The groups that form the “crown of mammals”, informally called “true mammals”, comprise the groups currently alive. Before describing each of the modern groups of mammals, it is essential to understand their respective ancestors in common.
Since this group has living members, DNA analysis can be applied in an attempt to explain the evolution of characteristics that do not appear in fossils. This effort often involves molecular phylogenetics, a technique that has become popular since the mid-1980s.
Mammals also exhibit unique features, called autepomorphism. These unique features serve to clearly distinguish and diagnose a taxon. Among the main autapomorphies of the Mammalia class are, obviously, the mammary glands, lactation and breastfeeding, mandatory viviparity (except in the case of monotremados) and the presence of hair.
Of course, there are several other specific characteristics of the group that would require further study. Here, we will highlight the origin of each group and their respective ancestors.
Monotract
The group’s evolutionary history begins with the Ausktribosphenidae, the name of the group where monotremados are included. Mammals usually have triangular or V-shaped molars (called tribosphenics), a form of dentition that is characteristic only in placental and marsupials.
The group of monothermates appeared in the middle of the Cretaceous and is evidenced in the fossil record of Australia, since it was only connected to Antarctica. Placentaries originated in the Northern Hemisphere and were confined to it until continental drift formed land links from North America to South America, from Asia to Africa, and from Asia to India (the Cretaceous map shows how the southern continents were separated.
The Australosphenida clado includes the Ausktribosphenidae and Monotremata. The australophenid basal group is represented by the genus Asfaltomylos dating from the final half of the Jurassic (175-163 million years in the Patagonian region). It presents characteristics in common with the two groups (Rauhut et al, 2002).
Theinolophos of Australia is the oldest known monotremata and according to a study published in 2008, suggesting that it was not a basal monotremata (most primitive and ancestral of all), but a complete Platypus (platypus). It was dated 123 million years ago, so the lineages of Platypus and Echidna diverged very early (Rowe et al, 2008).
A study published in 2009 suggested that while Theinolophos was a type of platypus, it was also a basal monotremate representative of radiation that gave rise to modern ones.
The semi-aquatic lifestyle of the platypus prevented them from being overcome by the marsupials that colonized Australia millions of years ago. The genetic evidence determined that the echidnas diverged from the platypus lineage between 48 and 19 million years ago, when they made their transition from semi-aquatic to terrestrial lifestyle.
Metatheria – Marsupials
The best known characteristic of marsupials is their method of reproduction: the mother develops a structure in her uterus that provides nutrients to the embryo. Koala embryos and wombats additionally form placent-like organs that connect them to the uterine wall, although the organs are smaller than those presented by placental mammals and it is not certain that they transfer nutrients from the mother to the embryo.
The pregnancy period is very short, from four to five weeks. The embryo is born at a very early stage of development, and is usually less than 5 cm long. It has been suggested that a short pregnancy is necessary to reduce the risk of the mother’s immune system reacting against the embryo. The newborn marsupial uses its forearms to rise to a nipple, which is contained inside the pouch in the mother’s belly.
The mother feeds the baby by contracting muscles in her breast area, since the baby is too fragile to suck.
Although some marsupials look very much like some placentaries (such as the “newly-extinguished” tilacin or “marsupial lobe”), anatomically marsupials have characteristics that distinguish them from placentaries. Tilacin, for example, has four molars, while no known placental has more than three.
All have a pair of palatal fenestras, openings in the lower part of the skull (in addition to the smaller openings in the nostril).
Marsupials also have a pair of marsupials (called “epipubic bones”) that support the bag in females. But these are not exclusive to marsupials, since they were found in multitubercled, monotremed and even some Eutherios fossils.
It is probably a bone with characteristics that descend from a common ancestor that disappeared at some point after the ancestry of living placental mammals and that diverged from marsupials (Novacek et al, 1997). Some researchers believe that the original function of the epipubic bones was to assist in locomotion, favoring the fixation of muscles that are associated with the thighs (White, 1989).
One of the oldest known Metatherios is the Sinodelphys szalayi, which lived in China about 125 million years ago. A 2003 study presented the S. szalayi fossil from the Yixian Cretaceous formation in China, which provides sufficient morphological data to be classified as a basal marsupial. It shares derived characteristics such as tooth formation and pulse and ankle structures.
It is believed that the divergence of metatheria and euthery occurred in Asia about 125 million years ago, followed by the evolution of the Deltatheroida basal group in Asia and North America about 120 to 100 million years ago and then the diversification of the Paleocene to the ancestor closest to the current marsupials of South America (Paleos).
The first definitive marsupial fossil belongs to the mining Peradectes species, of the Montana Paleocene is dated back about 65 million years. From this point of origin in Laurasia, marsupials spread to South America, which was connected to North America up to about 65 million years ago. In the Laurasian land masses ancient marsupial groups such as Herpetotheriidae and Peradectidae remained alive from mid-Miocene to the end.