The tracks of a ghost

January 7, 2010 • 4:48 am

by Matthew Cobb

One-time US Secretary of State Donald Rumsfeld actually got it right about one thing: there are unknown unknowns – things we don’t know we don’t know. A cracking example of an unknown unknown –transformed by discovery into a known known – has just appeared in Nature: the earliest known trackways produced by a tetrapod – a four-limbed animal that is part of the same lineage as dinosaurs, cats and us.

In 2006, Jerry’s Chicago colleague, Neil Shubin, discovered Tiktaalik, a lobe-finned fish that pulled itself out onto the land about 375 MY ago, exactly filling a gap in the fossil record. A year later, Polish paleontologist Grzegorz Niedzwiedzki discovered some traces of footprints in the disused Zachelmie quarry in the Holy Cross Mountains. At first he thought they were made by dinosaurs, but he eventually realised that the rock they were found in was far, far older than the earliest dinosaur.

These exquisitely-preserved traces not only date to 397 MY (22 MY years earlier than Tiktaalik) above all they clearly show the marks of feet and toes. They were not made by a lobe-finned fish. They were made by tetrapods. And big ones at that – some of the traces (there are around a dozen of them) suggest the animals were up to 2 metres long.

Left: Laser scan of trace. Right top: drawing of left hind limb of known fossil, Ichthyostega, bottom of Acanthostega, both of which show similarities to the limb that left the trace.

Some of the traces show clear examples of parallel and alternating limb movement, like those made by modern reptiles or amphibians.

Left: trackways; animal is presumed to be traveling from bottom to top. Middle: graphic representation of traces. Right: generic tetrapod showing how its gait might have created the traces. Source: Nature

We don’t know exactly what made the traces, because there are no skeletons to go with them. But the fact that they show such clear traces of toes shows that the current view that tetrapods evolved at most 385 MY ago is wrong. Behaviour has trumped anatomy – we can see what the animal did, even if we don’t (yet) know exactly what animal made it. For the moment, these animals are “ghost fossils” – they must have existed, but we don’t know what they were.

Evolutionary tree of living and fossil lobe-finned fishes, and tetrapods. The new tracks are given in green. Dashed red lines = “ghost fossils”. 1 = earliest articulated tetrapod skeletons; 2 = earliest isolated tetrapod bones; 3 = earliest known tetrapodomorph fish; 4 = possible earlier tetrapodomorph fish. Source: Nature

It’s not only the date that’s got people excited, it’s also the location where these fossils were found. Not an obscure Polish quarry, but the tropical tidal mud-flats that made up the rock that was eventually dug up nearly 400 MY later. It was previously thought that the first steps onto land – like those by Tiktaalik – were made in brackish ponds. The authors correctly write that their discoveries “force a radical reassessment of the timing, ecology and environmental setting of the fish-tetrapod transition, as well as the completeness of the body fossil record”.

Here is a terrific video produced by Nature to explain the discovery. It includes a great scene where they bring a model of Tiktaalik to the cloud-shrouded gloomy quarry and show that it couldn’t have made the footprints… (For reasons too boring to mention, this is taken from The Guardian website.)

[vodpod id=ExternalVideo.913408&w=425&h=350&]

There are some other fascinating points raised by this research.

First, Swedish paleontologist Per Ahlberg, who helped guide the discovery into the pages of Nature, writes: Niedzwiedzki made his amazing finding by looking in the “wrong” place (“everyone” knew that the tetrapod transition to land took place at another time, in different kinds of rocks): “If you’re thinking of applying to a research council for a grant to do that, you are virtually certain to be turned down. But you need to have the opportunity to do what might seem to be crazy things. It’s only by doing this kind of stuff that wildly unexpected things can be discovered.”

Given the current debate in the UK over government proposals requiring research to have social and economic “impact”, this is a telling comment. Many (most?) of the  most stunning scientific discoveries have been made by accident, or without any expected “impact”.

Second, these animals were not the first to venture onto the land. Invertebrates got there first. The earliest know terrestrial arthropods were Trigonotarbids, tiny spider-like animals that ventured onto the land around 400 MY ago, and are now preserved in the Rhynie chert in Scotland. Something clearly happened to the Earth’s ecosystem at this time that made it possible for a wide range of organisms to colonize the land. Within 30 MY or so, the dry surface of our planet was teeming with walking and crawling things.

Finally, as I write this, I have just finished marking some student essays about the evolution of odorant binding proteins (OBPs). These are enigmatic molecules that all terrestrial animals have in their noses (or antennae), and which – in ways we don’t really understand – help us to smell. One of the things these OBPs do is to help smells get through the watery barrier that protects our smell neurons.

Fish, strikingly, do not have OBPs, but they do have a sense of smell, which only works in the water. So we can be pretty certain that when the first animals stuck their noses into the air, they couldn’t smell. Per Ahlberg suggests that the Polish “ghost fossils” would have been coming onto the land to nibble at the flotsam and jetsam left by the tide – dead fish and so on. Maybe so, but if they were, they wouldn’t have been able to smell those rotting fish.

Or, to put it another way, any animal that could produce a molecule that would help to detect those stinking fish would be at an immediate advantage – natural selection presumably led to the rapid evolution of these molecules and of this key sense. Curiously enough, the deep phylogenomics of OBP genes might tell us something important about those first steps onto the land, and about the “ghost fossils” that left these amazing traces.

h/t: Ray Moscow

First posted over at the z-letter

34 thoughts on “The tracks of a ghost

  1. This is one of those wonderfully exciting instances that makes being a scientist so much fun. But try explaining that to someone who is not interested – “oh, they found some animals walking on land 20 million years earlier than they expected to, so what?”

    On the other hand, I’ve heard that Niedzwiecki is in his early twenties, which makes me wonder what the hell I’ve been doing with my life!

  2. Jerry,

    Can you comment in how this affects the status of Tiktaalik?

    As I understand it, a conservative view would not necessarily place Tiktaalik in a direct lineage with land animals. But T. could be a “cousin” to the actual transitional species. Sorry if that’s expressed clumsily; I’m not a biologist.

    Second, is it possible/plausible that there could have been two exoduses to the land?


      1. Guillermo, I’m not a vertebrate paleontologist, but your interpretation would concur with my own. It would indeed be nice to know what Shubin thinks. Well, Neil?

  3. Wow, there’s something so energizing about reading real science. The discussion of the genuine uncertainties, the inferences and insights, the possibilities of new discoveries to come… I don’t know what it is, but it puts a huge grin on my face. Thanks Matthew, always great to read of new work!

  4. “Natural selection led “presumably” to the rapid (sic) evolution…. ? tsk ..tsk..tsk…lets review Evo 101 books.

    1. I stand by my formulation – a) I presume it was natural selection because I have no proof that any other force was involved, but, who knows, sexual selection may have driven the evolution of OBPs… Plus natural selection is not the only way you get evolution – it’s the only way you can reliably get adaptation. b) I presume it was rapid because i) all studied terrestrial vertebrates have OBPs, so nothing ended up without them and ii) there would be such a massive selective advantage in being able to smell those smells. But, of course, I could be wrong. Those are my presumptions…

    1. Not as far as I’m aware. People have looked in fish in general, and found none. That was the starting point for my (gratuitous) speculation.

      1. Huh. Well it would certainly be interesting to look at lungfish as the gross anatomy of their olfactory system contrasts with ray-finned fish quite a lot. Then again, lungfish don’t forage out of the water to my knowledge – but obviously there may be other roles for aerial chemoreception. It would also be interesting to look at mudskippers which do forage out of the water although at first glance they look like visual predators and perhaps stem-tetrapods were as well.

        As a sort of silly not entirely on topic rant: I think there is an unfortunate tendency sometimes to take Danio or Takifugu (which I’m guessing are likely some of the ‘fishes in general’) as “representative basal (i.e. non-tetrapod) vertebrates” — but this is not the case, teleosts bear very little resemblance to most Paleozoic fishes including the lineage which gave rise to us. I’m not saying Paleozoic fish had OBPs, just that we should be very cautious in using living fish as a model for our own ancestors.

  5. interesting but now i wantto know what a sarcopterygian is? a “flesh-flying” critter? like a hot chicken wing?

  6. The video wonderfully captures the thrill of great discovery. But was the fiberglass Tiktaalik really necessary? And can’t Nature come up with some more recent salamander footage?

  7. Creationists will have a field day spinning this one…

    Is it possible that the oldest specimens of any of the tetrapod ancestors haven’t been found?

    1. Yes, they do. They are very different. As far as we know, no aquatic organisms have OBPs, but they do have a sense of smell.

  8. “Directed research” is an old myth that will simply not die. Only imbecile managers can talk about such a thing because it simply does not exist. We’ve probably all heard the mantra: “we’re focusing our research to produce a bigger socio-economic impact”. The words are out of an English dictionary, but they mean nothing in any language – it is an excellent example of Orwellian Newspeak.

    When we look at the history of discoveries (and there are so many discoveries that no one can hope to know any more that a tiny fraction of the history), even directed research has few great outcomes. Perhaps the most successful and sustained directed research is in the semiconductor industry. Many discoveries there have been applicable elsewhere (in fact pretty useless to the industry supporting the research) and many useful discoveries have come about due to other (non-semiconductor industry) researchers pursuing other interests. So even in the most successful example the productive yields overall are low nor can the research thrive in isolation (that is, no contribution from other researchers doing their own thing). The mere act of suggesting a “greater impact” (however that is measured – I propose measuring it by dropping heavy objects on managers) betrays an impassable gulf of ignorance; such absurd claims also trivialize research. New discoveries don’t come easily.

    An anachronistic view is always useful. Let’s go back in time before the era of Max Plank, James Clerk Maxwell, and Michael Faraday. Someone says “hey, wouldn’t it be great to be able to talk to someone far far away as if they were standing right in front of you?” There we go – there is an objective – now how will research dollars achieve it?

    Come to think of it, if anyone wanted directed research with a guaranteed large “socio-economic impact”, simply pour more money into weapons development and start more wars – easy.

    1. If we forget education/habituation (:-o), the large theories, and the attempts of direction that you speak of, we would arrive at an evolutionary process as the basic one. Which goes hand in hand with what you see locally in a lab or on a lab bench.

      So there is a lot of predictive power in what you describe, I’m sure.

  9. Another one for my alma mater. [Ahlberg is at Uppsala University.] Envy me.

    But wow! Let me say that again, “wow!” This makes so much more sense IMHO, predicting from the general availability of tide water beaches and pointing to some form of prediction of the correlation with other terrestrial animals.

    Now I’m so curious about the ecological changes around that time. Most likely something held terrestrial establishment back before, than promoted a likely diversified change then. If the above tetrapods were beach combers, I google some early Trigonotarbids (Palaeocharinus) as predators.

    As it is too cold to venture outside, I can as well indulge in meaningless speculation.

    Googling I find: “The Silurian (443 to 417 million years ago) was a time when the Earth underwent considerable changes that had important repercussions for the environment and life within it. The Silurian witnessed a relative stabilization of the earth’s general climate, ending the previous pattern of erratic climatic fluctuations. One result of these changes was the melting of large glacial formations. This contributed to a substantial rise in the levels of the major seas. […] It is also at this time that our first good evidence of life on land is preserved, including relatives of spiders and centipedes, and also the earliest fossils of vascular plants.”

    Um, so “previous pattern of erratic climatic fluctuations”, “melting of large glacial formations”. Doesn’t seem too bad, but clearly something happened with the environment that affected climate. And that is as far as googling gets me.

  10. “The authors correctly write that their discoveries “force a radical reassessment of the timing, ecology and environmental setting of the fish-tetrapod transition, as well as the completeness of the body fossil record”.”

    Is it really correct the the discovery necessarily means a “radical reassessment of the … ecology and environmental setting”?

    If the maker of the tracks was already fully ‘walking’ rather than crawling like Tiktaalik would, then these tracks were made well after the point where this animal’s ancestors made their transition from water to land. If separated temporally, why not ecologically? ISTM possible that the transition was made in the presently thought environments and that this descendent had expanded its range to the outright marine (presumably not an enormous leap from a brackish delta).

    Certainly, moving the transition millions of years earlier is a surprising thing and the attitude of Ahlberg, finding excitement rather than fright in having to re-evaluate much of a career shows the difference between creationist and scientific attitudes.

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