not the most basal, mysticete, and our analysis suggests that it is representative of an initial stage of mysticete evolution in which teeth were functional analogs to baleen. In later lineages, the diastema between teeth increased—in some cases, markedly so [6]—and may mark a stage at which the balance of the oral fissure shifted from mostly teeth to mostly baleen. When placed in a phylogenetic context, our new taxon indicates that filter feeding was preceded by raptorial feeding and that suction feeding evolved separately within a clade removed from modern baleen whales.

  6. Mano Singham says

    Owlmirror,

    Thanks for those articles and the comments. My fascination with whale evolution continues to grow.

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