Mammal-like reptiles, also known as synapsids, are a group of ancient reptiles that share characteristics with modern mammals. They lived during the Permian and Triassic periods and are considered ancestors of mammals. These creatures had features such as differentiated teeth and a more advanced jaw structure, which allowed for varied diets. Unlike typical reptiles, mammal-like reptiles had a more upright posture and some even displayed early signs of fur. Their evolutionary adaptations helped them survive in diverse environments, paving the way for the rise of true mammals after the mass extinction event at the end of the Cretaceous period.