Before the K-T extinction the land was dominated by the dinosaurs but the other groups – non-dinosaurian reptiles, mammals and the birds – coexisted with them. The survivors of the Cretaceous extinctions had the opportunity to evolve into the areas vacated by the dinosaurs. The surviving reptiles did not take their opportunity, the mammals and birds did.
Mammals and birds have the ability to learn from their experiences and to manipulate conditions to make them more favourable. They can be cooperative, gathering into flocks or herds. They are also endothermic (‘within heating’), i.e. they generate their own body heat, and this gives them access to cooler habitats not available to ectotherms (‘outside heating’) such as the reptiles.
Birds have retained the reptilian reproductive system of laying eggs. Mammals that lay eggs are called monotremes (‘single hole’), of which only the echidna (from Greek mythology) and the duckbilled platypus (‘flat foot’) have survived.
Mammals that give birth to live young without using a shelled egg are called therians (‘beasts’) which include eutherian (‘true beast’) and placental (‘flat cake’) mammals; and metatherians (‘other beasts’) which include the marsupial (‘pouch’) mammals and their ancestors. Placentals and marsupials emerged as separate groups by the Late Cretaceous.
Marsupials give birth very early and the essentially helpless embryo crawls from the mother’s birth canal to a pouch called the marsupium where it attaches to a nipple, often for weeks or months depending on the species. When the newborn animal grows larger it detaches itself from the nipple but stays in the pouch, gradually venturing out to forage.
The placenta supplies the foetus with nutrition, oxygen and hormones and passes waste products to the mother for disposal. This means that the foetus stays attached and grows for a much longer time in the mother’s uterus. When the young placental mammal is born it is still highly dependent on its mother. During this time its contact with its parents provides training for its survival.
During the Triassic the continents were grouped together in the supercontinent Pangaea and mammals were able to radiate to all parts of the world. In the Jurassic, Pangaea began its break up into Laurasia (North America, Greenland, Europe and Asia) and Gondwana (South America, Africa, Australia, India and Antarctica), and the mammals’ further radiations were restricted to the island continent they happened to be on.
In the Early Cretaceous, Gondwana broke into three continents: South America/Africa, India and Australia/Antarctica. Marsupials probably evolved on Laurasia and spread south to Gondwana before the breakup. When they reached the southern end of South America there was still a land bridge and they (together with monotremes) were able to cross to Antarctica/Australia.
The placentals also arose on Laurasia and later spread to South America, Africa and India. By the time the placentals had spread to the southern end of South America there was no longer a land bridge to Antarctica/Australia. This was significant because wherever placentals went they out-competed the marsupials and they (apart from opossums) died off (Tingamarra?). During the Palaeocene (‘older new’ life), Laurasia split into North America, Greenland and Eurasia.
Placentals can be divided into eight major groups: carnivores (‘flesh-eaters) – cats, dogs, seals; chiropterans (‘hand wings’) – bats; insectivores (‘insect eaters’) – hedgehogs, moles, shrews; lagomorphs (‘hare form’) – hares, pikas, rabbits; primates (‘first rank’) – apes, humans, monkeys, tarsiers; rodents (‘gnawing animals’) – mice, porcupines, rats; ungulates (‘hoofed animals’) – cattle, hippopotamuses, horses; and xenarthrans (‘strange joints’) – anteaters, armadillos, sloths.
The mammals that appeared in the Early Palaeocene were not very different from those that were forced to live in the undergrowth and trees at the time of the dinosaurs during the Cretaceous, i.e. small insectivores; multituberculates (‘many cusps’), extinct rodent-like animals; and some early primates including the plesiadapids (named after the later adapids).
The condylarths (‘knuckle jointed’) are among the most distinctive of the Palaeocene mammals. The absence of large herbivores since the Cretaceous extinction triggered an evolutionary radiation of the condylarths that resulted in groups of ungulates that formed the dominant herbivores in most of the Cenozoic animal communities.
Great waves of herbivorous mammals came in the Late Palaeocene. The uintatheres (‘Uinta Mountains + beasts’) appeared in the Late Palaeocene but became extinct by the Late Eocene. The later uintatheres were rhino-sized and had three pairs of strange bony protuberances on their heads, which may have been sheathed in horn.
Several types of carnivorous mammals also appeared, and included the creodonts (‘flesh tooth’) and the mesonychids (‘middle claws’). The creodont oxyaenids (‘sharp’ or ‘drawn-out hyenas’) were superficially cat-like but unlike true cats they walked on flat feet. Mesonychids were closely related to artiodactyls (even-toed ungulates). Later mesonychids had tiny hooves on each of the four toes on each foot. Some very large flightless birds called diatrymas (‘canoes’) lived in the Palaeocene and Eocene. Standing two metres (6 ft) tall with massive legs and huge powerful beaks, fully grown diatrymas could have killed many mammals of their time. The skeletons of all birds are designed for lightness and these birds despite their great size would have been very swift animals with great endurance.
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