However, in the period from 31 Ma ago (in the Oligocene) to the present, cetaceans underwent a significantly more rapid sustained increase in body mass (a rate of increase in body mass0.259 of a factor of 3.2 per million years) than achieved by any group of terrestrial mammals. The green anaconda, an aquatic constrictor, is the heaviest snake, weighing up to 97.5 kg (215 lb) or more. , 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. The saltwater crocodile is the largest living reptile and a dangerous predator of humans. Over the next several centuries grass increased; sclerophyll vegetation increased with a lag of another century, and a sclerophyll forest developed after about another thousand years. In addition, accompanying domestic dogs may have competed with native carnivores, and the loss of keystone megaherbivore species may have triggered cascades of extinction through changes in habitat and vegetation and loss of a prey base for megacarnivores. The deep-diving ocean sunfish is the largest bony fish, but its skeleton is mostly cartilaginous. It has been suggested that the increasing thickness of avian eggshells in proportion to egg mass with increasing egg size places an upper limit on the size of birds. Despite having avoided extinction for now, many of the still extant megafauna are greatly reduced in distribution and abundance, often rendering them functionally extinct when considering large-scale ecosystem structure and processes. Indeed having domesticated megafauna around might cause a civilization to advance in unexpected ways because they are no longer dependent on human labor, teams of horses, or the post-industrial inventions of backhoes and cranes to perform large engineering projects. Famously, in the Cretaceous–Paleogene extinction event the non-avian dinosaurs and most other giant reptilians were eliminated. For other uses, see, Nonavian dinosaur size was not similarly constrained because they had a different relationship between body mass and egg size than birds. Accented within geometric shapes are the scenes of daily life roughly 12,000 years ago, set amid anthropomorphic and zoomorphic representations of humans and now-extinct megafauna. 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). The term megafauna is very rarely used to describe invertebrates, though it has occasionally been used for some species of 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. This is also the period when megafaunal flightless herbivorous gastornithid birds evolved in the Northern Hemisphere, while flightless paleognaths evolved to large size on Gondwanan land masses and Europe. Humans and climatic changes are scientists’ favored culprits when explaining megafaunal collapses worldwide, especially since human arrival on islands has almost always ushered in dramatic biodiversity losses. Megafauna are simply big animals. Other common uses are for giant aquatic species, especially whales, any of the larger wild or domesticated land animals such as larger antelope and cattle, as well as dinosaurs and other extinct giant reptilians. Pristichampsus), large snakes (e.g. 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. The term megafauna generally describes animals above a certain weight threshold, and it can be divided into four categories. Intriguingly, the islands' megafauna crashed in … The Nile perch, one of the largest freshwater fish, is also a damaging invasive species. 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.  The ancestors of cetaceans are believed to have been the semiaquatic pakicetids, no larger than dogs, of about 53 million years (Ma) ago. Would humans quickly exterminate man-eating predators? After early humans left Africa and entered the subcontinent, these hominins and megafauna co-existed for at least 20,000 years before extinctions.  Another study examined the change in the methane concentration in the atmosphere at the end of the Pleistocene epoch after the extinction of megafauna in the Americas. The first hints of abnormal rates of megafaunal loss, after hundreds of millions of years of almost continuous abundance, appear around 1 million years ago in Africa and Southern Eurasia. , An analysis of Sporormiella fungal spores (which derive mainly from the dung of megaherbivores) in swamp sediment cores spanning the last 130,000 years from Lynch's Crater in Queensland, Australia, showed that the megafauna of that region virtually disappeared about 41,000 years ago, at a time when climate changes were minimal; the change was accompanied by an increase in charcoal, and was followed by a transition from rainforest to fire-tolerant sclerophyll vegetation. 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