Mammals (more properly Mammaliformes) appear in fossil record the same time as dinosaurs, in Late Triassic.

Mammals are very advanced therapsids synapsids.

True mammals (Mammalia) found from Middle Jurassic onward.

Like birds, many of the features that characterize modern mammals don't fossilize:

• Warm-blooded
• Covered with fur
• Sweat glands
• Mammary glands
• Parental care of young

On the other hand, some mammalian features are preservable:

• Only two sets of teeth: deciduous (“baby”) and permanent
• Highly specialized teeth divided into incisors, canines, premolars, and molars
  • Teeth highly distinctive, can recognize a species from only one or two teeth
• Lower jaw comprised only of dentary; jaw joint is dentary-squamosal, not quadrate-articular
• Post-dentary lower jaw bones moved up into middle ear
• And more

Many features limited to Mammalia among living amniotes were probably found in their closest non-mammalian therapsids relatives.

Mesozoic mammals very small (shrew-to-house cat sized, with a badger-sized marsupial from the very end of the Cretaceous); mammals only become large AFTER extinction of non-avian dinosaurs.

Many groups of Jurassic and Cretaceous mammaliforms; four main clades within Mammalia:

Monotremata (monotremes):

• Oldest fossils Early Cretaceous; survive today in Australasia as platypus and echidna
• Still lay eggs (only living mammals to do so)
• Very simple mammary glands
• Today's monotremes have lots of primitive features, but many specializations of their own
• No evidence that monotremes were ever a dominant group of mammals

Marsupalia (marsupials):

• Oldest fossils Late Jurassic; survive today in opossums of the Americas and great diversity in Australasia (and in Cenozoic were even more diverse in South America)
• Birth barely-formed young that develop in pouch
• During Mesozoic were very common mammals in both Gondwana and Laurasia
• Largest mammals of Mesozoic were North American marsupials (badger-sized)

Multituberculata (multituberculates):