Puijila, the walking seal – a beautiful transitional fossil

By Ed Yong | April 22, 2009 1:00 pm

Puijila_animatic.jpg

Blogging on Peer-Reviewed ResearchSeals and sea-lions gracefully careen through today’s oceans with the help of legs that have become wide, flat flippers. But it was not always this way. Seals evolved from carnivorous ancestors that walked on land with sturdy legs; only later did these evolve into the flippers that the family is known for. Now, a beautifully new fossil called Puijila illustrates just what such early steps in seal evolution looked like. With four legs and a long tail, it must have resembled a large otter but it was, in fact, a walking seal.

Natalia Rybczynski unearthed the new animal at Devon Island, Canada and worked out that it must have swam through the waters of the Arctic circle around 20-24 million years ago. She named it Puijila darwini after an Inuit word referring to a young seal, and some obscure biologist. The skeleton has been beautifully preserved, with over 65% of the animal intact, including its limbs and most of its skull.

Puijila is a massive boon for biologists trying to understand the evolution of pinnipeds, the group that includes seals, sea lions and walruses. It’s not itself a direct ancestor, having branched off the evolutionary path that led to modern pinnipeds. It did, however, retain many of the same features that a direct ancestor would have had. “Puijila is a transitional fossil,” Rybczynski explains. “It gives us a glimpse of what the earliest stages of pinniped evolution looked like, before pinnipeds had flippers. And it suggests that in the land-to-sea transition, pinnipeds went through a freshwater phase.”

This familiar group evolved from land-dwelling carnivores and their closest living relatives are the bears and the mustelids (otters, weasels, skunks and badgers). For other marine mammals like whales and dolphins, the fossil record has given us dramatic visuals for the gradual transformation from land-dweller to full-time swimmer. But for pinnipeds, that transition is much murkier because until now, the earliest known seal Enaliarctos already had a full set of true flippers. Puijila changes all of that.

In the Origin of the Species, the ever-prescient Darwin wrote, “A strictly terrestrial animal, by occasionally hunting for food in shallow water, then in streams or lakes, might at last be converted into an animal so thoroughly aquatic as to brave the open ocean”. This year, on the 150th anniversary of the book’s publication, the walking seal that bears his name pays a fitting tribute to Darwin’s insight.

Puijila.jpg

Puijila was just over a metre in length and had a long tail. Its four legs were short but strong, and would have been attached to its trunk by powerful muscles. The bones of its toes were somewhat flattened, which strongly suggests that they were webbed. In many ways, its skeleton was very similar to a modern otter’s but the shape of its skull and teeth mark it out as a seal.

As examples, its bottom jaw had four incisors rather than the standard six of other carnivores. And it had a large “infraorbital foramen” – a hole beneath each eye through which whisker nerves would have passed. The wide hole suggests that, like modern seals, this animal had sensitive whiskers.

Based on these features, Rybczynski built a new family tree that included Puijila and other prehistoric carnivores, which rewrote some of their relationships. According to Rybczynski’s analysis, Puijila and Enaliarctos belong to a small family of early pinnipeds that also includes Potamotherium (formerly thought to group with the weasel-like mustelids) and Amphicitceps (previously classified with the bears).

Puijila_skeleton.jpg

Compared to all other pinnipeds, it was the least specialised for life in the water, but it lived at the same time as the distinctly seal-like Enaliarctos. “In its time, Puijila would have been a ‘living fossil’,” says Rybczynski. It was a relict of a previous body plan that had since been refined for superior swimming.

Rybczynski thinks that Puijila swam using all four of its feet and it wouldn’t have been anywhere as graceful as its modern cousins. Most otters swim by thrusting simultaneously with their hind legs and using their long tails. Seals also use their hind flippers by waving them from side-to-side, while sea lions rely on their front pair to fly through the water. Puijila‘s four-legged swimming stroke represented a style that could easily have given rise to either of the techniques used by today’s pinnipeds.

Puijila‘s discovery also tells us about where the earliest seals may have evolved. It wouldn’t have been suited to life at sea and probably lived and hunted in freshwater, suggesting that seals made the transition from land to sea via rivers and lakes. It lived in the Arctic and 23 million years ago, this region was cool and temperate, with freshwater lakes that would have frozen over in winter. Rybczynski speculates that these conditions may have driven the ancestors of seals to experiment with coastal environments. Free of ice, these would have provided them with opportunities to hunt even in the depths of winter.

Update: The Canadian Museum of Nature have developed an entire microsite dedicated to the new find! I’m really impressed – it’s got some fantastic detail, great use of Flash and 3D animations. Go over and see Puijila up close.

Also have a look at Brian’s excellent take at Laelaps.

Reference: Nature doi:10.1038/nature07985

Images: Painting by Mark Klingler; 3D reconstruction by Alex Tirabasso; skeleton laid out by Martin Lipman;

More on transitional fossils:

PS: I guarantee that someone, somewhere will get these details wrong, so two points are worth clarifying:

  • Puijila is not the “ancestor of seals”
  • This does not mean that seals “evolved from otters/bears” or other such nonsense

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Comments (32)

  1. Wow! I was just reading my marine mammal book the other day, specifically the section about pinnipeds, and I thought, “when are they going to find a pinniped version of Ambulocetus?” Turns out I didn’t have to wait long!

  2. tbell

    but wait! if seals evolved from otters, then WHY are there still otters around? answer me that, silly evolutionist person…

  3. J-Dog

    Yay! Two more gaps for the DI and ID Creationists to get confused about! Hilarity will ensue. Thanks for the post – it was great.

  4. Great stuff. Small suggestion….Insert words to this effect:
    “that shares an ancestor with modern seals, and since it retains many features of that ancestor we can infer what that ancestor may have looked like”
    approximately here at the ^
    Rybczynski unearthed the new animal ^ at Devon Island
    And then the next nine or ten references that strongly imply that this is an ancestor to the seals will not have to be undone in the last half of the writeup!!!!! :)
    It is a great write-up, though. Nice fossil as well.

  5. Ed Yong

    Done, after a fashion. Fair point Greg.

  6. http://en.wikipedia.org/wiki/Puijila
    The lead author of the discovery is a personal friend. Great work.

  7. That’s exciting and for no logical reason but partisanship, I’m glad the fossil was found by a fellow Canadian!

  8. MAL

    Can we also give a shout out to one of the co-authors, Mary Dawson (Carnegie Museum paleomammalogist), who is a pioneer for women in paleontology and a true scientific hero? I consider it a great privilege in my life to have gotten to know her, and you won’t regret having a chat with her at the next SVP, for those of you that attend! She was asked recently, prior to the lifting of the press embargo, about this new paper coming out and slyly looked at the questioner while replying: “All I can say is, my lips are sealed.”

  9. MattK

    Awesome. The 3D reconstruction of skull makes it seem like this animal had some pretty heavy hardware on the front end.
    Oh and I just checked out the CMN site; it is fantastic.
    On CBC radio today it was suggested that the first piece of the skeleton was found because one of the researchers (a grad student i think) was kicking around (literally) after her group’s ATVs had run out of gas. One of those fortuitous things I suppose. Wasn’t there a story like that about the Burgess Shale (someone fell off a horse or something??) that turned out to be false? It’s been awhile since I read Wonderful Life.

  10. MattK

    Actually, now I see that the CMN site has the running-out-of-gas story too. It was Liz Ross (a field tech) who found it.

  11. She named it Puijila darwini after an Inuit word referring to a young seal, and some obscure biologist.
    Heh, nice. :D

  12. Nit, but one that can confuse: Origin of Species, not Origin of the Species.
    I just finished the book, btw. Thanks for both that and this blog; I’ve mined it several times when putting together lectures for my behavioural neuro class this semester :-) .

  13. Ed Yong

    Awesome. The 3D reconstruction of skull makes it seem like this animal had some pretty heavy hardware on the front end.

    Don’t underestimate the hardware that modern seals and sea lions have. Check out Paul Nicklen’s leopard seal pics – here’s a taster from the Wildlife Photographer of the Year competition. I once interviewed this man about whether swimming with these animals was a scary experience. He said, “Their threat display, when they take your head in their jaws, is a bit nerve-wracking, but you just have to stay calm…”
    Update: I should probably mention that one of Nicklen’s photos is really very graphic (and could very well be used as the main image for the F*ck You Penguin blog. If you’re squeamish, don’t click on that link.

  14. MattK

    Wow. Those photos were killer.

  15. JFKman

    Thanks for the science even a History major can understand…

  16. Ed Yong

    Welcome Fark readers. The person who posted “This doesn’t explain bananas, does it? No, I didn’t think so.” made me snort coffee through my nose.

  17. cwatson

    reply to tbell
    but wait! if seals evolved from otters, then WHY are there still otters around? answer me that, silly evolutionist person
    my answer
    location, location, location thats the answer. different locations have different needs and dangers.
    same reason why humans dont look the same. location,location, location

  18. Great writeup! I’m sharing it with my students since this has been a topic of discussion recently.

  19. Greg

    reply to tbell
    Why are otters still around?
    There is still a place in nature where the otter can survive. Just because a species changes doesn’t mean the original line dies off.
    Imagine if one of your children moved to China. In three or four generations that part of your lineage would begine to look diffrent than the part of your family that stayed here. Both branches of your family continue to survive. One branch will evolve into somthing a little diffrent because of its location.

  20. Ed Yong

    Er folks, I think tbell was joking?
    Obviously good on you for the defence, but the satire filter probably needs ratcheting one notch up ;-)

  21. Niles

    All hail the holy FARK for he is the bringer of amusement and truth.
    Bananas were bred selectively to look like that. Just like poodles and pomeranean punt doggies. The imaginary level 80 sky wizard did not touch a wolf and say ‘be thy meek, tiny, domesticated, and the general mass of a football.’
    Or perhaps since your religion based off of a jewish zombie that is his own father that gets nailed to a giant tree that you telepathically tell him that he is your savior that you are saved. And also that the world whose recorded civilization goes back more than forty thousand years suddenly gets cut down to 6000.

  22. RHawley

    It seems to be a recurring theme that major groups of marine animals got their start in freshwater. Jawless fish, bony fish, whales and now seals…are rivers like training grounds for the more demanding environment of the sea?

  23. Ed Yong

    Are rivers like training grounds for the more demanding environment of the sea?

    That sounds about right to me. In freshwater, you have to adapt to moving about in water, perceiving prey, holding your breath and so on. The sea has all of that plus the additional challenge of high concentrations of salt, and a much larger scale.

  24. A lurker

    “Answers” in Genesis has replied. It seems that some newspapers have called the fossil “otter-like” thus combined with some more quote mining of newspaper accounts this evidence for evolution has been disproved. Yeah, that the ticket.

  25. Natalia Rybczynski

    Thank you Ed for writing such a nice article on Puijila! This crater carnivore still has alot more to tell us about the evolution of our flippered friends. Back to work…

  26. Dave

    Wow A Lurker, your lack of intelligence just made me spit up my drink that I just took a sip of. Its not wonder you are a religious person because you believe “newspaper accounts” of everything you read, do you also believe in everything in the National Inquirer?
    Disprove evolution Lurker, when you do i’ll break the reality to you that believing in god, and a fictional book is the same as believing in Clifford… Show me Clifford Lurker..

  27. MartinB

    Dave, I think the use of words like “quote mining” and the final statement should make it clear that A lurker did not agree with the link he posted.

  28. Sanjo in Canada

    Um, yeah… I too am quite confident that A lurker’s comment was tongue-in-cheek. I would also note the use of quotes – “Answers” in Genesis XD And not all religious people are creationist nutbars… Clifford is cool though. Well anyway. Thanks to the author for this great article. This is really exciting!

  29. SteveF

    I’m definitely not a creationist but neither do I believe the evolution stuff. The handy thing about creationists is the excellent ways they show the fallacy of the evolutionists. If macro evolution exist, we would see the process today with animals that transverse to other types of animals. I’m still waiting on a much better theory.

  30. MattK

    SteveF, I suggest that you hold your breath while you wait. I hear that it helps.

  31. SteveF, here’s how quickly the process can happen. Now, granted, it’s still a lizards (as PZ laments), but if a carnivorous lizard can become not only herbivorous but develop specific gut adaptations to deal with heavy herbivory in 30-odd years, I mean…it happens pretty quickly.

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