A Mysterious Fossil Points to the Origins of Lizards and Snakes
The ancestors of today’s squamates were lost in time. Now paleontologists have identified the earliest known example: Meet Megachirella.
Some 240 million years ago, a storm rolled over the tropical edge of the supercontinent of Pangea and battered the coastal islands. Rain lashed forests of horsetails and ferns, sweeping plants and animals out to sea and burying them in silt.
Among the victims was a small reptile. At the time, it didn’t look like much. But recently in the journal Nature, an international team of scientists identified the animal, called Megachirella, as the earliest known ancestor of the squamates, the group containing lizards, snakes and wormlike amphisbaenians.
With 10,000 distinct species, squamates comprise one of the largest orders of land vertebrates on the planet. They’re found on six continents, have adapted to a dizzying array of habitats and range in size from 25-foot pythons to chameleons smaller than a pencil eraser. Extinct members of the group include massive snakes like Titanoboa and seafaring mosasaurs, which rivaled small whales in size.
Squamates “inhabit almost every single environment,” said Tiago Simões, a paleontologist at the University of Alberta, who coauthored the study. “They learned how to climb on glass, how to swim, how to glide. They can even run over water.”
But the origin of squamates has long been an enigma. The order belongs to a larger group called lepidosaurs, a category that also includes a sister lineage called the rhynchocephalians — represented today only by the humble tuatara, a small reptile in New Zealand.
The two groups diverged from a common ancestor in the distant past, but when and how that happened has been a mystery. Genetic data had long suggested that squamates originated somewhere during the Early Triassic Period, 252 to 247 million years ago, said Adam Pritchard, a paleobiologist at the Smithsonian National Museum of Natural History.
But when it came to fossil evidence, the only lepidosaurs anyone could find from the Triassic Period were the rhynchocephalians. The earliest known squamate fossils hailed from the later Jurassic period, leaving a frustrating 75 million-year gap between the shared ancestor and the appearance of what are now some of the planet’s most familiar animals.
“It’s as if we have definitive evidence of a family that had two siblings,” Dr. Pritchard said. “One sibling, you know a lot about their early life and how they grew up. And the other one, you have no information whatsoever about their childhood.”
At first, Megachirella didn’t appear to offer much insight.
The specimen was found by an amateur fossil hunter in 1999 in an outcrop of limestone in the Dolomite mountains of northern Italy. The slab, which dated to the middle Triassic Period, contained the front half of a small, slightly squashed skeleton with a large head and heavy forelimbs.
The remains were beautifully articulated, but somewhat hard to make out. When scientists formally named it in 2003, they described it as a primitive lepidosaur. Available technology wasn’t sufficient to probe deeper.
But in 2014, Dr. Simões — who specializes in untangling the evolutionary history of living and extinct squamates — came across a paper on the fossil that caught his interest. Looking at the anatomical figures, he knew right away that he was likely looking at a lizard — and a very early lizard.
He contacted one of the paper’s authors, Massimo Bernardi, a paleontologist at the Museum of Science in Trento, Italy, and suggested that they run the fossil through high-resolution micro CT scanning in order to better see the compressed bones. Dr. Bernardi, who had been thinking along similar lines, agreed.
Under the high-tech gaze of the scanners, the scientists were able to digitally reconstruct the small crushed bones, picking out fine details of anatomy invisible to the naked eye. They found that portions of the animal’s braincase more closely resembled that of a modern iguana than those of other Triassic reptiles; so did the arrangement of teeth and the lack of perforations in the lower part of Megachirella’s vertebrae.
Even more intriguing were the animal’s wrist bones, shoulders and arms, more characteristic of those seen in living lizards. But Megachirella had some vestigial bits of anatomy that are absent in modern lizards and snakes: gastralia, belly-ribs still found in tuataras and crocodiles, and a small cheek bone called the quadratojugal.
Both Megachirella’s appearance in the middle Triassic and the later remains of rhynchocephalian fossils make a strong case that the earliest squamates emerged toward the end of the Permian, a period 270 million years ago that was abruptly shattered by one of the worst known mass extinctions. At the dawn of the following Triassic Period, over 90 percent of life on the planet had been wiped out.
The surviving animal groups rapidly diversified throughout the emptied world, said Michelle Stocker, a paleobiologist at Virginia Tech. The trees filled with chameleon-like drepanosaurs and eventually with the first pterosaurs; the oceans soon gave rise to the first marine reptiles; archosaurs morphed into bipedal crocodiles and skulking dinosaurs.
Lepidosaurs scurried along in the shadows, undergoing their own explosion in diversity. And while rhynchocephalians fared well for tens of millions of years, Dr. Pritchard said, it now seems that squamates gained on them, becoming even more common and diverse.
Somewhere in the Jurassic, 75 million years after Megachirella, the first recognizable lizards emerged, still bearing some of the anatomical markers of their distant ancestor.
“The Permian extinction opened the way for the origin of these lineages, and opened up opportunities for colonizing land and sea,” Dr. Simões said.
“A lot of the modern fauna in the world today — in terms of reptiles especially — originated at some point in the Triassic,” he added. “The first mammals are from the Triassic, the first dinosaurs are from the Triassic, and now we know the first lizards are from the Triassic, too.”
Megachirella’s mixture of archaic and modern anatomy is also a clue to which of today’s lizard families were the first to emerge, Dr. Simões said. This is a contentious topic. Since 2014, DNA-based family trees have suggested that geckos, of all things, were the earliest group of modern lizards to evolve.
But scientists who looked primarily at fossils championed the large and diverse iguanian family — a group that includes chameleons, iguanas and agamas — as the earliest modern lizards. Dr. Simões incorporated data from Megachirella’s anatomy into a larger data set he had assembled by studying squamate species from 50 museum collections in 17 countries. The resulting tree suggested that geckoes were indeed the first of the surviving lizard families to emerge, in the Jurassic period (though there’s no evidence they acquired their distinctive toe-pads until the Cretaceous period).
Iguanian fossils show up in the Cretaceous, and the genetic data groups them closely with monitor lizards and snakes, Dr. Simões said. By the end of the Cretaceous, the rhynchocephalians had all but vanished, leaving only the tuatara. But the squamates continued to diversify into their present array of forms.
“The amazing thing is that those few surviving lineages became very diverse,” Dr. Simões said. “Imagine how diverse the world must have been in the Mesozoic, with all the uncut branches of the reptile family tree still around.”
For Dr. Stocker, part of the value of Megachirella lies in the fact that so much information was pulled from its bones long after the fossil was discovered and described — a testament to the value of the dusty specimens lying in museum collections worldwide.
“Even the information we can get out of a fossil today, it might pale in comparison to what technology might let us understand about that extinct animal in the future,” Dr. Stocker said. “So it’s important that we take care of these fossils now for future researchers.”
There are still a lot of questions about both Megachirella and the winding path that squamates took to the present day. After all, a huge gap remains in the family history, and many extinct types have unclear links to modern species.
“We have more time between Megachirella and the other squamates than between humans and the last dinosaurs,” Dr. Simões said. “Explore the first 75 million years of any other family and you see an amazing amount of diversity in body plans and types.”
“If we keep looking in this period, we’re going find amazing things that we never dreamed of, or things that we never thought lizards could do.”