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Unformatted text preview: LAB # 10 - MAMMALS Class Mammalia According to modern biological classification, mammals are warm-blooded, hairy vertebrates which suckle their young. Since none of these features are preserved in the fossil record, it would seem difficult to distinguish the older mammal groups from their therapsid ancestors. However, mammals can be distinguished paleontologically by the fact that their lower jaw ( mandible ) consists of a single bone (the dentary ) which is received by the squamosal bone of the skull. In reptiles two different bones (the articular of the mandible and quadrate of the skull) are involved in jaw articulation. The teeth of mammals are much more complex than the simple, peg- or blade-like teeth of other tetrapods. They are used extensively in identification of fossil mammals, since a species of that class can typically be determined on the basis of a single, isolated molar tooth! Mammals may be divided into three modern subclasses: the Monotremata, Marsupialia and Eutheria. In addition, there are a number of Mesozoic mammal types that cannot be placed within the modern subclasses. These include triconodonts, symmetrodonts, panthotheres and multituberculates; groups primarily known from tiny isolated teeth of Jurassic or Cretaceous age. Monotremes The monotremes are curious, toothless, egg-laying mammals very similar in anatomy to the therapsid reptiles. They are very rare in the fossil record and are largely confined to the Pleistocene and Recent of Australia. They seem to represent an archaic sideline of mammalian evolution. Only two genera of monotremes are alive today, the duckbill platypus and spiny echidna. Marsupials The Marsupialia (Cretaceous- Recent) are characterized by the presence of a pouch ( marsupium ) in which the young develop until they are able to fend for themselves. Although currently most species are restricted to the continent of Australia, they were formerly a very widespread and rather important group. The American Oppossum, Didelphis , is very similar to marsupial specimens found in the Cretaceous of North America. Among the earliest remains that have been attributed to marsupials are remains found in the Lower Cretaceous Antler's Formation of North-central Texas, equivalent to our Twin Mountains-Glen Rose-Paluxy Formations. However, recent study of specimens from the Paluxy Formation in Stephenville indicates that there are significant differences in the North-central Texas mammals that may indicate that they belong to a group similar to, but distinct from, the marsupials....
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This note was uploaded on 01/16/2012 for the course GEOL 106 taught by Professor Dr.phillipmurphy during the Fall '10 term at Tarleton.
- Fall '10