Mammals originated from reptiles but the fossils show that the reptiles that gave rise to mammals were Synapsids that diverged from the main reptilian stock almost at the base of phylogeny during Permian period. Hence, mammalian relationship with the extant reptiles is remote. Sailbacks or Pelycosaurs from North America and Russia were primitive synapsids, without a false palate and heterodont dentition. The large sail on their back was perhaps a thermoregulatory device.
Synapsids were heavy-bodied, large-headed, stumpy-legged creatures, adapted to various types of habitats and food. Some were rodent-like seed-eaters, some insectivores. Cynodonts were dog-sized carnivorous therapsids known from all over Gondwanaland and dominant predators.
Mammalian characters in Therapsids
Increased feeding and respiratory rate due to the presence of heterodont dentition and diaphragm indicates high metabolism and endothermy in therapsids. They had developed many mammalian characteristics due to which they became dominant predators as well as herbivores of Permian period.
Majority of therapsids died out in the Triassic mass extinction. Those survived were small insectivores, the size of a small cat that thrived in hidden habitats away from the reach of the mighty dinosaurs and gradually evolved into archaic mammals. This mode of life developed acute sense of smell, sight and hearing in early mammals and consequently brain too enlarged to refine their wits that was so necessary to escape dinosaur predators of that time.
MAMMALS OF THE MESOZOIC
Mesozoic mammals were small rabbit-sized insectivores, herbivores or seed eaters, partially arboreal and active animals that were not allowed to proliferate due to the presence of ubiquitous dinosaurs. Cretaceous mass extinction eliminated most of them, leaving only small insectivorous pantotherians which found all ecological niches vacant after the extinction of dinosaurs. That led to the unusually rapid evolution in Palaeocene epoch when 7 distinct mammalian groups were differentiated, namely, insectivores, carnivores, primates, ungulates, rodents, edentates and cetaceans. By Pleistocene, mammals had become giant rulers of the earth, when another mass extinction eliminated the largest of them. Today mammals are on the decline. The mesozoic mammals were the following:
Triconodonts. They were the size of cat and had three sharp cusps on the grinding teeth. Ex. Priacodon.
Symmetrodonts. Cusps were three but were symmetrically placed in a triangle. Ex. Spalacotherium.
Pantotheres. Upper molars triangular with two cusps and lower molars with three cusps. Ex. Amphitherium.
Docodonts. Molars were nearly squarish having 4 cusps for grinding grasses. Ex. Disacus.
Multituberculates. Rodent-like herbivores with chisel-like incisors and broad molars with rows of grinding cusps. Diastema was present and canines were absent. Ex. Plagiaulax.
THE COENOZOIC PLACENTALS
Early branching of marsupials, placentals and perhaps monotremes had been accomplished before the end of cretaceous period. During early coenozoic period at least 7 mammalian groups had already been differentiated, namely, insectivores, carnivores, primates, ungulates, rodents, edentates and whales.
1. Insectivores. Shrew-like insectivores lived in hidden situations. Ex. hedgehogs, shrews etc.
2. Carnivores. They probably evolved from insectivores and continued with their ancestral carnivorous habit. Creodonts were archaic carnivores that rapidly diverged from earlier insectivores. Ex. cats, dogs, tiger, lion
3. Primates. Their evolutionary process can be traced through tree shrews, lemurs, tarsiers, monkeys.
4. Ungulates. Condylarths were primitive ungulates of Palaeocene and Eocene epochs that gave rise to all modern ungulates. Ex. Perissodactyla such as horse and Artiodactyla such as cow, buffalow, deers, goat, sheep etc.
5. Rodents. Their fossils are known from the late Palaeocene strata that possessed chisel-like incisors.They are related to Lagomorpha. Ex. Rats, mice, guinea pigs etc.
6. Edentates. Evolutionary origin of armadillos and South American anteaters is not known.
7. Whales. Earliest known whales of the middle Eocene were completely aquatic with fore limbs modified as flippers but whales evolved from mesonychids in Palaeocene.
8. Proboscideans. They evolved from some marsh-dwelling ancestor that was forced to migrate to land due to arrival of dry conditions. Sirenia and Hyracoidea are related orders that evolved from a common ancestor. Ex. Elephants.