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Topic: Placentals

	Eutheria - Wikipedia, the free encyclopedia
		is a taxon (specifically, an infraclass) nearly containing the placental mammals.
		The closest living relatives of the eutheres are part of the subclass Marsupialia.
		The Eutheria are distinguished from other mammals in that the fetus is nourished during gestation via a placenta while, in general, this is not the case with other mammals (Bandicoots are a conspicuous exception to this rule).
	en.wikipedia.org /wiki/Placental (302 words)

	Placental Mammals (Site not responding. Last check: 2007-11-01)
		Placental mammals are a rather diverse group, with nearly 4000 described species, mostly rodents and bats (photos at left).
		Placental mammals all bear live young, which are nourished before birth in the mother's uterus through a specialized embryonic organ attached to the uterus wall, the placenta.
		The placenta is derived from the same membranes that surround the embryos in the amniote eggs of reptiles, birds, and monotreme.
	www.ucmp.berkeley.edu /mammal/eutheria/placental.html (196 words)

	Some notes on the history and diversity of placental mammals
		Placental mammals provide something like 94% of all existing mammalian species; rats to whales and shrews to you.
		Both placentals and marsupials are descendants of the first therian; a common ancestor not shared by the monotremes.
		Apart from a couple of weirdoes, the maximum quantity for placentals is: (lowers), three, one, four and three; (uppers): three, one, four and three respectively.
	www.geocities.com /trevor_dykes/placentals.htm (2572 words)

	f01 Living mammals are placentals (eutheria), marsupials, and monotremes
		Placental embryos, nourished by a placenta, complete their development (gestate) in their mother’s uterus and are born by her labor.
		Placentals that had spread to South America were edentates (Figure d01iii) and the herbivorous ungulate (hoofed) condylaths.
		For example, modern placental mammals do not have small bones called epipubic bones that in living marsupials and monotremes are attached to both sides of the pubis.
	geowords.com /histbooknetscape/f01.htm (1615 words)

	Encyclopedia :: encyclopedia : Mammal (Site not responding. Last check: 2007-11-01)
		Phylogenetically, Mammalia is defined as all of the descendants of the last common ancestor of monotremes (e.g., echidnas) and therian mammals (placentals and marsupials).
		The traditional view that no placental mammals reached Australasia until about 5 million years ago when bats and murine rodents arrived has been challenged by recent evidence and may need to be reassessed.
		It should however be noted that these molecular results are still controversial because they are not reflected by morphological data and thus not accepted by many systematists.
	www.hallencyclopedia.com /Mammal (1832 words)

	OPOSSUM
		Marsupials differ from the placental mammals by the presence of a fur-lined pouch (marsupium) on the abdomen of most of the marsupial female.
		While North and South America were separated by water (for 60 million years), the placentals exterminated the marsupials in North America, while marsupials, in the absence of carnivorous placentals, dominated South America and Australia.
		When the Isthmus of Panama landbridge was re-established 3 million years ago, the placental carnivores from North America quickly traveled south and established dominance, wiping out many of the herbivorous and all of the carnivorous marsupials in South America (including the marsupial version of the saber-toothed tiger).
	www.bobpickett.org /order_didelphimorphia.htm (2324 words)

	Thylacoleo - About Australia and the Marsupials (page 3)
		How the marsupials established themselves in early Australia without the placentals arriving simultaneously, and competing with them for ecological niches and becoming the dominant mammalian fauna, is a question of particular interest to zoologists.
		Placentals are known from the Tertiary of Australia as well, including some whales and a bat from the Middle Miocene (15 million years ago), and rodents of the family Muridae from the Early Pliocene (4-5 million years ago).
		Condylarths are an ancient group of placental mammals known from various other regions of the world, and the presence of Tingamarra in Australia indicates that placentals were indeed present there at the time when the continent broke away from Antarctica.
	www.naturalworlds.org /thylacoleo/introducing/about_marsupials_3.htm (878 words)

	Thylacoleo - About Australia and the Marsupials (page 2)
		Unlike placentals, marsupials are born in a semi-embryonic state, and in most species they are then protected within an abdominal pouch where they are milk-fed until much more developed.
		How the monotremes relate to the marsupials and placentals is less certain, and has been the subject of considerable debate.
		Many Australian marsupials are analogues to the placentals of other continents in that they occupy the same types of ecological niches.
	www.naturalworlds.org /thylacoleo/introducing/about_marsupials_2.htm (917 words)

	Fossil Gallery: Placentals (Site not responding. Last check: 2007-11-01)
		Placental mammals are the most diverse group of mammals.
		Despite this diversity, many groups of placentals have gone extinct, such as the Creodonta, Condylarthra, Desmostylia, and Embrythopoda.
		In placental mammals, the embryo develops internally in an organ called the placenta that provides gas exchange, nourishment, and waste removal for the embryo.
	www.paleoportal.org /fossil_gallery/taxon.php?taxon_id=92 (178 words)

	The Making Of The Cat - 3
		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.
	www.netpets.com /cats/reference/info/catevolution3.html (1564 words)

	Eutheria (Site not responding. Last check: 2007-11-01)
		Molecular and morphological supertrees for eutherian (placental) mammals.
		Resolution of the early placental mammal radiation using Bayesian phylogenetics.
		Phylogenetic relationships among cetartiodactyls based on insertions of short and long interspersed elements: hippopotamuses are the closest extant relatives of whales.
	tolweb.org /tree?group=Eutheria&contgroup=Mammalia (2253 words)

	Lecture 3
		Eutherians or Placentals are viviparous and have a highly developed placenta that facilitates respiratory and excretory exchange between maternal and fetal circulatory systems.
		The female reproductive system possesses a pair of ovaries that produce gametes (eggs) and hormones, and oviducts (fallopian tubes) into which the eggs are released.
		Placental fetuses have a Zona Pellucida (early) and Trophoblast cells (later) that surround the fetus and effectively prevent immuno-rejection.
	www.bsu.edu /web/temorrell/ZOOL_446/Lecture_3.htm (881 words)

	ABC - Science - Beasts - Mammals' Family Tree
		Placental mammals, like ourselves, carry their young inside their body until they are almost fully formed.
		There are a huge number of placental mammals alive today, but recent DNA analysis has given us a great insight into how they are related to each other.
		As the southern continents began to split apart, the 'Xenarthra' were restricted to South America, and the 'Afrotheria' to Africa.
	www.abc.net.au /beasts/familytree/placentals.htm (226 words)

	Carnegie Museum of Natural History News
		The new fossil is the most primitive known relative to all placental mammals, and provides rare evidence for the earliest history of placental mammals.
		Eomaia was a shrew-sized (about 5 inches or 14 cms long and weighed 20 to 25 grams) placental mammal that co-existed with large sauropod dinosaurs and the carnivorous therapod dinosaurs of the Cretaceous.
		The fossil was found in early 2000, in northeastern China in the famed feathered dinosaur quarry in the Liaoning Province.
	www.carnegiemnh.org /news/02-mar-apr/042502eomaia.html (676 words)

	Marcupials, CARM
		If a new placental line were to evolve from marsupials today, they would be less divergent from marsupials and, thus, would retain more marsupial characteristics than the already established placental line.
		Australia, into which the first terrestrial placentals were thought to have migrated only 5 million years ago, is their main remaining stronghold.
		That bone structure is different in some areas between placentals and marsupials should be unsurprising; if they were the indistinguishable, we wouldn't distinguish the marsupials from the placentals.
	www.carm.org /evolution_archive/marcupials.htm (3542 words)

	News in Science - Kangaroos have a turbocharged 'boing' - 27/07/2004
		Marsupials, including kangaroos, have traditionally been believed to be primitive mammals because their basal metabolism, or resting energy rate, is about a third lower than that of placental mammals.
		Marsupials, and in particular kangaroos, may even be more efficient than placental mammals because of their ability to "rev up" very quickly.
		Marsupials and placentals, which comprise the theria or "advanced mammals", diverged in the early Cretaceous period about 125 million years ago.
	www.abc.net.au /science/news/stories/s1162587.htm (419 words)

	Uncommon Descent » Marsupials and Placentals: a case of front-loaded, pre-programmed, designed evolution?
		The principal difference between the marsupial and placental mammals is the rate of gestation, or the length of time the offspring is carried in the uterus.
		The extended maturation time in placentals, as opposed to all other vertebrates, is a result of the placenta, which allows nutrients to travel from the mother’s system to the embryo and waste to be carried away.
		The same concept is applied to the parallel evolution of marsupial and placental mammals: similar environments and subsistence patterns place similar selective constraints on marsupial and placental mammals in different locations, resulting in strikingly similar anatomical and physiological adaptations, despite relatively non-homologous ancestry.
	www.uncommondescent.com /index.php/archives/1313 (5641 words)

	Paleocene mammals of the world
		It is widely known that placental mammals originated in the Mesozoic as small, unspecialized forms that can best be compared to the insectivores of the modern mammal fauna.
		Instead, it has been proposed that the living insectivores and their close fossil relatives form a separate branch on the evolutionary tree of the placental mammals, just like the rodents or the primates, and should not be mixed up with unrelated archaic mammals.
		This is in fact an exceptional case, since all other placental mammals of the Paleocene epoch, with the exception of armadillos, belong to families no longer existing today.
	www.paleocene-mammals.de /insectivores.htm (7964 words)

	Science & Technology at Scientific American.com: Chinese Fossil May Be Mother of All Placental Mammals (Site not responding. Last check: 2007-11-01)
		Image: MARK A. Researchers have unearthed the fossilized remains of what may be the mother of all placental mammals, so-named for the placenta that nourishes the young during gestation.
		The 125-million-year-old specimen is the earliest and most primitive known representative of the placental group, to which the vast majority of living mammals--humans among them--belong.
		She cautions, however, against generalizing about all early placentals on the basis of this one skeleton (the next oldest placental skeletons are some 50 million years younger than Eomaia).
	www.sciam.com /article.cfm?chanID=sa003&articleID=000F30B7-117C-1CD0-B4A8809EC588EEDF (367 words)

	The Natural History of Marsupials
		The marsupials and placentals diverged from a pantotheran stem stock in the late Cretaceous.
		Alphadon (marsupials can be distinguished from placentals by their dentition - marsupials have 3 premolars and 4 molars whilst placentals have 3-5 premolars and 3 molars).
		Apparent polar wander paths for Australia and Antartica are identical prior to the Miocene - indicating that before the Miocene Australia/Antarctica acted as a coherent unit i.e.
	www.talkorigins.org /faqs/marsupials.html (431 words)

	Howcomyoucom.com The Earliest Known Relative Of Marsupial Mammals
		Modern marsupials and their extinct relatives make up an important mammalian lineage, known as metatherians, consisting of mammals that are more closely related to modern marsupial mammals (such as opossum, kangaroos and koala) than to placentals (such as humans, rodents and whales).
		Modern marsupials are a significant part of the larger metatherian mammal group, and are the descendants of the extinct metatherians that lived during the age of dinosaurs, known as the Mesozoic.
		Marsupials and placentals are both therians mammals characterized by live-birth fetuses, yet they have different reproductive strategies.
	www.howcomyoucom.com /selfnews/viewnews.cgi?newsid1071204261,48201,.shtml (959 words)

	Tasmanian Tigers, CARM
		Since there was no competition from placentals, marsupials were able to evolve to fill all of the niches filled by placentals on the other continents.
		As Mockingbird1 pointed out, the introduction of a placental predator to Australia has not only threatened prey species, but has caused the extinction of marsupial predators that were unable to compete.
		Most modern mammals--including ourselves--are placentals, in which the developing young are nurtured inside the uterus.
	www.carm.org /evolution_archive/tasmanian_tigers.htm (3544 words)

	Aerobic characteristics of red kangaroo skeletal muscles: is a high aerobic capacity matched by muscle mitochondrial ...
		placentals, and differed from that in bipedal humans.
		with quadrupedal placental mammals have shown this to be broadly
		the muscles of the placentals the capillary supply is variable.
	jeb.biologists.org /cgi/content/full/207/16/2811 (5200 words)

	SMU Research Magazine 2003: Preserved in Amber
		It is significant because it lived about the time when placental mammals and marsupials diverged from a common ancestor.
		Marsupials and placentals could have evolved from Slaughteria, or an animal like it.
		To date, it has been unclear how the remarkably different systems of tooth replacement used by modern mammals (marsupials and placentals) developed, Winkler says.
	www.smu.edu /newsinfo/research/2003/researchnews6.html (386 words)

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