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Mammals are thought to have evolved from the class Reptilia particularly within the order Therapsida of the subclass Synapsida or “mammal-like reptiles” (Novak 1999). Adelobasileus cromptoni from the late Triassic of Texas about 225 million years old was said to be the earliest known mammal (Lucas and Hunt, 1990). However, with respect to several important characters, Adelobasileus shows an intermediate condition between cynodonts and mammals and its status as mammals has been doubted (Lucas and Luo 1993). The recent discovery of Hydrocodium wui provides evidence for the earliest known mammal in 195 million year old Cretaceous Chinese sediments, suggesting that the mammals evolved 45 million year earlier than previously recognized (Lou et al. 2001). The small body size of Hydracotherium also reveals that there was more evolutionary diversity among early mammals than had been previously known (Lou et al. 2001). It is also speculated that in as early as Jurassic period mammals were occupying all available ecological niches (Wyss 2001). The present day diversity in mammals is undoubtedly the result of its early radiation through adaptation for different niches. The diversification of placental mammal orders is now unambiguously dated before the K/T boundary (Murphy and O’Brien, 2001; Tavare, et al. 2002; Springer et al. 2003). During the K/T boundary there were extremely harsh environmental conditions that probably led to large scale extinctions (Officer et al. 1986). Subsequent to the K/T transition and Gondwana split up there was cosmopolitanization of mammals that resulted in a shared lineage of old world mammals distributed in Madagascar and Penninsular India (Krause, et al. 1999). There is also evidence suggesting that the Asian ancestral mammals gave rise to at least one of the group of mammals that first appeared in North America about 55 million years ago (Beard, 2002). Therefore, the Asian region has played a significant role in mammalian radiation in the terrestrial ecosystems of the world.

Eisenberg (1981) and Novak (1999) give a good review of the evolutionary history of mammals. Modern mammals (ca. the last 5,000 years) have been placed under 28 orders, 146 families, 1,192 genera and 4,809 species (Novak 1999). An earlier comprehensive review of the taxonomy and distribution of mammal species of the world was given by Wilson and Reeder (1993). They reported 4629 species of living and recently extinct mammals. These living and recently extinct species were distributed among 26 orders, 136 families and 1135 genera. The great majority of the mammal species of the world inhabit terrestrial environments; only 2.5 % of the species occupy marine habitats. Rodentia (Rats, Squirrels and allies) is the largest mammalian order with 2015 species, more than twice the 925 species described for the next largest order Chiroptera (Bats). Both these orders are found naturally on all major land masses except Antarctica (Wilson et.al. 1996). Members of five other orders are also found in terrestrial environments of most regions: Insectivora – shrews and allies (7 families, 428 species), Carnivora – dogs, cats, bears, etc., (11 families, 271 species), Primates – apes, monkeys, lorises and allies (13 families, 233 species), Artiodactyla – deer, bison, etc., (10 families, 220 species), and Lagomorpha – rabbits and hares (2 families, 80 species). The Perissodactyla – elephants, hippos, rhinos, horses and allies (3 families, 18 species), which are native to both the New World and Old World, are also widely distributed.

The mammalian fauna of the Nearctic include 10 orders, 37 families and 643 species (14% of all the species; Cole et al. 1994). The greatest diversity of mammals in the New World is found in the Neotropical region, where 12 orders, 50 families and 1096 species occur (Cole et al. 1994). Eighty percent of the species of the neotropics are endemic to the region. The Palearctic...
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