Jurassic Period: The Age of Dinosaurs
The Jurassic Period, which lasted from approximately 201 to 145 million years ago, is well-known as the Age of Dinosaurs. It is during this period that reptiles ruled the Earth, and iconic dinosaurs like the Tyrannosaurus rex, Stegosaurus, and Brachiosaurus roamed the land. However, amidst the dominance of these reptiles, the first birds and mammals also began to emerge.

Evolution of Birds: From Reptiles to Avians
1. Archaeopteryx: The earliest known bird, Archaeopteryx, lived during the late Jurassic Period. It possessed a mix of reptilian and avian features, such as feathers, wings, and a beak, but also had teeth, claws on its wings, and a long bony tail. This transitional species provides evidence for the evolution of birds from small, carnivorous dinosaurs.

2. Feather Adaptations: Over time, feathers evolved to become more efficient for flight. They developed a streamlined shape, lightweight structure, and better insulation capabilities. These adaptations allowed birds to take to the skies and exploit new ecological niches.

3. Avian Characteristics: Birds gradually acquired unique avian characteristics, such as a keeled sternum for flight muscle attachment, a beak for feeding, and modifications in their respiratory system to support high-energy flight.

Evolution of Mammals: From Synapsids to Placental Mammals
1. Synapsids: Mammals belong to a group of animals called synapsids, which includes modern mammals as well as their extinct relatives. Synapsids first appeared in the late Carboniferous Period and evolved alongside reptiles.

2. Therapsids: During the Permian Period, a sub-group of synapsids called therapsids emerged. These mammal-like reptiles possessed some mammalian characteristics, such as differentiated teeth and a more upright posture.

3. Early Mammals: By the Triassic Period, true mammals had evolved. They exhibited distinct mammalian features, including fur or hair, specialized teeth for different diets, and mammary glands for nourishing their young.

4. Rise of Placental Mammals: The diversification and dominance of placental mammals, the group to which humans belong, occurred during the Cenozoic Era, specifically in the Eocene and Oligocene periods. Placental mammals are characterized by their reproductive adaptation of a placenta, which allows for more efficient nourishment of developing embryos.

Therefore, the evolution of birds and mammals occurred during the Jurassic Period (for birds) and continued into the Eocene and Oligocene periods (for placental mammals).