 Mammalia class has about 4,500 species › Very diverse (ranges from small rodents to huge elephants)  Greatest range of any group of vertebrates

 Tooth structure is a way to classify  Can also classify by # and kinds of bones in the head  Most important way to categorize is by the way they reproduce and develop

 MONOTREMES: egg laying mammals › Reproductive and urinary systems open into a cloaca › Lay soft shelled eggs › Nourished by mother’s milk  Duckbill platypus, spiny anteaters › Found in Australia and New Guinea

 MARSUPIALS: bear live young that complete their development in an external pouch › Fertilized egg develops into an embryo inside the mother’s reproductive tract › Embryo crawls across fur into a marsupium (pouch)  Nourished by mother’s milk

 PLACENTAL MAMMALS: nutrients, CO 2, and wastes are exchanged btwn embryo and mother through the placenta  Stays in uterus much longer (few weeks to a few years)  Mammals nurse young

 Mammals migrated freely during the Paleozoic Era (supercontinent)  After continental drift, the mammals became isolated from one another  Much convergent evolution  similar ecological opportunities on different continents showed examples of convergent evolution (see example) › Similar form and function