All of this made it difficult to know where to start. Because of gaps in the fossil record, paleontologists don't know what creatures were the ancestors of pterosaurs.

Hyracotherium is an extinct species of a very small horse-like ungulate which lived approximately 55 to 45 million years ago – from the Early Eocene Period through the Middle Eocene Period. Hyracotherium (/ ˌ h aɪ r ə k oʊ ˈ θ ɪər i ə m,-k ə-/ HY-rək-o-THEER-ee-əm; "hyrax-like beast") is an extinct genus of very small (about 60 cm in length) perissodactyl ungulates that was found in the London Clay formation. The enormity of the task convinced me to put it off for a few months, but finally I began, searching through "Fossil Horses" ( MacFadden, 1992 ). The first pterosaurs appear in the fossil record around 210 million years ago. Hyracotherium was the first name given to this genus, based on specimens found in England.Later, based on specimens found in North America, it was mistakenly named again; this time as “Eohippus,” which means “dawn horse.”Eohippus is thus a secondary junior synonym of Hyracotherium, and in the international rules of naming of organisms, Hyracotherium has priority because it was named first. It was first discovered during the early 19th century and was given the name Hyracotherium – which means “hyrax-like beast” in Greek – by Richard Owen in 1841. Horse - Horse - Evolution of the horse: The evolutionary lineage of the horse is among the best-documented in all paleontology. As well, Hyracotherium is the 'generic' name for several species, just as Equus is the generic name for horses, zebras, and asses. The evolution of the horse, a mammal of the family Equidae, occurred over a geologic time scale of 50 million years, transforming the small, dog-sized, forest-dwelling Eohippus into the modern horse.
The history of the horse family, Equidae, began during the Eocene Epoch, which lasted from about 56 million to 33.9 million years ago. During the early Eocene there appeared the first ancestral horse, a hoofed, browsing mammal designated correctly as Hyracotherium … Paleozoologists have been able to piece together a more complete outline of the evolutionary lineage of the modern horse than of any other animal.