Quetzalcoatlus

Quetzalcoatlus Temporal range: Late Cretaceous (Maastrichtian),
Restored Quetzalcoatlus skeleton displayed in quadrupedal stance, Houston Museum of Natural Science
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Order:	†Pterosauria
Suborder:	†Pterodactyloidea
Family:	†Azhdarchidae
Subfamily:	†Quetzalcoatlinae
Genus:	†Quetzalcoatlus Lawson, 1975
Type species
†Quetzalcoatlus northropi Lawson, 1975
Other species
†Q. lawsoni Andres and Langston Jr., 2021
Synonyms
Pteranodon gigas Lawson, 1972

Quetzalcoatlus (/kɛtsəlkoʊˈætləs/) is a genus of azhdarchid pterosaur that lived during the Maastrichtian age of the Late Cretaceous in what is now North America. The first specimen, recovered in 1971 from the Javelina Formation of Texas, United States, consists of several wing fragments. It was made the holotype of Quetzalcoatlus northropi in 1975 by Douglas Lawson. The generic name refers to the Aztec serpent god of the sky, Quetzalcōātl, while the specific name honors Jack Northrop, designer of tailless fixed-wing aircraft. The remains of a second species were found between 1972 and 1974, also by Lawson, around 40 km (25 mi) from the type of Q. northropi. In 2021, these remains were assigned the name Quetzalcoatlus lawsoni by Brian Andres and (posthumously) Wann Langston Jr.

Quetzalcoatlus northropi has gained fame as a candidate for the largest flying animal ever discovered, though estimating its size has been difficult due to the fragmentary nature of its type specimen. Wingspan estimates over the years have ranged from 5.2–25.8 m (17–85 ft), though this has more recently been narrowed down to around 10–11 m (33–36 ft), based on extrapolations from more complete members of the Azhdarchidae, the family of which Quetzalcoatlus belongs to. The smaller and more complete Q. lawsoni had a wingspan of around 4.5 m (15 ft). The proportions of Quetzalcoatlus were typical of azhdarchids. It possessed a very long neck and beak, shortened non-wing digits that were well adapted for terrestrial locomotion, and a very short tail.

Historical interpretations of the diet of Quetzalcoatlus northropi have ranged from scavenging to skim-feeding like the modern skimmer bird. However, more recent research has found that it most likely hunted small prey on the ground, in a similar way to storks and ground hornbills. This has been dubbed the terrestrial stalking model and it is a common feeding behavior among other large azhdarchids. On the other hand, the second species, Q. lawsoni, appears to have been associated with alkaline lakes, and a diet of small aquatic invertebrates has been suggested. Similarly, while Q. northropi seems to have been fairly solitary, Q. lawsoni appears to have been highly gregarious (social).

For years it was uncertain how Quetzalcoatlus took off. Early models using a bipedal stance, such as that of Sankar Chatterjee and R.J. Templin in 2004, were heavily reliant on a relatively light weight (about 70 kg (150 lb) in Chatterjee and Templin's case), and even then, struggled to explain how takeoff was achieved. Based on the work of Mark P. Witton and Michael Habib in 2010, it now seems likely that pterosaurs, especially larger taxa such as Quetzalcoatlus, launched quadrupedally, using the powerful muscles of their forelimbs to propel themselves off the ground and into the air.

Discovery and species

Cast of the holotype humerus

Early history

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 Douglas A. Lawson, who was then a geology graduate student from the Jackson School of Geosciences at the University of Texas, Austin. He would then go on to become a doctoral student at the University of California, Berkeley. The specimen he discovered 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. In a thesis, Lawson gave the remains the provisional name of "Pteranodon gigas", describing it as being "nearly twice as large as any previously described species of Pteranodon".

In 1972, Lawson discovered a second Big Bend site around 40 km (25 mi) from the first. Field crews from the University of Texas, led by Wann Langston Jr. of the Texas Memorial Museum, investigated the site between 1972 and 1974, and unearthed three fragmentary skeletons of much smaller individuals. In 1975, Lawson announced the original find in an article in Science. At the time, a detailed description was underway, though this never saw publication. Instead, he published a letter to the same journal a few months later, in which he made the original specimen (TMM 41450-3) the holotype of a new genus and species, Quetzalcoatlus northropi. The genus name refers to the Aztec feathered serpent god, Quetzalcōātl. The specific name honors John Knudsen Northrop, the founder of Northrop Corporation, who drove the development of large tailless flying wing aircraft designs resembling Quetzalcoatlus.

Taxonomic history

The holotype specimen of Q. northropi was not properly described and diagnosed until 2021, and until then, the status of the genus Quetzalcoatlus was noted as problematic. Mark Witton et al. noted that the holotype of Q. northropi represents elements which are typically considered undiagnostic to generic or specific level and that this complicates interpretations of azhdarchid taxonomy. For instance, Witton et al. suggested that the Q. northropi type material is of generalized enough morphology to be near identical to that of other giant azhdarchids, such as the overlapping elements of the contemporary Romanian giant azhdarchid Hatzegopteryx. Assuming that Q. northropi could be distinguished from other pterosaurs (i.e., if it was not a nomen dubium), the possibility of it representing the same taxon as Hatzegopteryx was noted. However, Witton et al. also noted that the skull material of Hatzegopteryx and then-unnamed Q. lawsoni differ enough that they cannot be regarded as the same taxon. These issues could only be resolved by Q. northropi being demonstrated as a valid taxon and its relationships with Q. lawsoni being investigated. An additional complication to these discussions is the likelihood that huge pterosaurs, such as Q. northropi, could have made transcontinental flights, suggesting that locations as disparate as North America and Europe could have shared giant azhdarchid species.

Skull reconstruction of Q. lawsoni

Initially, it was assumed that the smaller specimens of Quetzalcoatlus 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 Wann Langston Jr. 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 4.5–5.5 m (15–18 ft). This species was named Q. lawsoni in 2021, in honor of the genus' original describer.

Subsequent discoveries

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 Tyrannosaurus rex specimen. Despite their association, the vertebra shows no evidence of scavenging. The bone came from an individual azhdarchid pterosaur estimated to have had a wingspan of 5–5.5 m (16–18 ft). However, while initially assigned to Quetzalcoatlus, Brian Andres and Wann Langston Jr (2021) regarded BMR P2002.2 as a putative azhdarchiform.

Description

Size

See also: Pterosaur size

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

In 1975, Douglas Lawson compared the wing bones of Quetzalcoatlus northropi's holotype to equivalent elements in Dsungaripterus and Pteranodon, and suggested that it represented an individual with a wingspan of around 15.5 m (51 ft), though presented alternative estimates of 11 m (36 ft) and 21 m (69 ft). Estimates put forward in subsequent years varied dramatically, ranging from 5.2–25.8 m (17–85 ft), owing to differences in methodology. From the 1980s onwards, estimates were narrowed down to 11–12 m (36–39 ft). More recent estimates based on greater knowledge of azhdarchid proportions place its wingspan at 10–11 m (33–36 ft). Q. lawsoni, was estimated to have a wingspan of around 4.5 m (15 ft). In 2022, Gregory S. Paul estimated that Q. lawsoni had a wingspan of 5 m (16 ft) and a body length of 3.5 m (11 ft).

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

Body mass estimates for giant azhdarchids are extremely problematic because no existing species shares a similar size or body plan, and in consequence, published results vary widely. Crawford Greenewalt gave mass estimates of between 30–440 kg (66–970 lb) for Q. northropi, with the former figure assuming a small wingspan of 5.2 m (17 ft). A majority of estimates published since the 2000s have been substantially higher, around 200–250 kg (440–550 lb). In 2021, Kevin Padian and his colleagues estimated that Q. lawsoni would have weighed 20 kg (44 lb). In 2022, Gregory S. Paul estimated that Q. lawsoni had a body mass of 65 kg (143 lb).

Skull

A complete skull of Quetzalcoatlus is not known, so reconstructions necessarily draw from eight specimens of Q. lawsoni that preserve skull elements. The skull of Q. lawsoni was about 94–96 cm (37–38 in) long. Like other azhdarchoids, Quetzalcoatlus had a long, toothless beak that consisted largely of the premaxilla and maxilla. At the base of the beak, formed from the premaxilla, was a small crest. Though its exact form has yet to be determined, it is possible, based on what is preserved, that two distinct morphotypes exist: one with a humped premaxillary crest and a tall nasoantorbital fenestra (the opening combining the nasal and antorbital fenestra), and one with a more semicircular premaxillary crest and a shorter nasoantorbital fenestra. While early reconstructions of Quetzalcoatlus depicted it with a short, blunt beak, owing to the attribution of remains from Wellnhopterus, the beak is actually long and slender. The tip is not preserved, so it is not clear how the beak ends. The beak likely had a gape of around 52 degrees. Typically among azhdarchids, the nasoantorbital fenestra is very large, with more than 40% of its height being above the orbit. The orbit is small and obovate (an inverted egg shape).

Skeletal reconstruction of Q. lawsoni.

Postcrania

Quetzalcoatlus had nine elongated cervical vertebrae that were compressed dorsoventrally (up-and-down), and accordingly better suited to dorsoventral motion than lateral (side-to-side) motion. However, lateral motion was still extensive, and the neck and head could swing left and right about 180 degrees. The vertebrae at the base of the neck and the pectoral girdle are poorly preserved. The first four dorsal vertebrae are fused into a notarium. Most other dorsal vertebrae are absent, except for those integrated into the sacrum. Seven true sacral vertebrae are preserved. No caudal vertebrae are preserved. The pelvis of one Q. lawsoni specimen (TMM 41954-57) is large compared to that of other specimens, with deep posterior (rear) emargination and no preserved symphisis. This suggests sexual dimorphism similar to that suggested for other monofenestratans (i.e. Darwinopterus, Anhanguera and Nyctosaurus).

Based on the limb morphology of Q. lawsoni, related azhdarchids such as Zhejiangopterus and other pterosaurs, Quetzalcoatlus was likely quadrupedal. 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 humerus was short and robust, with considerable mobility, whereas the femur was more gracile. The wing finger may have been held between the body and proximal limb elements whilst on land. The first digit (digit I) is the smallest, and digit III is the biggest, with the exception of the wing finger. Like in all pterosaurs, forelimb musclature was extensive, and flapping power came from several muscle groups on the torso, forearm and manus. Terrestrial locomotion in Quetzalcoatlus likely involved a pacing gait, with both limbs on a given side of the body moving at the same time. The forelimbs were probably lifted off the ground first to avoid collision with the hindlimbs.

Classification

A comparison of Quetzalcoatlus cervical vertebrae on the left with the Phosphatodraco holotype on the right

When describing Quetzalcoatlus in 1975, Douglas Lawson and Crawford Greenewalt opted not to assign it to a clade more specific than Pterodactyloidea, though similarities to Titanopteryx (later Arambourgiania) were noted earlier that year. In 1984, Lev Alexandrovich Nessov erected the subfamily Azhdarchinae within Pteranodontidae to comprise Azhdarcho, Quetzalcoatlus, and Titanopteryx. Unaware of the subfamily, in the same year, Kevin Padian erected the family Titanopterygiidae to accommodate Quetzalcoatlus and Titanopteryx. Two years later, in 1986, he synonymised the two taxa and elevated Azhdarchinae to family level, forming the family Azhdarchidae. In 2003, the clade Azhdarchoidea was defined by David Unwin, and azhdarchids were determined to form a clade with Tapejaridae. Quetzalcoatlus now represents the type genus of the subfamily Quetzalcoatlinae, alongside other very large azhdarchids such as Arambourgiania and possibly Hatzegopteryx, as well as smaller taxa like Zhejiangopterus.

Below is a cladogram showing the phylogenetic placement of Quetzalcoatlus within Neoazhdarchia from Andres and Myers (2013).

Neoazhdarchia

Thalassodrominae  Thalassodromeus sethi Tupuxuara leonardii Tupuxuara longicristatus  Dsungaripteridae  Domeykodactylus ceciliae Dsungaripterus weii Noripterus complicidens Noripterus parvus  Chaoyangopteridae  Eoazhdarcho liaoxiensis Shenzhoupterus chaoyangensis Chaoyangopterus zhangi Jidapterus edentus Radiodactylus langstoni  Azhdarchidae  Azhdarcho lancicollis TMM 42489 Zhejiangopterus linhaiensis Arambourgiania philadelphiae Quetzalcoatlus northropi Quetzalcoatlus lawsoni

Below is a cladogram showing the phylogenetic placement of Quetzalcoatlus within Azhdarchidae, from Andres (2021).

Azhdarchidae	Azhdarchinae Azhdarcho Albadraco Aerotitan Mistralazhdarcho Quetzalcoatliinae Aralazhdarcho Phosphatodraco Eurazhdarcho Zhejiangopterus Cryodrakon Wellnhopterus Hatzegopteryx Arambourgiania Quetzalcoatlus lawsoni Quetzalcoatlus northropi

Ortiz David et al. (2022), however recovered Quetzalcoatlus as sister to Thanatosdrakon, apart from the other giant azhdarchids (though they still nested within Quetzalcoatlinae).

Paleobiology

Feeding

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, similarly to the marabou stork (which will scavenge, but is more of a terrestrial predator of small animals), but 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, 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, making it 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 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.

Quetzalcoatlus northropi is found in plains deposits, likely being a solitary hunter. Q. lawsoni, however, is found in great numbers in facies that likely represent alkaline lakes. It may have lived like modern gregarious wading birds, feeding on small invertebrates such as annelids, crustaceans and insects that inhabit environments.

Flight

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. 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). 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. 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.

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. 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. This study proposed that large pterosaurs most likely utilized a short burst of powered flight to then transition to thermal soaring. 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. Studies of Q. northropi and Q. lawsoni published in 2021 by Kevin Padian et al. instead suggested that Quetzalcoatlus was a very powerful flier.

Launching

Early interpretations of Quetzalcoatlus launching relied on bipedal models. In 2004, Sankar Chatterjee and R.J. Templin used a 70 kg (150 lb) model and utilised a running launch cycle powered by the hind limbs, in which Q. northropi was only barely able to take off. In 2008, Michael Habib suggested that the only feasible takeoff method for a 200–250 kg (440–550 lb) Quetzalcoatlus was one that was mainly powered by the forelimbs. In 2010, Mark Witton and Habib noted that the femur of Quetzalcoatlus was only a third as strong as what would be expected from a bird of equal size, whereas the humerus is considerably stronger, and affirmed that an azhdarchid the size of Quetzalcoatlus would have great difficulty taking off bipedally. Thus, they considered a quadrupedal launching method, with the forelimbs applying most of the necessary force, a likelier method of takeoff. In 2021, Kevin Padian et al. attempted to resurrect the bipedal launch model, using a comparatively light weight estimate of 150 kg (330 lb). They suggested that Quetzalcoatlus' hind limbs were more powerful than previously suggested, and that they were strong enough to launch its body as high as 8 ft (2.4 m) off the ground without the aid of the forelimbs. A large breastbone would support the necessary muscles to create a flight stroke, allowing Quetzalcoatlus to gain enough clearance to begin the downstrokes needed for takeoff. Padian et al. also suggested that the legs and feet were likely tucked under the body during flight, as in modern birds.

Paleoenvironment

Quetzalcoatlus is known from the Lancian portion of the Javelina Formation, in a fauna dominated by Alamosaurus. It co-existed with another azhdarchid known as Wellnhopterus, as well as an additional pterosaur taxon, suggesting a relatively high diversity of Late Cretaceous pterosaur genera. The depositional environment represents a floodplain which was probably semi-arid, analogous in terms of climate and flora to the coastal plains of southern Mexico, consisting of an evergreen or semideciduous tropical forest. These forests consisted largely of angiosperm trees such as Javelinoxylon, conifers related to the modern Araucaria, and woody vines, with a closed canopy in excess of 30 m (98 ft) in height. The remains of both Quetzalcoatlus species are found in association with freshwater environments. Quetzalcoatlus lawsoni, in particular, is strongly associated with abandoned channel and lake facies, which are rare in the Javelina Formation. These facies preserve a diverse fauna of gastropods and bivalves, though the vertebrate fauna known from other aquatic environments belonging the Javelina, such as crocodiles, fishes and turtles, are absent. This suggests that the environment was inhospitable compared to normal stream channels, and high carbonate precipitation suggests that the water may have been highly alkaline. Eggshell fragments that may be attributable to Quetzalcoatlus suggest that they may have nested around alkaline lakes.

Cultural significance

Models on London's South Bank for the Royal Society's 350th anniversary exhibition

In 1975, artist Giovanni Caselli depicted Quetzalcoatlus as a small-headed scavenger with an extremely long neck in the book The evolution and ecology of the Dinosaurs by British paleontologist Beverly Halstead. Over the next twenty-five years prior to future discoveries, it would launch similar depictions colloquially known as "paleomemes" in various books, as noted by Darren Naish.

In 1985, the US Defense Advanced Research Projects Agency (DARPA) and AeroVironment used Quetzalcoatlus northropi as the basis for an experimental ornithopter unmanned aerial vehicle (UAV). They produced a half-scale model weighing 18 kg (40 lb), with a wingspan of 5.5 m (18 ft). Coincidentally, Douglas A. Lawson, who discovered Q. northropi in Texas in 1971, named it after John "Jack" Northrop, a 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.

In 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 over 10 m (33 ft), were intended to help build public interest in science. The models were created by scientists from the University of Portsmouth.

See also

• List of pterosaur genera
• Timeline of pterosaur research

External links

• Quetzalcoatlus at EarthArchives.org
• "Quetzalcoatlus - Flying monsters". YouTube. Paleo Studio. December 29, 2013.
• "The Biggest Thing That Ever Flew". YouTube. PBS Eons. August 14, 2017.
• "Quetzalcoatlus - Ancient Animal". YouTube. North 02. October 25, 2019.
• "Going out on a limb for Quetzalcoatlus". YouTube. thebrainscoop. June 9, 2020.