Egg-Laying Mammals or Prototherians
The subclass Prototheria contains the egg-laying mammals, which are the most ancestral forms in the class Mammalia. There are only five extant species grouped into two families and a single order, the Monotremata. Despite bearing fewer species than most mammalian genera, the prototherians are so unique among mammals that there is little question of them being...um..different.
Prototherians probably split from the lineage leading to other mammals sometime in the Mesozoic. They retain many characters of their therapsid ancestors (for example, a complex hip structure, laying of eggs rather than bearing live young, have one opening for excretion and urination etc.). The skulls of monotremes are almost birdlike in appearance, with a long beak and smooth external appearance. Monotremes have several important mammalian characteristics, however, including fur (but they lack whiskers), a four chambered heart, a single dentary bone, three middle ear bones, and the ability to produce milk(lactate).
There are only a five species that still exist in this suborder and they are found in Australia and New Guinea. Old fossils found in Argentina indicate that once monotremes ranged a bit further that the boundaries of the Wallace Line.
Prototherians can be traced back to a lineage that began from reptile-like creatures approximately 190 million years ago. This group split from the ancestors of Eutherian and Metatherian a long time before those became placental and therefore more adapted to diverse climates and harsher environments. This group looks like a combination of reptiles, birds and "true mammals". This bizarre array of combinations makes them very interesting and also very hard to pin down to one class. It is only based on modern genetics we are able to put them firmly within mammals because they have the same Y-chromosome based determination of the sex of an individual.
This group is mostly solitary with at least one species(Short-beaked Echidna) being territorial. Echidnas live on the ground and eat ants,termites and worms wheras platypuses spend most of their time in the water using their beaks to dig out worms from the stream beds. All extant species are exceptional diggers, using their powerful limbs to dig shelters. These burrows are a defence against predators as well as a great place to lay their leathery eggs. They can stay dormant for long periods of time if food sources are scarce.
Very little is known about the mating habits of prototherians except that they are found in pairs during breeding season. The young platypi use a milk tooth to break out of their shell after 12 days of incubation and the mother provide them milk and rears them for quite some time before they are able to fend for themselves.
Hearing, touch and smell are all important to some degree in prototherians. The platypus may even use electrical signals to find prey hidden underneath the stream or riverbed. A good sense of hearing is also important for Echidnas as they forage along the forest floor looking for prey. Very simple sounds that can emanate from beaks are sometime used but we do not known their functions well enough to discern whether they are mating calls or for protection of territory.
These are primitive creatures in that they have not had to evolve too much from the original ancestors so one is quite amazed to note that they have few protections against predation. Echidnas have spines, very much in the hedgehog fashion and functioning in a similar way. Platypi have a bit of venom that they can inject from the spur of their hind-feet. Although, venom is a rarity for mammals to possess, it is not much against sophisticated predators like man. These animals are not aggressive in nature and will not attack if left unmolested...one wishes some of our friends were that way too.
Due to human intervention and the wholesale destruction of habitat at least three species of Echidna are at the brink of extinction. They are listed as critically endangered by the International Union for the Conservation of Nature (IUCN) redlist. Platypi were being hunted extensively for their fur but recent conservation efforts have helped steady their populations.