Mammals and their closest relatives (more properly Mammaliformes, or sometimes "Mammaliaformes") 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 cheek teeth (premolars and molars)
• Lower jaw comprised only of dentary; jaw joint is dentary-squamosal, not quadrate-articular
  • Post-dentary lower jaw bones moved up into middle ear, as both phylogenetic sequence and developmental biology show
• Respiration using diaphragm (so that dorsal vertebrae are divided into thoracic and lumbar sections)
• And more

Many features limited to Mammalia among living amniotes were probably found in their closest non-mammalian therapsids relatives. For example, we can't say for certan when warm-blood, fur, sweat & mammary glands show up. We can determine a few of these, though:

• Evidence for parental care in basal synapsids, as well as more derived therapsids
• Division of teeth into incisiors, canines, and cheek teeth in early therapsids
• Diaphragm breating in more advanced therapsids
• At around the same level, possible evidence for fur
• Predator-prey ratios of therapid communities suggest elevated (or at least non-cold blooded) metabolisms

Living mammals are divided into three clades:

• Monotremes: the last surviving egg-laying mammals. Rare and surviving only in Australasia
• Marsupials: reproduce by young born live but very poorly developed, which then suckle and grow within a pouch. Dominate Australasia, common in South America, present in early Cenozoic Europe and North America
• Placentals: stay in the womb until more developed, fed by a placenta. The dominant group in the Americas, Africa, and Eurasia.

However, mammal diversity in the Mesozoic was MUCH different. Many groups of Mesozoic mammals have long since died out. And some Mesozoic mammal groups survived the end of the Cretaceous, but have since died out.

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

Oldest mammaliforms of the Late Triassic and Early Jurassic were fairly small. But by Middle and Late Jurassic, there were already some specialized mammals:

• Gliders
• Burrowers
• Paddle-tailed, web-footed fish eaters

Some major groups of Jurassic and Cretaceous mammals:

Prototheria (sometimes called "Australosphenida"; monotremes and their extinct relatives):

• Generally relatively stumpy legs and tails, even in modern forms
• Oldest from Middle Jurassic onward
  • Oldest monotremes proper from Early Cretaceous (perhaps actually a platypus! it has the same electrosensory apparatus); survive today in Australasia as platypus and echidna
• Prototheres were never a dominant group, but were once moderately common in Gondwana (present in Asia, too)
• Retain ancestral sprawlling posture and egg-laying reproduction
• Insectivores?

Eutriconodonta (eutriconodonts):

• Range from tiny to opposum-sized to badger-sized
• Relatively common mammals from the Jurassic and Cretaceous in Laurasia and northern Africa
• Omnivores and carnivores: at least one has gut contents containing the remains of baby Psittacosaurus
• In their time, were major parts of the larger mammal fauna: ecologically comparable to opposums and raccoons in the modern U.S.
• Wiped out at end of Cretaceous
• Mode of reproduction unknown

Allotheria (allotheres):