<P> The therapsids came under increasing pressure from the dinosaurs in the Jurassic; the mammals and the tritylodontids were the only survivors of the line by the end of the period . </P> <P> The main points to the transition from reptile to bird are the evolution from scales to feathers, the evolution of the beak (although independently evolved in other organisms), the hallofication of bones, development of flight, and warm - bloodedness . </P> <P> The evolution of birds is thought to have begun in the Jurassic Period, with the earliest birds derived from theropod dinosaurs . Birds are categorized as a biological class, Aves . The earliest known species in Aves is Archaeopteryx lithographica, from the Late Jurassic period . Modern phylogenetics place birds in the dinosaur clade Theropoda . According to the current consensus, Aves and Crocodilia are the sole living members of an unranked clade, the Archosauria . </P> <Table> <Tr> <Td> Coelurosaurs </Td> <Td> <P> Template: Clad </P> </Td> </Tr> <Tr> <Td> </Td> </Tr> </Table>

Which class is thought to have been descended from dinosaurs