All major Cretaceous mammalian lineages, including monotremes ( egg-laying mammals ), multituberculates, marsupials and placentals, dryolestoideans, and gondwanatheres survived the K – Pg extinction event, although they suffered losses.

The relationships between the three extant divisions of mammals ( monotremes, marsupials, and placental mammals ) was long a matter of debate among taxonomists.

Most morphological evidence comparing traits such as number and arrangement of teeth and structure of the reproductive and waste elimination systems favors a closer evolutionary relationship between marsupials and placental mammals than either with the monotremes.

Marsupials and placental mammals split from the monotremes during the Cretaceous Period.

Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth.

Wagler went so far as to classify Pterodactylus, along with other aquatic vertebrates ( namely plesiosaurs, ichthyosaurs, and monotremes ), in the class Gryphi, between birds and mammals.

Instead, homologues with eutherian sex chromosomes lie on the platypus chromosome 6, which means that the eutherian sex chromosomes were autosomes at the time that the monotremes diverged from the therian mammals ( marsupials and eutherian mammals ).

The fossil record shows that monotremes have been present in Australia since the Early Cretaceous 145 – 99 MYA, and that marsupials and placental mammals date from the Eocene 56 – 34 MYA, when modern mammals first appeared in the fossil record.

The monotremes differ from other mammals in their methods of reproduction ; in particular, they lay eggs instead of giving birth to live young.

In mammals ( other than monotremes ), the cochlea is extended still further, becoming a coiled structure in order to accommodate its length within the head.

The rectum of mammals ( other than monotremes ) is derived from the cloaca of other vertebrates, and is, therefore, not truly homologous with the " rectum " found in these species.

The testes of the non-boreotherian mammals such as the monotremes, armadillos, sloths, elephants remain within the abdomen.

It is argued that those mammals with internal testes, such as the monotremes, armadillos, sloths, elephants, and rhinoceroses, have a lower core body temperatures than those mammals with external testes.

Echidnas and the platypus are the only egg-laying mammals, known as monotremes.

All mammals except the monotremes, the xenarthrans, the pangolins, and the cetaceans have up to four distinct types of teeth, with a maximum number for each.

For example, the dentition of the Early Cretaceous monotreme Steropodon is similar to those of Peramus and dryolestoids, which suggests that monotremes are related to some pre-tribosphenic therian mammals, but, on the other hand, the status of neither of these two groups is well-established.

The name is from the Latin for snail shell, which is from the Greek κοχλίας kokhlias (" snail, screw "), from κόχλος kokhlos (" spiral shell ") in reference to its coiled shape ; the cochlea is coiled in most mammals, monotremes being the exceptions.

In addition Eomaia has epipubic bones extending forwards from the pelvis, which are not found in any placental, but are found in all other mammals-non-placental eutherians, marsupials, monotremes and mammaliformes – and even in the cynodont therapsids that are closest to mammals.

Unique among the synapsids, however, most mammals are viviparous and give birth to live young rather than lay eggs, the exception being the monotremes.

" All extant mammals " includes monotremes, such as the duck-billed platypus, despite its residual egg-laying habit.

Australia is home to two of the 5 known extant species of monotremes and has numerous venomous species, which include the Platypus, spiders, scorpions, octopus, jellyfish, molluscs, stonefish, and stingrays.

Hatching takes place after ten days ; the young echidna, called a puggle, then sucks milk from the pores of the two milk patches ( monotremes have no nipples ) and remains in the pouch for 45 to 55 days, at which time it starts to develop spines.

One of the major differences between placental and nonplacental eutherians is that placentals lack epipubic bones, which are present in all other fossil and living mammals ( monotremes and marsupials ).

Both are conventionally united as infraclasses within the subclass Theria ( Parker and Haswell, 1897 ), which contains all living mammals except monotremes.

The corpus callosum is found only in placental mammals ( the eutherians ), while it is absent in monotremes and marsupials, as well as other vertebrates such as birds, reptiles, amphibians and fish ( other groups do have other brain structures that allow for communication between the two hemispheres, such as the anterior commissure, which serves as the primary mode of interhemispheric communication in marsupials, and which carries all the commissural fibers arising from the neocortex ( also known as the neopallium ), whereas in placental mammals the anterior commissure carries only some of these fibers ).

These characters appeared to unite monotremes with a range of Mesozoic fossil orders ( Morganucodonta, Triconodonta, Docodonta and Multituberculata ) in a broader clade for which the name Prototheria was retained, and of which monotremes were thought to be only the last surviving branch ( Benton 2005: 300, 306 ).

In a further reappraisal, the molars of embryonic and fossil monotremes ( living monotreme adults are toothless ) appear to demonstrate an ancestral pattern of cusps which is similar to the triangular arrangement observed in therians.

In theory, the Prototheria is taxonomically redundant, since Monotremata is currently the only order which can still be confidently included, but its retention might be justified if new fossil evidence, or a re-examination of known fossils, enables extinct relatives of the monotremes to be identified and placed within a wider grouping.

According to the L ' Encyclopédie du Marsupilami, they are monotremes like the platypus and echidna, which explains why they lay eggs while having mammalian features.

In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg ( Latin, ovum ) is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing embryo can survive on its own.

All amphibians, birds, reptiles, and monotremes possess this orifice, from which they excrete both urine and feces, unlike placental mammals, which possess two or three separate orifices for evacuation.

The cloacal region is also often associated with a secretory organ, the cloacal gland, which has been implicated in the scent marking behavior of some reptiles, amphibians and monotremes.

This is one of the features of marsupials ( and monotremes ) that suggest their basal nature, as the amniotes from which mammals evolved possessed a cloaca, and the earliest animals to diverge into the mammalian class would likely have had this feature too.

Along the rivers, there were fish, frogs, salamanders, lizards, crocodiles, turtles, pterosaurs, crayfish, clams, and monotremes ( prototherian mammals, the largest of which was about the size of a rat ).

The allantois functions similarly in monotremes, which are egg-laying mammals.

For example, all gargoyle females lay eggs just as monotremes do.

* Oviparous organisms, including most insects and reptiles, monotremes, dinosaurs and all birds lay eggs that continue to develop after being laid, and hatch later.

In reptiles, birds, and monotremes, the main part of the oviduct is a muscular tube, capable of considerable distension to transport the large eggs that are produced.

Like the other extant monotremes, the short-beaked echidna lays eggs ; the monotremes are the only group of mammals to do so.

Despite all the mammalian traits that Megazostrodon acquired, it is thought that they still laid leathery eggs like their reptilian cousins, similar to the extant monotremes.

Like modern monotremes, this mammal laid eggs.