THE TRACKWAYS OF A GHOST

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, 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 videos no longer available.

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.

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