|MadSci Network: Evolution|
The distinguishing charcteristic that unites mammals and gives the class Mammalia its name is the presence of mammary glands which produce milk to nourish the young mammals after they are born. Modern mammals are divided into three groups: the Eutheria, or placental mammals, which includes most of the mammals around today; the Marsupials, which have pouches (marsupia) for raising their young and have the broadest representation in Australia; and the Monotremes, or egg-laying mammals, of which the platypus and two species of echidna are the only living members.
The problem with identifying mammals in the fossil record is that it is impossible to determine whether the extinct animals had mammary glands, so other criteria must be used to characterize fossilized animals as mammalian or pre-mammalian. The solution is to examine the skeletons of the different types of mammals. Skeletal structures which are common to all three types of mammals must have appeared in their common ancestors, while skeletal structures which are different between eutheria, marsupials, and monotremes can be used to identify when each branch broke away from the others. The major differences between mammals and other land vertebrates are:
When paleontologists find fossils, they use these criteria to determine whether a fossil is more like a reptile or more like a mammal. If a fossil has a synapsid skull with a varied dentition, a single jaw bone, and is missing its lumbar (stomach) ribs, then it is classified as a mammal. If the fossil has uniquely marsupial characteristics not found in other mammals, then it is positively classifed as a marsupial, the same is true for eutheria and monotremes.
The group of extinct synapsid reptiles which are the ancestors of mammals are the Therapsids. These were large, wolf-like reptiles that ruled the Permian period before the arrival of dinosaurs in the Mesozoic Era. During the Triassic period, decendents of the therapsids evolved more and more mammalian charcteristics. A group of fossils from the late Permian / early Triassic period, the Cynodonts, shared many characteristics with modern mammals, and may have been warm-blooded and furry. Later around the late Triassic / early Jurassic, several fossilized "proto-mammals" (including Adelobasileus cromptoni, Sinoconodon, Kuehneotherium, and other) lie at the evolutionary border between reptiles and mammals, sharing traits with both. Since there is no way to determine whether these extinct animals laid eggs and/or produced milk, it is open to debate whether they were reptiles or mammals. If they produced milk to feed their young, then they were mammals from birth to death; evolutionary transitions only occur between generations, not within a single animal.
I have included several links to other sites within the above text, several of which give a more detailed account of the fossil record and the evolution of mammals (I searched AltaVista, using THERAPSID as a search term to find many of these sites), although much of this comes from a course on Vertebrate Structure (WashU artsci/FL97/L41/L41311), which I took as an undergraduate. Our textbook was:
Functional Anatomy of the Vertebrates, by W.F. Walker, Jr., Saunders College Publishing (CBS), 383 Madison Ave., New York, NY 10017.
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