Final answer:
During the Triassic Period, mammals were small, nocturnal animals that evolved from therapsids like cynodonts, featuring transitional traits between reptiles and modern mammals. The mass extinction at the end of the Triassic allowed for the rise of dinosaurs, and much later, the end of the Cretaceous led to the dominance of mammals following the extinction of dinosaurs.
Step-by-step explanation:
Mammals During the Triassic Period
During the Triassic Period, mammals were primarily small, nocturnal creatures likely with features distinct from those of the dominant reptiles of the time. Mammals evolved from a group known as the therapsids, which had begun to exhibit mammalian traits such as differentiated teeth, legs positioned under the body, and some degree of endothermy. Cynodonts, a subgroup of therapsids, showcased many of these evolving features, although they were not considered true mammals yet. A significant transition occurred with the morganucodonts, which had 'transitional' jaws combining reptilian and mammalian characteristics and were diphyodonts—meaning they had two successive sets of teeth.
Moreover, the mass extinction at the end of the Triassic Period gave rise to the flourishing of dinosaurs in the Jurassic Period, where mammalian evolution continued albeit with a still relatively small size. The survival and evolution of these mammal-like creatures during the Triassic were possibly aided by their nocturnal and arboreal habits, which helped them avoid competition and predation from the dominant reptilian fauna. As the Triassic Period ended and the Jurassic Period began, these animals would give rise to the major groups of mammals observed in the fossil record.
It is also noteworthy that another mass extinction at the end of the Cretaceous Period, approximately 65 million years ago, led to the extinction of the dinosaurs, paving the way for mammals to become the dominant land animals in subsequent periods.