The evolution of mammals from synapsids (mammal-like "reptiles") was a gradual process that took approximately 70 million years, beginning in the mid-Permian. By the mid-Triassic, there were many species that looked like mammals, and the first true mammals appeared in the Early Jurassic. The earliest known marsupial, Sinodelphys, appeared 125 million years ago in the Early Cretaceous, around the same time as Eomaia, the first known eutherian (member of placentals' "parent" group); and the earliest known monotreme, Teinolophos, appeared two million years later. After the Cretaceous-Tertiary extinction wiped out the non-avian dinosaurs (birds are generally regarded as the surviving dinosaurs (see Feathered dinosaurs and the Origin of birds)) and several other mammalian groups, placental and marsupial mammals diversified into many new forms and ecological niches throughout the Tertiary, by the end of which all modern orders had appeared.
From the point of view of phylogenetic nomenclature, mammals are the only surviving synapsids. The synapsid lineage became distinct from the sauropsid ("reptile") lineage in the Late Carboniferous period, between 320 and 315 million years ago, and were the most common and largest land vertebrates of the Permian period. But in the Triassic period a previously obscure group of sauropsids, the archosaurs, became the dominant vertebrates and one archosaur group, the dinosaurs, dominated the rest of the Mesozoic era. These changes forced the Mesozoic mammaliforms ("nearly mammals") into nocturnal niches, and may have contributed greatly to the development of mammalian traits such as endothermy, hair and a large brain. Later in the Mesozoic mammals spread into other ecological niches, for example aquatic, gliding and even preying on dinosaurs.
Most of the evidence consists of fossils. For many years fossils of Mesozoic mammals and their immediate ancestors were very rare and fragmentary, but since the mid 1990s there have been many important new finds, especially in China. The relatively new techniques of molecular phylogenetics have also shed light on some aspects of mammalian evolution by estimating the timing of important divergence points for modern species. When used carefully, these techniques often, but not always, agree with the fossil record.
Although mammary glands are the signature feature of modern mammals, little is known about the evolution of lactation, and virtually nothing is known about the evolution of another distinctive feature, the neocorte] region of the brain. Most study of the evolution of mammals centers around the development of the middle ear bones from components of the ancestral amniote jaw joint. Other much-studied aspects include the evolution of erect limb posture, a bony secondary palate, fur and hair, and warm-bloodedness.
- Robert L. Carroll, Vertebrate Paleontology and Evolution, W. H. Freeman and Company, New York, 1988 ISBN 0-716-71822-7. Chapters XVII through XXI
- Nicholas Hotton III, Paul D. MacLean, Jan J. Roth, and E. Carol Roth, editors, The Ecology and Biology of Mammal-like Reptiles, Smithsonian Institution Press, Washington and London, 1986 ISBN 0-87474-524-1
- T. S. Kemp, The Origin and Evolution of Mammals, Oxford University Press, New York, 2005 ISBN 0-19-850760-7
- Zofia Kielan-Jaworowska, Richard L. Cifelli, and Zhe-Xi Luo, Mammals from the Age of Dinosaurs: Origins, Evolution, and Structure, Columbia University Press, New York, 2004 ISBN 0-231-11918-6. Comprehensive coverage from the first mammals up to the time of the K-T mass extinction.
- Zhe-Xi Luo, "Transformation and diversification in early mammal evolution", Nature volume 450 number 7172 (13 December 2007) pages 1011–1019. doi:10.1038/nature06277. A survey article with 98 references to the scientific literature.
- The Cynodontia covers several aspects of the evolution of cynodonts into mammals, with plenty of references.