Insights into tetrapod and lizard evolution

Palaeontologists using high-tech methods more commonly associated with medical or crime-scene labs are prying ever-more sophisticated knowledge out of fossils, in the process refining our knowledge of the origins of important groups of modern animals.

In a paper published in the journal Nature, for example, a team led by Jean Goedert of the Université de Lyon, France, used sulfur isotopes in the fossilised bones of the earliest four-limbed organisms, known as tetrapods, in order to determine the conditions in which these creatures evolved.

Although famous for being the first to walk on land, these animals were primarily aquatic, with gills and a powerful tail for swimming. For decades, palaeontologists have argued about what type of water they lived in, with some believing they came from freshwater and others believing they came from marine environments.

To resolve the question, Goedert’s team turned to an analysis of an isotope of sulfur known as sulfur-34, the second most common of the element’s four stable isotopes.

On average, it represents about one atom in 24 of any given sulfur sample. But it is substantially more plentiful in seawater than fresh — a factor that allows ecologists studying modern organisms to determine their home environments.

These ecological studies, however, have used soft tissues, where sulfur is relatively common, rather than bones, where it is not.

In order to determine if the same method can be used to determine the origin of fossils, Goedert’s team analysed sulfur-isotope ratios in 51 tetrapod specimens from two locations dating to the Devonian Period, about 365 million years ago. They then did similar analyses on modern animals, ranging from fish to crocodiles and turtles, verifying that their method could distinguish freshwater from salt-water species.

Based on this, they concluded that the tetrapods came from brackish environments – estuaries and deltas in which freshwater and seawater.

These environments, Goedert and his colleagues conclude, required the animals to cope with widely varying levels of brackishness — an adaptation that may have given them the versatility they needed to progress onto the land and become the ancestors of every four-limbed creature we know today, including ourselves.

In another paper in the same issue of Nature, a team led by Tiago Simões, a palaeontologist at the University of Alberta in Canada used an entirely different technique to study a 240-million-year-old fossil of a species known as Megachirella wachtleri, collected many years ago in the Italian Alps.

Most of the fossil was still encased in rock, but by putting it in a device akin to a medical computed tomography (CT) scanner, Simões was able to probe its hidden parts, allowing him to see the animal’s anatomy in three dimensions.

“It’s not very different from what happens in a medical scanner,” he says.

In fact, he adds, it’s actually possible to use proper medical scanners for such analyses, if the fossil is large enough.

“Those machines are basically measuring difference[s] in density,” he says. “In the human skeleton you get contrasts between bone and other tissues. In the case of a fossil, it’s reading the difference in density between the preserved remains of the animal and the rock matrix where it’s embedded.”

Smaller fossils like this one, which is only a few centimetres long, require specialised equipment called a micro-CT scanner that can provide greater resolution, but the concept is the same.

In the best machines, he explains, resolution is on the order of microns. It’s even possible to peer inside a fossil, not only imaging every bone individually, but mapping the interior of the skull in order to determine the shape of ancient animals’ brains.

“That’s called palaeoneurology,” he says.

In the case of Megachirella, the CT scans found features indicating that the animal belonged to a taxonomical order known as squamates — a group of reptiles that includes modern lizards and snakes.

Megachirella is the oldest known lizard,” Simões says.

That’s important, he adds, because it’s 240 million years old — 72 million years older than the earliest previously known squamate.

“Lizards and snakes are one of the most diverse faunas in the world today,” he says. “They have more than 10,000 species. So trying to understand their origins [is] important.”

Stephanie Pierce, a vertebrate palaeontologist at Harvard University in Massachusetts, US, says that both papers are interesting and exciting. The tetrapod paper, she says, is important because it helps explain how the creatures managed to appear on many different continents — something that would be difficult if they were confined to freshwater environments separated by oceans.

The squamate paper, she adds, pushes the origin of snakes and reptiles back to before an event known as the Permian extinction, indicating that rather than evolving after that event wiped out most terrestrial animals some 252 million years ago, the group appears to have survived the extinction and prospered in the aftermath.

But what unites the two studies is the big picture they provide of modern palaeontology.

“These are two very different studies using two different techniques,” Pierce says, “but they are really bringing palaeontology to the modern age. It’s not just a rock hammer and a hat. These are biologists, geochemists, all these people coming together to try and understand and reconstruct the evolution of life. That’s pretty exciting stuff.”

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