Nine years ago, a team of fossil-hunters led by Philip Gingerich from the University of Michigan uncovered something amazing - the petrified remains of an ancient whale, but one unlike any that had been found before. Within the creature's abdomen lay a collection of similar but much smaller bones. They were the fossilised remains of a foetal whale, perfectly preserved within the belly of its mother. Gingerich says, "This is the 'Lucy' of whale evolution."
The creatures are new to science and Gingerich have called them Maiacetus inuus. The genus name is an amalgamation of the Greek words "maia" meaning "mother" and "ketos" meaning "whale", while Inuus, the Roman god of fertility, gave his name to the species.
The foetus's teeth were the first to be uncovered and only as the surrounding (and much larger) bones were revealed, did Gingerich realise what his team had found - the first ever foetal skeleton of an ancestral ancient whale (see video). Alongside the mother and calf, the group also discovered another fossil of the same species in even better condition. Its larger size and bigger teeth identified it as a male.
This trio of skeletons is so complete and well-preserved that Gingerich likens them to the Rosetta Stone. They provide an unparalleled glimpse at the lifestyle of an ancient whale before the group had made the permanent transition to the seas. How it gave birth, where it lived, how it competed for mates - all these aspects of its life are revealed by these beautiful new finds.
Maiacetuswasn't quite like the whales we know and love. It was an intermediate form between the group's earliest ancestors and the fully marine versions that swim about today. For a start, still had sturdy hind legs that were good for swimming but would have allowed it to walk on land.
Another piece of evidence tells us that Maiacetus was definitely amphibious - its foetus was facing backwards in the womb. If the mother had lived long enough to give birth (and judging by the foetus's size, that wasn't far off), the infant would have greeted the world face-first. No living whale or dolphin does that - all of their young emerge backwards, leading with their tails, to minimise the risk of drowning in the event of a prolonged labour. A head-first delivery means that Maiacetus gave birth as a landlubber.
Whales are so beautifully adapted to life in the water that their relationships to other mammals aren't immediately clear. Thankfully, their evolution has been beautifully charted by a series of "transitional fossils" documenting the change in their bodies over massive gulfs of time.
They evolved from deer-like ancestors, hoofed mammals that lived on land and occasionally ventured into the water. Early members of the family included Pakicetus, a meat-eater with long, hooved legs, a dog-like snout, and a distinctive inner ear that only whales and their kin possess. From there, the family became gradually more comfortable in the water, with later species like Ambulocetus having powerful tails and back legs that were clearly adapted for swimming.
These adaptations became even more extreme in the protocetids, a group that included species like Rodhocetus. They had seal-like bodies and possibly tail flukes like modern whale, but they still kept powerful hind legs to support their weight on land. Later whales like Basilosaurus or Dorudon were very different. Their hind legs were tiny - larger than those of modern whales, but useless for walking. Their hip bones were also disconnected from their spines. They were fully marine animals.
The newly discovered Maiacetus was a protocetid - several changes away from its original hooved ancestors, but not as thoroughly adapted of ocean life as Basilosaurus. Gingerich believes that it fed in the sea before coming ashore to rest, mate and give birth. Its teeth are suited for eating fish. Its legs were built to power swimming and support its weight on land, but they wouldn't have let it swim very far, or granted it with much terrestrial agility. These legs constrained the animal to the boundary between land and sea - jack-of-all-trades, but master of none.
The foetus was alone in the womb, which suggests that Maiacetus (like modern whales) devoted its energy to rearing a single infant during every found of breeding. As I've mentioned, the baby would have been delivered head-first while mum was safe on land. The foetus as large and its teeth were well-developed, with the growth of its permanent teeth already underway. Among mammals, advanced chompers like these are a sign that the calf would have emerged from the womb as a mobile and capable youngster - just as deer fawns can run soon after they're born.
The male skeleton was about 12% larger than the female and in the flesh, the animal would have weighed about 39% more. Compared to other marine mammals, this size difference between the two genders is actually relatively small. It suggests that males didn't have to compete too brutally for mates for those that do (like elephant seals) grow to enormous proportions that dwarf the fairer sex. These fighters hoard females in harems, and the fact that Maiacetus didn't suggests that it couldn't. Perhaps food and shelter were widely spread out commodities that were impossible to hoard and defend.
All in all, Gingerich's latest finds are among his most alluring yet. The remains of these three individuals have lasted through 48 million years of compression and today, they paint an incredibly vivid picture of the life of an ancient species. The fact that they are whales is the icing on the cake. This group's story is one of the most beautifully illustrated in the field evolution and every new discovery is a welcome one.
Reference: PLoS ONE Gingerich PD, ul-Haq M, von Koenigswald W, Sanders WJ, Smith BH, et al. (2009) New Protocetid Whale from the Middle Eocene of Pakistan: Birth on Land, Precocial Development, and Sexual Dimorphism. PLoS ONE 4(2): e4366. doi:10.1371/journal.pone.0004366
Update: For other accounts of this story, I'd highly recommend reading what Carl Zimmer (Loom) and Brian Switek (Laelaps) have to say on it. Both bloggers have written extensively on whale evolution before and I used their pieces as a lot of background reading for this story.
More on transitional fossils:
- Whales evolved from small aquatic hoofed ancestors
- 'Missing link' flatfish has eye that's moved halfway across its head
- Heroes in a half-shell show how turtles evolved
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Damn it! You and Switek both posted this before me, as I thought there was an embargo present. Curses! Beautiful fossil though!
Amazing!
This is lovely and timely. I've been looking at the evolution of the whale with my dtrs. I'm going to show this to them tomorrow. They'll be fascinated by the foetus. (When they were pre-schoolers they had me pretend to go into labour complete with sound effects while they slid down the twisty slide in the playground, ie the birth canal. But I draw the line at being the ancestral whale mother).
It's an exciting discovery Ed, though I can't help thinking that your comment "Its teeth are suited for eating fish. Its legs were built to power swimming and support its weight on land, but they wouldn't have let it swim very far, or granted it with much terrestrial agility. These legs constrained the animal to the boundary between land and sea - jack-of-all-trades, but master of none. " does Maiacetus a disservice. The description isn't far off how you'd describe a seal or (stretching it a little) otter, so Maiacetus may have been very well adapted to its coastal environment, perhaps even the "master" of it.
Paul - Fair point. Perhaps a bit of hyperbolic prose on my part. Will change later.
Lilian - I love that you show the blogposts to your kids ;-)
Zach - Bora tells us that the PLoS embargoes lift automatically if the paper is published slightly ahead of schedule. We only posted by 90 mins tops.
Ed,
Might I recommend linking this as a Trackback?
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0004…
Thanks for the links, Ed! I'm glad you find some of my other posts helpful. I should have just sent you the chapter on whales I'm working on (which now needs to be updated, of course).
I am a HS physics teacher, and I was amazed by this reporting. Thank you for the good service you provide to me who loves knowledge.
Wow! Great story. Thank your for such excellent writing and use of hypertext.
If you tried to send a trackback - don't worry. TOPAZ upgrade saved them elsewhere and will automatically put them on the paper once the process is over.
When you're talking about mammals, it's apparently considered OK to describe any old transitional fossil as "ancestral", even though (a) it's almost certainly not an ancestor of anything living, and (b) if it was, you would have no way to demonstrate it. I see that as a convenient shorthand, but it seems that if you step back to the Mesozoic and say "ancestor", or even "possible ancestor", you will assuredly be scalped.
Yeah it's meant to be a shorthand. But you're right - it could be misinterpreted. Tell you what - let's take a poll. Anyone else think that "ancestral" in this context is a misfire. Is this one of the things that science writers do that pisses off scientists? Let me know so I can continue/refrain as appropriate.
Actually sod it - I've mulled it over and Nathan's right. "Ancestral" is ambiguous verging on wrong. Out it goes.
Me, I think the interesting distinction is between "might have been ancestral" and "was not ancestral because...". Invariably (unless the intermediate form is just a damn tooth or something) there's some collection of derived features not present in the next taxon up (shovelwise). Exactly what those features are makes an interesting sidelight: "they fused some wrist bones, but it didn't save them!"
I have read that mammalogists are particularly sloppy about this sort of thing. Is that your impression? (I think it was one of the Zachs who mentioned the phenomenon.)
Dave Hone hosted an interesting, if slightly apoplectic, exploration of the issues at http://archosaurmusings.wordpress.com/2008/12/23/anchiornis-a-new-basal… . You're mentioned there, by the way, entirely non-ironically.
Heh. Interesting discussion over there. On balance, I think David's right though and I value accuracy in science writing above all else. What's interesting to me is that I specifically avoided this mistake in an earlier post on Indohyus, another key fossil in the whale evolution story.
Disappointing, then, that it's cropped up here. All I can really say is that I, just like any other science writer/journalist, will inevitably make mistakes. Even with the best will in the world, it's going to happen.
I will, however, always correct stuff here if the right reasons are given. It's part of the ethics of blogging and part of what I think makes the medium strong. You get feedback from much more learned and informed people than yourself, which can only help people like us to be better at our jobs. Only an idiot would turn a deaf ear to that.
So thanks Nathan. And for the compliment over at Archosaur Musings too. Appreciated.
It's hard to know what you're agreeing with Dave about.
You feel it's better to splash a bucket of facts on ignorant reporters, and let them try to put together a coherent story, and then complain when they fail, than to give them an engaging story, including some background and context, that they can run as is?
I meant specifically that I think he's right about the incorrect use of the word "ancestral".
OK, everybody agrees on that. Apologies for misunderstanding.
Great story. Thanks for the interesting read.