Pterosaurs - uncertain ancestry
The very first pterosaurs from the Triassic are true pterosaurs, fully capable of flight, and as such no intermediate animal, between a pterosaur and a possible ancestor, is currently known. This means that palaeontologists are still a little unsure as to the ancestry of pterosaurs. Currently most scientists think that pterosaurs belong to a group of reptiles known as archosaurs, which also includes crocodiles, dinosaurs and birds. Pterosaurs are probably closely related to dinosaurs, but they are not members of the group Dinosauria.
The pterosaur skeleton displays numerous modifications that enabled them to fly. Their skeletons were very lightly constructed and most of their bones were hollow and enclosed an air sac system connected to the lungs, as in birds. The bones of the arms were elongated, especially those of the hand and one of the fingers. The bone was composed of many microscopically thin layers stacked together like a spirally bound plywood tube.
Sometimes the bones had cross-sectional shapes that provided added strength, such as D, T and ? shapes. This combination of shape and cross lamination allowed the bones to be both light and strong, but where extra strength was required pterosaurs had thickened bone or internal cross-struts. The bones of the shoulder girdle were robust and an expanded sternum allowed for the attachment of powerful flight muscles.