Genus of istiodactylid pterosaur from the Early Cretaceous
Istiodactylus is a genus of pterosaur that lived during the Early Cretaceous period, about 120 million years ago. The first fossil was discovered on the English Isle of Wight in 1887, and in 1901 became the holotype specimen of a new species, "O. latidens" (Latin for "wide tooth"), in the genus "Ornithodesmus". This species was moved to its own genus, "Istiodactylus", in 2001; this name is Greek for "sail finger". More specimens were described in 1913, and "Istiodactylus" was the only pterosaur known from three-dimensionally preserved fossils for much of the 20th century. In 2006, a species from China, "I. sinensis", was assigned to "Istiodactylus", but it has also been suggested to belong to a different genus.
"Istiodactylus" was a large pterosaur; estimates of its wingspan range from . Its skull was about long, and was relatively short and broad for a pterosaur. The front of the snout was low and blunt, and bore a semicircle of 48 teeth. The triangular teeth were closely spaced, interlocked, and formed a "razor-edged" outline. The lower jaw also had a tooth-like projection that occluded with the teeth. The skull had a very large naso-antorbital opening (which combined the antorbital fenestra and the opening for the bony nostril) and a slender eye socket. Some of the vertebrae were fused into a notarium, to which the shoulder blades connected. It had very large forelimbs, with a wing-membrane distended by a long wing-finger, but the hindlimbs were very short.
Until the 21st century, "Istiodactylus" was the only known pterosaur of its kind, and was placed in its own family, Istiodactylidae, within the group Ornithocheiroidea. "Istiodactylus" differed from other istiodactylids in having a proportionally shorter skull. The distinctive teeth of "Istiodactylus" indicate that it was a scavenger that may have used its teeth to sever morsels from large carcasses in the manner of a cookie cutter. The wings of "Istiodactylus" may have been adapted for soaring, which would have helped it find carcasses before terrestrial carnivores. "Istiodactylus" is known from the Wessex Formation and the younger Vectis Formation, which represent river and coastal environments that were shared with various pterosaurs, dinosaurs, and other animals.
History of discovery.
In 1887, the British palaeontologist Harry G. Seeley described a fossil synsacrum (fused vertebrae attached to the pelvis) from the Wessex Formation of the Isle of Wight, an island off the coast of southern England. Though he compared it with those of dinosaurs and pterosaurs, he concluded that it belonged to a bird (though more dinosaur-like than any known bird), which he named "Ornithodesmus cluniculus". The British geologist John W. Hulke suggested later that year that "Ornithodesmus" was a pterosaur, finding it similar to fossils that he had seen before, but Seeley disagreed. 
In his 1901 "Dragons of the Air", the first popular book about pterosaurs, Seeley reported another specimen (NHMUK R176 at the Natural History Museum in London, formerly BMNH), found by the Reverend William Fox in Atherfield on the Isle of Wight, and acquired by the British Museum in 1882. Specimen NHMUK R176 had been assigned to the species "Ornithocheirus nobilis" by the English naturalist Richard Lydekker in 1888, but Seeley considered it another species of the genus "Ornithodesmus", which he now considered pterosaurian. Seeley named the new species "O. latidens"; "latus" is Latin for "wide" and "dens" means "tooth", a name originally used by Fox and his friends. Seeley presumably assigned the new species to the existing genus due to similarities between their sacra, but with little explanation.
Specimen NHMUK R176 is a poorly preserved skeleton consisting of the back of the skull, a neck vertebra, the sternum, the sacrum, the right humerus, the notarium, the left humerus, part of the radius and ulna, carpals, metacarpals, and wing phalanx bones. Though Seeley did not designate a holotype specimen for "O. latidens", he described and illustrated parts of NHMUK R176, which makes the 1901 naming valid according to the International Commission on Zoological Nomenclature, and the specimen is considered the holotype today. Later researchers have been mystified by the fact that Seeley described the jaws and teeth of "O. latidens" and named it after the latter, when the only specimen available in 1901, NHMUK R176, does not appear to have had these elements. Only the back of the skull was listed as present by Lydekker in 1888, but it was rumoured that a set of jaws had been lost from Fox's collection, so it is possible that Seeley had examined them prior to this.
In 1913, the British amateur palaeontologist Reginald W. Hooley described two more specimens of "O. latidens", collected from the sea after a rockfall near Atherfield Point on the Isle of Wight in 1904, originating from the Vectis Formation. The first of these, NHMUK R3877, was collected as three blocks and consists of a skull, neck and trunk vertebrae, a shoulder blade, an ischium, and parts of the forelimbs. The second specimen, NHMUK R3878, was collected in one block, and includes parts of the pectoral girdle and forelimbs. These specimens represent the most complete remains of Cretaceous pterosaurs found in England, and NHMUK R3877 was one of the only known three-dimensionally preserved pterosaur skeletons for much of the 20th century (pterosaur bones are often flattened compression fossils). Hooley discussed "O. latidens" in detail, and placed the genus "Ornithodesmus" in its own family, Ornithodesmidae. 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. 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.
New genus and assigned species.
In 1993, the British palaeontologists Stafford C. Howse and Andrew C. Milner concluded that the holotype sacrum and only specimen of "O. cluniculus" did not belong to a pterosaur, but to a maniraptoran dinosaur (this conclusion had also been reached independently by the British palaeontologist Christopher Bennett). They 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", with one writer even treating the original species as a synonym of the newer. As a definite species of pterosaurs, "O." "latidens" therefore required a new genus name. In 2001, Howse, Milner, and David Martill moved "O." "latidens" to the new genus "Istiodactylus"; the name is derived from Greek "istion", "sail" and "daktylos", "finger", referring to the wings of large pterosaurs. They also named the new family Istiodactylidae, with "Istiodactylus" as the sole member.
Additional "Istiodactylus" specimens were later found on the Isle of Wight, including IWCMS 2003.40, a dentary fragment that may belong to a juvenile, and isolated teeth found through screen washing from 2002 and onwards. During the early 21st century, new types of istiodactylids were reported from China. In 2006, Brian Andres and Ji Qiang named a second species of "Istiodactylus", "I. sinensis", from the Jiufotang Formation of China (from Greek "sino", pertaining to China), based on a partial skeleton. They found it very similar to "I. latidens", though much smaller, with a wingspan of , and more teeth. In 2006, Lü Junchang and colleagues concluded that "I. sinensis" was a junior synonym of the istiodactylid "Nurhachius ignaciobritoi" from the same formation. In 2008, Lü and colleagues instead found "Longchengpterus zhaoi" to be the sister species of "I. sinensis", and suggested that these two may belong to the same species.
In 2012, the British palaeontologist Mark P. Witton reported the "rediscovery" of a jaw-piece that belonged to specimen NHMUK R3877, found while he was visiting the museum to photograph the skull. The piece had been neglected in a drawer for a century, perhaps removed shortly after Hooley's 1913 article, and had apparently not been fully prepared until decades later. Though Hooley included an illustration of the piece, it was not incorporated into his reconstruction of the skull, and the skull length he proposed had consequently been repeated by other researchers. Witton attempted to fit the skull together to see if the rediscovered piece would be a bridging element, and sought confirmation from other pterosaur researchers at the museum. He concluded that the jaw-piece represented almost the entire missing portion of the skull (which might only be missing millimetres), and suggested that the skull would have been much shorter than previously assumed, making "I. latidens" very distinct from other istiodactylids.
Witton's updated skull reconstruction was different enough from the skull of "I. sinensis" for him to conclude they did not belong in the same genus, but he advised against placing "I. sinensis" in its own genus. Witton pointed out that the two "Istiodactylus" species had been grouped with "Liaoxipterus brachyognathus" in previous studies, and that "I. sinensis" may instead have been a species in the genus "Liaoxipterus", or even the same species, since they were found in the same formation and had few differences. He provisionally retained the existing taxonomy, awaiting further investigation of the issue. Witton stated that "Istiodactylus" specimen NHMUK R3877 remains the most well-preserved istiodactylid skeleton, and that some details of the group's anatomy are known only from it. In 2022, the Chinese palaeontologist Yizhi Xu and colleagues stated that the difference between "Liaoxipterus" and both species of "Istiodactylus" was limited, and that their interrelationship warranted further study.
Howse and colleagues suggested in 2001 that the now missing holotype jaws of "I. latidens" may be specimen CAMMZ T706, which was recognised in 1982 by the English palaeontologist Jenny A. Clack, but had no documented history prior to the mid-1960s. In 2021, the Russian palaeontologist Alexander O. Averianov and colleagues suggested that the front ends of an istiodactylid snout and mandible at the Vernadsky State Geological Museum in Moscow, specimen SGM 1810–01, could also be the missing holotype, since analysis of pollen from its matrix showed that it likely came from the Vectis Formation, where "I. latidens" has been found. It may have entered the Russian collection when a curator of this museum, the Russian geologist Alexey Pavlov and his wife the Ukrainian paleontologist Maria V. Pavlova, visited England in 1888 for the Geological Congress, and again in 1891, possibly acquiring the specimen then. Averianov and colleagues performed CT scans on the specimen, revealing its anatomical details.
Description.
"Istiodactylus" was quite a large pterosaur, with estimates of its wingspan ranging from . This makes it the largest known member of its family, Istiodactylidae. Some isolated pterosaur wing-bone fragments that may belong to this genus indicate a wingspan as large as . The most completely known skull is fragmentary but is thought to have been about in length, based on a long-lost fragment of its jaw reported in 2012. Before this, the skull had been estimated to be long. At , the jaws were less than 80 percent of the skull's length, which is short for a pterodactyloid pterosaur. As a pterosaur, "Istiodactylus" would have been covered in hair-like pycnofibres, and had extensive wing-membranes, which were distended by a long wing-finger.
The skull of "Istiodactylus" was relatively short and broad compared to most other pterosaurs, and it had a short, low snout-region in front of the nostrils. Much of the skull was occupied by very large naso-antorbital fenestrae (openings which combine the antorbital fenestra and the bony nostril). Unusually, this opening extended past the jaw joint and the back of the mandible. The orbit (eye socket) was reclined and slender, and was capped at the front by a tuberosity. The hind part of the skull was relatively tall, and the skull table bore a low crest or ridge at the front. The snout identified in 2021 revealed that "Istiodactylus" had a palatal ridge as in other pterosaurs, a feature previously unknown in this genus. The mandibular symphysis (where the two halves of the lower jaw connected) was abbreviated, and the mandible was deepest where the mandibular rami (halves of the lower jaw) diverged. The maxilla of the upper jaw was very slender, and only deep. The tip of the beak