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.[5] 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,[6] 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.[7] P. compressirostris later became the type species of a newly created genus called Lonchodectes (meaning "lance biter") in a review by English paleontologist Reginald Walter Hooley in 1914.[8] Confusingly, this species was also long regarded, incorrectly, as the type species of Ornithocheirus.[9]

In 1861, further pterosaur specimens were found in the UK, and were given the new species Pterodactylus simus by Owen.[10] 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 1870, Seeley reassigned the species Pterodactylus cuvieri as Ornithocheirus cuvieri.[11][8] In 1874, Richard Owen proposed 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 O. simus, which was later reassigned by Owen as Criorhynchus simus.[12]

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. In the same study, Rodrigues & Kellner also reviewed the species Pterodactylus giganteus, and reassigned it to a newly created genus called Lonchodraco, this resulted in a new combination called Lonchodraco giganteus.[13]

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.[14] Seeley also reported another specimen found on the same site. He then considered it another species of Ornithodesmus. In 1901, Seeley named this new species as O. latidens, meaning "wide tooth".[15] Later, Reginald Hooley discussed O. latidens in detail, based on specimens he had found, which led Ornithodesmus to be placed within a new family called Ornithodesmidae.[16] Paleontologist Charles William Andrews however, had expressed doubts as to whether O. latidens belonged in the genus Ornithodesmus, as the vertebrae of the specimen of that genus was based on differed markedly from those of Hooley's specimen.[17]

In 1993, the British paleontologists 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.[18] Howse, Milner, and David Martill in 2001, moved "O." latidens to a new genus called Istiodactylus. They had also named a new family called Istiodactylidae, with Istiodactylus as the only member.[19]

Comparison between the holotypes of Tropeognathus mesembrinus and Ornithocheirus simus

Other important ornithocheiromorph discoveries include the anhanguerids Tropeognathus and Anhanguera from the Romualdo Formation in Brazil.[20][21] 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.[22] In 1989, Brazilian paleontologist Alexander Kellner considered it an Anhanguera mesembrinus,[23] then a Coloborhynchus mesembrinus by Veldmeijer in 1998,[24] and then a Criorhynchus mesembrinus in 2001 by German paleontologist Michael Fastnacht.[25] 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.[26][27] In 2013 however, Taissa Rodrigues and Alexander Kellner concluded that Tropeognathus would be valid again, and containing only T. mesembrinus, the type species.[13]

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. 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.[28]

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).[4] 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.[29] Though its jaws measured only 28.5 centimeters (11.2 in), which was less than 80 percent of the skull's length.[17] Anhanguerids and 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, for example, Tropeognathus mesembrinus, had a normal wingspan of about 8.26 meters (27.1 ft), and 8.70 meters (28.5 ft) as the maximum estimate.[30] Another species which was impresively large is Coloborhynchus capito, 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).[31] However, this species may belong to a different genus called Nicorhynchus.[32]

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

Most anhanguerids bore distinctive convex "keeled" crests on their snout and underside of their mandible, this was well developed in several genera such as Tropeognathus.[33] 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.[34]

Other anhanguerids 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.[35] Some of the teeth were extensively restored and enlarged until the wider front of the jaws showed very large and robust teeth projecting outward. 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.[36] 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.[37]

Even though most ornithocheiromorphs didn't have a cranial crest like the closely related pteranodontids, there were some exceptions, this included Caulkicephalus and Ludodactylus.[38] Caulkicephalus had a rounded snout, very similar to that of Ornithocheirus and Anhanguera, and therefore it is placed within either Anhangueridae or Ornithocheiridae, depending on the author.[39][40] Caulkicephalus was also a large pterosaur, with wingspan estimates of around 5 meters (16 ft).[40]

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.[41] The neck of ornithocheiromorphs was typically relatively long and robust, being longer than the torso in some derived clades.[42] The neural spines of ornithocheiromorph cervical vertebrae were generally tall and spikelike.[37][34] In some genera such as Tropeognathus and Istiodactylus, up to six dorsal vertebrae are fused into a notarium.[43] In some genera such as Anhanguera, four to seven sacral vertebrae are fused into a synsacrum.[34] 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.[44] Anhanguera, another well-known genus, had a shorter tail, with broad caudal vertebrae that bore a "duplex" cross-section similar to Pteranodon.[34]

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.[45] Despite the structure length, the processes of these rod-like forms indicate that the hindlimb muscles attached to them were limited in strength.[46] 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.[47][45]

Below is a cladogram showing the results of a phylogenetic analysis presented by Longrich et al. (2018). In the analysis, they placed the genus Hongshanopterus as a basal member, and also found Guidraco as the sister taxon of the more derived Ludodactylus, meaning that Guidraco is placed in a more derived position, which contradicts the earlier analysis by Andres et al. in 2014. Longrich et al. also included the genus Siroccopteryx in their analysis, which is the sister taxon to Coloborhynchus.[50]

Life restoration of a Ludodactylus in flight

In 2019, several new species of ornithocheiromorphs were found, and the former species Ornithocheirus wiedenrothi was renamed as Targaryendraco wiedenrothi.[39] The description of Iberodactylus in Spain also made some paleontologists reclassify the genus Hamipterus in a newly named family called Hamipteridae.[51] The ornithocheirid Cimoliopterus was also reclassified as well, and it is currently grouped with Aetodactylus and Camposipterus in the clade Targaryendraconia, specifically to its own family, the Cimoliopteridae.[39] However, an analysis by Jacobs et al. (2019) recovers both Camposipterus and Cimoliopterus within the Ornithocheiridae again, using a new data matrix not including Targaryendraco.[52] The previously recovered basal eupterodactyloid Haopterus was reclassified due to the description of Mimodactylus, and is placed in a new family called Mimodactylidae.[53] However, a more recent analysis using the data in the description of Mimodactylus has found Haopterus as a basal member of the more inclusive group Istiodactyliformes.[54] Many recent analyses have also recovered several ornithocheirids, including Tropeognathus, Coloborhynchus, and Caulkicephalus within the family Anhangueridae, meaning that they were more closely related to Anhanguera than to Ornithocheirus.[51][39][54][53][32]

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

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

Ornithocheiromorphs were originally regarded as piscivorous creatures, feeding mainly on small and mid-sized fish.[36] Some paleontologists even suggested details on how these pterosaurs caught fish, some of which included dipping their beaks close to the water for prey.[36] 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.[16] 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.[55] An analysis by 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.[29]

Another ornithocheiromorph that possessed 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,[56] though this conclusion isn't considered entirely accurate.[57]

Anhanguerids 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.[36] Another difference that can be seen in more primitive ornithocheiromorphs their eyes being proportionally smaller compared to the assumed predatory and more advanced anhanguerids, this again adds to the fact that the more primitive groups were most likely scavengers, and later got more successful within the food chain, leading the later, more advanced groups to be the dominant fish hunters during the early Late Cretaceous.[58]

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.[59] 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, traveling very long distances without flapping while at the same time flying close the surface of the water with exploited wind speed, and without the necessity of thermals.[60] 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.[29][61] It is also possible that ornithocheiromorphs ran (but not walked) bipedally, or that they used a hopping gait.[59] 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.[62][16]