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Mammals The final group of Mesozoic reptiles, the synapsids, would not normally have attracted attention. They were small inconspicuous quadrupeds with only one claim to fame: they developed mammalian characteristics. One group, the theriodonts, became the ancestor of all mammals. As reptiles, the synapsids became extinct 170 million years ago.
About 225 million years ago, the theriodonts evolved into the pantotheres, the first monotremes. The first monotremes were small, insectivorous, shrew-like creatures about 6 inches long.
Monotremes are mammals, but barely so, and survive today only as the platypus and the echidna found in Australia and New Guinea. They have very poor internal temperature control, being only somewhat warmblood- ed, are the only mammals to produce venom, are the only mammals to lay eggs, and, though milk-producing, are the only mammals without teats the milk is secreted directly though the skin and lapped by the young).
About 200 million years ago, the pantotheres evolved into metatheres, the first marsupials. Unlike a monotreme, which lays eggs, a marsupial gives birth to live young. These young are very premature, and must crawl into a marsupium (pouch) where they attach themselves to teats and receive nourishment while they continue to develop towards self-sufficiency. The kangaroo and opossum, among others, are today's surviving marsupials. The first marsupials were not much different in appearance from their monotreme forebears, being shrew-like in appearance and about 6-8 inches long.
With marsupialism, a mother no longer had to provide all the early nourishment for her young in the yolk of an egg, but could nourish her young as she herself was nourished--sort of child-bearing on time payments. The young also had the advantage of being able to flee danger, via mom's legs, whereas an egg is easy prey.
Good as marsupialism is, it still exposes the young to the world when they are most vulnerable: a new-born marsupial is little more than an embryo, (a newborn opossum is about the size of a bee, a kangaroo a little over an inch long). This problem was corrected by the evolution of the metatheres into eutheres, the placentals, about 100-80 million years ago, in the northern hemisphere.
The placenta is a complex organ allowing nutrients in the mother's bloodstream to be passed to the fetus' bloodstream, with waste products passed in the reverse direction, while not allowing a direct connection between the bloodstreams. The placenta of a marsupial is very primitive and inefficient, hence the premature birth, whereas that of the placentals is a truly wondrous organ. The young could now remain within the mother's womb, receiving nourishment directly from her, until relatively well developed and more ready to face life.
The marsupials and placentals were both drastic improvements over the monotremes, and seemed to have divided the planet between them: for a while marsupials dominated the southern hemisphere while placentals dominated the northern. As the placentals grew more numerous they gradually forced out the less-efficient marsupials: Today, the only significant marsupials left worldwide are the opossums, which survive because they are so fecund.
The dominance of placentals is firmly established except in Australia and a few surrounding islands, which had broken from the Asian continent after the marsupials had dominated the south but before the placentals had spread down from the north. In pre-colonial Australia marsupials were to be found in all the mammalian ecological niches (there is even a marsupial "cat") except for the aborigines (who arrived by boat), the dingos (wild dogs, which arrived with the aborigines), the bats (which flew in), and the surviving monotremes (which defy logic all around). Modern man has introduced many other species of placental, most notably the rabbit and the mongoose, and the long-delayed marsupial/placental struggle is now taking place in Australia, with the marsupials losing.
Near Cats The Cretaceous Period and the Mesozoic era came to an abrupt halt with the Cretaceous Great Dying, 65 million years ago. Suddenly, the Earth finds itself with virtually all of its dominant species wiped out: no more dinosaurs, pterosaurs, or plesiosaurs [Nessie?], and very little of anything else. The Cenozoic era had arrived.
Of those few creatures which survived the Cretaceous Great Dying, one was a small, active, adaptable, shrew-like euthere, about 7-8 inches long, who then experienced rapid radial evolution. By 60 million years ago one of its many newly-evolved descendants was miacis, who ate flesh and was among the first truly carnivorous mammals.
Miacis was somewhat martin-like in appearance. His distinguishing characteristic was his teeth, which set the basis for all modern carnivores. He had a dental plan with incisors, canines, premolars, carnassials, and molars in each jaw. The carnassials were a new invention, being designed specifically for the cutting of flesh in a scissor-like action. Modern cats and dogs have carnassials, humans do not. These advanced teeth were fundamental in the demise of other predators, allowing him to make more kills and to better digest his prey, both of which meant more and larger miacids and fewer others.
Miacis was a short-term creature, quickly evolving under the pressure of competition into several different miacids, each of which went on to become a differing type of carnivore. By 45 million years ago, one of these differing creatures was profelis, the forerunner of all cats.
By 40 million years ago profelis had evolved into hoplophoneus and dinictis. The primary differences between hoplophoneus and dinictis were in jaw structure. In hoplophoneus the upper canines increased drastically in length to become stabbing weapons, with corresponding changes in the jaw hinge to allow the mouth to open extra widely. In dinictis the upper and lower canines became more balanced and the jaw hinge developed more muscle. Both were halfway between a cat and a civit in appearance, long in the body and tail, short in the legs; both had definitely cat-like heads; and both were plantigrade: modern cats are digitigrade and walk on their toes, good for running, while people are plantigrade and walk upon their whole foot, good for standing.
About 25 million years ago, hoplophoneus had evolved into smilodon, the famous saber-toothed tiger. Smilodon was definitely a cat in appearance, walking upon his toes and all, but had a somewhat flattened head with a small brain pan (he wasn't very bright). Smilodon was the end of his line, and vanished some 12,000 years ago. The exaggerated tooth structure of the hoplophoneans and especially smilodon was a response to the evolution of the titanotheres, the giant mammals of the early Cenozoic. These animals were huge, with correspondingly thick and/or shaggy coats, which the dagger-like canines of the saber-toothed tiger could pierce to deliver a killing blow. The largest of the titanotheres, and the largest land mammal ever, was the ground sloth baluchitherium, which stood 18 feet at the shoulder (the height of a tall giraffe), and whose head reached 26 feet off the ground.
Real Cats While hoplophoneus was evolving into smilodon, dinictis was also evolving. Dinictis itself had one seemingly trivial, but really very fundamental characteristic: it had three eyelids. Modern cats, and many related species, have three eyelids, the third being the haw, or nictitating membrane.
Dinictis evolved into pseudailurus, which was definitely a cat in appearance, not too different from some of the more extreme species of modern cats. Its teeth were identical in structure to those of the modern cat and it was digitigrade, walking on its toes (though not quite as well as the modern cat), but it still had a small brain pan.
Some 18 million years ago, the oldest of the modern genera of cats evolved from pseudailurus: acinonyx. The modern cheetah is the only species of acinonyx surviving today and is actually little changed from its early ancestors. Some 12 million years ago, pseudailurus had evolved into felis, the modern lesser cats. Two of the first modern cats to appear were felis lunensis, Martelli's cat, and felis manul, Pallas' cat. These cats had larger brains, surprisingly human- like in structure, and were in all ways true modern cats. Martelli's Cat has become extinct, but Pallas' Cat is still very much with us, the oldest living species of genus felis.
By 3 million years ago, the last of the modern genera of cats evolved, panthera, the greater or roaring cats, to which the tigers, lions, leopards and their kin belong.
Somewhere between the First and Second Ice Ages, 900,000 to 600,000 years ago, a very special cat, felis sylvestris, made its appearance, and is still with us as the European Wildcat. During the Second Ice Age, the glaciers moved down from the north, driving him southward. At the same time, the Mediterranean and Black Seas were greatly reduced in size, providing many land bridges to the south into Africa and to the east around the foot of the Urals into Asia, allowing him to extend his domain into those regions.
As the ice receded the seas rose and the climates changed, the immigrant species became isolated from each other by water, deserts, and mountains. Over time, those species of wildcat isolated in Africa became the Sand Cat, the African Wildcat, the Forest Cat, and the Black-Footed Cat, while the Asian version became the Chinese Desert Cat. There were, of course, several other subspecies that, for one reason or another, didn't survive the changing landscape and climate.
One of felis sylvestris' many offshoots was felis lybica, the African Wildcat. He is still with us, but, more importantly, he is the immediate and primary ancestor of all domestic cats.
R. Roger Breton
Nancy J Creek
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