Quetzalcoatlus

Size comparison of a human to Q. northropi (green) and Q. lawsoni (blue)

When it was first named as a new species in 1975, scientists estimated that the largest Quetzalcoatlus fossils came from an individual with a wingspan as large as 15.9 m (52 ft). Choosing the middle of three extrapolations from the proportions of other pterosaurs gave an estimate of 11 m, 15.5 m, and 21 m, respectively (36 ft, 50.85 ft, 68.9 ft). In 1981, further advanced studies lowered these estimates to 11–12 m (36–39 ft).[10]

Comparison of Q. northropi with a Cessna 172 light aircraft.

More recent estimates based on greater knowledge of azhdarchid proportions place its wingspan at 10–11 m (33–36 ft).[8] Remains found in Texas in 1971 indicate that this pterosaur had a minimum wingspan of about 11 m (36 ft).[11] Generalized height in a bipedal stance, based on its wingspan, would have been at least 3 m (9.8 ft) high at the shoulder.[3]

Body mass estimates for giant azhdarchids are extremely problematic because no existing species share a similar size or body plan, and in consequence, published results vary widely.[3] Generalized weight, based on some studies that have historically found extremely low weight estimates for Quetzalcoatlus, was as low as 70 kg (150 lb) for a 10 m (32 ft 10 in) individual. A majority of estimates published since the 2000s have been substantially higher, around 200–250 kg (440–550 lb).[12][13]

In 2021, Kevin Padian and his colleagues estimated that Q. lawsoni would have weighed 20 kg (44 lb) for individuals with a wingspan of 4.5 m (15 ft).[14] In 2022, Gregory S. Paul estimated that Q. lawsoni had a wingspan of 5 m (16 ft), body length of 3.5 m (11 ft), and body mass of 65 kg (143 lb).[15]

Skull reconstruction of Q. lawsoni

Skull material from Q. lawsoni shows that Quetzalcoatlus had a very sharp and pointed beak. That is contrary to some earlier reconstructions that showed a blunter snout, based on the inadvertent inclusion of jaw material from another pterosaur species, as this material was named as the holotype of a genus of short-snouted azhdarchid, Wellnhopterus, in 2021.[1] A skull crest was also present but its exact form and size are still unknown.[6]

Artist's impression of a group of Quetzalcoatlus feeding on the ground

There have been a number of different ideas proposed about the lifestyle of Quetzalcoatlus. Because the area of the fossil site was 400 km (250 mi) removed from the coastline and there were no indications of large rivers or deep lakes nearby at the end of the Cretaceous, Lawson, in 1975, rejected a fish-eating lifestyle. Instead, he suggested that Quetzalcoatlus scavenged, similar to the marabou stork (which will scavenge, but is more of a terrestrial predator of small animals), but then on the carcasses of titanosaur sauropods, such as Alamosaurus. Lawson had found the remains of the giant pterosaur while searching for the bones of this dinosaur, which formed an important part of its ecosystem.[4]

In 1996, Lehman and Langston rejected the scavenging hypothesis, pointing out that the lower jaw bent so strongly downwards that even when it closed completely a gap of over 5 cm (2.0 in) remained between it and the upper jaw, very different from the hooked beaks of specialized scavenging birds. They suggested that, with its long neck vertebrae and long toothless jaws, Quetzalcoatlus fed like modern-day skimmers, catching fish during flight while cleaving the waves with its beak.[19] While this skim-feeding view became widely accepted, it was not subjected to scientific research until 2007 when a study showed that, for such large pterosaurs, it was not a viable method because the energy costs would be too high due to excessive drag.[20] In 2008, pterosaur workers Mark Witton and Darren Naish published an examination of possible feeding habits and ecology of azhdarchids. Witton and Naish noted that most azhdarchid remains are found in inland deposits far from seas or other large bodies of water required for skimming. Additionally, the beak, jaw, and neck anatomy are unlike those of any known skimming animal. Rather, they concluded that azhdarchids were more likely terrestrial stalkers, similar to modern storks, and that they probably hunted small vertebrates on land or in small streams. Though Quetzalcoatlus, like other pterosaurs, was a quadruped when on the ground, Quetzalcoatlus and other azhdarchids have fore and hind limb proportions more similar to modern running ungulate mammals than to their smaller cousins, implying that they were uniquely suited to a terrestrial lifestyle.[3]

Quetzalcoatlus northropi is found in plains deposits, being likely a solitary hunter, while Q. lawsoni is found in great numbers in alkaline lakes, and likely lived like modern gregarious wading birds.[21]

Restoration of a Quetzalcoatlus in flight

The nature of flight in Quetzalcoatlus and other giant azhdarchids was poorly understood until serious biomechanical studies were conducted in the 21st century. A 1984 experiment by Paul MacCready used practical aerodynamics to test the flight of Quetzalcoatlus. MacCready constructed a model flying machine or, ornithopter, with a simple computer functioning as an autopilot. The model successfully flew with a combination of soaring and wing flapping.[22] The model was based on a then-current weight estimate of around 80 kg (180 lb), far lower than more modern estimates of over 200 kg (440 lb).[23] The method of flight in these pterosaurs depends largely on their weight, which has been controversial, and widely differing masses have been favored by different scientists. Some researchers have suggested that these animals employed slow, soaring flight, while others have concluded that their flight was fast and dynamic.[3] In 2010, Donald Henderson argued that the mass of Q. northropi had been underestimated, even the highest estimates, and that it was too massive to have achieved powered flight. He estimated it in his 2010 paper as 540 kg (1,190 lb). Henderson argued that it may have been flightless.[23]

Quetzalcoatlus skeleton mounted in a flying pose at the Royal Ontario Museum.

Other flight capability estimates have disagreed with Henderson's research, suggesting instead an animal superbly adapted to long-range, extended flight. In 2010, Mike Habib, a professor of biomechanics at Chatham University, and Mark Witton, a British paleontologist, undertook further investigation into the claims of flightlessness in large pterosaurs. After factoring wingspan, body weight, and aerodynamics, computer modeling led the two researchers to conclude that Q. northropi was capable of flight up to 130 km/h (80 mph) for 7 to 10 days at altitudes of 4,600 m (15,000 ft). Habib further suggested a maximum flight range of 13,000–19,000 km (8,000–12,000 mi) for Q. northropi.[24] Henderson's work was also further criticized by Witton and Habib in another study, which pointed out that, although Henderson used excellent mass estimations, they were based on outdated pterosaur models, which caused Henderson's mass estimations to be more than double what Habib used in his estimations and that anatomical study of Q. northropi and other big pterosaur forelimbs showed a higher degree of robustness than would be expected if they were purely quadrupedal.[13] This study proposed that large pterosaurs most likely utilized a short burst of powered flight to then transition to thermal soaring.[25] However, a study from 2022 suggests that they would only have flown occasionally and for short distances, like the Kori bustard (the world's heaviest bird that actively flies) and that they were not able to soar at all.[26]

Studies of Q. northropi and Q. lawsoni published in 2021 by Kevin Padian and colleagues instead suggested that Quetzalcoatlus was a very powerful flier, it had a large breastbone to support the necessary muscles for creating the flight stroke, and that it would probably have used its powerful hind legs to launch as high as 8 ft (2.4 m) when taking off, allowing it to gain enough height and momentum to begin the necessary downstrokes needed for takeoff. This same study also suggests that Quetzalcoatlus had limited walking motion in its wings, with the limbs on each side of the body being moved together and the forelimbs being lifted out of the way of the hindlimbs. This study also suggests that the wings of pterosaurs were only attached to the body, with the legs and feet being tucked underneath, much like how modern birds tuck their legs beneath their own bodies in flight.[14]