During the Cenozoic era the branch of monotreme mammals gradually declined. Their variety gradually became lesser, and the habitat area narrowed steadily. In human epoch the area of monotremes included only Australia and New Guinea. But in these districts monotremes were competitive enough, and strongly kept their place in ecosystems. Before the occurrence of people in Australia the sheep-sized giant echidna existed in this continent. But human activity had quickly resulted in extinction of these animals. Having survived in human epoch, monotremes had proved their viability. In Neocene representatives of this group of animals had formed a number of new species. The apopheosis of evolution of this order of mammals is the largest species of ever living monotremes, the inhabitant of rainforests at the north and east of Meganesia. This animal is named stegoechidna. The appearance of stegoechidna causes memoirs on epoch of dinosaurs. This is a large four-footed animal, growing to about one and a half meters high at a shoulder and weighting up to 200 kgs. It has a massive constitution and thick short tail. Back and tail of this animal bristle up with corneous prickles and scales, providing reliable protection against an attack of predator. Along the back of animal pointed and long corneous spikes of reddish color grow, gradually transforming to reddish plates on tail and hips. The skin of this animal about 2 sm thick is covered with short and rough black wool. The top layer of skin cornificates, forming additional protection. Head of stegoechidna is narrow and long, with slightly curved “Roman” snout and small mouth. The bottom jaw is mostly long and thin, and only in its basis bones extend. In these places on bottom jaw thick corneous plates, with which help the animal crushes forage, develop. At this animal there are small eyes, and also external auricles are not present. Sight and hearing at stegoechidna are weak, but sense of smell is very keen. This animal lives in underbrush and moves, supporting on external part of hand and having turned up huge forepaw claws. With their help animal digs ground in searches of forage. As against to its smaller relatives, this animal is omnivorous, and the significant part of ration of stegoechidna is made of vegetative food – tubers and roots of plants. Animal digs them out by monstrous claws of forepaws, takes by small mouth and pounds between ridge corneous plates growing on jaws and tongue basement. Also it willingly eats earthworms, grubs and fruits fallen from trees. Stegoechidna has very small brain, and its behaviour differs in primitiveness. It leads a solitary way of life, and shows interest to relatives only during the breeding time. These animals breed at presence of enough of food, usually in late spring or in the beginning of summer. The female lays three large eggs with dense shell into the brooding pouch forming at this time. The incubation lasts about two months. Newborn cubs eat milky secretions of skin within three months, and leave a pouch rather well developed. Having left the pouch, they follow the mother and eat larvae of insects and other invertebrates, yet become advanced enough to digest vegetative food. Young animals abandon mother at the age of about 5 months. They reach sexual maturity at the 6-th year of life. Life expectancy of stegoechidna reaches 60 – 70 years. The adult animal does not have enemies – only few predators can bite through its thick skin. However, the young animals have recently left the pouch are protected much more poorly, than adult individuals – corneous scales and spikes at them develop within approximately one month after they leave a parental pouch. At this age the greatest number of animals of this species perishes.