Final answer:
The oldest known bird is D. Archaeopteryx, which lived during the Jurassic period and is an important transitional fossil linking dinosaurs and birds.
Step-by-step explanation:
The oldest known bird is D. Archaeopteryx. This species is a well-known transition from dinosaurs to birds and lived during the Jurassic period. Archaeopteryx is considered an important fossil in understanding the evolution of birds from theropod dinosaurs.
As part of the archosaur clade, birds evolved from theropods, specifically from a group of bipedal predators. Archaeopteryx displayed a mixture of traits from both its dinosaur ancestors and modern birds. It possessed feathers similar to those used for flight in current avian species, a jaw with teeth, and a long bony tail, features indicative of its status as a transitional species.
The discovery of Archaeopteryx has been pivotal in the study of avian evolution, providing conclusive evidence that birds are highly modified theropods and, more broadly, part of the dinosaur lineage. This connection is further supported by numerous skeletons sharing characteristics in the hip and wrist bones, as well as the structure of the wishbone.