The class Mammalia is so big and diverse that it is hard to get a grasp of how it evolved. From elephants to shrews, from anteaters to whales, the diversity of form is mind boggling. However if one looks at the orders of mammals, the evolution of the group becomes clearer. We still have a lot to learn but the basic picture is now fairly well understood.
Mammals evolved from Therapsid reptiles during the first age of dinosaurs (Triassic Period). These animals, the cynodonts, were rather dog-like in form and from their teeth were predators. They lurked around under the shadows of the dinosaurs but really didn't take off until the dinosaurs went extinct at the end of the Jurassic period. It is thought that the cynodonts were nocturnal in order to avoid their ectothermic dinosaur predators and this may have led to that great mammalian 'inventions' of hair and endothermy. It may have been this that allowed them to survive the catastrophe that killed the dinosaurs. Whatever the reason, with the extinction of the major life forms of the Jurassic, the stage was set for the mammals to diversify and evolve into all the modern forms.
The most primitive living mammals are the echidnas and platypus of the order Monotremata. These animals have specialisations for their life styles but still show their reptilian roots in several ways: they lay eggs, their body temperatures are quite low, their milk glands are still primitive, without teats, and they have a cloaca rather than separate openings for wastes and reproduction. Nonetheless, they are clearly mammals, with fur, milk production and with control of their internal body temperature even if it is low. The egg stage is quite short and after that, the young are raised by their parents in a very mammalian way.
The Order Marsupialia were once thought to be the next evolutionary step in mammals, with a pouch instead of a placenta. However embryological studies show that some marsupials do have a simple, primitive placenta for the foetus, but this does not develop and instead the young are born at a very undeveloped state and finish their development in the pouch. So Marsupials are probably an offshoot of the main line of mammals. The earliest marsupial fossils date back about 125 million years. Marsupials occur in South America but their main area of influence was Australia where they radiated out to inhabit many different niches: there are marsupial carnivores, herbivores, insectivores and scavengers, tree climbers, ground dwellers, hoppers, runners and even gliders. They inhabit all ecosystems and niches within those ecosystems and show interesting examples of convergent evolution with placentals, such as the remarkable similarity between marsupial and placental moles.
The most primitive living group of placental mammals is the order Insectivora and these animals, the shrews, are the closest living descendents to the original, insectivorous, mammalian ancestors. Insectivore families include moles and hedgehogs as well as over two hundred species of shrews. They have simple teeth, basic five fingered limbs and other primitive features, as well as more advanced features to suit their modern lifestyles, such as spines in hedgehogs.
From the shrews came several distinct lines that filled very different ecological niches. The first is the order Carnivora, which contains the dog, cat, bear, raccoon, hyena, civet and weasel families. These animals share the characteristics of paws with claws and specialised flesh-tearing teeth. One group of Carnivores went back to the seas and became the Order Pinnipedia ("flipper feet"). These animals include the seals, sea lions and walruses. Separately, a group of weasels went back to the water to become the otters and one went on to become the sea otter. All share a carnivorous lifestyle.
Another order that descended directly from Insectivores is our Order, the Primates. Primates are specialised descendants of insectivores. The primates developed a forest dwelling lifestyle that led to the development of grasping hands and arms, binocular vision, and hind limbs adapted for climbing as well as walking. Most primates are omnivorous, eating a variety of plant and animal foods. The families of primates include apes and monkeys, the more primitive lemurs, plus tree shrews and flying lemurs and perhaps even flying foxes. Most are forest animals, but our line left the shrinking forests for the grasslands during the great Miocene drought, stood upright, grasped tools, tamed fire and the world hasn't been the same since.
Bats, order Chiroptera, are the descendants of Order Insectivora that specialised in flight in order to keep up with their main food source, the insects. Some taxonomists now think the Chiroptera are actually polyphyletic, with the microbats being descended from insectivores while the larger, more vegetarian Flying Foxes may actually be descended from Primates that took to the air much later.
For the predators to become so numerous and diverse, there had to be a lot of herbivores. The most successful order of herbivores is the Rodentia, which includes the rats and mice, squirrels and gophers, beavers, porcupines, capybara, chinchillas and mole-rats. They are still relatively primitive although their teeth show adaptations for eating vegetation. A closely related group is the Order Lagomorpha, the rabbits.
Just as dinosaurs started out with small herbivores and then went big, so did the mammals. The largest group of highly evolved herbivores is the Order Artiodactyla, the even-toed ruminants. They have specialised teeth, guts and legs. They have gone from the original mammalian five toes to either four or two, hence the name even-toed. This huge diverse Order contains the families of the deer, antelope, cattle, sheep, goats, pigs,camels, hippos and giraffes. A second line, the Odd-toed ungulates or Order Perissodactyla, has been less successful but still important and includes the horses, asses and zebras plus tapirs and rhinos.
These advanced herbivorous ungulates evolved from more primitive ungulates that are represented today by the hyraxes, aardvaarks and elephants. These are the living descendants of a group called the Condylarthra, which appear about sixty million years ago in the fossil record and are the ancestors of the Artiodactyls and Perissodactyls. These more specialised groups have lost their original five toes but the elephants and the hyraxes still have this characteristic along with other primitive characteristics of the first large mammalian herbivores.
One offshoot of the Artiodactyls, probably from the hippo line, became the modern Order Cetacea, the whales and dolphins. Superficially resembling fish because they live in the same watery environment, a look at their skeletons reveals their mammalian nature as much as their warm blood, placentas and milk glands. Under their flippers, whales and dolphins retain their five fingered forelimbs and the remains of their lost hind limbs can be seen in the skeletons of the more primitive species.
There are some other small orders of Mammals left, such as the Edentates: anteaters, sloths, armadillos and pangolins. They are similar in strucutre due to similar life styles but are probably polyphyletic in origin so they are placed in two separate orders: Pholidota for the pangolins and Edentata for the rest. They diverged from the main mammalian line about 60 million years ago and have retained many primitive characteristics.
Modern mammals represent only a small proportion of all the mammalian species that have lived over the last 60 million years. Many became extinct as conditions changed. Unfortunately, unless we change our ways, we will be the cause of the extinction of many more. Already 30 species of Australian marsupials have joined the ranks of the extinct. It is to be hoped that we can learn from our mistakes and stop this trend before we lose more of these amazing, beautiful, unique animals.
Reference: Macdonald, D. 1989 The Encyclopedia of Mammals