Teeth have played an important role in the evolution of mammals as they help determine the specific diet organism can have. Those who are born without teeth are often those who grow a single set of baby and eventually adult teeth. Being born without teeth proves advantageous as it allows organisms to nurse on the milk of their mother. By nursing, it allows for faster growth and higher metabolisms which enables activity in cold, darker environments.
The earliest forms of mammals experienced this as having teeth and jaw-closing muscles proved to be more efficient as opposed to the dangerous method of biting and swallowing foods. Cusps on molars as well as tribosphenic molars aided in this as the ability to grind and tear apart foods prevented choking. Those whose teeth best suited the available food sources in various environments had the most fitness and best chances of survival. This led to these traits to be selected for in future mammals.
Along with the variation in teeth and in turn diet, the article mainly attributes the rise of mammals to several environmental occurrences. Mammaliaform forerunners first originated at the start of the Triassic period, following a mass-extinction set off by a volcano which wiped out many giant amphibians and reptiles. The Triassic was eventually superseded by the Jurassic when Pangea split and ecosystems began to collapse. Certain mammaliaforms survived the event and prospered as they were able to inhabit niches others could not. However, dinosaurs still dominated during the Jurassic and Cretaceous periods until an asteroid triggered extreme, long-term climatic change. Dinosaurs were ill-suited for the change in environment, unlike a few species of early mammals which evolved into those we see today.
Evidence of evolution can be derived from fossil records of early mammals in the Jurassic period. One I found most interesting includes the Agilodocodon as it helped scientists see an early form of arboreal life. Each fossil found from the Jurassic period proved to be interesting as characteristics of each organism can be seen in organisms alive today. For example, the Castorocauda had a beaver-like tail, the Volaticotherium can be likened to a flying squirrel, and the Fruitafossor had teeth like those of an armadillo.