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Ancient fossil rewrites the hiss-tory of lizards and snakes

By Research Associate Dr. Alessandro Palci

Research Associate Dr. Alessandro Palci with a copy of Nature showcasing his work on fossil lizards and snakes

It’s quite odd how sometimes things unexpectedly come back from our past in circuitous ways. It was the summer of 2000 when, still an undergraduate student, I was touring the Dolomites as part of a field trip organised by the department of Geological Sciences of the University of Trieste (Italy).

It was a sunny spring day, and I was looking down at some Jurassic dinosaur footprints, when I first heard of a small fossil reptile that had recently been found nearby, in 240 million-years-old rocks. The rumour was that someone was already working on its description. I won’t deny that I wished I could be that someone. Unfortunately, I was only an undergraduate student at the time, and a long way to go before my first palaeontological paper would be published.

The fossil was described and published by Silvio Renesto and Renato Posenato in 2003. It turned out that the fossil was preserved in rocks that originated on the bottom of a warm shallow sea. The little reptile was one of very few terrestrial organisms that had been entombed in those marine sediments, which makes its discovery incredibly fortuitous.

Renesto and Posenato named the fossil after its relatively large hands: “mega” and cheiros” are ancient Greek words for “large” and “hand”, respectively. They then classified the reptile as a member of the Lepidosauromorpha, a broad group of reptiles that includes lizards, snakes, tuataras, and many other more distantly related reptiles. Ten years later, in 2013, Megachirella was re-described by Silvio Renesto and Massimo Bernardi, and the authors confirmed the classification of Megachirella as some kind of lepidosauromorph based on a phylogenetic analysis, but did not resolve its affinities very precisely.

Another four years went by, and in the summer of 2017 I flew to Calgary, to attend the 77th annual meeting of the Society of Vertebrate Paleontology. International meetings are a great opportunity for networking, but also for catching up with old friends spread around the world. One of these friends of mine is Tiago Simões, who was finishing his PhD in Systematics and Evolution at the University of Alberta. He told me that he was working on an interesting fossil reptile, what he suspected could be the oldest fossil lizard. He also told me that he was looking for someone who could work on its CT scan data. At the time I had no idea what the mysterious fossil was, but I had considerable experience working on CT scan data, so I offered to help him out.

It turned out that the fossil of Megachirella had been scanned at the ICTP Elettra synchrotron facility in Trieste, Italy, and that was the data that Tiago wanted me to work on. Only now I realize how crazy all this is, it was that very same little fossil reptile I heard about in the Dolomites seventeen years before, and it had somehow finally found its way into my hands. The CT scan data provided a wealth of new information about the little fossil, and this helped us place Megachirella in quite a different and much more interesting light.

The three-dimensional digital images generated from the CT scan data allowed us to look through the rock, at parts of the skeleton that were inaccessible before, without incurring in the risk of damaging this unique fossil by removing the embedding rock mechanically. What we discovered was that Megachirella already shows modifications in its skull and limbs that are shared with modern lizards (like for example a triradiate squamosal at the back of the  skull, a mobile quadrate bone bearing the upper jaw joint, an ulnar patella in the elbow, and the loss of the first distal carpal in the wrist), but it also retains some extra bones that were commonplace in early reptiles, and were lost much later in lizard evolution (like gastralia along the belly, and a quaratojugal in the cheek region). We included this new information in a an extensive phylogenetic dataset that Tiago had put together during the course of his PhD, and it turned out that Megachirella is indeed a stem-lizard. The oldest and most primitive lizard known to date, a finding that earned us the cover of the latest issue of Nature.

Megachirella gives us a new window on what the first lizard probably looked like and helps us understand how the anatomy of earlier reptiles gradually changed over time, in little steps, to produce the body shape of lizards that is familiar to us today.

This fossil is important not only because its anatomy bridges a gap between two major groups of reptiles, much like Archeopteryx, but also because it extends the fossil record of lizards back in time by about 70 million years, into the middle Triassic (~240 million years ago). That lizards must have been present in such deep time had been predicted by some molecular studies of modern lizards, based on the speed at which DNA strands accrue mutations over time; however, the oldest known fossil lizards before Megachirella were frustratingly “young”, coming from rocks that originated in the Jurassic, only about 167 million years ago.

Megachirella tells us that lizards were indeed around in the Triassic, and based on a new estimate of the rate of evolution of lizards, we can now place the origin of these reptiles in the late Permian.

A life scene in the Dolomites region, Northern Italy, about 240 million years ago, with Megachirella wachtleri walking through the vegetation. (Photo: © Davide Bonadonna.)

So, next time you are out in the field looking for fossils and see a lizard scampering around, think of Megachirella, and that the evolutionary history of this amazing group of reptiles started 260 million years ago, long before the first dinosaurs appeared!

(Adapted from the original article appearing in The Conversation on 31.05.18)

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