How whales started swimming (new players, new hypothesis)

Earlier
here and here we looked at the whale-tenrec connection.

Figure 1. Maiacetus is a basal whale with legs and it is also a giant tenrec. Compare to Hemicentes in figure 2 and remember that another tenrec, Limnogale, has a long tail.

Figure 1. Maiacetus is a basal whale with legs and it is also a giant tenrec. Compare to Leptictidium in figure 2. Hind limbs are shown in running pose. Both go back while swimming.

Spoiler alert:
Pre-whales, like Maiacetus (Fig. 1), started swimming by converting terrestrial bipedal hopping, as in Leptictidum, (Figs. 1,2) to simultaneous hind limb paddling which promoted lumbar flexion and extension with the large tail snaking up and down in waves as in living whales. Here’s how it happened in a bit more detail:

Figure 1. Leptictidium is known as a hopper. Here it nests with whales. Combine the two and when Lepticitidium jumps in the water, it continues hopping. That long, long tail is homologous to the long, long tail in Zeuglodon.

Figure 2. Little Leptictidium is widely considered a terrestrial hopper, like a kangaroo. Here it nests with whales. Combine the two and put Lepticitidium  in water, it continues hopping.

  1. Leptictidium was a bipedal tenrec that nests as a sister to land (stem) whales and aquatic whales (Fig.3).
  2. On land, Leptictidium might have hopped, like a kangaroo. Or maybe not.
  3. In water, Leptictidium let its legs extend backward as it undulated its supple spine and greatly elongated robust tail (Fig 1) in the manner of sea otter (see video). Of course all four legs contributed to propulsion and steering.
  4. Over time, the longer tail and lumbar region became more efficient, reducing the need for drag-inducing hind legs. The pelvic connection was already loose at this stage. So were the leg joints.
  5. Leptictidium was preserved with a complete pelage of thick fur. That disappears, of course, in whales.
Figure 2. Elements of Leptictidium nasutum SMF ME 1143 from Storch and Lister 1985 (labeled L. auderiense at Wikipedia). Head + torso = 30cm.

Figure 3. Elements of Leptictidium nasutum SMF ME 1143 from Storch and Lister 1985 (labeled L. auderiense at Wikipedia). Head + torso = 30cm.

Current and traditional thinking from Wikipedia: 
“Leptictidium (Early Eocene, 50 mya) is on a short list of bipedal mammals. Combination of primitive eutherian traits (prepubic bones) with derived adaptations such as the powerful hind limbs and long tail. Adapted to a forest ecosystem, Leptictidium is considered omnivorous and its lineage became extinct 35 mya. 

“The ankles and the sacroiliac joint were quite loosely fixed, while the pelvis had a flexible joint with only one coccygeal vertebra. The anteorbital muscle fenestrae in their crania suggest they probably had a long and mobile snout, similar to that of elephant shrews.  Its dentition was quite small in comparison to the size of the mandible and the animal as a whole.

“Studies of the bone structure of Leptictidium have yielded contradicting information: its leg articulations appear too weak to have supported the shock of repeated jumps, but its long feet were obviously adapted for jumping rather than running.”

That extinction statement is not exactly true
as both tenrecs and whales continue this lineage, though neither are bipedal nor do they have such proportions anymore. But this is news that the Wiki writer was not aware of.

It’s interesting and supportive of the present hypothesis
that the leg, ankle and hip joints of Leptictidium are considered “loose”. On whales, of course, the sacral connection to the pelvis is long gone. The Messel pit, from which several fossils were found, was a former lake. Maybe Leptictidium was not a hopper, but a swimmer. The long flexible snout hypothesized for Leptictidium could have been used as a snorkel to keep it underwater while breathing. And so, this is likely the origin of the cetacean blow hole.

Traditional renderings of whale swimming origins
usually employ a wolf-sized predator at the water’s edge. Now we can reduce that to a rabbit-size or smaller predator/insectivore.

If you still like hippos for whale ancestors 
as DNA, prior morphological studies and several helpful paleontologists suggest, with hippos (or the earlier ungulate Elomeryx) you don’t get a long supple lumbar area, a long tail, long flat feet with long toes, a long narrow snout with small triangular teeth or really anything else whales are famous for, except, perhaps, blubber.

This heretical discovery
should be seen as one more nail in the coffin of DNA studies that try to draw cladograms from their unsupportable results. We’ve seen similar untenable cladograms nesting turtles as archosaurs and glyptodonts as armadillos.  It’s time to put our thinking caps on and see hard evidence for what it is. Only morphological studies, and good ones, will recover gradual accumulations of derived taxa. Put all such studies under the magnifying glass to see if they make sense. Let’s put DNA studies on the shelf until they produce results that confirm morphological studies.

Figure 5. Mammal cladogram with tenrecs and stem whales highlighted. At present, Leptictidium is the closest outgroup to stem whales.

Figure 3. Mammal cladogram with tenrecs and stem whales highlighted. At present, Leptictidium is the closest outgroup to stem whales, represented here by Maiacetus.

This hypothesis could not have been recovered 
without the data and skeletons provided by Phillip Gingerich, Hans Thewissen and other stem whale discoverers over the last few decades. Phylogenetic analysis based on morphological data permitted seeing Leptictdium in a new light, as a swimmer, not a hopper with loose joints.

References
Storch G and Lister AM 1985. Leptictidium nasutum, ein Pseudorhyncocyonide aus dem Eozän der “Grube Messel” bei Darmstadt (Mammalia, Proteutheria). Senckenbergiana lethaea 56:1-37.

wiki/Pakicetus
Whale video 1
Whale video 2
Whale video 3

 

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18 thoughts on “How whales started swimming (new players, new hypothesis)

  1. “Only morphological studies, and good ones, will recover gradual accumulations of derived taxa. Put all such studies under the magnifying glass to see if they make sense. Let’s put DNA studies on the shelf until they produce results that confirm morphological studies.”

    And with that, you consign yourself to the dustbin of science. You can keep adding mammals to your analysis, but unless you enforce a molecular constraint there’s really no point. I tried to help, really I did. But you think all of the molecular phylogeneticists in the world are wrong. Just think- what’s being inherited from ancestors? It’s not the large scale structure, it’s the genetic code.

  2. We disagree here. Mickey, please don’t consign yourself to the dustbin of science. You’ll never trace gradually accumulating traits with DNA. I already have with morph studies. You put your faith in combinations of phosphate compounds that have not been confirmed with morphological studies, which is the only way you’re going to understand those combinations and what they mean.

    • David, we often disagree for various reasons. But I don’t see how you can make comments like, “Morphology trumps genetics. Ultimately you’re going to have to show a gradual accumulation of traits. Snakes still have the gene for legs. Humans still have the gene for gills and a tail,” or, “Let’s put DNA studies on the shelf until they produce results that confirm morphological studies,” or even, “You’ll never trace gradually accumulating traits with DNA. I already have with morph studies. You put your faith in combinations of phosphate compounds that have not been confirmed with morphological studies.”

      Is this what you truly believe about molecular data? I can’t believe that anyone who has ever taken even an introductory biology class could misunderstand genetics and inheritance in such a manner. I am not saying this to be mean or what not; I genuinely don’t understand how you can reject something so well understood and fundamental to biology out of hand by basically hand-waving that their results don’t match yours. Unless you are Lamarkian, you must understand that genetics underlies the growth, development, etc. of those morphological traits you talk about, right? That random mutation (which is exactly what is traced in molecular studies) is what drives the minute changes we see in these structures and those changes, amplified over time, are the long-term morphological trends we track as paleontologists, yes?

      Ironically I have heard almost exactly the same criticism, but reversed, from genetics grad students about morphological data. They are both halves to the same puzzle. If your results don’t fit the data, don’t throw out the data…

  3. Thanks, Rob, 1. DNA is lost in fossils. 2. DNA does not replicate tree topologies recovered from fossils. 3. DNA topologies make sense only when they compare humans to humans, slugs to slugs, etc. For some reason they stop making sense when crossing larger clades as compared to morphology. I wish it were not so… but it is so. So I agree with the rest of your thoughts. I don’t know what breaks down over larger phylogenetic distances and I never will because I can’t replicate DNA experiments with my lack of expertise and equipment.

    • “For some reason they stop making sense when crossing larger clades as compared to morphology.”

      Do you have any evidence for this beyond the fact that the results don’t agree with your tree, which obviously is the golden standard that all must be judged by.

      “I can’t replicate DNA experiments with my lack of expertise and equipment.”

      Then maybe you should start paying attention to people who can study these things? This is basically the same as comment you made when I suggested you read Dave Bapst’s paper. You send it was completely over your head and so you decided to ignore it.

  4. Neil,
    Ultimately, for anyone other than a DNA specialist with their equipment, the hypothesis of relationships cannot be replicated in an experimental test. On the other hand, not only can I test morphology using MacClade and PAUP, but afterwards I can see the synapomorphies, the patterns and that’s the gold standard, especially when it comes to fossil taxa, which is kryptonite to DNA.

      • Yes, it is an argument against it. I can’t use DNA for fossils. It’s like trying to change a lightbulb with a hammer. The only proof that DNA works across a broad range of clades will be when the DNA results match the morphology results. That has not happened yet. And not every DNA study agrees with every other study. With 740 taxa, each of which has an opportunity to nest with any of the 739 taxa, the LRT is getting stronger and stronger.

  5. Thanks, Rob and Neil. One more try…

    “The only proof that DNA works across a broad range of clades will be when the DNA results match the morphology results.”

    How about this one, David? Reeder et al. (2015) looked into morphological vs. molecular support in Squamates. They found that “the clade of snakes, anguimorphs, and iguanians (Toxicofera)” was actually supported by six unambiguous morphological characters, even though it’s not the most parsimonious option morphologically. On the other hand, none of the 46 genes tested supported an Iguania-Scleroglossa split. So just like with the Eocene afrotherians, here we have DNA explaining some non-molecular data, while morphology has no explanation for why no genes match it. Why would all the genes happen to find Afrotheria, or Toxicofera, or Archelosauria?

    Seriously, David- do you have any way to explain why so many independent variables would all converge to find the same wrong topology? You can’t just hand wave away so much data just because it doesn’t match your result.

    Reference- Reeder TW, Townsend TM, Mulcahy DG,
    Noonan BP, Wood PL Jr., Sites JW Jr., et al. (2015)
    Integrated Analyses Resolve Conflicts over
    Squamate Reptile Phylogeny and Reveal
    Unexpected Placements for Fossil Taxa. PLoS ONE
    10(3): e0118199. doi:10.1371/journal.pone.0118199

    • To be fair, Dave’s hardly the only one who’s this skeptical of molecular phylogenies (Prothero 2004, 2013; Averianov & Lopatin 2014).

      • Would be easier to respond if you included full citations, but of course there are biologists who think this way. There are also those who still use key characters over cladistics, but the field doesn’t pay attention to them. You could use the example of Gauthier et al.’s (2012) well done morphological squamate analysis that found all the legless taxa to group together (which they named Krypteia). They had about as good of a reply as to why they got different results than molecular analyses as David does, and they got the same kind of scientific follow-up. Which is to say, basically none. No one’s looking for the legged ancestor to Krypteia, just like no one’s looking for those tenrec-whale missing links (should we tell David about Potamogale, the semi-aquatic tenrecid?). The dustbin of science.

      • The ‘morphology vs. molecules’ problem is muuuuch more complex (in most cases) than either side presents it here…

  6. I dropped a comment on this PLoS ONE paper over a year ago at their site. I will raise the issues I see now and then.1) They don’t have a sufficient list of pre-Squamates (including Proquamata and Tritosauria including Huehuecuetzpalli, Pterosauria, etc.) in their outgroup. That may be one reason why Iguania does not split at the Squamate base. 2) I see they nest the very derived tiny burrowing Leptotyphlops (famous for jaws that move laterally more than dorsoventrally) nests at the base of their Squamata along with aquatic and sometimes giant mosasaurs (with their expandable mid jaw joints). Why aren’t you raising a ruckus about that clear breach of “gradually accumulating characters?” Generally at the base of clades you get plesiomorphic taxa, ones suitable to be the great grand dads of all the bizarre kids that ultimately arise. That’s not happening with the Reeder et al. study, BUT it DOES happen at the large reptile tree.

  7. Thanks for the referral to Potamogale. Skeletals are online. I’m used to other workers not paying attention to my work, even my published work. Paleontology runs its own course. Everyone is in this Science to discover things. If someone does discover something, others are not happy because that’s one less brass ring to grab. And so it goes.

    • Nonsense. There is no shortage of scientific discoveries to make, and if somebody else discovers something *well-supported*, others are thrilled that the field has been advanced. A scientist might be perturbed if something they were specifically working on got scooped, but it doesn’t hurt an astrophysicist if a zoologist identifies a new species.

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