Quetzalcoatlus

Quetzalcoatlus (/kɛtsəlkoʊˈætləs/) is a genus of azhdarchid pterosaur that lived during the Maastrichtian age of the Late Cretaceous in North America. The type specimen, recovered in 1971 from the Javelina Formation of Texas, United States, consists of several wing fragments and was described as 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 a 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 Q. northropi locality. In 2021, these remains were assigned to the name Quetzalcoatlus lawsoni by Brian Andres and (posthumously) Wann Langston Jr., as part of a series of publications on the genus.

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 the only known specimen. While wingspan estimates over the years have ranged from 5.2–25.8 m (17–85 ft), more recent estimates hover around 10–11 m (33–36 ft). Previously, smaller azhdarchid specimens were referred to the species Q. lawsoni, though other researchers have questioned the placement of this species within the genus.

Historical interpretations of the diet of Quetzalcoatlus 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 hypothesis and is thought to be a common feeding behavior among large azhdarchids. On the other hand, the disputed 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 is speculated to have been fairly solitary, Q. lawsoni appears to have been highly gregarious (social). Azhdarchids like Quetzalcoatlus were highly terrestrial by pterosaur standards, though even the largest were nonetheless capable of flight. 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 (from a four-legged posture), using the powerful muscles of their forelimbs to propel themselves off the ground and into the air.

The genus Quetzalcoatlus is based on fossils discovered in rocks pertaining to the Late Cretaceous Javelina Formation in Big Bend National Park, Texas. Remains of dinosaurs and other prehistoric life had been found in the area since the beginning of the 20th century. The first Quetzalcoatlus fossils were discovered in 1971 by the graduate student Douglas A. Lawson while conducting field work for his Master's degree project on the paleoecology of the Javelina Formation. This field work was supervised by Wann Langston Jr., an experienced paleontologist who had been doing field work in the region since 1938 and since 1963 led expeditions through his position as curator at the Texas Science and Natural History Museum. The two had first visited the park together in March 1970, with Lawson discovering the first Tyrannosaurus rex fossil from Texas. Returning in 1971, Lawson discovered a 3 ft (0.91 m) bone while investigating an arroyo on the western edge of the park, and returned to Austin with a 1 ft (0.30 m) section of it. He and Langston then identified it as a pterosaur fossil based on its hollow internal structure with thin walls. Returning in November 1971 for further excavations, they were struck by the unprecedented size of the remains compared to known pterosaurs. The initial material consisted of a giant radius and ulna, two fused wristbones known as syncarpals, and the end of the wing finger. Altogether, the material comprised a partial left wing from an individual (specimen number TMM 41450-3) later estimated at over 10 m (33 ft) in wingspan. Lawson described the remains in his 1972 thesis as "Pteranodon gigas", and diagnosed it as being "nearly twice as large as any previously described species of Pteranodon". As a thesis is not recognized as a published work by the International Code for Zoological Nomenclature (ICZN), "Pteranodon gigas" is not a valid name. Further field work at the site was conducted in March 1973, when fragments were found alongside a long and delicate bone connected to an apparently larger element. This fossil was left in the ground until April 1974, when they fully excavated the larger element, a humerus. Due to the close association of discovered remains, Langston felt confident there were nothing more to be found at the site. Several later excavations of the site have indeed been unsuccessful.

Lawson announced his discovery in the journal Science in March 1975, with a depiction of the animal's size compared to a large aircraft and a Pteranodon gracing the cover of the issue. Lawson wrote that it was "without doubt the largest flying animal presently known". He illustrated and briefly described the remains known at the time, but did not offer a name and indicated that a more extensive description was in preparation that would diagnose the species. In May, he submitted a short response to his original paper to the journal, considering how such an enormous animal could have flown. Within the paper, he briefly established the name Quetzalcoatlus northropi, but did still not provide a diagnosis or a more detailed description, which would later cause nomenclatural problems. Though not specified in the original publication, Lawson named the genus after the Aztec feathered serpent god Quetzalcōātl, while the specific name honors John Knudsen Northrop, the founder of Northrop Corporation, who drove the development of large tailless Northrop YB-49 aircraft designs resembling Quetzalcoatlus. The discovery of the giant pterosaur left a strong impression on both the scientific community and the general public, and was reported on throughout the world. It was featured in Time Magazine and appeared on the cover of Scientific American in 1981 alongside an article on pterosaurs by Langston. The species would become referenced by over 500 scientific publications, with Quetzalcoatlus northropi becoming the single most cited pterosaur species and Quetzalcoatlus the fourth most cited pterosaur genus after Pteranodon, Rhamphorhynchus, and Pterodactylus, much older genera with many more species than Quetzalcoatlus.

Prior to the announcement of the discovery, Langston had returned to Big Bend with a group of fossil preparators in February 1973, primarily aiming to excavate bones of the dinosaur Alamosaurus. One of the preparators, a young man named Bill Amaral who went on to be a respected field worker, had been skipping his lunches to conduct additional explorations of the area. He came across some additional fragments of pterosaur bone on a different portion of the ridge, around 50 kilometers away from the original site. Two more new sites quickly followed nearby, producing many fragments which the crew figured could be fit back together, in addition to a complete carpal and intact wing bone. Langston noted in his field notes that none of these bones suggested animals as large as Lawson's original specimen. Further remains came from Amaral's first site in April 1974, after Lawson's site had been exhausted; a long neck vertebra and a pair of jawbones appeared. Associated structures were initially hoped to represent filamentous pycnofibres, but were later confirmed to be conifer needles. Near the end of the 1974 season, Langston stumbled over a much more complete pterosaur skeleton; it consisted of a wing, multiple vertebrae, a femur and multiple other long bones. They lacked time to fully excavate it, leaving it in the ground until the next field season. This area, where many smaller specimens began to emerge, came to be known as Pterodactyl Ridge. Two of the smaller individuals were reported in the first 1975 paper by Lawson, presumed to belong to the same species, though Langston would begin to question the idea they belonged to Q. northropi by the early 1980s. Excavations continued in 1976, and eight new specimens emerged in 1977; in 1979, despite complications due to losing the field notes from 1977, Langston discovered another new site that would produce an additional ten specimens. Most importantly, a humerus of the smaller animal was finally found, which Langston considered of great importance to understanding Quetzalcoatlus. Several further new localities followed in 1980, but 1981 proved less successful and Langston began to suspect the ridge may have been mostly depleted of pterosaur fossils. There was similarly little success in 1982, and visits during 1983 and 1985 proved to provide the last substantive discoveries of Quetzalcoatlus fossils. Langston returned in 1989, 1991, 1992, and 1996, but only found isolated bones and fragments. Eventually a handful of additional specimens were discovered by former student Thomas Lehman. A visit to Lawson's initial site during 1991 showed that all traces of excavation had by now eroded away. Langston would visit Big Bend for the last time in 1999, having concluded the pterosaur expeditions to focus on the excavation of two skulls of Deinosuchus, another famous fossil of the area.

The expected further description implicated by Lawson never came. For the next 50 years, the material would remain under incomplete study, and few concrete anatomical details were documented within the literature. Much confusion surrounded the smaller individuals from Pterodactyl Ridge. In a 1981 article on pterosaurs, Langston expressed reservations whether they were truly the same species as the immense Q. northropi. In the meantime, Langston focused on the animal's publicity. He worked on a life-sized gliding replica of Quetzalcoatlus northropi with aeronautical engineer Paul MacCready between 1981 and 1985, promoting it in a dedicated IMAX film. The model was created to understand the flight of the animal — prior to Lawson's discovery such a large flier wasn't thought possible, and the subject remained controversial at the time. Furthermore, the model was intended to allow people to experience the animal in a more dynamic manner than a mere static display or film. Around this time he also created a skeletal mount of the genus that was exhibited at the Texas Memorial Museum.

The next scientific effort of note was a 1996 paper by Langston and pterosaur specialist Alexander Kellner. By this time, Langston was confident the smaller animals were a separate species. A full publication establishing such a species was still in preparation at the time, but due to the importance of the skull material for the understanding of azhdarchid anatomy, the skull anatomy was published first. In this publication, the animal was referred to as Quetzalcoatlus sp., a placeholder designation for material not assigned to any particularly valid species. Once again, the planned further publication failed to materialize for decades, and Quetzalcoatlus sp. remained in limbo. A publication on the bioaeromechanics of the genus was also planned by Langston and James Cunningham, but this failed to materialize and the partially completed manuscript later became lost. Ultimately, a comprehensive publication on Quetzalcoatlus sp. would not appear before Langston's death in 2013. By this point he had produced many notes and individual descriptions, but had not begun writing any formal manuscript that could be published.