Transitional fossils are important remnants of organisms that exhibit traits common to both ancestral and descendant groups. They provide evidence for the process of evolution by showing how species have changed over time. For example, the Archaeopteryx is a well-known transitional fossil that displays features of both dinosaurs and modern birds. These fossils help scientists understand the gradual changes that occur in species, supporting the theory of evolution. By studying transitional fossils, researchers can trace the lineage of various organisms and gain insights into how complex traits developed, illustrating the connections between different forms of life throughout Earth's history.