Megafauna

In terrestrial zoology, megafauna (from Greek μέγας megas 'large' and New Latin fauna 'animal life') are large or giant animals. The most common thresholds used are weight over 40 kilograms (90 lb) or 44 kilograms (100 lb) (i.e., comparable or larger in mass than a human) or over a tonne, 1,000 kilograms (2,205 lb) (i.e., having a mass comparable to or larger than an ox). The first of these include many species not popularly thought of as overly large, such as white-tailed deer and red kangaroo.Some Paleozoic sea scorpions (Eurypterus shown) were larger than a human.Dunkleosteus was a 10 m (33 ft) long toothless armored predatory Devonian placoderm fish.Sail-backed pelycosaur Dimetrodon and temnospondyl Eryops from North America's Permian.Pliosaur Pliosaurus (right) harassing the filter feeder fish Leedsichthys during the Jurassic.Macronarian sauropods; from left, Camarasaurus, Brachiosaurus, Giraffatitan, Euhelopus.Tyrannosaurus was a 12.3 m (40 ft) long theropod dinosaur, an apex predator of west North America.Indricotheres, the land mammals closest to sauropods in size and lifestyle, were Asian rhinos.The Late Miocene teratorn Argentavis of South America had a 7 m (23 ft) wingspan.Reconstructed jaws of C. megalodon (Baltimore).Deinotherium had downward-curving tusks and ranged widely over Afro-Eurasia.Titanis walleri, the only terror bird known to have invaded North America, was 2.5 m (8.2 ft) tall.Hippo-sized Diprotodon of Australia, the largest marsupial of all time, became extinct 40,000 years ago.Megalania, a giant carnivorous goanna of Australia, might have grown to 7 metres long.Glyptodon, from South America's Pleistocene, was an auto-sized cingulate, a relative of armadillos.Macrauchenia, South America's last and largest litoptern, likely had a short trunk like a saiga.American lions exceeded extant lions in size and ranged over much of N. America until 11,000 BP.Woolly mammoths vanished after humans invaded their habitat in Eurasia and N. America.Haast's eagle, the largest eagle known, attacking moa (which included the tallest bird known).The eastern gorilla is the largest and one of the more endangered primates on the planet.The most common tiger subspecies, Bengal tigers are endangered by poaching and habitat destruction.Polar bears, among the largest bears (consistent with Bergmann's rule), are vulnerable to global warming.The critically endangered black rhinoceros, up to 3.75 metres (12.3 ft) long, is threatened by poaching.Wild Bactrian camels are critically endangered. Their ancestors originated in North America.Unlike woolly rhinos and mammoths, muskoxen narrowly survived the Quaternary extinctions.Hippos, the heaviest and most aquatic even-toed ungulates, are whales' closest living relatives.The sperm whale, the largest toothed whale and toothed predator, has the biggest brain.The orca, the largest dolphin and pack predator, is highly intelligent and lives in complex societies.The cassowary, the heaviest non-African bird, can run at 50 km/h through dense rainforest.The saltwater crocodile is the largest living reptile and a dangerous predator of humans.The Komodo dragon, an insular giant and the largest lizard, has serrated teeth and a venomous bite.The green anaconda, an aquatic constrictor, is the heaviest snake, weighing up to 97.5 kg (215 lb) or more.The deep-diving ocean sunfish is the largest bony fish, but its skeleton is mostly cartilaginous.The Nile perch, one of the largest freshwater fish, is also a damaging invasive species.The great white, the largest macropredatory fish, is more endangered than the tiger.The manta, a filter feeder, is the largest ray at up to 7.6 m across, yet can breach clear of the water.Examination of a 9 m giant squid, an abyssal giant and the second largest cephalopod. In terrestrial zoology, megafauna (from Greek μέγας megas 'large' and New Latin fauna 'animal life') are large or giant animals. The most common thresholds used are weight over 40 kilograms (90 lb) or 44 kilograms (100 lb) (i.e., comparable or larger in mass than a human) or over a tonne, 1,000 kilograms (2,205 lb) (i.e., having a mass comparable to or larger than an ox). The first of these include many species not popularly thought of as overly large, such as white-tailed deer and red kangaroo. 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 term is especially associated with the Pleistocene megafauna – the land animals often larger than modern counterparts considered archetypical of the last ice age, such as mammoths, the majority of which in northern Eurasia, the Americas and Australia became extinct within the last forty thousand years. It is also commonly used for the largest extant wild land animals, especially elephants, giraffes, hippopotami, rhinoceroses, and large bovines. (Of these five categories of large herbivorous mammals, only bovines are presently found outside of Africa and southern Asia, but all the others were formerly more wide-ranging.) Megafaunal species may be subcategorized by their trophic position into megaherbivores (e.g., elephants), megacarnivores (e.g., lions), and, more rarely, megaomnivores (e.g., bears). Other common uses are for giant aquatic species, especially whales, any larger wild or domesticated land animals such as larger antelope and cattle, as well as numerous dinosaurs and other extinct giant reptilians. The term is also sometimes applied to animals (usually extinct) of great size relative to a more common or surviving type of the animal, for example the 1 m (3 ft) dragonflies of the Carboniferous period. Megafauna – 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. These characteristics, although not exclusive to such megafauna, make them vulnerable to human overexploitation, in part because of their slow population recovery rates. 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. 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. 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. 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. 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. 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. Starting from just a few kg before the event, maximum size had reached ~50 kg a few million years later, and ~750 kg by the end of the Paleocene. This trend of increasing body mass appears to level off about 40 Ma ago (in the late Eocene), suggesting that physiological or ecological constraints had been reached, after an increase in body mass of over three orders of magnitude. However, when considered from the standpoint of rate of size increase per generation, the exponential increase is found to have continued until the appearance of Indricotherium 30 Ma ago. (Since generation time scales with body mass0.259, increasing generation times with increasing size cause the log mass vs. time plot to curve downward from a linear fit.) Megaherbivores eventually attained a body mass of over 10,000 kg. The largest of these, indricotheres and proboscids, have been hindgut fermenters, which are believed to have an advantage over foregut fermenters in terms of being able to accelerate gastrointestinal transit in order to accommodate very large food intakes. A similar trend emerges when rates of increase of maximum body mass per generation for different mammalian clades are compared (using rates averaged over macroevolutionary time scales). Among terrestrial mammals, the fastest rates of increase of body mass0.259 vs. time (in Ma) occurred in perissodactyls (a slope of 2.1), followed by rodents (1.2) and proboscids (1.1), all of which are hindgut fermenters. The rate of increase for artiodactyls (0.74) was about a third that of perissodactyls. The rate for carnivorans (0.65) was slightly lower yet, while primates, perhaps constrained by their arboreal habits, had the lowest rate (0.39) among the mammalian groups studied. Terrestrial mammalian carnivores from several eutherian groups (the artiodactyl Andrewsarchus - formerly considered a mesonychid, the oxyaenid Sarkastodon, and the carnivorans Amphicyon and Arctodus) all reached a maximum size of about 1000 kg (the carnivoran Arctotherium and the hyaenodontid Simbakubwa may have been somewhat larger). The largest known metatherian carnivore, Proborhyaena gigantea, apparently reached 600 kg, also close to this limit. A similar theoretical maximum size for mammalian carnivores has been predicted based on the metabolic rate of mammals, the energetic cost of obtaining prey, and the maximum estimated rate coefficient of prey intake. It has also been suggested that maximum size for mammalian carnivores is constrained by the stress the humerus can withstand at top running speed.