The evolutionary history and systematics of the Carnivora are clouded in controversy, as the fossil record is patchy and incomplete. In spite of this limitation it is remarkable what paleontologists, evolutionary biologists and geneticists have managed to uncover in the way of the early history of mammals. A major breakthrough has been the development of accurate methods to date fossils.
About 65 million years ago (mya) the dinosaurs, which were the dominant animals on Earth, underwent a rapid and mass extinction. At this time the mammals were small shrew-like creatures. With the extinction of the dinosaurs many ecological vacancies, known as niches, opened up, including that of predator, and the mammals quickly filled many of them. The early mammalian predators were marsupials, mammals whose young develop in a pouch, the ancestor of which was a small, opossum-like creature with a pointed snout and large ears. These early marsupial carnivorous creatures soon evolved into all shapes and sizes and dominated the southern continents for 30 million years.
Meanwhile, placental mammals were evolving in the northern continents. Instead of their young developing in a pouch after being born, placental mammals grow their young inside them, in a womb. One of these placental mammals was a squirrel-sized creature called Cimolestes that lived on insects. A very important feature possessed by Cimolestes was a flattening of the cheek teeth providing the beginning's of a
scissor action. Over several millions of years these teeth became refined to slice meat in what became the carnassial shear. This feature was inherited by two separate groups of animals. One gave rise to the modern Carnivora, the other to a group known as the Creodonts. At first the Creodonts dominated as the earth's meat eaters. In the fossil record from 55 to 35 mya a number of cat-, dog-, bear- and hyena-like animals are found, some even with saber teeth, but none of these were true Carnivora. Then the fossil record shows a change; more Carnivora species are found and fewer and fewer Creodonts.
It is not known for sure why this replacement of Creodonts by Carnivora took place. The carnassial shear in the Carnivora was situated more to the front of the mouth than in the Cre-odonts. This meant that the teeth further back in the mouth could still be used for feeding on other foods, for example on vegetable matter. Perhaps the Carnivora could be more flexible in diet and therefore exploit more ecological niches, both meat eating and vegetable, than the Creodonts, who had no teeth behind their carnassial shear and so could only eat meat. Support for this idea comes from evidence of climatic change during the demise of the Creodonts. The earth became cooler and more seasonal. This may have led to a situation where prey became less available, but fruit crops and insects more abundant due to the seasonal bloom.
The early Carnivora, known as miacids, were small and rather unspectacular, many resembling the genets of today. The major division into dog- and cat-like Carnivora took place some 55 mya and all the modern carnivore families had evolved by 7 mya. Among the cat-like Carnivora were the saber-toothed cats that dominated the carnivore scene from 26 to 2 mya. As the Carnivora moved south, they out-competed the marsupial predators mentioned earlier. Today, only a handful of their descendants such as the Tasmanian devil and quoll survive in Australia. Perhaps the best known was the thylacine or Tasmanian wolf that was exterminated about 70 years ago by bounty hunters.
Traditionally Carnivora are divided on the basis of their anatomy and behavior into two suborders, terrestrial carnivores (Fissipedia) and marine carnivores (Pinnipedia). This subdivision is incorrect, for blood serum analyses have shown that the pinnipeds are closely related to bears and evolved from a single bear-like ancestor. Today most scientists involved in the field of carnivore classification recognize 10 families in the two major divisions; the cat-like and the dog-like Carnivora. The former are the Viverridae (civets and genets), Herpestidae (mongooses), Felidae (cats) and Hyaenidae (hyenas). The latter are the Ursidae (bears), Otariidae (eared seals—fur seals and sea lions—although the most recent classification puts the walrus in a separate family, the Odobenidae), Canidae (dogs), Procyonidae (a collection of mainly South American carnivores including the raccoons and coatis and a taxonomic group that is still surrounded by much controversy), Mustelidae (otters, badgers, skunks, weasels and polecats), and Phocidae (true seals—elephant seals, monk seals, leopard seals, etc.).
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