Why is the evolution from reptiles to mammals described as mosaic evolution?

The evolution from reptiles to mammals is described as mosaic evolution because the distinct characteristics separating these two groups did not emerge simultaneously. Instead, these traits evolved at different rates and in response to various interrelated environmental and physiological conditions.1

Throughout the transition period, this process resulted in intermediate forms that possessed a unique combination of both reptilian and mammalian features. Because these attributes—such as specialized dentition, endothermy, and structural changes to the jaw and ear—developed independently and at different times, the resulting evolutionary pattern is termed "mosaic".1

This phenomenon is common when major new adaptive types originate, making it difficult to draw a sharp line between ancestral and descendant groups. Consequently, scientists often rely on specific osteological markers, such as the articulation of the jaw, to classify fossil species, even though the full suite of mammalian characteristics likely emerged gradually over millions of years.1