Temporal range: Late Cretaceous, 68–66Ma
|Restored skeleton in quadrupedal stance|
Quetzalcoatlus (//) was a pterodactyloid pterosaur known from the Late Cretaceous of North America (Maastrichtian stage, about 68–66 million years ago), and one of the largest known flying animals of all time. It was a member of the Azhdarchidae, a family of advanced toothless pterosaurs with unusually long, stiffened necks. Its name comes from the Mesoamerican feathered serpent god Quetzalcoatl.
Skull material (from the unnamed smaller species) shows that Quetzalcoatlus had a very sharp and pointed beak, contrary to some earlier reconstructions that showed a blunter snout, based on the inadvertent inclusion of jaw material from another pterosaur species, possibly a tapejarid or a form related to Tupuxuara. A skull crest was present but its exact form and size are still unknown.
When it was first discovered, scientists estimated that the largest Quetzalcoatlus fossils came from an individual with a wingspan as large as 15.9 meters (52.2 feet), choosing the middle of three extrapolations from the proportions of other pterosaurs that gave an estimate of 11, 15.5 and 21 meters respectively (36 feet, 50.85 feet, 68.9 feet). In 1981, further study lowered these estimates to 11–12 meters (36–39 feet). More recent estimates based on greater knowledge of azhdarchid proportions place its wingspan at 10–11 meters (33–36 ft).
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. While some studies have historically found extremely low weight estimates for Quetzalcoatlus, as low as 70 kilograms (150 lb) for a 10-meter individual, a majority of estimates published since the 2000s have been higher, around 200–250 kilograms (440–550 lb).
Discovery and species 
The first Quetzalcoatlus fossils were discovered in Texas, from the Maastrichtian Javelina Formation at Big Bend National Park (dated to around 68 million years ago) in 1971 by a geology graduate student from the University of Texas at Austin's Jackson School of Geosciences, Douglas A. Lawson. The specimen consisted of a partial wing (in pterosaurs composed of the forearms and elongated fourth finger), from an individual later estimated at over 10 m (33 ft) in wingspan. Lawson discovered a second site of the same age, about forty kilometers from the first, where between 1972 and 1974 he and Professor Wann Langston Jr. of the Texas Memorial Museum unearthed three fragmentary skeletons of much smaller individuals. Lawson in 1975 announced the find in an article in Science. That same year, in a subsequent letter to the same journal, he made the original large specimen, TMM 41450-3, the holotype of a new genus and species, Quetzalcoatlus northropi. The genus name refers to the Aztec feathered serpent god Quetzalcoatl. The specific name honors John Knudsen Northrop, the founder of Northrop, who was interested in large tailless flying wing aircraft designs resembling Quetzalcoatlus. At first it was assumed that the smaller specimens were juvenile or subadult forms of the larger type. Later, when more remains were found, it was realized they could have been a separate species. This possible second species from Texas was provisionally referred to as a Quetzalcoatlus sp. by Alexander Kellner and Langston in 1996, indicating that its status was too uncertain to give it a full new species name. The smaller specimens are more complete than the Q. northropi holotype, and include four partial skulls, though they are much less massive, with an estimated wingspan of 5.5 meters (18 ft).
The holotype specimen of Q. northropi has yet to be properly described and diagnosed. Where the known remains overlap, it has been considered by Mark Witton and colleagues (2010) to be indistinguishable from its Romanian contemporary Hatzegopteryx. If Q. northropi is complete enough to be distinguished from other pterosaurs (i.e., if it is not a nomen dubium), Hatzegopteryx may represent the same animal. It is likely that huge pterosaurs such as Q. northropi would have had very large, transcontinental ranges, making its presence in both North America and Europe unsurprising. Mark Witton et al. pointed out that the skull material of Hatzegopteryx and Q. sp. differ enough that they cannot be regarded as the same animal, making it likely that Q. sp., if not identical to Quetzalcoatlus northropi, represents a distinct genus.
An azhdarchid neck vertebra, discovered in 2002 from the Maastrichtian age Hell Creek Formation, may also belong to Quetzalcoatlus. The specimen (BMR P2002.2) was recovered accidentally when it was included in a field jacket prepared to transport part of a tyrannosaur specimen. Despite this association with the remains of a large carnivorous dinosaur, the vertebra shows no evidence that it was chewed on by the dinosaur. The bone came from an individual azhdarchid pterosaur estimated to have had a wingspan of 5 - 5.5m (16.5 – 18 ft).
Quetzalcoatlus was abundant in Texas during the Lancian in a fauna dominated by Alamosaurus. The Alamosaurus-Quetzalcoatlus association probably represents semi-arid inland plains. Quetzalcoatlus had precursors in North America and its apparent rise to widespreadness may represent the expansion of its preferred habitat rather than an immigration event, as some experts have suggested.
There have been a number of different ideas proposed about the lifestyle of Quetzalcoatlus. Because the area of the fossil site was four hundred kilometers 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 suggesting that Quetzalcoatlus scavenged like the Marabou Stork, 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.
In 1996, Thomas 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 five centimeters 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. 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. In 2008 pterosaur workers Mark Paul 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 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.
The nature of flight in Quetzalcoatlus and other giant azhdarchids was poorly understood until serious biomechanical studies were conducted in the 21st century. One early (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; however, the model was half scale based on a then-current weight estimate of around 80 kg, far lower than more modern estimates of over 200 kg. The method of flight in these pterosaurs depends largely on 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. 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. Henderson argued that it may have been flightless.
However, most 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 a further investigation into the claims of flightlessness in large pterosaurs. After factoring wingspan, body weight, and aerodynamics, a sophisticated computer program led the two researchers to conclude that Q. northropi was capable of flight "up to 80 miles an hour for 7 to 10 days at altitudes of 15,000 feet". Mike Habib further suggested a maximum flight range of 8,000 to 12,000 miles for Q. northropi. Henderson's work was further criticized by Habib, who pointed out that although Henderson used excellent mass estimations, they were based on outdated pterosaur models, and that anatomical study of Q. northropi and other large pterosaur forelimbs show a higher degree of robustness than would be expected if they were purely quadrupedal. Habib believes that large pterosaurs most likely utilized a short burst of powered flight in order to then transition to thermal soaring.
Cultural significance 
Quetzalcoatlus has been featured in documentaries, both in cinemas and on television, since the 1980s. The Smithsonian project to build a working model of Q. northropi was the subject of the 1986 IMAX documentary On the Wing, shown at the National Air and space museum in Washington, D.C.. It has also been featured in television programs such as the BBC's Walking with Dinosaurs in 1999 and Dangerous, Ltd.'s Clash of the Dinosaurs in 2009. The later program featured traits invented by the producers to heighten entertainment value, including a depiction of Quetzalcoatlus with the ability to use ultraviolet vision to locate dinosaur urine when hunting in the air. It was also depicted in the 2011 documentary March of the Dinosaurs, where it was erroneously depicted as a clawless, bipedal scavenger, and in the 2009 series Animal Armageddon, where it was correctly portrayed with pycnofibres. In the "Return to Jurassic Park" bonus feature of the 2011 Blu-ray release of the Jurassic Park film series, John R. Horner describes Quetzalcoatlus as the pterosaur that most accurately represented and matched the size of the pterosaurs that are featured in the films.
In June 2010, several life-sized models of Q. northropi were put on display on London's South Bank as the centerpiece exhibit for the Royal Society’s 350th anniversary exhibition. The models, which included both flying and standing individuals with wingspans of 30 feet (9.1 m), were intended to help build interest in science among the public. The models were created by scientists from the University of Portsmouth, including David Martill, Bob Loveridge and Mark Witton, and engineers Bob and Jack Rushton from Griffon Hoverwork. The display presented to the public the most accurate pterosaur models constructed at the time, taking into account anatomical and footprint evidence based on skeletal and trace fossils from related pterosaurs.
In 1985, the U.S. Defense Advanced Research Projects Agency (DARPA) and AeroVironment used Quetzalcoatlus northropi as the basis for an experimental ornithopter UAV. They produced a half-scale model weighing 40 pounds (18 kg), with a wingspan of 18 feet (5.5 m). Coincidentally, Douglas A. Lawson, who discovered Q. northropi in Texas in 1971, named it for John "Jack" Northrop, a famous developer of tailless flying wing aircraft in the 1940s. The replica of Q. northropi incorporates a "flight control system/autopilot which processes pilot commands and sensor inputs, implements several feedback loops, and delivers command signals to its various servo-actuators." It is on exhibit at the National Air and Space Museum.
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