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Changes in tree canopy supported the earliest gliding lizards

There are only three known fossil specimens of the long extinct gliding lizard named Coelurosauravus elivensis. Dated to the late Permian Period – between 260 million to 252 million years ago – this unusual reptile inhabited the wetlands and forests of Madagascar when conditions were temperate, warm and humid. The species is striking because of its patagium – a membranous flap of skin stretching between front and hind limbs – that enabled it to glide; this makes it the oldest known gliding reptile in the world. 

The three fossils were discovered between 1906 and 1907, and the species was described to science in 1926. Since then, however, scientists have debated about how exactly the patagium was supported and used, and how this lizard moved about in it environment. A team of researchers has now examined in great detail the three fossils of C. elivensis from Madagascar, as well as a number of related specimens, all belonging to the same reptile family (Weigeltisaurida), in order to understand more about this unusual lizard. 

Experts from the French National Museum of Natural History in Paris (Muséum national d’Histoire naturelle) and the Staatliches Museum für Naturkunde Karlsruhe in Germany further hypothesize that this remarkable lizard most likely evolved its flying ability in response to changes in the forests of the time.  

“Pennsylvanian [late Carboniferous] forests, while taxonomically and vertically heterogeneous, had rather open canopy strata with spatially separated arborescent taxa resulting in little crown overlap. In contrast, Cisuralian [early Permian] forests show evidence of denser communities suggestive of more continuous canopy strata. Such change in forest structure could explain why no gliders have been reported prior to weigeltisaurids although several arboreal or scansorial amniotes have been described from Pennsylvanian and Cisuralian deposits,” said study lead author Valentin Buffa, from the Centre de Recherche en Paléontologie at the French Natural History Museum.

“These dragons weren’t forged in mythological fire, they simply needed to get from place to place. As it turned out, gliding was the most efficient mode of transport and here, in this new study, we see how their morphology enabled this.”

The researchers were particularly interested in understanding how all the bits of the C. elivensis skeleton fitted together, and so they focused on the postcranial portions – the body, including the torso, limbs, and remarkable gliding apparatus, the patagium. Previous analysis of the reptile had assumed that its patagium was supported by bones that extended from the ribs, as they do in modern gliding lizards of the genus Draco that are found in Southeast Asia.

Since there was no complete specimen among the fossils, the researchers had to piece together the existing parts of the fossils in order to get a better picture of the lizard’s skeletal structure. Their thorough investigation suggested that the patagium of C. elivensis was not supported by extensions from the ribs. Instead, the approximately 29 pairs of long, rod-shaped bones that supported the gliding membrane were proposed to originate either simply from the animal’s skin and trunk musculature, or from its gastralia.

Gastralia are dermal bones found in the skin covering the stomach region of some reptiles, including the crocodilians, the tuatara and the dinosaurs. If these bones in C. elivensis were modified gastralia, they would have been positioned more towards the lower lateral surface of the body, and not towards the dorsal side as is seen in modern Draco lizards. The researchers concluded that the bony rods were flexible and that furling and unfurling of the flight membrane would probably have been controlled by the abdominal muscles. 

Combining these findings with others derived from the bone structure observed in the fossils, the researchers came up with a more refined vision of how this agile creature moved through its arboreal habitat.

“Sharp, curved claws and compressed body form support the idea that C. elivensis was perfectly adapted to moving vertically up tree trunks. The similarity in length of the forelimbs and hind limbs further indicates that it was an expert climber – their proportional length assisted it in remaining close to the tree’s surface, preventing it from pitching and losing its balance. Its long, lean body and whiplike tail, also seen in contemporary arboreal reptiles, further supports this interpretation,” explained Valentin Buffa.

An analysis of the similarities and differences between C. elivensis and living gliding lizards from the genus Draco found that the fossil species was likely less efficient at gliding than modern Draco species due to its larger body size. Its estimated wing loading of around 107.9 N/m2 is 4.5 times higher than that of the Draco species, which would likely lead to a substantial drop in height per glide and correspond with a steep descent angle of more than 45 degrees.

C. elivensis does bear a striking resemblance to the contemporary genus Draco,” said Valentin Buffa. “While its habits were likely similar to those of its modern counterpart, we do see subtle differences though.”

“Like Draco lizards, Coelurosauravus was able to grasp its patagium with its front claws, stabilize it during flight, and even adjust it, allowing for greater maneuverability. An additional joint in one finger, though, may have enhanced this capability. This may have been a necessary compensation for the lower positioning of the patagium, which likely made it more unstable.”

The research is published in the Journal of Vertebrate Paleontology.

By Alison Bosman, Staff Writer

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