Platypus in the context of "Terrestrial animal"

Knuckle-walking is a form of quadrupedal walking in which the forelimbs hold the fingers in a partially flexed posture that allows body weight to press down on the ground through the knuckles. Gorillas and chimpanzees use this style of locomotion, as do anteaters and platypuses.

Knuckle-walking helps with actions other than locomotion on the ground. Gorillas use fingers for the manipulation of food, whereas chimpanzees use fingers for the manipulation of food and climbing. In anteaters and pangolins, the fingers have large claws for opening the mounds of social insects. Platypus fingers have webbing that extends past the fingers which aids in swimming, thus knuckle-walking is used to prevent stumbling. Gorillas move around by knuckle-walking, although they sometimes walk bipedally for short distances while carrying food or in defensive situations. Mountain gorillas use knuckle-walking plus other parts of their hand—fist-walking does not use the knuckles, using the backs of their hand, and using their palms.

The beak or bill is an external rostrum structure found mostly in birds. A beak is used for pecking, grasping, and holding (in probing for food, eating, manipulating and carrying objects, killing prey, or fighting), preening, courtship, and feeding young. The terms beak and rostrum are also used to refer to a similar mouth part in some ornithischians, pterosaurs, cetaceans, dicynodonts, rhynchosaurs, anuran tadpoles, monotremes (i.e. echidnas and platypuses, which have a bill-like structure), sirens, pufferfish, billfishes, and cephalopods.

Although beaks vary significantly in size, shape, color and texture, they share a similar underlying structure. Two bony projections–the upper and lower mandibles–are covered with a thin keratinized layer of epidermis known as the rhamphotheca. In most species, two holes called nares lead to the respiratory system.

Monotremes (/ˈmɒnətriːmz/) are mammals of the order Monotremata. They are the only mammals still in existence which lay eggs, rather than bearing live young. The five extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified by structural differences in their brains, jaws, digestive tracts, reproductive tracts, and other body parts, compared to the more common mammalian types. Although they are different from other living mammals in that they lay eggs, female monotremes are like other mammals in that they nurse their young with milk.

Monotremes have been considered by some authors to be members of Australosphenida, a clade that contains extinct mammals from the Jurassic and Cretaceous of Madagascar, South America, and Australia, but this categorization is disputed and their taxonomy is under debate.

Echidnas (/ɪˈkɪdnəz/), sometimes known as spiny anteaters, are quill-covered monotremes (egg-laying mammals) belonging to the family Tachyglossidae /tækiˈɡlɒsɪdiː/, living in Australia and New Guinea. The four extant species of echidnas and the platypus are the only living mammals that lay eggs and the only surviving members of the order Monotremata. The diet of some species consists of ants and termites, but they are not closely related to the American true anteaters or to hedgehogs. Their young are called puggles.

Echidnas evolved between 20 and 50 million years ago and descend from an amphibious, platypus-like monotreme.

The pseudoautosomal regions or PARs are homologous sequences of nucleotides found within the sex chromosomes of species with an XY or ZW mechanism of sex determination.

The pseudoautosomal regions get their name because any genes within them (so far at least 29 have been found for humans) are inherited just like any autosomal genes. In humans, these regions are referred to as PAR1 and PAR2. PAR1 comprises 2.6 Mbp of the short-arm tips of both X and Y chromosomes in humans and great apes (X and Y are 154 Mbp and 62 Mbp in total). PAR2 is at the tips of the long arms, spanning 320 kbp. The monotremes, including the platypus and echidna, have a multiple sex chromosome system, and consequently have 8 pseudoautosomal regions.

The Ornithorhynchidae /ɔːrˌnɪθəˈrɪŋkɪdiː/ are one of the two extant families in the order Monotremata, and contain only the platypus and its extinct relatives. The other family is the Tachyglossidae, or echidnas.

Within the Ornithorhynchidae are the main Cenozoic genera Ornithorhynchus and Obdurodon, and several potential stem-genera dating back to the Late Cretaceous, of which the oldest is possibly Dharragarra. Although fossil evidence suggests the presence of ornithorhynchids in the Cretaceous, phylogenetic evidence has largely found that they and the Tachyglossidae only diverged during the Cenozoic, although this varies based on the specific constraints used.