Pterosaurs and birds were both competing for space in the Mesozoic skies. However, the pterosaurs diverged as an order before the birds, and this was a clear advantage. If the number of currently defines species are plotted on a graph by geological age, an interesting trend is observed.
Pterosaurs emerged in the Late Triassic, becoming the dominant flying creatures of the Era. The birds are first seen in the Middle Jurassic, and start off as localised communities, as did the pterosaurs many millions of years earlier. As bird species increase throughout the Cretaceous, the pterosaur species are seen to peak in numbers and then fall into decline. By the end of the Cretaceous, pterosaurs were specialised as very large and wide ranging flying animals - most of the smaller pterosaur species having disappeared towards the end of the Cretaceous.
Birds clearly had an ecological advantage in most niche environments, eventually forcing pterosaurs into decline. By the time of the Mass Extinction Event at the end of the the Mesozoic, There were only a handful of pterosaur species left, and these were too highly specialised to survive as an order of animals. The changes in ambient temperature and atmospheric oxygen content that resulted from the meteor impact and ensuing lava flows would have been biologically catastrophic, making life impossible for the large pterosaurs.
