Land-Dwelling Dinosaur Relatives Bridge Gap to Origin of Pterosaurs
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Pterosaurs were highly successful reptiles that lived between 210 and 65 million years ago. They were the first vertebrates to evolve powered flight, but their origin has remained an unresolved enigma in paleontology since the 19th century. With the help of pterosaur skulls and skeletons that were unearthed in North America, Brazil, Argentina, and Madagascar in recent years, a team of paleontologists has now demonstrated that flightless dinosaur precursors called lagerpetids (family Lagerpetidae) are the sister group of pterosaurs.

Ixalerpeton, a species of lagerpetid that lived in what is now Brazil approximately 233 million years ago. Image credit: Rodolfo Nogueira.

Pterosaurs are deeply rooted in popular culture, are frequently portrayed in books and films, and include the largest flying animals ever known. Their evolutionary history spans more than 150 million years, ending at the close of the Mesozoic era.

The oldest pterosaurs are from the Upper Triassic epoch (219-201.3 million years ago) of Europe and North America and they diversified into several groups by the Middle-Late Jurassic epochs.

The oldest recognized pterosaurs already had a highly specialized body plan linked to their ability to fly, which was conserved in all pterosaus.

“The first pterosaurs were found during the 18th century and, from that time, it was observed that they had an anatomy, a body plan that was very different from that of other known reptiles, having wings formed by membranes, supported by an hyperdeveloped fourth finger of the hand (the ring finger in humans), which is a unique feature of pterosaurs,” said lead author Dr. Martín Ezcurra, a paleontologist at the Museo Argentino de Ciencias Naturales, CONICET and the University of Birmingham.

“Since then, and during the next 250 years, one of the main mysteries of vertebrate paleontology was who were the closest terrestrial relatives of these flying reptiles and thus be able to understand how this evolutionary transition occurred.”

“The origin of the flying reptiles was one of the great enigmas of paleontology and evolutionary biology as a whole,” said co-author Dr. Federico Agnolin, a paleontologist at the Museo Argentino de Ciencias Naturales, CONICET and the Universidad Maimónides.

“Now, we know that there was an intermediate step in the lagerpetids, terrestrial reptiles that look similar to a lizard, which could not fly, but that, in this study, we could already observe some evolutionary steps in their brain and inner ears that were adaptations that later would have allowed pterosaurs to develop flight.”

The lagerpetids were small, wingless reptiles that lived across Pangea during much of the Triassic period, from 237 to 210 million years ago.

In their research, Dr. Ezcurra, Dr. Agnolin and their colleagues used micro-computed tomographic (μCT) scanning to examine fragile skull bones (jaws, skull roofs and braincases) and associated skeletal remains of lagerpetids.

They determined that the lagerpetid brains and sensory systems had many similarities with those of pterosaurs.

“Some of these delicate fossils were collected nearly 80 years ago, and rather than destructively cutting into the first known skull of a lagerpetid species called Dromomeron gregorii, we were able to use this technology to carefully reconstruct the brain and inner ear anatomy of these small fossils to help determine the early relatives of pterosaurs,” said co-author Dr. Michelle Stocker, a researcher in the Department of Geosciences at Virginia Tech.

One stark and mystifying finding was that the flightless lagerpetids had already evolved some of the neuroanatomical features that allowed the pterosaurs to fly, which brought forth even more information on the origin of flight.

“From the CT scans, we discovered that lagerpetids shared a number of unique characteristics with pterosaurs, which allowed them to have greater agility,” Dr. Ezcurra said.

“One of these characteristics is that in the cerebellum of lagerpetids and pterosaurs, there is a structure called flocculum, highly developed in both, which is interpreted as an evolutionary adaptation that gave them a greater capacity for processing information.”

“For example, between the eye, the rest of the head and the members of the body, so they could have greater visual coordination.”

Now that lagerpetids are deemed the precursors of pterosaurs, paleontologists can say that pterosaurs evolved at the same rate as other major reptile groups, thanks to the newly-discovered ‘middle man.’

“Flight is such a fascinating behavior, and it evolved multiple times during Earth’s history,” said co-author Dr. Serjoscha Evers, a researcher in the Department of Earth Sciences at the University of Oxford and the Department of Geosciences at the University of Fribourg.

“Proposing a new hypothesis of their relationships with other extinct animals is a major step forward in understanding the origins of pterosaur flight.”

Some questions still remain in this evolutionary mystery. Now that lagerpetids are the closest relatives of pterosaurs, why are they still lacking some of the key characteristics of pterosaurs, including the most outstanding of those — wings?

“We are still missing lots of information about the earliest pterosaurs, and we still don’t know how their skeletons transformed into an animal that was capable of flight,” said co-author Dr. Sterling Nesbitt, a researcher in the Department of Geosciences at Virginia Tech.

The findings appear in the journal Nature.

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M.D. Ezcurra et al. Enigmatic dinosaur precursors bridge the gap to the origin of Pterosauria. Nature, published online December 9, 2020; doi: 10.1038/s41586-020-3011-4

This article is based on press-releases provided by Virginia Tech and CTyS-UNLaM Agency.

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