Transitional fossils are those that demonstrate the evolutionary relationship between two groups of organisms. They demonstrate how one group of organisms evolved into another over time, providing evidence for the process of evolution. These fossils are known as "connecting links" because they help to connect the evolutionary histories of various groups of organisms.

Archaeopteryx, a reptile-like bird fossil, is considered a transitional fossil because it possesses characteristics of both reptiles and birds. This fossil discovery provided evidence for the evolution of birds from reptiles.

In this way, transitional fossils aid scientists in understanding how species evolved over time and the relationships between different groups of organisms.