Kangaroos, koalas, platypuses, and wombats: Why does Australia retain these supposedly antiquated mammals? According to the Sherwin-Williams model, marsupials, advanced mammals themselves million years ago, migrated into Gondwana ahead of placentals. They simply got on board the Antarctic-Australian landmass before it broke away from the rest of Gondwana.
Placentals arrived too late—the Australian ship had already sailed. That theory made a lot of sense until the late s, when some revealing fossils began turning up in various parts of the old Gondwana—Patagonia, Madagascar, and Australia.
The new evidence, once again, came in the form of jawbones and teeth—a particular type known as tribosphenic molars. Such teeth work like a mortar and pestle, a further improvement on the slicing teeth of earlier mammals. The ancestor of marsupials and placentals had tribosphenic teeth. Thus the discovery in the Southern Hemisphere of tribosphenic teeth as old as million years, or 25 million years older than any found in the north, complicates the north-south model.
Some explain the presence of these southern tribosphenic teeth by saying they must have developed independently in both hemispheres. Others say the innovation was too intricate to have evolved twice and that mammals must have evolved in the south, with subsequent generations moving north.
The tribosphenic controversy gets even deeper in Australia, where the husband-and-wife team of Tom Rich of the Museum of Victoria and Pat Vickers-Rich of Monash University have turned up three different mammals with tribosphenic teeth dating back million years. The Riches say that these mammals weren't simply on the way to becoming placental, they were placental—something like hedgehogs, in fact. Opponents of the Riches' theory argue that placentals—and certainly not the relatively advanced hedgehogs—were not supposed to be anywhere near Australia so long ago.
Eomaia, that early forerunner of placentals, lived in Asia. If the Riches are right, we have to rethink how placentals traveled from Asia to the Southern Hemisphere. Rather than traveling down the Americas, Eomaia may have found an island-hopping shortcut to Australia.
Or perhaps placentals were widespread much earlier than we think now, and there's just no record of them. They could even have originated in Gondwana and spread out from there. Placentals, suggest the Riches, might even have become extinct with the dinosaurs in Australia, making room for the marsupials to move in later.
Rich himself concedes, "Most radical ideas are wrong. It's wise to be wary of them—especially when they are your own. Even more radical to many paleontologists has been the marriage of plate tectonics evidence and the placental family tree proposed by evolutionary geneticist Mark Springer and his colleagues. Springer is part of a new generation of researchers who examine the strands of an animal's DNA rather than scraping dirt from fossils at a dig.
These molecular biologists read the sequences of genes in a living animal's DNA like an evolutionary history book. The scientists can then determine how closely these animals are related genetically and how long ago their ancestors diverged. Troubling as it is to many paleontologists, Springer's reading of mammals' genetic history fits remarkably well with what geologists now know about the breaking up and subsequent motion of ancient continents.
The oldest group of living placental mammals, according to Springer and his colleagues, arose in Africa just before the continent finished breaking away from the rest of Gondwana around million years ago. Springer calls these animals afrotheres. They include elephants, aardvarks, manatees, and hyraxes. When Africa floated off, it carried these animals away to evolve on their own for tens of millions of years. The fossil record for Africa from this period is almost blank.
Nevertheless, Emmanuel Gheerbrant, a researcher for the National Center for Scientific Research in France, speculates that Africa "must have been a laboratory for some very peculiar animals. One species Gheerbrant has discovered from this period in Africa is the oldest and most primitive known member of the elephant group, the proboscideans.
The million-year-old fossil of Phosphatherium escuilliei was discovered in Morocco. It was the size of a fox, and although it lacked a trunk, it had many dental and cranial features strikingly similar to modern elephants. Paleontologists had long thought elephants were one of the younger modern groups, evolving from ungulates that originated in Asia.
But Gheerbrant's fossil, like the genetic evidence, suggests that proboscideans are in fact one of the oldest of the modern ungulate mammals. Today hyraxes resemble guinea pigs. But 35 million years ago hyraxes took many forms. Some were the size of rhinoceroses; others had long legs like gazelles. Most mammals on the African ark began to disappear around 20 million years ago, after Africa came into contact with the rest of the world again.
But Africa wasn't the only ark. An ancient seaway split South America from Eurasia and North America for millions of years, and South America became home to what geneticist Springer calls xenarthrans, another group of placental mammals. South America's fossil record during its isolation is far better than Africa's, and includes such xenarthrans as sloths, armadillos, and anteaters.
Springer's data, in other words, indicate that the most recent common ancestor of placental mammals is Gondwanan. Contrary to more than a century of northern chauvinism, the northern continents have the youngest placental mammals. One group, the laurasiatheres, includes seals, cows, horses, whales, and hedgehogs. The other group, euarchontoglires, includes rodents, tree shrews, monkeys, and humans.
These genetic findings reveal more than simply which came first. They also redefine relationships among placental mammals. For one, anatomists have always assumed that bats were in the same superorder as tree shrews, flying lemurs, and primates.
But genetic data place bats with pigs, cows, cats, horses, and whales. The data further show that these superorders of living mammals started to diversify much earlier than the fossil record suggests. What gets fossilized is a record of an animal's shape. But geneticists contend that genes in an organism's mitochondria, the parts of a cell that are used to trace and date lineages, can be evolving rapidly without changing what would be left behind in the fossil record.
Birds have a slow rate, yet they can evolve physically very rapidly. However surprising the claims of geneticists seem at first, paleontologists and DNA researchers are finding that their theories can be complementary. Some stunning new fossils have confirmed a previously controversial DNA finding about whales.
Most paleontologists long believed that whales and dolphins—or cetaceans—descended from an extinct line of carnivorous mammals that for unknown reasons became aquatic between 50 and 45 million years ago. At the time of these fossils' discovery, molecular biologists were maintaining that new DNA work indicated the cetaceans were actually aligned closely with artiodactyls, an order that includes even-toed ungulates such as pigs, camels, deer, and hippopotamuses.
Paleontologists first dismissed this unlikely connection because nothing in the fossil record supported it. Then in September two teams of fossil hunters published finds that backed up the claims of the biologists. A group led by Hans Thewissen of Northeastern Ohio Universities College of Medicine found two species of the earliest known whales in million-year-old deposits in Pakistan. Both had ear bones unique to whales, but the legs and anklebones of artiodactyls.
Almost simultaneously, a group from the University of Michigan led by Philip Gingerich announced similar fossils from Pakistan that had the same dual traits. The evolutionary transition among major groups of mammals is rarely illustrated so clearly. And no other discoveries have linked fossils to DNA findings with such precision. Until 65 million years ago dinosaurs dominated the land.
The oceans swarmed with huge sharks and voracious marine reptiles. The dinosaurs and other large predators occupied the richest and most obvious evolutionary niches, keeping mammals at the margins.
Then an event occurred whose scale is still hard to comprehend. An object six miles 9. That impact may have been one of many over the next several hundred thousand years, each adding to the destruction. The temperature reached degrees in parts of the world. They suddenly found themselves in a world without large carnivores. Restraints were off.
Within , years they were diversifying and growing bigger. Still, the majority of mammals didn't get much larger than a pig until the Eocene epoch, which began about 55 million years ago. Then a rapid increase in global temperature encouraged the spread of forests around the world—even near both Poles.
This abundance of rich vegetation opened yet more ecological niches for mammals to exploit. Mammal diversity soared. One of the newcomers in the fossil record was our own order, the primates. The earliest primates belonged to the lemur branch.
Today lemurs are confined to the island of Madagascar, where one species made it from Africa perhaps 50 million years ago, probably on rafts of storm-tossed debris. A few million years later, more advanced primates appear in the fossil record of eastern Asia.
These higher primates are anthropoids—monkeys, apes, and humans. Chris Beard, a specialist in primate origins at the Carnegie Museum of Natural History, has unearthed in China what may be the earliest known example, called Eosimias.
These creatures evolved in the mid-Eocene as the world was cooling and concentrated in the midlatitudes where forests remained lush.
Beard says they "must have been frenetic little animals. Kind of caffeinated. They probably ate all the time. When you are that small, you have to.
They probably lived in troops and maybe never left the tree they were born in. About 34 million years ago smarter, bigger, and more aggressive monkeys evolved.
Catopithecus, one of many anthropoids his team has uncovered, has a skull the size of a small monkey's, a relatively flat face, and a bony enclosure at the rear of its eye sockets. It is the first anthropoid to show the same arrangement of teeth humans have—two incisors, one canine, two premolars, and three molars—leading Simons to argue, "This is the first chapter of human history. At the start of the long Miocene epoch— The world was warming again and more seasonal climate patterns may have emerged.
At higher latitudes, forests gradually gave way in many places to grassland meadows and savannas. Because grass is abrasive, some mammals developed new dentition. Horses, for instance, emerged as little leaf-eaters in the forests but later developed molars that are much better adapted to eating grass. Horses' crowns extend into the jawbones. As the crown gets ground down, new crown will emerge from the jaw to replace it.
Early in the Miocene, Africa's long isolation ended when it and Arabia came back into contact with Eurasia. That's when the ancestors of many mammals we think of as native to Africa arrived there.
First came the ancestors of antelope, cats, giraffes, and rhinos. Later, around ten million years ago, North American mammals—camels, horses, and dogs—began to arrive. Almost every animal that roams the Serengeti today is a relative newcomer to the continent.
Africa gave back as well. Apes moved into Eurasia and flourished. Elephants and their relatives spread across the globe, reaching as far as the tip of Patagonia. But geology and climate changed the world once again for mammals as the Miocene drew to a close.
The Earth grew colder and drier still. Ice caps formed in the Arctic. The Sahara began taking over North Africa, and savannas spread across much of the continent. The changing climate restricted the range of the primates to the equatorial zone. The surviving apes became larger and more specialized.
Then, around seven million years ago, at least one offshoot of the African apes began walking on two legs. As that bipedal ape evolved into what would become us, other mammals came and went.
Most had to adapt to yet another global climate change about 2. Its formation blocked east-west ocean circulation and encouraged the Gulf Stream to grow stronger. As the Gulf Stream pumped more warm water closer to the North Pole, precipitation increased. Heavy snows became glaciers two miles three kilometers thick, which advanced and retreated in a series of more than 20 ice ages.
Because big bodies retain heat better, many mammals, such as the woolly mammoth, grew larger. Even in the temperate zones of Australia, animals became immense.
Australia was soon home to big meat-eating kangaroos, wombat-like creatures the size of trucks, and a marsupial lion twice as big as a leopard. The animal would starve to death in a fruit and veggie shop.
Those big mammals, such as the marsupial lion and the killer kangaroo, disappeared between , and 20, years ago. Few controversies rage more fiercely in paleontology than why the megafauna vanished—not just in Australia but also in North America, where mammoths, horses, camels, and dozens of other large Ice Age mammals all vanished by about 11, years ago. Many scientists cite climate change. Others say it was humans, arguing that newly arrived Homo sapiens killed off the giants with their spears.
We humans may or may not have killed off the giant mammals of the Ice Age. But we are unquestionably threatening innumerable species today, as we expand relentlessly into ever more of their habitats. Signs of this encroachment appear all around the world. Manatees in Florida chopped up by boat propellers. Rhinos in the Ngorongoro Crater poached. Vast rain forests in Southeast Asia obliterated. All this done by the most intelligent of mammals.
Evolution has given us this gift of intelligence, but are we too smart for our own good? If somehow we could rewind time to the dawn of anthropoids, what different path could we have gone down? One answer lies some 5, miles 8, kilometers from the Serengeti's vibrant mammalian spectacle, in the rain forests of Indonesia, Borneo, and the Philippines.
There lives the tarsier, which the Carnegie Museum's Beard cites as an example of the primate road not taken. They're the primates' version of an owl. But they're the nearest living relatives of higher primates. Tarsiers share a common ancestor with all anthropoids. We know this because like all higher primates, tarsiers lack a tapetum lucidum—the reflective layer in the eyes of nocturnal animals. The tapetum lucidum is critical to vision in low light levels and is what makes the eyes of night creatures glow when a flashlight shines on them.
Unlike most of their anthropoid relatives, tarsiers went back to a nocturnal lifestyle at some point and had to compensate by evolving enormous, spooky eyes.
Shine a flashlight in a lemur's eyes at night, and they'll glow back at you. A tarsier's won't. For humans, tarsiers represent what might have been. But because of us, today they're hard to find. Hunting, development, and the destruction of the rain forests have constricted the tarsiers' habitat. In Borneo, where tarsiers are considered bad luck, few villagers worry about that.
Another local, Lemon Ales, agrees. Other villagers regard the tarsier as totems, because the small agile creatures sometimes are seen in rice paddies holding on to the rice stems, as if guarding them. Lemon and I head into the forest at twilight. The world looked like this in the Eocene, when primates were evolving. The forest is full of the same kinds of fruit-bearing trees that helped primates thrive in the vast forests then emerging around the world. The heavy air seems to press moisture into my skin, and my pores fight back with gushes of sweat.
With flashlights we stumble on for several hours in the dark. But the tarsiers never show up. Or maybe they do: The locals warn that if the creatures don't move, you won't see them.
The Singapore zoo has tried to make sure its patrons won't be similarly disappointed. Order Therapsida 1. Morganucodontidae - basic triconodont molar - may have evolved triconodons, docodonts, and monotremes. Kuehneotheriidae - triangular molars - may have given rise to the therians symmetrodonts, pantotheres, marsupials, and eutherians.
Order Multituberculata - first appeared in the late Jurassic period to Tertiary - first mammalian herbivores - wide spread in both the old and new worlds - Ecologically equivalent to rodents - strongly built lower jaw with attachment for powerful jaw muscles - 2 or 3 incisors - diastema in front of premolars, 3 parallel cuspules - olfactory lobes enlarged.
Icaronycteris index - from Eocene beds in Wyoming the oldest known undoubtable bat material first described by Jepsen in claws on the first 2 digits of the hand fairly short, broad wings Other fossils prove insectivory during Eocene - moth scales in gut Late Eocene and Oligocene deposits in France have shown evidence for Microchiroptera families Emballonuridae , Megadermatidae , Rhinolophidae , and Vespertilionidae Megachiroptera appeared in Oligocene in Italy.
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