the situation in most later mammals, the upper and lower molars did not occlude properly when they first met; as they wore against each other, however, their shapes were modified by wear to produce a precise fit. A 2020 study suggests that the metabolism of Morganucodon was significantly slower than that of comparably sized modern mammals, and that it had a life-span more similar to that of reptiles, with the oldest specimen having a lifespan of 14 years. Thus it likely did not possess the fully endothermic metabolism seen in current mammals. Species Classification Morganucodon is the type genus for the order Morganucodonta, a group of generally similar mammaliaforms known from the Late Triassic to Late Jurassic epochs, with one possible member (Purbeckodon) dating to the Early Cretaceous. All were small and likely insectivorous. Morganucodon is the best preserved and best understood member of Morganucodonta. There is currently controversy about whether or not to classify Morganucodon as a mammal or as a non-mammalian mammaliaform. Some researchers limit the term "mammal" to the crown group mammals, which would not include Morganucodon and its relatives. Others, however, define "mammals", as a group, by the possession of a special, secondarily evolved jaw joint between the dentary and the squamosal bones, which has replaced the primitive one between the articular and quadrate bones in all modern mammalian groups. Under this definition, Morganucodon would be a mammal. Nevertheless, its lower jaw retains some of the bones found in its non-mammalian ancestors in a very reduced form rather than being composed solely of the dentary. Furthermore, the primitive reptile-like jaw joint between the articular and quadrate bones, which in modern mammals has moved into the middle ear and become part of the ear ossicles as malleus and incus, is still to be found in Morganucodon. Morganucodon also suckled (it may have been the earliest animal to do so), had only two sets of teeth and grew rapidly to adult size and stopped growing thereafter, all typical mammalian traits. Phylogeny See also Evolution of mammals References Morganucodonts Prehistoric cynodont genera Late Triassic synapsids Triassic synapsids of Europe Jurassic synapsids Transitional fossils Rhaetian first appearances Middle Jurassic extinctions Fossil taxa described in 1949 Taxa named by Walter Georg Kühne