Event 18

220-200 million years ago

Thrinaxodon and pups
A mammal-like reptile, Thrinaxodon, and her pups. This carnivore lived about 245 million years ago during the Triassic. It combines apparent mammalian traits such as a fur coat and likely warm-bloodedness along with reptilian traits such as egg-laying and various features of its skeleton.
Credit: Illustration by John Sibbick, Courtesy of John Sibbick Illustration. This image is used with permission. All rights reserved.

Most people don’t realize it, but the class of animals that we belong to, the mammals, evolved from mammal-like reptiles at the same time as the emergence of early dinosaurs, around 200 million years ago. Mammals and dinosaurs therefore co-existed for some 135 million years before dinosaurs went extinct. Although small and insignificant at first (and probably nocturnal, or moving and feeding at night), they would later become a major player in the History of Life. These early mammals established the foundation of what would become the human condition.

Today’s mammals, as contrasted to reptiles, are characterized by being warm-blooded (being able to maintain a constant body temperature, making them more active in cooler weather), bearing their young live rather than laying eggs, nourishing their young by producing milk (with mammary glands, thus their name), having larger brains, having hair on their bodies, and having sweat glands in the skin, and each side of the lower jaw (mandible) composed of only one bone rather than several. Most mammals also show more parental care for their young than reptiles do.

Ceratosaurs and Apatosaurus
Early dinosaurs co-evolved with the early mammals. This shows the carnivorous dinosaur Ceratosaurus in the foreground eyeing a herd of Apatosaurus dinosaurs (formerly known as Brontosaurus) around 150 million years ago.
Credit: Illustration by John Sibbick, courtesy of John Sibbick Illustration. This illustration is used with permission. All rights reserved.

One important early fossil mammal form, Megazostrodon, has been found in southern Africa around 200 million years ago (Late Triassic). It was similar in size and form to modern shrews (about 4 inches in length), probably hunted insects and very small reptiles, and chewed them up with teeth that showed another hallmark of mammals: differentiated teeth for different functions. These teeth were the incisors, canines, premolars, and molars or cheek teeth; this basic pattern is still retained in our own species today. These early mammals also had a first set of deciduous teeth (“milk teeth”) that were shed as the animal grew, to be replaced by the permanent adult dentition. These early mammals were probably still egg-laying (like the modern primitive mammals, the platypus and the spiny anteater of Australia) and not yet milk-producing, but nonetheless show many of the anatomical features in their skeletons, especially in their skulls, that denote their mammalian status.


The fossil record shows a detailed transition from reptiles to mammal-like reptiles to mammals between 300 million and 100 million years ago.


Although the early mammals were a relatively insignificant class of animals during the Age of Reptiles, they would ultimately come to inherit the earth with the extinction of the dinosaurs. The early mammals’ small size, warm-bloodedness, and ability to live underground probably sustained them through the catastrophic times that were to come. Mammals were able to diversify into an astonishing range of forms adapted to the ground, to the trees (like our early mammalian ancestors), to the air, to the rivers, lakes, and seas, and to tunneling in the earth. The anatomical pattern (e.g. differentiated teeth) and physiological pattern (warm-bloodedness, milk-producing) of these early mammals would be the foundations of the human condition. This foundation was established by about 200 million years ago.

Today there are 19 orders of mammals (including ours, the Primates) comprising some 4,000 living species around the world. All of these lines can be traced back to the earliest mammals.






This is a YouTube video, “Highway of Life: The Ancestors of All Mammals.”

An easy to understand survey of the evolution of mammals.

This is a basic information site on the differences between reptiles and mammals.


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