On the origin of Pterosaurs

The recently discovered and selected bones characterizing the lagerpetid body plan. From Ezcurra et al., 2020

In 1784, Cosimo Alessandro Collini, a former secretary of Voltaire and curator of the natural history cabinet of Karl Theodor, Elector of Palatinate and Bavaria, published the first scientific description of a pterosaur. The specimen came from one of the main sources of such fossils, the Late Jurassic lithographic limestones of northern Bavaria, and Collini, after much deliberation, interpreted it as the skeleton of an unknown marine creature. In 1801, on the basis of Collini’s description, George Cuvier identified the mysterious animal as a flying reptile. He later coined the name “Ptero-Dactyle”. This discovery marked the beginning of pterosaur research.

Pterodactylus antiquus, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA (From Wikipedia Commons)

Pterosaurs were the first flying vertebrates. Their reign extended to every continent and achieved high levels of morphologic and taxonomic diversity during the Mesozoic, with more than 200 species recognized so far. During their 149 million year history, the evolution of pterosaurs resulted in a variety of eco-morphological adaptations, as evidenced by differences in skull shape, dentition, neck length, tail length and wing span. The oldest-known pterosaurs appear in the fossil record about 219 million years ago. Most Triassic pterosaurs are small but already had a highly specialized body plan linked to their ability to fly: shoulder girdle with strongly posteroventrally enlarged coracoid braced with the sternum and laterally facing glenoid fossa; forelimb with pteroid bone and hypertrophied fourth digit supporting a membranous wing; and pelvic girdle with prepubic bone and strongly developed preacetabular process.

Due to the fragile nature of their skeletons and the absence of fossils with transitional morphologies, the origin of pterosaurs is one of the most elusive questions in vertebrate paleontology. They have been hypothesized to be the close relatives of a wide variety of reptilian clades. Now, a new study published in Nature indicates that lagerpetids are the sister group of pterosaurs.

A partial skeleton of Lagerpeton (Image Credit: Virginia Tech/Sterling Nesbitt)

Lagerpetids are small to medium-sized (less than 1 m long), cursorial, non-volant reptiles from Middle–Upper Triassic of Argentina, Brazil, Madagascar, and North America. Previous studies of lagerpetid anatomy was mostly limited to vertebrae, hindlimbs and a few cranial bones, but new fossil discoveries over the past few years have greatly increased the understanding of this group. Based on the anatomical information from Lagerpeton chanarensis (from the Chañares formation, Argentina), Ixalerpeton polesinensis (from the Santa Maria Formation, Brazil), Kongonaphon kely (from Morondava Basin, Madagascar), and Dromomeron spp. (from North America), an international team lead by Martin Ezcurra from the Museo Argentino de Ciencias Naturales in Buenos Aires, Argentina, elucidated their relationship to pterosaurs. The recognition of this group as the sister taxon to pterosaurs provides clues to study the origin of Pterosauria, its specialized body plan and flying abilities.

Time-calibrated reduced strict consensus tree focused on Pterosauria and Lagerpetidae. From Ezcurra et al., 2020.

The team found at least 33 skeletal traits suggesting an evolutionary link between lagerpetids and pterosaurs. The anterior region of the lagerpetid dentary is ventrally curved similar to those of the early pterosaurs like Austriadactylus. Lagerpetids and pterosaurs also share a unique inner ear morphology among archosaurs, characterized by taller than anteroposteriorly long semicircular canals. The semicircular canals detect head movements and a larger radius increases the sense of equilibrium. The cranial endocasts of D. gregorii and Ixalerpeton reveal strongly developed cerebellar floccular lobes, which resemble the even more developed floccular lobes of pterosaurs. The flocculus plays a key a role in coordinate eye movements, and tends to be enlarged in taxa that rely on quick movements of the head and the body. This condition in Pterosaurs has been hypothesized to be important for information processing related to flight.

Lagerpetids and pterosaurs also share similarities in hand, leg, ankle and pelvic bones. For example pelvic girdles of Lagerpeton and Ixalerpeton have a long pubo-ischiadic contact that extends ventrally up to the level of the anterovental margin of the pubis, as is the case in several early pterosaurs.

 

References:

Ezcurra, M.D., Nesbitt, S.J., Bronzati, M. et al. Enigmatic dinosaur precursors bridge the gap to the origin of Pterosauria. Nature (2020). https://doi.org/10.1038/s41586-020-3011-4