In a small office north of London, Stephanie Pierce from the Royal Veterinary College is watching a movement that hasn’t been seen for 360 million years. On her computer, she has resurrected the long-extinct Ichthyostega – one of the earliest four-legged animals to creep about on land. By recreating this iconic beast as a virtual skeleton, Pierce has shown that while it looked like a giant salamander, it couldn’t possibly have walked like one. It had some of the planet’s earliest bony legs, but they weren’t very good at taking steps.

Ichthyostega hails from the Devonian period, a time in Earth’s history when swimming transformed into walking. Fish invaded the land and evolved into the first tetrapods—four-limbed animals that include amphibians, reptiles, birds and mammals.  Muscular fins used for steering and balance evolved into legs for walking.

Since its discovery over 50 years ago, Ichthyostega has been an icon of this pivotal transition. Some 300 specimens have been found but many are incomplete, flattened or distorted. Pierce’s new model provides the best look yet at the animal’s skeleton. “It makes Ichthyostega a bit more tangible,” she says. “It’s not just a fossil laying in a rock now. It’s an animal that’s coming to life.”

Pierce built her virtual skeleton by putting dozens of Ichthyostega specimens in powerful CT-scanners, choosing only the best preserved ones out of the 300 or so in existence. “The front end of the animal was mainly composed from one beautifully preserved specimen called ‘Mr Magic’,” she says.

It was painstaking work. These fossils are so old that chemically, they are almost identical to the rocks around them. By eye, the bones stand out. To the scanners, they blend in. Pierce spent over two years scanning the specimens and building her model, but the results were worth it. “This has been on the wish-list for years,” says Michael Coates, who studies tetrapod evolution at the University of Chicago.

Those boots weren’t made for walking…

The model showed that Ichthyostega’s shoulders and hips were oddly restricted. They could move back and forth, and up and down, but they couldn’t rotate about their long axis. Hold your arm out and rotate your palm so it faces up then down—Ichthyostega’s shoulder couldn’t do that.

Most modern tetrapods need long-axis rotation in order to walk. Without it, their legs can’t be thrown forward or pulled backward. Ichthyostega’s limitations meant that despite having four limbs, it probably couldn’t have taken a step. It hind feet would never have been planted flat against the ground or supported its weight. It had invaded the land, but it wasn’t striding across it.

“It highlights the fact that the earliest tetrapods are not just ‘gigantic salamanders’, despite a vague similarity in outline,” says Per Ahlberg from Uppsala University. “The limbs and girdles are very different from anything now living.”

Pierce thinks that Ichthyostega moved by paddle with its front limbs, using powerful muscles and flexible elbows to make rowing motions. The closest living analogue is probably the mudskipper – a fish that drags itself along muddy land with its front fins (as in the video below).

Pierce also compared Ichthyostega’s joints and limbs to those of other living animals with sinuous bodies and interesting gaits, including a salamander, crocodile, seal, otter and platypus. Compared to these modern species, Ichthyostega’s hips and shoulders were similarly flexible in most planes of movements, but along their long axis, they could barely rotate.

Some scientists think that the tetrapods evolved limbs before they could walk, and their first members lived in shallow water. Others think that it’s the other way round, and that muscular limbs, hips and shoulders evolved while fish still had fins. The virtual Ichthyostega supports the former idea, since it had limbs but couldn’t walk. But Coates cautions against “fitting a smooth transition from swimmers to walkers.” He says, “Evolutionary transitions needn’t follow linear routes. Ichthyostega probably represents one of multiple experiments among the first tetrapods with limbs, trying-out life in the shallows.”

So… what made those tracks?

Other early tetrapods had similar shoulders and hips, so they probably had the same limitations too. John Hutchinson, who led the new study, plans to find out. His lab is busy reconstructing other early tetrapods including Acanthostega, one of Ichthyostega’s contemporaries, and Pederpes, a later model.

But Ahlberg notes that Ichthyostega had a very unusual and rigid spine, and may not have been representative of other early tetrapods. “Other tetrapods are known to have had more flexible spines” he says, “and this probably allowed them to overcome the limitations of their shoulders and hips”.

This might explain why Ahlberg and others have discovered tracks that pre-date Ichthyostega by around 20 million years, and had become fairly common by the time it evolved. Many of these tracks showed precisely the kind of salamander-like movements that Ichthyostega was apparently incapable of making. They were clearly made by early four-legged tetrapods, and to this date, we don’t know what made them.

Pierce agrees that the final word on Ichthyostega’s movements will have to wait until she can animate its entire skeleton. “The ultimate goal would be to try and create some sort of dynamic movement,” she says. She has applied for a grant to do just that, to model the motions of the entire animal, and compare them to salamanders or crocodiles. “That’s going to take so much time, but it’ll be very interesting,” she says.

PS: I want to point out that in researching this story, I spent a good minute on my living room floor trying to walk without long-axis rotation. It was really hard, and I looked like an idiot. I did a similar thing when I was writing about hummingbird wing movements for Nature. I’m going to christen this Method Science Journalism.

Reference: Pierce, Clack & Hutchinson. 2012. Three-dimensional limb joint mobility in the early tetrapod Ichthyostega. Nature http://dx.doi.org/10.1038/nature11124

Image by Julie Molnar

More on tetrapods:

Fossil tracks push back the invasion of land by 18 million years

Fish fins and mouse feet controlled by the same ancient genetic switch

Experienced divers know that rising too quickly can be a fatal mistake. The changing pressure yanks previously dissolved gases out of one’s blood and forms tiny bubbles, like the fizz in a newly opened can of soda. Depending on where they emerge, the bubbles can cause everything from a rash (the skin) to seizures (the brain). To avoid this condition, known as decompression sickness or “the bends”, divers rise slowly.

Then again, if you’re being chased by a gigantic prehistoric shark, you may have no choice.

(more…)

Yutyrannus, by Brian Choo

Meet the largest feathered animal in history – an early version of Tyrannosaurus rex, clad in long, fuzzy filaments. This newly discovered beast has been named Yutyrannus huali, a mix of Mandarin and Latin that means “beautiful feathered tyrant”. And its existence re-opens a debate about whether the iconic T.rex might have been covered in feathers.

“This is a tremendously important fossil. Paleontologists have been waiting for a gigantic feathered theropod to turn up for some time,” says Lindsay Zanno from the Field Museum. Larry Witmer from Ohio University, agrees. “The big thing is the one-two punch of being huge AND feathered,” he says.

(more…)

In Australia, an ancient murder mystery is coming to a riveting conclusion, thanks to an unusual clue – not a fingerprint, or a bloody weapon, but fungal spores preserved in fossilised dung. The spores belong to Sporormiella, a fungus that only grows in mammal and bird droppings. Large plant-eaters provide it with vast banquets. In turn, the fungus reveals how many big vegetarians were living in the neighbourhood.

Scientists have used these spores to study the deaths of the giant animals that once grazed North America – “megafauna” such as mammoths, woolly rhinos, and more. Now, it’s Australia’s turn. By using Sporormiella records, along with other buried clues, Susan Rule from the Australian National University has found strong evidence that acquits a changing climate in the death of Australia’s giants. Instead, her study points the finger squarely at humans.

(more…)

---

For the longest time, artists could only speculate about what dinosaurs looked like. Sure, we could reconstruct their silhouettes from their bones, but the colour of their skin was a mystery. That’s not quite true anymore. Thanks to some well preserved fossils and some ingenuous detective work, scientists have started to assign the right palettes to these prehistoric reptiles.

The latest species to get this treatment is Microraptor. This Chinese dinosaur was about the size of a crow. Its body was covered in feathers. Long plumes on both its arms and legs gave it a distinctive four-winged, baggy-trousered look, and may have allowed it to glide or fly. And thanks to a new study by Quanguo Li form the Beijing Museum of Natural History, we know that Microraptor was probably black and certainly shiny.

It was a iridescent dinosaur, with the same metallic sheen that you see on today’s hummingbirds, peacocks, and swallows. If you travelled back in time and stumbled across Microraptor, you might think that you’d found a Cretaceous starling.

(more…)

“Get off!” “No, YOU get off!” “Okay, we let go on three. One… tw…” “Wait, wait, is that one, two, three and then we let go, or one, two, let go?” “One, two, let go.” “Okay, okay. One… two…” And they both died.

Here is a fatal accident, etched in stone. This fossil comes from Solnhofen in Germany and dates back to the Jurassic period. On the left is Rhamphorhynchus. It’s a pterosaur – one of many flying reptiles that flapped through the skies while the dinosaurs ruled the land. Its arm bones, which supported its leathery wings, stretch out to the left of the image, while its long, stiff tail points downwards. On the right is Aspidorhynchus, a predatory fish with a long, pointed snout.

On first glance, you might think that the fish tried to eat the pterosaur. But Eberhard Frey and Helmut Tischlinger have been studying the fossil in detail, and they think otherwise.

(more…)

In the desert of the United Arab Emirates, there is an unusual series of flat discs imprinted in the sand.  Each one is about 40 centimetres wide, and they snake off into the distance in several parallel lines, for hundreds of metres.

They are tracks. They were made by a herd of at least 14 early elephants, marching across the land between 6 and 8 million years ago. The track-makers are long dead, but in the intervening time, nothing has buried their tracks or eroded them away. Today, their social lives are still recorded in their fossilised footsteps.

(more…)

What do a leaping lizard, a Velociraptor and a tiny robot at Bob Full’s laboratory have in common? They all use their tails to correct the angle of their bodies when they jump.

Thomas Libby filmed rainbow agamas – a beautiful species with the no-frills scientific name of Agama agama – as they leapt from a horizontal platform onto a vertical wall. Before they jumped, they first had to vault onto a small platform. If the platform was covered in sandpaper, which provided a good grip, the agama could angle its body perfectly. In slow motion, it looks like an arrow, launching from platform to wall in a smooth arc (below, left)

If the platform was covered in a slippery piece of card, the agama lost its footing and it leapt at the wrong angle. It ought to have face-planted into the wall, but Libby found that it used its long, slender tail to correct itself (below, right). If its nose was pointing down, the agama could tilt it back up by swinging its tail upwards.

(more…)

A cat-like animal explodes from the long grass and leaps onto an antelope. Its huge bulk drags the target to the ground and its muscled forelegs pin it down. With two long sabre-shaped canine teeth, it stabs its victim in the throat, just the once, severing its blood vessels and windpipe. Death comes quickly.

The hunter could be Smilodon, a sabre-toothed cat that lived throughout North and South America, around one or two million years ago.

Or, it could be a nimravid, another group of hunters that looked like cats, but belonged to a separate, closely-related family. Some of them had sabre-teeth too, and they wielded these weapons between 42 and 7 million years ago, well before Smilodon or its relatives did.

Or, it could be Barbourofelis, a member of yet another group of sabre-toothed not-quite-cats, which lived between 16 and 9 million years ago. Its long sabres slipped into long grooves in its lower jaw, which looked like it was about to melt away.

(more…)

The sickle-shaped “killing claws” of dinosaurs like Deinonychus and Velociraptor have captured the imagination for decades. They were held aloft from the second toe, and were far bigger than the neighbouring claws. In Jurassic Park, Alan Grant tells an annoying child that the dinosaurs used their claws to disembowel their prey with slashing motions. That seems unlikely – they didn’t have a suitable cutting edge. Others have suggested that they were used for climbing onto larger prey.

But neither idea made sense to Denver Fowler from Montana State University, who has put forward a very different idea about how these animals used their infamous claws. He compared the feet of extinct dinosaurs like Deinonychus to those of living dinosaurs like eagles, hawks and other birds of prey. Both groups are known as “raptors” and Fowler thinks that they share more than their nicknames.

In his vision, which he calls the “ripper” model, Deinonychus killed small and medium-sized prey in a similar style to a hawk or eagle dispatching on a rabbit. Deinonychus leapt onto its target and pinned it down with its full body weight. The large sickle-shaped claws dug into its victim, gripping tightly to prevent it from escaping. Then, Deinonychus leant down and tore into it with its jaws. The killer claws were neither knives nor climbing hooks; they were more like anchors.

It’s a simple idea, but a potentially important one, for it casts Deinonychus’s entire body into a new light. Fowler thinks that it flapped its large feathered arms to keep its balance while killing a struggling victim. And its feet, which were adapted for grasping prey, would have given its descendants the right shape for perching on branches. Fowler says, “It really helps to make sense of the weird anatomy of these little carnivorous dinosaurs.”

(more…)