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Natural Science Forum / Biology / Paleontology / March 2005Origin of Placental Mammals
I am not an expert; however I was reading Tim Flannery's book Country, the book is about the evolution of kangaroos. >From pages 194-202 he talks about placental mammals originating in Australia (or generally Gowanda) around 110 million years ago. He cited a fossil finds of a placental mammal found in Australia dated to 115 million years ago, also he cited two independent studies of placental mammal DNA published in the Nature and Science journals. The DNA results show that Afrotheria mammals branched off about 90 million years ago, Then the Xenartha mammals branches off around 80 million and then all the other placental mammals branching off less than 80 million years ago from a common ancestor in Laurasia. If this is proven to be true, it might shed some light on the mysterious Gondwanatheria group of mammals. I wanted to know your opinions on this topic, which according to Flannery is a hotly debated topic in palaeontology. >I am not an expert; however I was reading Tim Flannery's book > Country, the book is about the evolution of kangaroos. [quoted text clipped - 14 lines] > I wanted to know your opinions on this topic, which according to > Flannery is a hotly debated topic in palaeontology. http://archives.cnn.com/2002/TECH/science/04/25/coolsc.mammal/?related PITTSBURGH, Pennsylvania (CNN) -- A 5-inch-long creature that looked more like a mouse than a man may be the earliest known member of the mammal group that includes humans, according to U.S. and Chinese scientists who have studied fossilized remains of the animal. The fossil, resembling a contemporary tree shrew, is believed to be 125 million years old. It was unearthed in China in 2000. Experts believe this early mammal, which weighed less than a pound, had long claws, lived in bushes and ate insects, much like modern, squirrel-like shrews. However, unlike its modern counterparts, the ancient animal had the additional task of dodging carnivorous dinosaurs. Scientists say this is the earliest known placental mammal, the type dominant today. In placental mammals, babies grow and are nourished inside the mother's body through an organ known as the placenta. > I am not an expert; however I was reading Tim Flannery's book > Country, the book is about the evolution of kangaroos. [quoted text clipped - 14 lines] > I wanted to know your opinions on this topic, which according to > Flannery is a hotly debated topic in palaeontology. I know placentals have been found in Australia, predating the discovery of marsupial fossils there, but I wasn't aware the country had been proposed as a site for placental evolution. As far as I know the breeding mode of the Gondwanatherses (a Jurassic group, definitely predating 110 million years ago) is unknown and their remains are mostly known from Argentina. Physiologically the shrews and tenrecs appear to be the most primitive extant placentals (tenrecs in particular appear to have only limited endothermic abilities, little more developed than those in echidnas), and I don't know of any remains resembling either from Australia. Philip Bowles > I know placentals have been found in Australia, predating the > discovery of marsupial fossils there, but I wasn't aware the country > had been proposed as a site for placental evolution. My correction before I read that book I never heard anything about Australian being a centre for placental evolution. > As far as I know > the breeding mode of the Gondwanatherses (a Jurassic group, definitely [quoted text clipped - 6 lines] > > Philip Bowles Some think Gondwanatheres were closely related to monotremes (like Dr Tom Rich), others see similarites with Xentharia group. I will find that DNA study of Placental mammals that was published in Nature and Science and share it. Hi! Some usefull information can be found in the article "In quest for phylogeny of Mesozoic mammals" Full article, abstrats, data matrix and search results are available from here: http://www.app.pan.pl/app47-1.htm (first article from top). Some interesting articles, like the one from Nature, 2002 concerning the earliest eutherian mammals can be downloaded from here: http://www.carnegiemnh.org/vp/cv/luo.htm Cheers, Dawid |
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