“Therianthropy” is a compound word derived from the Greek for beast and human. This term often has been used by archaeologists to describe animal-human figures found in prehistoric rock art, such as the cave painting of the Dancing Sorcerer at Les Trois-Frères in southwestern France.

A human (placental) holding two koalas, (marsupial), representing the two extant infraclasses of therian mammals. Theria (/ˈθɪəriə/; Greek: θηρίον theríon, wild beast) is a subclass of mammals amongst the Theriiformes (the sister taxon to Yinotheria).

Shapeshifters are not just the stuff of science fiction, though. We have them right here on earth. Some are innocuous, like a caterpillar turning into a butterfly. But there’s another shapeshifter that’s responsible for more than 400,000 deaths every year.

Placental mammals are found on all continents, in the air, and in the seas. Primates, cats, dogs, bears, hoofed animals, rodents, bats, seals, dolphins, and whales are among the dominant placental mammal groups today. Nearly 94% of all mammal species now are placental mammals (5,080 species out of 5,416).

Extant mammals are divided into three subclasses based on reproductive techniques (monotremes, marsupials, and placentals) consisting of 27 orders.

Mammal

Humans are also classified within: the subgroup of mammals called primates; and the subgroup of primates called apes and in particular the ‘Great Apes’

Placental mammals are a rather diverse group, with nearly 4000 described species, mostly rodents and bats (photos at left). The placental mammals include such diverse forms as whales, elephants, shrews, and armadillos.

The monotremes and marsupials are thus intermediate between the reptiles which lay shelled eggs but do not feed their young on milk and the Eutheria or placental mammals where an allantoic placenta is always present and the young are retained in the uterus up to an advanced stage of development.

Platypus are monotremes – a tiny group of mammals able to both lay eggs and produce milk. They don’t have teats, instead they concentrate milk to their belly and feed their young by sweating it out. It’s believed mammals evolved teats or nipples because it was a sterile way to deliver milk to their young.

Only two kinds of egg-laying mammals are left on the planet today—the duck-billed platypus and the echidna, or spiny anteater. These odd “monotremes” once dominated Australia, until their pouch-bearing cousins, the marsupials, invaded the land down under 71 million to 54 million years ago and swept them away.

The largest orders are the rodents, bats and Eulipotyphla (hedgehogs, moles, shrews, and others). The next three are the Primates (including humans, apes, monkeys, and others), the Artiodactyla (cetaceans and even-toed ungulates), and the Carnivora (cats, dogs, seals, and others).

The placentals include all living mammals except marsupials and monotremes. The true placenta of the placentals allows for a longer developmental period within the protection of the womb, a factor considered to have contributed to the evolutionary success of the group.