LATE TRIASSIC TO CRETACEOUS PERIOD: 220 - 65 million years ago
The first mammals emerged during the Mesozoic Period (248 to 65 million years ago) in the Late Triassic, 220 million years ago. These early mammals never gained a dominant presence and broad diversity until the Miocene Period, 15 million years ago. Of the Mesozoic Period, most people are familiar with the widely popular reptiles, namely dinosaurs. What is not commonly known is that during the age of the dinosaurs, small mammals were present scampering about the land no larger than your typical mouse or rat. It was not until the end of the dinosaurs 65 million years ago, that land mammals began to diversify and gain in size to immense proportions.
An interesting trait that early mammals developed is teeth with multiple cusps. This allowed for more efficient chewing, enabling the animal to extract higher nutritional benefit from its food. This in comparison to the simplistic peg, spike or knife-shaped teeth of the reptiles where food was swallowed whole. The precursors to mammals were mammal-like reptiles called Cynodonts. These hybrid type creatures had molars with three cusps in a row. The first true land mammals also had molars with three inline cusps and are called Triconodonts. Triconodonts are believed to have been carnivorous but it is possible they could have been insectivores. They ranged in size from a large cat to a small rodent and all died out at the end of the Cretaceous Period, 65 million years ago.
Another group of Mesozoic land mammals are the Multituberculates. These animals first emerged in the Jurassic, 160 million years ago and survived the mass extinction in the Cretaceous, surviving all the way up until the Oligocene Period, 35 million years ago. Their dentition resembled a rat with large premolars and long, multi-cusped chewing molars. The Multituberculates were also the first mammals to live in trees like the modern squirrel. In the Northern Hemisphere during the Late Cretaceous, more than half of typical land mammalian species were Multituberculates. This group of Mesozoic mammals were the most diverse in size ranging from a very small mouse to a beaver. It is believed they were omnivorous.
Other groups of early mammals include the Monotremes (egg-laying mammals that suckle their young), Metatherians which were marsupials, and the Eutherians also known as Placentals (presence of the placenta allows the mother to nourish the developing embryo in the womb without forcing a developmentally early birth). Monotremes and Eutherians first existed during the Cretaceous but are survived today. Metatherians also emerged in the Early Cretaceous but many died out in the Paleogene Period with only crown marsupials surviving to the present. Most mammals living today are Eutherians including humans. Monotremes on the other hand are rare, survived by few animals such as the Duck-billed Platypus and Spiny Anteater of Australia.
The end of the Cretaceous marks the end of the Mesozoic Era. Mammals survived this profound mass extinction where the dinosaurs could not. This new age is called the Cenozoic Era which begins after the Cretaceous, 65 million years ago. Into this age, mammalian diversity and size exploded. Cenozoic translates into "new life" and best describes the amazing new creatures that this era was about to bring.