The Order of Monotremes (Monotremata), belonging to the infraclass of Prototeri (Prototheria), like that of the Marsupials (Marsupialia), afferent to the infraclass of (Metatheria), include some of the most interesting and original mammals, for the zoologist.
These animals are so different from the common mammals Euteri (Eutheria), that since their discovery, there are still many novelties and original information that characterize them.
With the exception of the opossums and cenolesti of the New World, monotremes and marsupials are found only in the Australian region and in some surrounding islands, such as Tasmania and New Guinea.
The discoverers and first explorers of the Australian coasts saw very few mammals during their first explorations; in truth, they had not penetrated much inside, but in spite of this, what they report in their diaries and documents is very little.
Abel Tasman, who in 1642 visited the southern coast of Tasmania (the name of this island was later given in his honor), calling it "Terra Van Diemen", from the name of the governor of Batavia, he noticed "wild animal traces, not different from those left by the claws of a tiger or some similar creature ... "; it is not excluded that he and his crew had come across traces left by the Tasmanian devil (Sarcophilus harrisii).
For a long period of time, oddly enough, the study of Australian fauna did not arouse great interest among zoologists, compared instead to the great interest in flora by botanists.
In reality, this depended on the large size of the Australian continent, compared to the small size instead of the British and Dutch colonies, which were being born in this immense territory, making it difficult to meet this original fauna.
Throughout the 17th and 18th centuries, the areas conquered by the colonists were used to build farms or penal colonies; only a few explorers and biologists tried to provide information on local flora and fauna, but while the former was more easily accessible and studied, the latter was almost a mystery.
The biologist William Dampier, in a first expedition he made in Australia in 1688, landed on the north coast and also reported that he had not seen any animal, except for some traces that seemed similar to that of a large mastiff.
Subsequently, in 1699, he revisited this continent, to map the "Bay of Sharks", on the west coast; this time, through his diaries and notebooks, he lets us know that he only encountered some type of procionid, different from those present in the West Indies, especially for the legs.
In fact, the scientist tells us that the front legs were very short, but they jumped on them; about a month later, it would seem that he had a meeting with some very skinny canids, almost skinned, in the wild.
Probably the animals he describes are the striped hare kangaroos and the dingo.
The legendary Captain Cook, in 1770, sailed along the entire east coast of Australia, from south to north, meeting kangaroos, possums and dingoes.
During the eighteenth century, some biologists, like John White, produced beautiful engravings on paper, the fauna and flora of those exotic places, as can be seen in the volume entitled "New South Wales" of November 18, 1788.
During the 19th century, other works were "Mammals of Australia", 1871 by J.L.G. Krefft, curator for a few years of the Sydney Natural History Museum; to these, followed at the beginning of the twentieth century, works by biologists of various nationalities, such as that of the French A.S. Le Souef "The wild animals of Australasia" and, by the Australian H. Burell with "The furred animals of Australasia".
At this point, zoologists' interest in the Australian teriofauna ceased, or at least dropped very much, since they were convinced that everything there was to discover about it had already been discovered and widely described.
This sort of Middle Ages of the Australian mammal fauna, ended at the end of the Second World War, when in fact the Australian biologists realized that the species of mammals described up to that time were absolutely not the only ones on the continent and at the same time, their biology had been studied only partially and superficially.
The Wildlife Research section of the Commonwealth Scientific and Industrial Research Organization was therefore established by Australian zoologists; moreover, Australian museums and zoos, such as natural parks and wildlife reserves, made a square to study the known fauna in detail and discover it again.
At this point, biological literature (which up to that time had recorded a heavy absence of articles on Australian mammals), was invaded by an explosive flow of scientific information on these animals.
To date, although the information has grown a lot, still much study awaits biologists, both in the field and in a controlled environment, for example in zoos, on all known species of Australian mammals.
Taxonomist biologists tend to divide the class of Mammals (Mammalia), taking note of the reproductive anatomical and physiological differences, as well as the phylogenetic distances, in three infraclasses (until the 70s of the last century, they were considered subclasses).
These are, the class of Prototeri (Prototheria), to which belongs the order of Monotremes (Monotremata), the class of Metateri (Metatheria), to which belongs to that of the Marsupials (Marsupialia), and the class of Euperi (Eutheria), also called good mammals or placental or placental mammals, to which the majority of mammals belong, including primates and human being.
Here we will turn to describe only monotremes.
The Monotremata, represent the only known order of the Prototeri infraclass (Prototheria); the term monotremes, derives from the Greek, and means a single hole or a single exit.
This definition comes from an anatomical adaptation in the members of this order, where there is a single hole, called the "cloaca", in which the ducts of the urogenital and digestive tract converge.
So defecation, lorination and reproduction, including how we will see egg laying, as they are oviparous, takes place by means of this.
This, we will discuss later, recalls a phylogenetic similarity with reptiles and birds, denouncing a certain archaic nature of the group.
These mammals, in fact, while showing extremely specialized adaptations towards the environment and the way of life, are primitive, since they retain some typical characteristics of reptiles.
The reptilian character, which differentiates them in a more concrete way from all the other mammals, is the fact of being oviparous, rather than viviparous like all the others; this is correlated with the structure of the reproductive system, which in fact is similar to that of the reptiles and birds themselves.
Many other details of the anatomy of these animals are closer to those of reptiles than to those of mammals and the most surprising is that of the "thoracic belt", which keeps the coracoid and interclavicular bones separate, while the "pelvic belt" , the epipubic bones are attached.
Numerous other points of the anatomy of the skeleton, as well as of the moles, are equally primitive; these have been studied by numerous biologists and the results have been collected in works of an encyclopedic character, such as the "Traité de Zoologie" edited by the famous French zoologist biologist P.P. Grassé, where they were presented admirably.
On the other side of the monotremes, it is markedly different from that of reptiles and instead looks very much like that of metaters; the prosencephalon or "cerebrum" (the anterior part of the central nervous system, or rather the brain itself), especially in the echidna (both the species of the genus Tachyglossus, and of the genus Zaglossus), is relatively large, despite being devoid of corpus callosum, that is, of that great bridge connecting the two hemispheres, which is characteristic in placental cells.
As zoologist biologist Wood Jones points out, monotremes are well endowed with cerebral cortex, but their brains still don't have the perfect connections.
The monotremes, they did not leave behind them, the ancestral reptilian characters, as did the other mammals; however, they do not represent an intermediate stage of the evolution of the metaterals and of the euters, but a parallel evolutionary line, which detached itself early from the original stock of the insectivorous reptiles.
Recent studies, carried out on the fossil remains of mammals with primitive reptile characters, suggest lipothesis that the monotremes strain detached itself from what gave rise to the other mammals, about 200 million years ago, in the Triassic period, it was Mesozoic o Secondary, at the level of reptilian organization.
These researches were not carried out on monotremes fossils, of which nothing has been found, dating back to before the Pleistocene, Quaternary, about two million years ago, but on fossils Triconodonti (Triconodonta).
The remains of these animals, which have been preserved as fossils, are generally their tiny teeth; modern methods of separating small fossils from their matrix have brought to light some bone fragments which measure only a few millimeters in total and which, after careful and conscientious examination, have provided the information given above.
The research also suggests that there is no evidence to support the erroneous but widespread opinion that there was a violent evolutionary explosion among mammals in the Tertiary.
The first satisfactory fossils of eutherian mammals appear in the Paleocene, Cenozoic, when almost all modern orders, many of which were extinct, had already differentiated: but mammals had had at least a hundred million damages, to differentiate and therefore, there is no reason to suppose that their evolution accelerated the pace at the beginning of the Tertiary, eighty million years ago.
The lack of fossil records takes away the possibility of knowing the evolution of mammals in the Mesozoic era, but the great variety of forms present at the end of quellera shows that there must have been no discontinuity.
The current monotremes are limited geographically to Australia, including Tasmania and New Guinea; the pleistocene fossil forms therefore, which belong to existing forms and are therefore not very ancient and it would be more correct to call them "sub-fossil" forms, were found only in Australia.
There are no elements to determine whether the monotremes were once more widely distributed (as were the triconodonts, also known in England and North America), or if, on the contrary, they were always confined to the Australian region.
Monotremes have the two diagnostic features of mammals: they have hairs and mammary glands.
Due to the remote separation of their lineage from that of other mammals, one might think that mammals evolved from their reptile ancestors twice.
On the other hand, the size of the chidna's brain and the similarity between the monotremes 'and metaterals' brains seem to show a closer relationship than that would imply.
A recent study, on the amount of DNA present in the somatic cells of monotremes, has also shown that these animals have a closer affinity with other mammals than with reptiles: this has been confirmed by measuring the total area occupied by their chromosomes.
The DNA content in monotremes is between 93 and 98% of that of euter mammals: the content in the only marsupials examined, genera Potorus is Didelphis, it is 81% and 94%.
In comparison in birds, it is about 50%; in snakes and lizards of 60-67%, in crocodiles and chelons of 80-89%.
In evaluating these results, it must be borne in mind, however, that the question of the amount of DNA becomes increasingly complicated through various types of polyploidy.
Electron microscope studies, carried out on echidna spermatozoa, have also revealed that, although the shape of these germinal elements is superficially similar to that of reptile spermatozoa, the structure is mainly that of mammals.
The discovery of a greater number of fossils may one day give us more information; in the meantime, there is no doubt that there is still much to learn from observing living forms.
The Order of Monotremes (Monotremata), is in turn divided into two families: the Tachiglossidi (Tachyglossidae), the echidne and the Ornitorinchidi (Ornithorhynchidae), the platypus.
Echidna porcupine - Tachyglossus aculeatus (photo http://deography.com)
Platypus - Ornithorhynchus anatinus