Pteranodon
While the first "Pteranodon" wing bones were collected by Marsh and Cope in the early 1870s, the first "Pteranodon" skull was found on May 2, 1876, along the Smoky Hill River in Wallace County (now Logan County), Kansas, USA, by Samuel Wendell Williston, a fossil collector working for Marsh. A second, smaller skull soon was discovered as well. These skulls showed that the North American pterosaurs were different from any European species, in that they lacked teeth and had bony crests on their skulls. Marsh recognized this major difference, describing the specimens as "distinguished from all previously known genera of the order Pterosauria by the entire absence of teeth." Marsh recognized that this characteristic warranted a new genus, and he coined the name "Pteranodon" ("wing without tooth") in 1876. Marsh reclassified all the previously named North American species from "Pterodactylus" to "Pteranodon". He considered the smaller skull to belong to "Pteranodon occidentalis", based on its size. Marsh classified the larger skull, YPM 1117, in the new species "Pteranodon longiceps", which he thought to be a medium-sized species in between the small "P. occidentalis" and the large "P. ingens". Marsh also named several additional species: Pteranodon comptus and "Pteranodon nanus" were named for fragmentary skeletons of small individuals, while "Pteranodon gracilis" was based on a wing bone that he mistook for a pelvic bone. He soon realized his mistake, and re-classified that specimen again into a separate genus, which he named "Nyctosaurus". "P, nanus" was also later recognized as a "Nyctosaurus" specimen.
Istiodactylus
In 1887, the British palaeontologist Harry G. Seeley described a fossil synsacrum (fused vertebrae attached to the pelvis) from 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. The missing jaws 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 1913, the British amateur palaeontologist Reginald W. Hooley described two more specimens of "O. latidens", collected from the sea after a rock fall near Atherfield Point on the Isle of Wight in 1904. 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.
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.