All modern birds lie within the crown group Aves (alternately Neornithes), which has two subdivisions: the Palaeognathae , which includes the flightless ratites (such as the ostriches ) and the weak-flying tinamous , and the extremely diverse Neognathae , containing all other birds.  These two subdivisions are often given the rank of superorder ,  although Livezey and Zusi assigned them "cohort" rank.  Depending on the taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800  to 10,050. 
The Mesozoic world was stranger than we ever imagined. It really was the Weird Feathered Thing planet. The biggest carnivores, with their nine-inch teeth, were more like roadrunners than lizards. The skies were full of pterosaurs, which were not dinosaurs, birds, or reptiles, in any sense that we understand the word reptile. Their wings were covered with fur or fuzzy feathers and they ranged from the size of a hummingbird to the wingspan, if not the weight, of a 747. Mark Witton’s book Pterosaurs and his blog show how some had crests like radio antennae, some walked around and fed like storks, and some had bold stripes on their impossible headdresses. Several modern forms of birds had already emerged—possibly, species not unlike chickens, ducks, parrots, and loons flew and swam around, surrounded by all those Weird Feathered Things. Only the mammals persisted like living fossils, remnants of a vanished age before the dinosaurs, furry and low-slung. As far as we know, no mammals were even as big as a German shepherd yet.
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