From Laura Klappenbach,
Your Guide to Animals / Wildlife.
FREE Newsletter. Sign Up Now!

Mammals appeared on Earth during the early Jurassic period (213mya-144mya). The first mammals were small and included animals such as:

• Eozostrodon - (210mya) among the first true mammals, pointed snout, quadruped, short legs, five-toes, long hair-covered tail,egg-laying, females fed their young milk
• Deltatheridium - (80mya) opossum-like, quadruped, canine teeth, insectivore, early marsupial characteristics
• Jeholodens - (125mya) long tail, quadruped, insectivore, primitive pelvis, possibly nocturnal
• Megazostrodon - (200mya) small quadruped, possibly insectovirous
• Triconodon - cat-sized insectivore and carnivore
• Zalambdalestes - shrew-like, quadruped, insectivore, long tail, large eyes (Source: 'Evolution')

Early mammals shared the planet with the dinosaurs. Some of the primitive mammals (which included early marsupials and insectivores) that lived during the latter part of the Cretaceous period (144mya-65mya) survived the global catastrophe that brought about the extinction of the dinosaurs. Some of the surviving marsupials gave rise to placental mammals, which include many of the mammals familiar to us today. The extinction of the dinosaurs marked the end of the Age of Reptiles and the dawn of the Age of Mammals (Source: Bryant).

During the 10 million years that followed the extinction of the dinosaurs, mammals diversified to occupy a variety of habitats (land, sea, air). Mammals evolved a range of feeding habits (carnivorous, herbivorous, insectivorous, omnivorous).

Mammalian Phylogeny
The evolutionary history of mammals (mammalian phylogeny) begins with a group of vertebrates that dates back over 300 million years: the synapsids (Synapsida). Present day mammals are but one branch of synapsids. Synapsids also include many extinct pre-mammalian species; among them, were the predecessors to present day mammals animals. Therapsida (mammal-like reptiles) are a type of synapsid that became the dominant land animals during the mid-Permian period (286mya-248mya). They are a step closer in the evolutionary line that lead to today's mammals. The cladogram below shows the evolutionary lineage from synapsids to present day mammals:

Synapsida
`--Therapsida
    `--Cynodontia
        `--Eucynodontia
            `--Mammaliformes
                `--Mammalia
                    |--Triconodonts †
                    `--+--+--Montremata
                         |    `--Multituberculata †
                         `--+--Marsupialia (marsupials)
                              `--+--Palaeoryctoids †
                                   `--Eutheria (placental mammals) (Sources: 'Therapsida' and 'Mammalia')

References

• 'The Evolution of Mammals' (On-line), EnchantedLearning.com. Accessed October 12, 2005.
• 'Mammalia' (On-line), Tree of Life. Accessed October 12, 2005
• 'Therpasida Overview' (On-line), PALEAEOS: The Trace of Life on Earth. Accessed October 12, 2005.
• 'Synapsida Overview' (On-line), PALEAEOS: The Trace of Life on Earth. Accessed October 12, 2005.
• Bryant P. J. 'The Age of Mammals' (On-line), Biodiversity and Conservation. Accessed October 12, 2005.