What it is
A dog, standing in for all mammals, you included, in the cave-mummy chapter, in a row beside a reptile and a bird.
Why it is there
A 289-million-year-old Captorhinus mummy from Oklahoma kept its rib cage, cartilage and all, built to breathe by moving the ribs. The scientists who studied it propose that this was the starting point for the rib-powered breathing of living reptiles, birds and mammals.