Subsequent to the Cretaceous–Paleogene extinction event that eliminated the non-avian dinosaurs about 66 Ma (million years) ago, terrestrial mammals underwent a nearly exponential increase in body size as they diversified to occupy the ecological niches left vacant. In terrestrial mammals Large terrestrial mammals compared in size to one of the largest sauropod dinosaurs, Patagotitan When normalized to generation length, the maximum rate of body mass decrease was found to be over 30 times greater than the maximum rate of body mass increase for a ten-fold change. A strikingly faster rate of change was found for large decreases in body mass, such as may be associated with the phenomenon of insular dwarfism. This is thought to reflect the emergence, during a trend of increasing maximum body size, of a series of anatomical, physiological, environmental, genetic and other constraints that must be overcome by evolutionary innovations before further size increases are possible. In an examination of mammal body mass changes over time, the maximum increase possible in a given time interval was found to scale with the interval length raised to the 0.25 power. One observation that has been made about the evolution of larger body size is that rapid rates of increase that are often seen over relatively short time intervals are not sustainable over much longer time periods. These characteristics, although not exclusive to such megafauna, make them vulnerable to human overexploitation, in part because of their slow population recovery rates. Megafauna animals – in the sense of the largest mammals and birds – are generally K-strategists, with high longevity, slow population growth rates, low mortality rates, and (at least for the largest) few or no natural predators capable of killing adults. The term megafauna is very rarely used to describe invertebrates, though it has occasionally been used for some species of invertebrates (which are on average much smaller than vertebrates) such as coconut crabs and Japanese spider crabs, as well as extinct invertebrates that were much larger than all similar invertebrate species alive today, for example the 1 m (3 ft) dragonflies of the Carboniferous period. Other common uses are for giant aquatic species, especially whales, as well as any of the larger wild or domesticated land animals such as larger antelope, deer, horse and cattle, as well as dinosaurs and other extinct giant reptiles. The megafauna is also categorized by the class of animals that it belongs to, which are mammals, birds, reptiles, amphibians, fish, and invertebrates. Megafaunal species may be categorized according to their dietary type: megaherbivores (e.g., elephants), megacarnivores (e.g., lions), and, more rarely, megaomnivores (e.g., bears). Wild equines are another example of megafauna, but their current ranges are largely restricted to the Old World, specifically Africa and Asia. Of these five categories of large herbivores, only bovines are presently found outside of Africa and southern Asia, but all the others were formerly more wide-ranging, with their ranges and populations continually shrinking and decreasing over time. Among living animals, the term megafauna is most commonly used for the largest extant terrestrial mammals, which includes (but is not limited to) elephants, giraffes, hippopotamuses, rhinoceroses, and large bovines. The term is especially associated with the Pleistocene megafauna – the land animals often larger than their extant counterparts that are considered archetypical of the last ice age, such as mammoths, the majority of which in northern Eurasia and the Americas became extinct within the last forty thousand years. In practice, the most common usage encountered in academic and popular writing describes land mammals roughly larger than a human that are not (solely) domesticated. The first of these include many species not popularly thought of as overly large, and being the only few large animals left in a given range/area, such as white-tailed deer, Thomson's gazelle, and red kangaroo. The most common thresholds to be a megafauna are weighing over 46 kilograms (100 lb) (i.e., having a mass comparable to or larger than a human) or weighing over a tonne, 1,000 kilograms (2,205 lb) (i.e., having a mass comparable to or larger than an ox). In zoology, megafauna (from Greek μέγας megas "large" and Neo-Latin fauna "animal life") are large animals. The African bush elephant (foreground), Earth's largest extant land mammal, and the Masai ostrich (background), one of Earth's largest extant birds For other uses, see Megafauna (disambiguation). This article is about living or extinct large animals.
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