"an altogether different perspective on defining mammalia is based on traditional palaeobiological practice. an arbitrary decision is made about which characters to select as defining characters... characters deemed appropriate are those reflecting the... fundamental mammalian biology. the essence of mammalian life is to be found in their endothermic temperature physiology, greatly enlarged brain, dentition capable of chewing food, highly agile, energetic locomotion, and so on. the organisms that achieved this grade of overall organisation are deemed to be mammalia... around the end of the triassic period, about 205 mya, a number of fossils are found of very small animals that have [most] of the skeletal characters of modern mammals".