Which well-known group is ancestral to all living birds? What evidence in the fossil record supports this conclusion?

The well-known group ancestral to all living birds is called the "Theropod dinosaurs."

Theropod dinosaurs were a diverse group of bipedal, primarily carnivorous dinosaurs that included many different shapes and sizes, ranging from small bird-like forms, such as the Velociraptors, to large apex predators like the Tyrannosaurus rex. They had hollow bones, a bipedal stance with their limbs held underneath the body, and many shared features with modern birds, such as a furcula (wishbone) and a three-toed configuration in their feet.

The key evidence supporting the conclusion that Theropod dinosaurs are ancestral to all living birds comes from various fossils found around the world, displaying a transition from these dinosaurs to birds. Some of these major findings include: a) Archaeopteryx: Discovered in Germany, this is arguably the most famous transitional fossil bridging the gap between dinosaurs and birds. It had feathers, a wishbone, and wings, but also retained some dinosaur features such as teeth, a long bony tail, and a hand structure with distinct fingers. b) Microraptor: A small, four-winged dinosaur discovered in China, Microraptor had feathered wings on both its forelimbs and hindlimbs. The fossil shows that flight likely developed gradually in small theropods before the appearance of more advanced birds. c) Sinosauropteryx: This dinosaur, discovered in China, provided the first direct evidence of feathers in theropod dinosaurs. The feathers were not used for flight but most likely served as insulation or for display purposes. d) Yixian Formation: Fossils from this region in China have provided a wealth of new species that further demonstrate the dinosaur-to-bird transition. Fossilized specimens found here include numerous small, feathered theropods like Anchiornis, Xiaotingia, and Aurornis. These fossil discoveries, along with many others, provide strong evidence that Theropod dinosaurs are indeed the well-known ancestral group to all living birds.