The evolution of mammalian traits took place bit by bit, over millions of years. So how do scientists decide which fossils are really mammals and which aren’t? By their features (teeth shape, ear bones, skeletal arrangement, etc.), and by their ancestry (their relationship to other animals in the tree of life.)
Take a look at just a few of the many marvelous creatures that form early branches on the mammal family tree!
Although it lived before mammals, Cynognathus had already acquired many—but not all—of the features that also occur in true mammals. For example, it may have been warm-blooded and have had whiskers and fur.
It also had carnivorous cheek teeth that sliced past each other to suggest chewing, an adaptation found almost exclusively in later mammals. And it had a secondary palate—the bony roof of the mouth that allows mammals to keep breathing while they eat or drink.
Ambulocetus was a “walking whale” that lived nearly 50 million years ago. This semi-aquatic whale shows some early stages of the transition from land-dwelling ancestors to today’s familiar, fully-marine whales.
If you compare your own arm with the flipper of a whale, at first glance, they look very different. But beneath the skin, the arrangement of the bones is startlingly similar. That’s because all mammals evolved from a common ancestor—an animal with four limbs, or a tetrapod—and retain the same basic skeletal pattern.
© AMNH/D. Finnin
We may think of mammals as quite large, but surprisingly, throughout history the average mammal weighed only about one-quarter pound. So while rats and mice may seem small compared to us, it’s we humans who are extremely large for a mammal.
The shrew-like 1.5-inch (3.8-cm) Batodonoides vanhouteni is the smallest land mammal that ever lived. It was so tiny that it could have perched on a pencil. With an estimated body weight of just 1.3 grams—less than a twentieth of an ounce—it weighed only about as much as a dollar bill.
© AMNH/D. Finnin
Generation after generation, individuals with an advantageous trait, or adaptation, will survive longer and produce more offspring, until most or even all members of the species possess it. This evolutionary process, called natural selection, can sometimes result in “extreme” traits such as odd headgear.
For example, it’s unusual for mammals to have an odd number of horns, but this extinct giraffe relative had five “ossicones,“ a type of bony knob unique to the giraffe family.
One of the oddest-looking mammals that ever lived is the extinct South American ungulate Macrauchenia. Along with a camel-like body and a giraffe-like neck, it had one of the most extreme of noses: a long, flexible trunk, similar to that of an elephant.
Macrauchenia is a prime example of convergent evolution: when completely different species living far apart evolve similar adaptations or traits because they live in similar environments.
Most mammals live on land, and some live in the sea. But only a rare few can glide through the air. And only one group—the bats—can truly fly. Bats are an extreme success story: among more than 5,400 living species of mammals, more than 1,100 are bats.
The earliest bat species found so far, Onychonycterys finneyi, reveals important clues about the evolution of flight in mammals: it had long fingers like modern bats but shorter forelimbs and longer hind limbs, a ratio that suggests a transitional stage.