Docodonta

Docodonta is an order or suborder of extinct mammaliaforms. They lived during the mid- to late-Mesozoic era. 
They had sophisticated molars, from which the suborder gets its name. In the fossil record, isolated teeth and bits of jawbones are found. 
Docodonts are not quite as closely related to the placentals and marsupials as the monotremes are, so they are not included among the crown-group mammals. 
Because of their molars and their lower jaw being a single bone (the dentary), they are generally regarded as mammals. However, those authors who limit the term "Mammalia" to the (living) crown group exclude docodonts. They use the term mammaliaformes instead.
Docodonts were primarily herbivorous or insectivorous. However, "Castorocauda", adapted to a semi-aquatic life, has teeth which suggest it ate fish.
"Castorocauda" is important because the first find was an almost complete skeleton (a real luxury in paleontology). It breaks the "small nocturnal insectivore" stereotype.
It was noticeably larger than most Mesozoic mammaliaform fossils, and gives absolutely certain evidence of hair and fur. Of course, this strongly suggests mammalian-type temperature regulation.