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.
Stenonychosaurus
A more complete skeleton of "Stenonychosaurus" was described by Dale Russell in 1969 from the Dinosaur Park Formation, which eventually formed the scientific foundation for a famous life-sized sculpture of "Stenonychosaurus" accompanied by its fictional, humanoid descendant, the "dinosauroid". "Stenonychosaurus" became a well-known theropod in the 1980s, when the feet and braincase were described in more detail. Along with "Saurornithoides", it formed the family Saurornithoididae. Based on differences in tooth structure, and the extremely fragmentary nature of the original "Troodon formosus" specimens, saurornithoidids were thought to be close relatives while "Troodon" was considered a dubious possible relative of the family. Phil Currie, reviewing the pertinent specimens in 1987, showed that supposed differences in tooth and jaw structure among troodontids and saurornithoidids were based on age and position of the tooth in the jaw, rather than a difference in species. He reclassified "Stenonychosaurus inequalis" as well as "Polyodontosaurus grandis" and "Pectinodon bakkeri" as junior synonyms of "Troodon formosus". Currie also made Saurornithoididae a junior synonym of Troodontidae. In 1988, Gregory S. Paul went farther and included "Saurornithoides mongoliensis" in the genus "Troodon" as "T. mongoliensis", but this reclassification, along with many other unilateral synonymizations of well known genera, was not adopted by other researchers. Currie's classification of all North American troodontid material in the single species "Troodon formosus" became widely adopted by other paleontologists, and all of the specimens once called "Stenonychosaurus" were referred to as "Troodon" in the scientific literature through the early 21st century.