Around 395 million years ago, a group of four-legged animals strode across a Polish coast. These large, amphibious creatures were among the first invaders of the land, the first animals with true legs that could walk across solid ground. With sprawling gaits and tails held high, they took pioneering footsteps. Their tracks eventually fossilised and their recent discovery yields a big surprise that could rewrite what we know about the invasion of land. These animals were walking around 18 million years earlier than expected.
The evolution of four-legged creatures - tetrapods - is one of the most evocative in life's history. It has been illustrated by a series of beautiful fossils that vividly show the transition from swimming with fins to walking on legs. These include Panderichthys, a fish with a large tetrapod-like head and a muscular pair of front fins. Tiktaalik expanded on these themes. Its head could turn about a solid neck. Its limbs had the fin rays of its fishy predecessors but clear wrist bones and basic fingers too. Tiktaalik could support itself on strong shoulder bones, bend its fins at the wrists, and splay out its hand-like bones.
These animals - the elpistostegids - have largely been seen as transitional fossils. Tetrapods supposedly evolved from these intermediate forms and eventually replaced them. You could draw all of their skeletons in the corner of a book and flick the pages to see the move from sea to land happen before your eyes. But the new fossil tracks tell a very different story. As one reviewer writes, they "lob a grenade into that picture".
The earliest true tetrapods so far discovered were around 375 million years old and the earliest elpistostegids hail from around 386 million years ago. But the Polish tracks are 10 million years older still. These dates suggest that the elpistostegids weren't transitional forms at all. They weren't early adopters of new biological technology, but late-surviving relics that stayed in their fish-like state while other species had evolved new bodies and, quite literally, run with them. Per Ahlberg, who led the study, says, "I've been working on the origin of tetrapods for about 25 years, and this is the biggest discovery I have ever been involved in. It is enomously exciting."
It would be tempting to cast animals like Pandericthys or Tiktaalik in the role of biological luddites, outstaying their welcome with outmoded bodies. But that would be an injustice. If these animals co-existed with tetrapods for at least 10 million years, it suggests that their bodies were stable, well-adapted structures in their own right. They weren't just brief flirtations with sturdiness on the way to full-blown walking.
Ahlberg discovered the fossil tracks along with researchers from Warsaw University, led by Grzegorz Niedzwiedzki. The tracks are found in the disused Zachelmie Quarry, nestled among Poland's Holy Cross Mountains. The area used to be part of a tidal plain. Rocks from the site and a few rare fossils allowed the team to confidently date the track-ridden layer to around 395 million years ago, the middle of the Devonian period.
Among this layer, Niedzwiedzki found several tracks of different shapes and sizes. He also found several isolated handprints and footprints that clearly show signs of toes and ankles. Many of these tracks (such as in the diagram at the top) were clearly made by an animal walking with powerful, diagonal strides, powered by sprawling right-angled limbs.
They weren't the work of any elpistostegid, whose straight limbs and backward-pointing shoulders and hips would have created far narrower tracks. Elpistostegids would also have dug long central troughs as they laboriously dragged themselves along. No such troughs exist at the quarry. This tells us that the track-makers strode along using strong hips and shoulders to hold their bodies and tails off the ground.
Niedzwiedzki thinks that they were undoubtedly tetrapods, and big ones too. The animal that created the tracks above was just 40-50 cm long. But some of the footprints were 15cm wide and hinted at creatures that were around 2.5 metres in length. The largest print is 26cm wide and its maker was probably a giant.
The implications of the Polish tracks are so controversial that reactions from other palaeontologists have been, understandably, mixed. Ted Daeschler and Neil Shubin, who discovered Tiktaalik, both find the study intriguing, but not definitive.
For Shubin, the deal-breaker would be identifying the animals that made the trackways and establishing where they sit on the evolutionary tree. He says, "The skeletal anatomy, let alone evolutionary relationships, of a trackmaker is hard to interpret from a track or print." For example, he says that a model of Tiktaalik's skeleton would produce a print much like the one in the paper if it's mushed into sand, and different consistencies or angles would produce an even closer match. He adds, "There is nothing in Tiktaalik's described anatomy that suggests it didn't have a stride."
Daeschler agrees that "trace fossils such as these presumed tracks... are a notoriously difficult class of evidence to interpret with full confidence". Nonetheless, he's keeping an open mind and a keen eye on future developments. "Paleontology is a lively field in which new discoveries constantly refine our knowledge of the history of life on earth," he says.
Jenny Clack, the Cambridge scientist who discovered Acanthostega, has seen the Polish tracks for herself and finds them more convincing. Her only reservation is that the detailed prints don't have any trackways to show how their maker moved, while the trackways themselves consist of blobs. "But so do lots of previously known tracks," she says. "If you'd found those in other deposits in the last part of the Devonian, you wouldn't have any qualms about them." She'd like to see trackways of the detailed prints but she's nonetheless excited. "It's going to change all our ideas about why tetrapods emerged from the water, as well as when and where."
On the question of where, many scientists have suggested that the invasion of land began at the margins of freshwater - at river banks, deltas, lakes or flooded forests. But the Zachelmire quarry wasn't any of these. Most likely, it was a shallow tidal flat or perhaps a saltwater lagoon. The first tetrapods didn't lurk in rivers, but trampled the mud of coral-reef lagoons.
Niedzwiedzki thinks that this revised locale makes a better staging ground for the invasion of land. Twice a day, the zone between high and low tide is awash with stranded marine animals that would provide a feast for marine creatures experimenting with life on land. He argues that life was driven aground by the rich availability of snacks.
This new setting for the rise of the tetrapods also helps to answer a big question raised by the tracks. If tetrapods were walking around 18 million years earlier than we thought, they and the elpistostgids must have large "ghost lineages" - periods when they must have existed but for which no fossils have been found. Actually, very few fossils have been found at Zachelmie Quarry at all. These sites, where tetrapods first marched onto land, may have been good at preserving footprints, but they haven't been equally kind to bones.
But why do the fossils that have been found make it look a lot like the elpistostegids preceded the tetrapods? That's more of a stumper, but Niedzwiedzki has a possible answer. He thinks that elpistostegids may have colonised new environments before their tetrapod peers (or at least those environments that would preserve their bones). It's a nice hypothesis, but for the moment, it's just that. There's no clear answer, although the hunt for new fossils or tracks will hopefully provide one.
Clack is certainly excited by the new doors opened by this discovery. "People are now going to start looking in different places from where they traditionally looked," she says. "The Polish trackways were only discovered by accident. Nobody had ever looked at these Devonian deposits in detail before. Now the same team are starting to look for body fossils and they've started to find some, but no tetrapods yet. I'm expecting stuff to come out from other parts of the world too, like China."
And here, even the sceptics agree. "All scenarios are intruiging, but we simply do not know for sure," says Shubin. "All the more excuse to continue to go out in the field and find skeletons!"
Reference: Niedzwiedzki. 2009.Tetrapod trackways from the early Middle Devonian period of Poland. Nature doi:10.1038/nature08623
All images: copyright of Nature
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