Ornithocheiromorpha

Associated and referred bones of Pterodactylus giganteus, now considered as Lonchodraco giganteus

The first specimens of ornithocheiromorphs were unearthed at a chalk pit near Burham in Kent, England. In 1846, British paleontologist James Scott Bowerbank named and described the remains found as Pterodactylus giganteus, as it was common at that time to assign any new described pterosaur species to Pterodactylus.[3] In the same chalk pit as P. giganteus, two other pterosaur species were discovered. The first was named in 1851 by Bowerbank as Pterodactylus cuvieri,[2] in honor of the prominent German naturalist and zoologist Georges Cuvier, while the second was described in the same year by British paleontologist Sir Richard Owen as Pterodactylus compressirostris.[10]

In 1861, further pterosaur specimens were found in the UK, and were given the new species Pterodactylus simus by Owen.[11] British paleontologist Harry Govier Seeley then created the new genus Ornithocheirus for the new species in the same year, the generic name translating as "bird hand" is due to the notion of the time that pterosaurs were the ancestors of modern birds. In 1869, Seeley reassigned Pterodactylus cuvieri into Ptenodactylus cuvieri, but in 1870, Seeley had realized that the generic name Ptenodactylus had been considered a nomen nudum, and therefore reassigned the species into Ornithocheirus cuvieri. Between 1869 and 1870, Seeley reassigned various species of pterosaurs, including P. cuvieri into newly named species of Ornithocheirus, therefore 27 in total.[12][13]

Richard Owen wasn't pleased when Seeley published his conclusions in his 1870 book The Ornithosauria, and he therefore considered the name Ornithocheirus to be inappropriate. In 1874, Owen created two new genera, Coloborhynchus, meaning "maimed beak" and Criorhynchus, meaning "ram beak". While Coloborhynchus consisted in a totally new type species, C. clavirostris, as well as two other species reassigned from Ornithocheirus, Criorhynchus consisted entirely of former Ornithocheirus species, including the type species, O. simus, which was later reassigned by Owen as Criorhynchus simus, as well as 5 other species. In the same year, Owen also reassigned O. cuvieri into Coloborhynchus cuvieri.[14] Seeley later disagreed and didn't accept Owen's position, and he therefore assigned Ornithocheirus simus as the type species of Ornithocheirus in 1881. Seeley had also named a new separate species called O. bunzeli in the same year.[15]

In 1914, English paleontologist Reginald Walter Hooley kept the name Ornithocheirus, but created two new genera: Lonchodectes, meaning "lance biter", and Amblydectes, meaning "blunt biter". This classification was rarely applied at the time, though.[16] The species Pterodactylus compressirostris was formerly transferred to Ornithocheirus in 1870 by Seeley, but it became the type species of Lonchodectes in Hooley's 1914 review of Ornithocheirus.[13] Confusingly, this species was also long regarded, incorrectly, as the type species of Ornithocheirus.[16]

Holotype of Pterodactylus cuvieri, now known as Cimoliopterus

In 2013, Brazilian paleontologists Taissa Rodrigues & Alexander Kellner made a deeper analysis on the species Pterodactylus cuvieri. In the analysis, they stated that it needed a separate genus, and assigning it to Ornithocheirus was inappropriate, therefore, they created the new genus called Cimoliopterus, with the new resulting combination Cimoliopterus cuvieri, though its type species still remains as P. cuvieri. The generic name Cimoliopterus combines Κιμωλία, Kimolia (meaning "chalk" in New Greek), in reference to the white clay of the island Kimolos, and the Latinised Greek word πτερόν, pteron, which means "wing". In the same year, Rodrigues & Kellner also reviewed the species Pterodactylus giganteus, and reassigned it to Lonchodectes, resulting in a Lonchodectes giganteus. Both paleontologists later concluded that the genus Lonchodectes and its type species L. compressirostris were nomina dubia, and therefore L. giganteus needed a new separate genus. Rodrigues & Kellner created the new genus Lonchodraco, and reassigned L. giganteus from Lonchodectes into the new genus, resulting in a new combination called Lonchodraco giganteus. The generic name combines Greek λόγχη, lonchē (meaning "lance"), with Latin draco, meaning "dragon".[17]

Skull fragments of Ornithodesmus latidens, which is currently known as Istiodactylus latidens

In 1887, Seeley had described new fossil remains from the Isle of Wight, an island off the coast of southern England. He thought it belonged to some kind of bird-like creature, which he named it Ornithodesmus cluniculus.[18] John Whitaker Hulke suggested later that year that Ornithodesmus was a pterosaur, finding it similar to fossils that he had seen before, but Seeley disagreed.[19] Seeley also reported another specimen found on the same site, which was then acquired in the British Museum in 1882. The specimen had been assigned to Ornithocheirus nobilis by Richard Lydekker in 1888, but Seeley considered it another species of Ornithodesmus, which he now considered a species of pterosaur instead of a bird. In 1901, Seeley named the new species as O. latidens, which means "wide tooth".[20]

Reginald Hooley later described two more specimens of O. latidens in 1913, which were collected in 1904 from the sea after a rockfall near Atherfield Point on the Isle of Wight. The first of the specimens was collected, and consisted of a skull, neck and trunk vertebrae, a shoulder blade, an ischium, and parts of the forelimbs. The second specimen was collected in just one block, and it includes parts of the forelimbs and pectoral girdle. These two specimens represent the most complete remains of Cretaceous pterosaurs found in England. Hooley also discussed O. latidens in detail, which led Ornithodesmus to be placed within its own family, Ornithodesmidae.[21] But then his article ended with a discussion wherein it was noted that the palaeontologist Charles William Andrews had expressed doubts as to whether O. latidens belonged in the genus Ornithodesmus, as the vertebrae of the specimen that genus was based on differed markedly from those of Hooley's specimen. The American palaeontologist Samuel W. Williston subsequently reviewed Hooley's article, disagreeing with some of his conclusions about the anatomy and classification of the animal.[22] After Hooley's monograph, little was written about the animal for the rest of the 20th century, and no similar pterosaurs were found for decades.[23]

In 1993 however, the British palaeontologists Stafford C. Howse and Andrew C. Milner concluded that the holotype sacrum and only specimen of O. cluniculus didn't belong to a pterosaur, but instead to a maniraptoran theropod dinosaur. They also pointed out that no detailed attempts had been made to compare the sacrum of O. cluniculus with those of pterosaurs, and that O. latidens had in effect been treated as the type species of the genus Ornithodesmus. Now as a definite species of pterosaur, "O." latidens thus required a new genus name.[24] Howse, Milner, and David Martill in 2001, moved "O." latidens to a new genus called Istiodactylus. The generic name means "sail finger", which refers to the wings of large pterosaurs. After moving "O." latidens into Istiodactylus, they had also named a new family called Istiodactylidae, with Istiodactylus as the only member.[25]

Comparison between the holotypes of Tropeognathus mesembrinus and Ornithocheirus simus

Other important ornithocheiromorph discoveries include the ornithocheirids Tropeognathus and Anhanguera from the Romualdo Formation in Brazil.[26][27] Tropeognathus was described with its type species, T. mesembrinus in 1987 by German paleontologist Peter Wellnhofer. The generic name is derived from Greek τρόπις, tropis, meaning "keel", and γνάθος, gnathos, meaning "jaw". The specific name is derived from Koine mesembrinos, "of the noontide", simplified as "southern", in reference to the provenance from the Southern hemisphere. The description then led to an enormous taxonomic confusion.[28] In 1989, Brazilian paleontologist Alexander Kellner considered it an Anhanguera mesembrinus,[29] then a Coloborhynchus mesembrinus by Veldmeijer in 1998,[30] and then a Criorhynchus mesembrinus in 2001 by German paleontologist Michael Fastnacht.[31] T. mesembrinus was then considered a junior synonym of Ornithocheirus simus by British paleontologist David Unwin in 2001, but he then proposed an Ornithocheirus mesembrinus in 2003.[32][33] In 2013 however, Taissa Rodrigues and Alexander Kellner concluded that Tropeognathus would be valid again, and containing only T. mesembrinus, the type species.[17]

The discovery of Anhanguera in 1985 made some authors assign several species of Ornithocheirus to the new genus, this is due to the similarities of Anhanguera and Ornithocheirus. One of which was Ornithocheirus cuvieri, formerly known as Pterodactylus cuvieri, was renamed by David Unwin in 2001 as Anhanguera cuvieri, though in 2013 it has been considered a synonym of Cimoliopterus.[32][17] Other species of Anhanguera include A. piscator, which is known from a nearly-complete skeleton, and was once proposed to belong to the genus Coloborhynchus, but it has recently been placed back into Anhanguera by Andres and Myers (2013).[34][35] Another species which is also known from a complete skeleton is A. spielbergi, which was also originally considered as a species of Coloborhynchus in 2003, though in 2018, it has been reassigned to the genus Maaradactylus, specifically to its own species, M. spielbergi.[36][37]

Skull of Hamipterus tianshanensis

A discovery in Asia, specifically northwestern China, was reported in 2006. The lake sediments allowed an exceptional preservation of fossils, and therefore paleontologists Qiu Zhanxiang and Wang Banyue started official excavations. Part of the findings consisted of dense concentrations of pterosaur bones, associated with soft tissues and eggs. The site represented a nesting colony that storm floods had covered with mud. Dozens of individuals could be secured to run into the many hundreds.[38] In 2014, a new species was named and described: Hamipterus tianshanensis. It was named by Wang Xiaolin, Alexander Kellner, Jiang Shunxing, Wang Qiang, Ma Yingxia, Yahefujiang Paidoula, Cheng Xin, Taissa Rodrigues, Meng Xi, Zhang Jialiang, Li Ning, and Zhou Zhonghe. The generic name Hamipterus combines that of the Hami region, with the word pteron, meaning "wing", and the specific name refers to the provenance from the Tian Shan, a mountain range.[38] These findings represented an impressively large concentration of pterosaur fossils, and together with the Pterodaustro[39] nesting colonies in Argentina, both are regarded as the most diverse fossil concentrations of pterosaurs ever known.[40]

Size of Istiodactylus latidens compared to a human

Ornithocheiromorphs were large pterosaurs, with wingspans normally ranging between 3 to 6 meters (9.8 to 19.7 ft).[6] Istiodactylus for example, had a wingspan ranging from 4.3 to 5 meters (14 to 16 ft), with the most complete known skull estimated to have been about 45 centimeters (1.48 ft) in length, based on a long-lost fragment of its jaw reported in 2012.[9] Though its jaws measured only 28.5 centimeters (11.2 in), which was less than 80 percent of the skull's length.[23] Ornithocheirids were typically larger than others of the group and were more successful within the food chain rather than other ornithocheiromorphs, one reason is because of their large size, Ornithocheirus itself had a wingspan measuring 4.5 to 6.1 meters (15 to 20 ft). Other ornithocheirids were also regarded with similar wingspans, though the largest is probably Tropeognathus mesembrinus, with a wingspan exceeding 7 meters (23 ft), and 8.2 meters (27 ft) as the maximum estimate.[5] Another genus which was impresively large is Coloborhynchus, with a total skull length that could have been up to 75 centimeters (2.46 ft), leading to an estimated wingspan of 7 meters (23 ft).[41] However, this species may belong to a different genus.[17] One of the smaller genera includes the boreopterid Boreopterus, which is estimated to have a wingspan of around 1.45 meters (4.8 ft), but some paleontologists consider it a synonym of Zhenyuanopterus, in this case measuring around 3.5 to 4 meters (11 to 13 ft) in length.[42][43]

Skull comparison between different species of ornithocheiromorphs; notice their different structures

Most ornithocheirids bore distinctive convex "keeled" crests on their snout and underside of their mandible, this was well developed in several genera such as Tropeognathus.[44] The similar Anhanguera possessed jaws that were tapered in width, but expanded into a broad, spoon-shaped rosette at the tip. The jaws are distinguished from its relatives by several differences in the crest and teeth: unlike its close relatives Coloborhynchus and Ornithocheirus, the crest on the upper jaw of Anhanguera didn't begin at the tip of the snout, therefore, it was set farther back on the skull.[45] Several ornithocheiromorphs such as the basal Lonchodectes, had long jaws similar to the ornithocheirids, but were mostly thinner and more elongated. Lonchodectes had many short teeth, and its jaws were compressed vertically. Comparisons between ornithocheirids such as Ornithocheirus led to a conclusion that it had a relatively narrower premaxilla compared to related genera like Coloborhynchus and Anhanguera.[31][46]

Some basal anhanguerians like Cearadactylus had its first preparations with many serious mistakes: the front of the snout and the lower jaws were confused leading to a reconstruction in which the anterior part of the head was upside down.[47] Some of the teeth were extensively restored and enlarged until the wider front of the jaws showed very large and robust teeth projecting outwards. With this arrangement, the maxilla was kinked, and its interlocking teeth suggested that Cearadactylus had a piscivourous diet, allowing the animal to keep hold of slippery fish.[7] Cearadactylus was somewhat similar to ornithocheirids like Coloborhynchus, but it lacked the "keeled" crest on its snout, and therefore it was suggested to be a more basal form of ornithocheiromorph pterosaur.[48][49] Another smaller genus similar to Cearadactylus is Guidraco. Its holotype skull has a length of 38 centimeters (15 in), which makes it smaller than other genera. The skull is very elongated however, and a hollow profile is seen, but not very pointed, as the upper edge and the line of the jaw run nearly parallel over most of their length.[50]

Even though most ornithocheiromorphs didn't have a cranial crest like the closely related pteranodontids, there were some exceptions, this included Caulkicephalus and Ludodactylus.[51] While Ludodactylus is suggested to be a more basal member of the group, Caulkicephalus is more similar to ornithocheirids.[52] This is due to its rounded snout, very similar to that of Ornithocheirus and therefore it was placed within Ornithocheiridae. Caulkicephalus was also a large pterosaur, with wingspan estimates of around 5 meters (16 ft).[52]

Life restorations of Tropeognathus in flight; notice their high aspect ratio

Most pterosaurs of this group had extensive wing-membranes, which were distended by a long wing-finger and would have been covered in hair-like pycnofibres.[53] Like the modern-day albatross, ornithocheiromorphs also had a high aspect ratio and low wing loadings, though this is typically in the larger and more advanced genera. Wing remains of the istiodactylid Nurhachius have been compared to those of modern soaring birds, which fly with little flapping, and may have been ideal for low-energy soaring, which is necessary when searching for carrion or fish.[54] The wings of istiodactylids also seem to have been shorter compared to the fish-eating ornithocheirids, and unlike them, which were more suited for taking off and landing, istiodactylids may have preferred a more terrestrial setting. This is also seen in many inland soaring birds, which have a lower aspect ratio compared those that soar over the ocean. Huge genera such as Tropeognathus and Ornithocheirus are considered to have flown similar to modern-day albatrosses, which consisted on travelling very long distances and rarely flapping. This technique is mostly seen in later pterosaur locomotion, such as the related and mentioned pteranodontids, though not so common in the larger azhdarchids such as Hatzegopteryx and Quetzalcoatlus.[8][6]

Ornithocheiromorphs had shoulder girdles of strong structure, which transferred the forces of a rarely flapping flight to the thorax. The shoulder girdle was probably covered by thick muscle layers, with the upper bone (shoulder blade) resulting in some kind of straight bar.[55] The shoulder blade was connected to a lower bone called the coracoid, which is relatively long in most ornithocheiromorphs. In the more advanced species, the shoulder is combined whole, with the scapulocoracoid, being almost vertically oriented, meaning that it fitted into a recess in the side of the notarium, while the coracoid likewise connected to the breastbone. Some species also had the shoulder joint moved from the shoulder blade to the coracoid. The breastbone is somewhat wide, and was formed by fused paired sterna. At its rear, a row of belly ribs or gastralia was present, covering the entire belly.[56] To the front of the breastbone, a long point called the cristospina, jutted obliquely upwards, and the rear edge of the breastbone was the deepest point of the thorax. The absence of clavicles or interclavicles had made paleontologists wonder if pterosaurs evolved in different ways than other archosaurs.[57][56]

Illustrations of various Istiodactylus bones, including the neck vertebrae and notarium

The vertebral column of ornithocheiromorphs was heavily pneumatized by an extensive system of air sacs, leaving prominent pneumatic foraminae.[58] The neck of ornithocheiromorphs was typically relatively long and robust, being longer than the torso in some derived clades.[59] The neural spines of ornithocheiromorph cervical vertebrae were generally tall and spikelike.[50][45] In some genera such as Tropeognathus and Istiodactylus, up to six dorsal vertebrae are fused into a notarium.[60] In some genera such as Anhanguera, four to seven sacral vertebrae are fused into a synsacrum.[45] The tail is not well-known in ornithocheiromorphs, however. Zhenyuanopterus, which is known for having 13 caudal vertebrae, formed one of the longest tails of any pterodactyloid.[42] Anhanguera, another well-known genus, had a shorter tail, with broad caudal vertebrae that bore a "duplex" cross-section similar to Pteranodon.[45]

A rotated pelvis of Anhanguera santanae, showing its right side

The pelvis of ornithocheiromorphs was of moderate size compared to the body as a whole, similar to other ornithocheiroids. The three pelvic bones were often fused, as seen in many species such as Anhanguera santanae, the ilium was long and low, and its front and rear blades projected horizontally beyond the edges of the lower pelvic bones.[61] Despite the structure length, the processes of these rod-like forms indicate that the hindlimb muscles attached to them were limited in strength.[62] The pubic bone was fused with the broad ischium into an ischiopubic blade, resulting in a narrow build. Sometimes, the blades of both sides were fused, closing the pelvis from below and forming the pelvic canal. The front of the pubic bones was also articulated with a unique structure, resulting in a pair of prepubic bones within. This formed a cusp covering the rear belly, and was located between the pelvis and the belly ribs. The hip joint of ornithocheiromorphs was not perforated and allowed considerable mobility to the leg, and suggests that it was vertical, as therefore had a function in breathing, compensating the relative rigidity of the chest cavity.[63][61]

The hindlimbs of ornithocheiromorphs were strongly built, yet relative to their wingspans, smaller than those of birds in terms of hindlimb-to-wingspan ratio. The hindlimbs were long in comparison to the torso length, and the thighbone was rather straight, with the head making only a small angle with the shaft.[64][63] This implied the legs not being held vertically below the body, but were somewhat sprawling within.[64] A tibiotarsus is mostly seen in many ornithocheiromorphs, this consisted on the shinbone (tibia) being fused with the upper ankle bones, resulting in a longer structure than the thighbone, which could attain a vertical position when walking.[65] Most of the toes of ornithocheiromorphs were long, though compared to earlier forms of pterosaurs, they were less robust, and were also connected to the ankle in a higher position than the other metatarsals. Often curved, the mobile clawless fifth toe consisted of two phalanges, this is seen some specimens, which showed membranes between the toes, and was concluded that this allowed them to function as flight control surfaces.[66]

Below is a cladogram showing the results of a phylogenetic analysis presented by Andres et al., 2014. In the analysis, they placed the former istiodactylid Hongshanopterus as a basal member, and also found the family Boreopteridae to belong within Anhangueria, though placed in a more basal position.[4][69]

In 2019, new species of ornithocheiromorphs were found, and the former species Ornithocheirus wiedenrothi was renamed as Targaryendraco wiedenrothi.[70] The discovery of Iberodactylus[71] in Spain also made paleontologists reclassify the genus Hamipterus[38] in a newly named family called Hamipteridae. The former ornithocheirid Cimoliopterus[2] was also reclassified as well, and it is currently grouped with Aetodactylus[72] and Camposipterus in the newly named clade Targaryendraconia,[70] specificaly to its own family, the Cimoliopteridae. The former istiodactylid Haopterus was reclassified due to the discovery of Mimodactylus, and it's placed as the sister taxon of it in a new family called Mimodactylidae.[73]

The cladogram below is a topology recovered by Pêgas et al. (2019). In the analyses, they found Ornithocheiromorpha to be synonymous with Lanceodontia, and recovered Ornithocheirae as the more inclusive group containing Ornithocheirus, Anhangueria and Targaryendraconia. They also reassigned most of the ornithocheirid genera into the family Anhangueridae, which resulted in a basal position for Ornithocheirus within Ornithocheirae.[70]

Recreation of a Mimodactylus group in flight

A hypothetical recreation of a group of Istiodactylus feeding on a carcass of a stegosaur

Ornithocheiromorphs were originally regarded as piscivorous creatures, feeding mainly on small and mid-sized fish.[7] Some paleontologists even suggested details on how these pterosaurs caught fish, some of which included dipping their beaks close to the water for prey.[7] Hooley for example, found that the beak of the well known Istiodactylus was similar to those of birds such as herons, storks, and skimmers, and suggested that Istiodactylus probably fed on fish, this was mainly based on his 1913 jaw reconstruction of the animal.[21] In 1991, Peter Wellnhofer compared the jaw endings of Istiodactylus with those of a duck, but he then noticed that it wasn't a "duck-billed pterosaur" or anything similar, even though it was popularly called that way.[74] Other diet suggestions for Istiodactylus were mostly based on the broad snout remains, as well as the very distinctive teeth; leading to the thought of a more terrestrial life and hunting methods, which is far different from the lifestyle of later genera such as the huge Tropeognathus. This conclusion was later followed by Howse and colleagues suggesting that the strong, yet bizarre teeth of Istiodactylus were more suited for scavenging carcasses rather than catching fish, this led the idea of a different feeding technique, which consisted in bitting and twisting the skull to remove large chunks out of the carcass, very different from the initial suggestions of how early ornithocheiromorphs fed. Witton in 2012 found that the teeth of Istiodactylus were unlike the recurved and enlarged teeth seen in the more derived ornithocheirids such as Ornithocheirus, he instead pointed out that it was more "razor-edged" and better suited for carrion rather than fish.[9]

Another ornithocheiromorph that possesed similar features to Istiodactylus is Liaoxipterus, which is known from a skull with several unique traits, including numerous peg-like teeth. The shape of its teeth indicated that Liaoxipterus was a possible insectivore,[75] though this conclusion isn't considered entirely accurate.[76] Liaoxipterus was also smaller in size compared to Istiodactylus and other ornithocheiromorphs, this is perhaps an indication that the more primitive members of this group (istiodactylids in this case) were smaller compared to more derived and advanced families such as the lonchodectids, ornithocheirids and targaryendraconians. This can also be seen in the size comparison between ornithocheirids and istiodactylids, with the wingspan of Tropeognathus for example, measuring almost double that of Istiodactylus.[26][9]

Cimoliopterus (right) stealing prey from a Lonchodectes (left), both were derived members of this group and possible fish hunters

Ornithocheirids like Tropeognathus and Coloborhynchus are considered to be fish-eaters, and had longer and sharper teeth compared to the more rounded teeth of Istiodactylus, though this is still sometimes disputed.[7] Another difference that can be seen in more primitive ornithocheiromorphs their eyes being proportionally smaller compared to the assumed predatory and more advanced ornithocheirids, this again adds to the fact that the more primitive groups (istiodactylids) were most likely scavengers, and later got more successful within the food chain, leading the later, more advanced groups (ornithocheirids and lonchodectids) to be the dominant fish hunters during the early Late Cretaceous.[7][77]

A 2017 study confirmed that most ornithocheirids were fish-eaters, though in a deeper analysis, paleontologists found carbon isotopes within several teeth, and later compared the enamel values of the ornithocheirid teeth between the aquatic and terrestrial consumers near the fossil site.[7] Pterosaur enamels from the Twin Mountains Formation were shown with lower enamel values in comparison to the aquatic consumers that lived near, suggesting a more terrestrial diet, this is mostly due to the similarly low enamel values found in carnivorous theropods such as Acrocanthosaurus, which mainly had a terrestrial diet. The enamels found in the Paw Paw Formation however, were described with higher values, in comparison to the nearby terrestrial consumers. Analyzing these enamel remains, paleontologists found out that the values seen were more comparable to the aquatic consumer values rather the terrestrial ones. These analyses are considered a very useful reference to the evolutionary changes between the earlier and later species of ornithocheirids, as well as other ornithocheiromorphs. The diet changes were also taken into account, with earlier genera such as Coloborhynchus having a varied diet, preying partially on fish, while also hunting on land. The later genera such as Uktenadactylus however, fed almost entirely on fish, and rarely hunted on land.[7] This can be demonstrated with the age difference between the Twin Mountains Formation and the Paw Paw Formation. The enamel remains found in the Twin Mountains Formation dated back to the earlier Aptian stage, while the remains found in the Paw Paw Formation dated back to the later Albian stage, resulting in a slight diet transition between more primitive and more derived genera.[7]

Diagrams of three different pterosaur genera, including an anhanguerid (bottom); notice the breathing motion (top two) and internal air sac system (bottom two)

Pterosaur studies in 2009 showed that several pterosaurs, including ornithocheiromorphs, had a lung-and-air-sac system and a precisely controlled skeletal breathing pump, which supports a flow-through pulmonary ventilation model in pterosaurs, analogous to that of birds. This breathing method is also seen is carnivorous dinosaurs such as Aerosteon, but there are currently no studies showing its relationships with pterosaurs.[78] A subcutaneous air sac system is present in at least some species of pterodactyloids, and this technique would have given a further reduced density to the living animal when breathing.[79] Similar to modern crocodilians, many ornithocheiromorph genera appeared to have had some kind of hepatic piston, regarding their shoulder-pectoral girdles, which were suggested to be too inflexible to move the sternum. This can also be seen in modern-day birds, which similarly possess strong gastralia, and therefore paleontologists erect the idea that their respiratory system had characteristics comparable to primitive archosaurs.[80]

Restoration of a flying Istiodactylus; notice its high aspect ratio

Ornithocheiromorphs, like other pterosaurs, are considered to have been skilled fliers as well as swift at moving on the ground. Footprints from several species show that most pterosaurs did not sprawl their limbs to a large degree, as in modern reptiles, but rather held the limbs relatively erect when walking, like dinosaurs. While footprints are yet to be known, it is likely that ornithocheiromorphs also walked erect.[66] Compared to other earlier pterosaurs such as rhamphorhynchids, ornithocheiromorphs had unusually uneven limb proportions, with the forelimbs resulting in a much longer scale compared to the hind limbs. Their close relatives, the pteranodontids, were also found with similar features, though they more likely flew like modern-day albatrosses rather than anything else. Paleontologists also suggest that they most likely spend long stretches of time sea fishing, travelling very long distances without flapping while at the same time flying close the surface of the water with exploited wind speed, and without the necesity of thermals.[65] This would likely have required them to use unique modes of locomotion compared to other pterosaurs, this can already be seen in earlier evolutions such as Istiodactylus and Nurhachius, with powerful musculature attachments and well-developed pectoral and upper arm bones.[9][54] It is also possible that ornithocheiromorphs ran (but not walked) bipedally, or that they used a hopping gait.[66] Many pterosaur researchers like Mike Habib have noted that the limb proportions of some ornithocheiromorphs such as Anhanguera are consistent with hopping, though the scavenging istiodactylids are probably still the best examples of pterosaurs with a more terrestrial setting.[81][21]