A New Fossil Species Represents the Ancient Precursor to Most Modern Reptiles

Reconstruction of Taytalura life in its natural habitat with the extinct conifer Rhexoxylon in Ischigualasto (Argentina) during the Upper Triassic, hiding from the primitive dinosaur Eodromaeus (in the background) inside the skull of a mammalian ancestor. Credit: Original artwork created by scientific illustrator Jorge Blanco

An international team of researchers describes a new fossil species representing the ancient precursor to most modern reptiles.

Lizards and snakes are a key part of most terrestrial ecosystems on earth today. Together with the charismatic New Zealand tuatara (a “living fossil” represented by a single living species), the squamates (all lizards and snakes) constitute the Lepidosauria – the largest group of terrestrial vertebrates on the planet today with about 11,000 species, and by far the largest modern group of reptiles. Squamates and tuataras have an extremely long evolutionary history. Their lineages are older than the dinosaurs, having originated and diverged from each other around 260 million years ago. However, the first phase of lepidosaur evolution, 260 to 150 million years ago, is marked by highly fragmented fossils that do not provide much useful data to understand their early evolution, leaving the origins of this group of very diverse animals in mystery for decades.

In a study published today (August 25, 2021) in Nature an international team of researchers describes a new species that represents the most primitive member of the lepidosaurs, Taytalura alcoberi, found at the end Trias deposits from Argentina. Discovered by lead author Dr. Ricardo N. Martínez, Universidad Nacional de San Juan, Argentina, and curator at the Instituto y Museo de Ciencias Naturales, Taytalura is the first ancient lepidosaur fossil preserved in three dimensions. It has allowed scientists to infer with great confidence its placement in the evolutionary tree of reptiles and helps close the gap in our knowledge of the origin and early evolution of lepidosaurs.

Reconstruction of the skull of Taytalura

Reconstruction of Taytalura’s skull based on high-resolution CT scans (left) and its placement in the reptile evolutionary tree (right). Credit: (left) Gabriela Sobral, Jorge Blanco and Ricardo Martínez; (Right) Tiago Simões

Martínez and co-author Dr. Sebastián Apesteguía, Universidad Maimónides, Buenos Aires, Argentina, performed high-resolution CT scans of Taytalura which confirmed that it was something related to ancient lizards. They then contacted co-author Dr. Tiago R. Simões, a postdoctoral researcher in the Department of Organismal and Evolutionary Biology at Harvard University, to help identify and analyze the fossil. Simões specializes in the study of these creatures and in 2018 published the largest existing dataset to understand the evolution of the main groups of reptiles (living and extinct) in Nature.

“I knew the age and locality of the fossil and could tell by looking at some of its external features that it was closely related to lizards, but it looked more primitive than a real lizard and that’s something quite special,” said Simões.

The researchers then contacted co-author Dr Gabriela Sobral, Department of Paleontology, Staatliches Museum für Naturkunde Stuttgart, Germany, to process the CT scan data. Sobral, a specialist in CT data processing, created a mosaic of colors for each bone in the skull allowing the team to understand the anatomy of the fossil in high resolution on a scale of just a few micrometers – about the same thickness as a human hair.

With the data from Sobral, Simões was able to apply Bayesian evolutionary analysis to determine the correct placement of the fossil in the reptile dataset. Simões had recently applied the Bayesian method – which was adapted from methods originally developed in epidemiology to study how viruses like COVID-19[female[feminine evolve – to accurately estimate the time and rates of anatomical evolution during the rise of tetrapods. Statistical analysis confirmed their suspicions that Taytalura was actually the most primitive member of the line that eventually gave rise to all lizards and snakes. “It’s not even a lizard in the evolutionary tree,” Simões said, “but it’s the very next thing over there, between the real lizards and tuataras, and all the other reptiles.”

“This beautifully preserved 3D fossil is truly an important find. It is the most complete fossil representing the earliest stages of lepidosaur evolution that we have to date. All other known fossils are too incomplete, making it difficult to classify them with certainty, but the complete and articulate nature of Taytalura makes his relationships much safer,” Sobral said.

Simões agreed: “Taytalura is a major point in the reptile tree of life that was previously missing. Because these fossils are so small, they are very difficult to preserve in the fossil record. And the candidate fossils we have are very fragmented and poorly preserved, so they don’t provide as much useful data for analysis.

Taytalura The skull reveals that early lepidosaurs looked much more like tuataras than squamates, and therefore squamates represent a major departure from this ancestral pattern. Additionally, it has unique dentition, unlike the teeth found in any group of living or extinct lepidosaurs. “What our analyzes tell us, besides some other anatomical features that we could see on it, in the skull in particular, is that this type of sphenodontian body, at least for the skull, is the ancestral model of lepidosaurs. The ancestral pattern seems to be more similar to tuataras,” Simões said.

Taytalura preserves a composition of features that we did not expect to find in such an ancient fossil. For example, it shows some features that we thought were exclusive to the tuatara group. On the other hand, it has made us wonder how ‘primitive’ certain features of lizards really are, and it will cause scientists to reconsider several points in the evolution of this group,” Sobral said.

“The almost perfectly preserved Taytalura The skull shows us details about the origin of a very thriving group of animals, including more than 10,000 species of snakes, lizards and tuataras,” Martínez said. “But it also highlights the paleontological importance of the paleontological site of the Ischigualasto Formation, known for preserving some of the most primitive dinosaurs known to the world. The extraordinary quality of fossil preservation at this site has allowed something as fragile and tiny as this specimen to be preserved for 231 million years.

“Unlike nearly all Triassic lepidosaur fossils found in Europe, this is the first early lepidosaur found in South America, suggesting that lepidosaurs may have migrated through very distant geographic regions early in their history. scalable,” Simões agreed.

“We are used to accepting that Mesozoic The era was a time of gigantic reptiles, huge proto-mammals and huge trees, and so we routinely look for fossils visible at human height, just by walking,” Apesteguía said. “However, most of the old components of the ecosystem were small, like today. There was a universe of fauna weaving between larger, clawed or hoofed legs. Taytalura teaches us that we were missing important information by not only looking for larger animals, but also by thinking that the origin of lizards only occurred in the northern hemisphere, as the evidence seemed to confirm so far.

While Taytalura is primitive, it is not the oldest of the lepidosaurs. The fossil is 231 million years old, but there are also fossils of real lizards dating back 11 million years. The team plans to explore older sites next in hopes of finding similar or different species of the same lineage that branch just before the origin of true lizards.

Reference: “A Triassic stem lepidosaur sheds light on the origin of lizard-like reptiles” by Ricardo N. Martínez, Tiago R. Simões, Gabriela Sobral and Sebastián Apesteguía, August 25, 2021, Nature.
DOI: 10.1038/s41586-021-03834-3

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