Posture, locomotion, and paleoecology of pterosaurs

Despite being studied for over 200 years, the posture, locomotion, paleobiology, and phylogeny of pterosaurs are still poorly understood due to lack of well-preserved, three-dimensional specimens. In this paper we investigate the posture, locomotion, and paleoecology of pterosaurs based on anatomy and biomechanics: how they walked, how they flew, and how they lived. We want to understand how evolution has adjusted their skeletal structures and movements to maximize performance. The limb joints of an exquisite skeleton of the Cretaceous pterodactyl Anhanguera piscator are analyzed to estimate the range of movement during terrestrial and aerial locomotion. On land, pterosaurs were quadrupedal knuckle walkers with laterally directed digitigrade manus and forwardly directed plantigrade pes. From this position, pterosaurs could stand on their rear legs and run bipedally in an upright posture for a short distance during takeoff and landing. Pterosaurs evolved two basic wing planforms over time: the basal "rhampho-rhynchoids" had broad wings in bat-like fashion where the patagium was attached to the ankle; in pterodactyloids, the wings became narrow and the patagium was anchored near the knee joint. The wingtips appear to have been more rounded to avoid stalling. The actinofibrils in the membrane would confer some stiffness to the wing to maintain a flatter camber, preventing it from billowing and tearing during flight. They would also facilitate the folding of the wing when not in use. The flight performance of pterosaurs is investigated using ten genera in a wide size spectrum during their 160 million years of evolution, where the body mass ranges from 0.015 kg to 70 kg and the wingspan from 0.4 m to 10.4 m. Thus the largest pterosaur in our study weighs about 4700 times more than the smallest species, and the longest wingspan is 25 times the shortest. Helicopter momentum stream tube theory has been adapted to estimate the scaling of aerial locomotion of pterosaurs and to minimize the complexities of animal physiology. The aerodynamic data were calculated using the two computer programs ANFLTPWR (animal flight power) and ANFLTSIM (animal flight simulation). Pterosaur wings were long and narrow, similar to those of seabirds, with high aspect ratios. However, they had relatively low wing loadings and low cruising speeds compared to seabirds with similar masses. Gliding performance, deduced from the polar curves, indicates that smaller pterosaurs such as Eudimorphodon, Pterodactylus, Rhamphorhynchus, and Dorygnathus had lower gliding airspeeds, with a gliding angle close to 4°. The giant Cretaceous pterodactyloids such as Pteranodon and Quetzalcoatlus were excellent soarers comparable to the albatross, human-powered planes, and sailplanes, with a gliding angle between 1° and 2°. The cruising speed for best gliding depends on size, increasing proportionally to mass and wing loading, from as low as 4 m/s for Eudimorphodon to 16 m/s for Quetzalcoatlus. The power curves, displaying maximum and minimum level flight speeds, show three different styles of flight. In the four smaller genera (Mass < 0.3 kg) such as Eudimorphodon, Pterodactylus, Rhamphorhynchus, and Dorygnathus, the available aerobic power (Pa) exceeds the required power at zero speed, and they were evidently capable of hovering flight. Tapejara, Nyctosaurus, Dsungaripterus, Anhanguera, and Pteranodon appear to be capable of steady level flight at aerobic power, but within a limited speed range. The sustained power output of giant pterodactyloids such as Quetzalcoatlus was not enough for continuous level flapping flight; however, they could improve their flying performance if they flew in formation. Apparently, extended flight for large pterodactyloids was by soaring; they flapped normally when taking off or landing. Takeoff from the ground was initiated by running and hopping. Although small pterosaurs apparently had sufficient available power for running takeoff from the ground or from the water, larger pterodactyloids such as Pteranodon and Quetzalcoatlus were limited in their takeoff capabilities and were unable to take off with maximum aerobic power. They needed short bursts of anaerobic power to take off from the ground with a headwind of 5 m/s. For Quetzalcoatlus, a takeoff from a 10° downward slope would be helpful especially when it ventured inland. As their running speed increased, low-amplitude flapping was used to accelerate to take off; pterosaurs leaped into the air and flapped their wings for flight. The long axis rotation of the humerus in the upstroke position would have been useful during takeoff and landing. The function of the cranial crest may have been linked to thermoregulation, sexual display, and species recognition. The large head of pterodactyloids was probably downturned during flight and was used as a steering device for turning the body. The ecology of pterosaurs was similar to those of modern seabirds, spending much time in coastal areas for feeding. Small and medium-size pterosaurs probably foraged by plunge diving like modern pelicans. Large pterodactyloids were probably active waders or surface riders during feeding, using their feet to propel while folding their wings sidewise. Arising as small animals in the Triassic, pterosaurs exhibit long-term phyletic trends toward increasing body size during the Cretaceous, but the trend is erratic. They became extinct at the end of the Cretaceous along with dinosaurs and other organisms when multiple asteroids crashed into the Earth, accompanied by the spectacular Deccan volcanism that had devastating effects on the ecology.