Living mammals are placentals (eutheria), marsupials, and monotremes < aardvark, opossum, platypus >
The evolutionary history of the Cenozoic mammals (placentals, marsupials [&] and monotremes) is better known than that of any other class of vertebrates. This is so because terrestrial sediments of this age are: fairly complete (except for Australia where there is little Cenozoic continental sediment.); accessible (near to the surface and exposed by gullying); and, easily worked (unconsolidated for the most part as exposed, for example, in the Calvert Cliffs State Park, Chesapeake Bay, MD).
The living members of the class Mammalia include only one order each of monotremes (subclass Prototheria, order Monotreata) and marsupials (subclass Theria, infraclass Metatheria, order Marsupialia) and many orders of placentals (subclass Theria, infraclass Eutheria).
Malcolm C. McKenna and Susan K. Bell in their Classification of Mammals: Above the Species Level (1989) list 425 families (70% extinct) in which, distributed among 24 extant and 22 extinct orders are some 5000 genera (79% extinct). In this new classification, twenty four hundred genera of extinct mammals have been added as a result of fossil finds since George Gaylord Simpson published his classification of mammals in 1945.
The characteristic difference between placentals (ninety percent of all living species of mammals) and marsupials is how the embryos of each are provided for. Placental embryos, nourished by a placenta, complete their development (gestate) in their mothers uterus and are born by her labor. Marsupial embryos develop in uterus to a "rice sized, premature form" that enables them to wriggle, slug-like, to the mothers abdominal pouch. There they attach their mouth to a lactating nipple to complete, over a period of months, their development.
Monotremes, by virtue of their hair and in having but one bone in their lower jaw, are mammals. The first to be described formally arrived in England in 1799. This, a platypus skin preserved in rum, a furry animal with a broad duck-like bill called by early British colonists in Australia a "water mole," initially struck Dr George Shaw as nothing more than a hoax and he took up a pair of scissors to reveal the stitches (there werent any!) attaching the bill to the skin. Twenty five years later Johann Meckel discovered the presence of mammary glands (which, Yann Barrandon, in 2003, can explain, develop from the same discrete accumulation of stem cells resting in the outermost cell layer of an embryo, as do sweat glands and hair follicles). "If these glands produce milk, let's see the butter," retorted one disbeliever. But indeed, from their mothers lactating nipples, the young lap milk, affirming the mammalian status of the platypus. "And therefore, went the reasoning," David Penny writes, "it must give birth to live young. There was no 'sense' in laying eggs if the young were to suckle even if the aborigines said that the platypus laid eggs. Thousands of platypuses were shot to settle the question. This is the dark side of zoological history, the 'search and destroy' attitude that turned zoological research into the killing fields. One Scottish naturalist, William Caldwell, returned home from Australia with the remains of more than 1,300 echidna (the other egg-laying mammal)." Monotremes are reptilian in several details of their skeleton (emphatically so, as described by Alfred Sherwood Romer (1894-1973), in the structure of their shoulder girdle). Living monotremes, the duckbill (Ornithorhynchus) and spiny anteater (Echidna), are native to Australia but there their fossil record is absent from before the Pleistocene.
During the Cenozoic, marsupials in the southern continents evolved in isolation from carnivorous placentals. Placentals that had spread to South America were edentates (Figure d01iii) and the herbivorous ungulate (hoofed) condylaths. Australia remained isolated until the late Pleistocene. South Americas isolation ended when the Panama land bridge formed in the Late Miocene-Early Pliocene and had been briefly compromised in the Oligocene (Figure d01iv). Africas isolation of marsupial animals ended during the Oligocene when also did Madagascars. Africas isolation of marsupial animals ended during the Oligocene when also did Madagascars (see Footnote f1.1). The existence of marsupials and no placentals in the southern continents before these times is not because marsupials originated in the southern continents. The marsupials there had stemmed from marsupials that had migrated from North America and Europe where placentals and marsupials existed early in the Cenozoic with equal diversity: eight families of each. The ancestral marsupials of the southern continents were thus escapees from competition and the physical isolation of the southern continents had allowed them their run of evolutionary experimentation.
The mammals of the world at the beginning of the Cenozoic were themselves survivors of a time of great extinctions when the last dinosaurs and many of their reptilian relatives went extinct. The so-called "Age of Mammals," which continues, began 65 million years ago.
However, for 100 million years before their deliverance, conservative stocks of ancestor mammals had survived the reptilian dominated world of the Mesozoic.
Classification of mammals of the Mesozoic is difficult as their fossils (in 1878, Marsh collected the first found Jurassic ones from dinosaur, Brontosaurus, yielding Como Bluff, Wyoming) are mostly extinct orders and clear differences between them is sometimes only the result of lack of information. For example, modern placental mammals do not have small bones called epipubic bones that in living marsupials and monotremes are attached to both sides of the pubis. This diagnostic distinction is assumed to hold for all ancient placental mammals. But not so. Recently, these same bones have been found in a Cretaceous-age mammal that (from its dentition) was an insect-eating placental (unearthed in Mongolia and called Ukhaatherium nessovi by its finder Mochael J. Novacek).
A diagnostic feature of ancestral placentals and marsupials and Mesozoic-monotremes are tribosphenic (Gr. sphene meaning wedge) molar teeth. These teeth aid efficient food processing which the high metabolic rate of tiny mammals demands. Anne Weil describes their action so: "As a tribosphenic mammal bites down, a large cusp on the upper molar settles, mortar-like, into a pestle-like basin of the lower molar. Simultaneously, notched shearing crests on the sides of the triangular upper molar scissor against those of the lower molars. This combination of shearing and grinding has long been considered a key innovation in the clade containing marsupial and placental mammals indeed, as the innovation that was possibly most significant in the spread and diversification of mammals."
Footnote f1.1 A land bridge hypothesis
In Madagascar human occupation is fast driving to extinction animals with a native ancestry that dates back to the Oligocene. These Madagascans, such as femurs, tenrecs, and unusual rodents, are found nowhere else on earth. However, DNA studies indicate that their ancestors diverged from their relatives in Africa (including the lemur related decedentshumanswho are causing their extinction) about twenty-five million years ago (Late Oligocene). How these ancestral forms got into Madagascar, where the evolutionary pressure on them for change was evidently less, is something of a mystery.
Madagascar, charming for such oddities as Dracula ants whose queens live by sucking the blood of their larvae, is an island separated from all other land masses, and retaining an ancient semi-arid baobab-dominated scrubland, since the Late Cretaceous. Its narrowest separation from other lands has been the two hundred fifty miles width of the Mozambique Channel that during the Late Jurassic opened as deep sea floor between East Africa and Madagascar. Placental Madagascans ancestors were not present in these land areas 160 million years ago. The traditional explanation is that they entered Africa in the Late Oligocene and crossed then to Madagascar as passengers aboard drifting logs or rafts of vegetation swept into the Mozambique Channel by African floods. But why, until the very recent arrival of humans and their introduced animals and plants, did passage to Madagascar not continue? Modern monkeys, wild cats and the dogs were in Africa by twenty million years ago and these did not make the trip to Madagascar. So, what ever had allowed the Madagascans to arrive was not long in operation.
As Robert A. McCall has said, "I can't believe primitive mammals, like femurs, were simply more suited to rafting than more recent groups, like monkeys." His explanation is that a land bridge linked Africa to Madagascar, sometime 26-45 million years ago, which provided a dry migration route for animals and plants.
Evoked to pop up and disappear on cue to provide explanations, no longer present land bridges demand close scrutiny.
In the Mozambique Channel more than a thousand feet of ocean covers the shallowest seamount crests. These are on a long ridge, the Davie Fracture Zone, which is a transform fault that displaced Madagascar from East Africa. McCall suggests that unrelated to that (long ago horizontal motion) reactivation of this fault zone, by compression, squeezed chunks of crust upward along the old fault which now makes up the Davie ridge. This tectonics is suggested to have resulted from stress that existed in the Indian Ocean crust when India slammed into Asia and be reciprocal to the crumpling that raised the Himalayan mountains. The Davie ridge does contain some continental rocks that deep sea drilling has located. At 3,300 to 5,000 feet below sea level, pitted limestones with knife-edge ridges, a sign of erosion by rain, and laterite, an iron-rich soil that usually turns up only in dry tropical areas, have been found. These samples do not prove Oligocene uplift which McCall would have. More likely they are evidence of small continental fragments that sank isostatically in the Late Cretaceous rifting that opened the Mozambique Channel in the Late Jurassic.