Till the Cenozoic, before the appearing of man and his satellites from among placentary mammals, marsupial mammals of Australia had developed practically all accessible ecological niches, having formed a plenty of various species. But one environment had remained completely inaccessible for them – water habitats were those ones. Among Australian marsupials there are no analogues of otters, seals and beavers. The brooding pouch, the refuge for young growth for long weeks, had made development of water inhabitancy impossible for marsupials. Therefore in Cenozoic in Australia the vacant place of large water vertebrates was shared by birds, turtles, platypuses and rodents. In Neocene marsupials had made one original attempt to develop water habitats. Floating plants covering with leaves and stalks a surface of reservoirs, became the house for one such creature. At the end of day and at the early morning on the surface of carpet of floating plants it is possible to notice this creature, managed to find the compromise between presence of brooding pouch and semi-aquatic habit of life. It is tiny marsupial shrew - one of tiniest mammals of Neocene. Length of body of this creature may be about 4 cm; tail is about 2 cm long. The body constitution of marsupial shrew is very graceful; these small mammals are similar to shrews with long legs. Back legs of this animal are little bit longer than front ones. Head is rather large, with wide rounded ears. The muzzle is long, extended to mobile proboscis, on each side of which thin whiskers stick up. The marsupial shrew is the descendant of one of small species of marsupial mice (Antechinus). As this animal lives in areas with a warm climate, its wool is short and rigid. It is colored light yellow, almost white color – it protects small mammal from overheat. The body temperature at the marsupial shrew is changeable: at night the animal is hidden among plants and runs into original catalepsy during which the body temperature lowers down to ambient temperature. At this time the marsupial shrew is rather languid, therefore it tries to find a reliable place where it will be not found by predator. Sometimes for spending the night the animal even gets into buds of water lilies which are closed for night, but do not immerse into the water. In hottest time of day marsupial shrew is also hidden in shadow to avoid overheat, but thus its temperature does not fall. Fingers and toes of animal are bordered by “brushes” of hairs, and marsupial shrew greases them with fat secretions of anal glands. Due to it hairs on paws become not moistened, that permits this tiny creature to live on the surface of reservoirs. It quickly runs on floating water plants, and even can run small distances on the surface of water, quickly splashing on it by paws like the basilisk lizard (Basiliscus) known in Holocene epoch. Basically, young animals and adult males which are smaller, rather than females approximately to 30 – 40% are able to do it. Males and females without cubs can even dive for some seconds. The female, especially one with cubs, does not risk to run on water and to dive. It keeps on floating plants where it is easier to find forage and to hide from enemies. At this creature there are very sharp teeth: marsupial shrew is a gluttonous predator, and devours for day twice more food, than it weighs itself. It eats insects and their larvae, and also small shrimps. The animal catches them, immersing head and forward part of body under water. Due to small size marsupial shrew catches larvae of mosquitoes in leaf axils of floating plants – this animal alongside with fishes and predatory insects controls the amount of mosquitoes in damp districts. Also it can catch fish fry hiding among plants, and even attacks small crabs. Marsupial shrew lives in conditions where seasonal prevalence of climate rather feebly marked; therefore it breeds the year round. The brooding pouch at it is reduced up to two longitudinal plicas in groin area, bordering nipples from sides. In litter it may be only 3 – 5 cubs developing very quickly. At monthly age they already creep on back of the female, and in two months abandon mother. One – two weeks later the female is ready to reproduction again. Sexual maturity at young animals becomes at the age of three months. Life expectancy of the marsupial shrew is more, rather than at placentary mammals of similar size with constant body temperature: it may amount about two years. It is the result of night catalepsy which is slowing down the “combustion” of its organism.