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From Wikipedia, the free encyclopedia
Mesozoic mammals is a term used to describe mammals and/or their relatives which existed during the Mesozoic era, spanning roughly 252 to 66 million years ago. This period, often called the “Age of Dinosaurs,” also marked the early diversification and ecological experimentation of mammals, which evolved alongside dominant reptilian megafauna. The Mesozoic era represents over two-thirds of mammalian evolutionary history and saw the origin of key mammalian traits, including endothermy, complex dentition, and diverse locomotor capabilities. Fossil evidence reveals that Mesozoic mammals occupied a range of ecological niches — from insectivores and carnivores to herbivores and even semi-aquatic or gliding forms — long before the extinction of non-avian dinosaurs. Although most Mesozoic mammals were small, typically the size of modern shrews or rats, their morphological diversity was immense and some species grew the size of certain medium sized mammals today and were even capable on taking on small dinosaurs.
Groups
The evolutionary history of Mesozoic mammals is complex and has long been a subject of debate among paleontologists. Central to this discussion is the question of what defines a “true mammal”. Depending on the definition used, the emergence of mammals can be placed either in the Late Triassic or the Early Jurassic period. If defined broadly to include all descendants of the common ancestor of Sinoconodon and living mammals (monotremes, marsupials, and placentals), then mammals originated around 220 million years ago during the Late Triassic. The oldest fossils of crown mammals either dates to the Middle Jurassic or Late Triassic. However, if mammals are defined more narrowly—by features such as the presence of a dentary-squamosal jaw joint and three middle ear bones—then their appearance may date slightly later, to the Early Jurassic. Fossil discoveries from China and South America have expanded understanding of mammalian evolution in the Mesozoic, revealing that many groups, such as Haramiyida and Australosphenida, complicate the distinction between “stem” and “crown” mammals. The discovery of arboreal haramiyids from the Jurassic of China shows that early mammals already exhibited highly specialized lifestyles and that crown-group mammals may have originated earlier than previously thought. By the Cretaceous, mammals such as eutriconodonts, multituberculates, and the first therians (the group including marsupials and placentals) had become widespread and diverse. These therians eventually gave rise to the modern mammalian lineages that would dominate after the end-Cretaceous extinction.
Below is a list of groups of mammals that are thought to have lived in the mesozoic
- Morganucodontidae (May not be true crown mammals)
- Docodonta
- Allotheria
- Haramiyida
- Multituberculata
- Eutriconodonta
- Symmetrodonta
- Trechnotheria
- Australosphenida (inlcuding Monotremata)
- Metatheria
- Eutheria
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