Tertiary period started about 70 million years ago. It was the start of the “age of mammals.” It produces adaptive radiation in the mammals. Extinction of many reptilian lineage also occurred in this period. The synapsid lineage of reptiles diverged from other amniote lineages in the Carboniferous period. Synapsid lineage formed the roots of the mammals.
1. Evolution of Pelycosaurs
Mammalian characteristics evolved gradually over a period of 200 million years. The early synapsids were the pelycosaurs.Some pelycosaurs were herbivores. The others were predators. They showed skeletal adaptations for predator’s life. The anterior teeth of the upper jaw were large. These teeth sere separated from the posterior teeth by a gap. The jaw closed and the anterior teeth of the lower jaw were adjusted in the gap. Their palate was arched. It strengthened the upper jaws. It also allowed air to pass over when prey is held in the mouth. The e legs were longer and slimmer than the earlier amniotes.
2. Evolution of therapsid
Some other successful mammal like reptiles arose from the pelycosaurs by the middle of the Permian period. They were a adverse group known as the therapsids.
(a) Predator therapsids: Some the rapsids were predators. Their teeth were concentrated it the front of the mouth. They had enlarged teeth for holding and tearing the prey. The posterior teeth were reduced in size and number. The jaws of some therapsids were elongated. These jaws produce a large biting force during closing of mouth.
(b)Herbivore therapsids: Others were herbivores. The teeth of the herbivorous therapsids were also mammal like. Some had a large space called the diastema. Diastema separates the anterior and posterior teeth. The posterior teeth had ridges (cusps) and cutting edges. These teeth were used to grind plant material. The hind limbs of therapsids were directly beneath the body. The limbs move parallel to the long axis of the body. The size and shape of the ribs also changed. It suggests that their trunk was separated into thoracic and abdominal regions. Their breathing mechanism was also similar to the mammals.
(c)Cynodont therapsids: The last therapsids were a group called the cynodonts. Some of these were as large as a big dog. But most of them were small like earliest mammals.
Fig: Members of the Subclass Svilapsida. (a) Dimetrodon was a 3 in long pelycosaur (b) Cynognathus was a mammal-like reptile
3. Evolution of first mammals
The first mammals were small in size. They had delicate skeletons. It undergoes following evolutionary processes.
(a) Evolution of feeding: The mammalian phylogeny is studied from their fossilized teeth and skull fragments. These studies suggest that the mammals of the Jurassic and Cretaceous periods were mostly predators. They fed on other vertebrates and arthropods. A few were herbivores. Some others had combined predatory and herbivorous feeding habits.
(b) Evolution of brain and sense organ: The structure of their middle ear was changed. Similarly, the regions of the brain which control hearing and olfaction were also changed. It indicates that these senses were important during the early evolution of mammals.
(c) Evolution of nocturnal habit: Most mammals have small size, well developed olfactory, auditory abilities and color vision. It suggests that the early mammals were nocturnal. But the larger dinosaurs and the smaller reptiles were diurnal. Thus the competition between early mammals and dinosaurs was reduced.
(d)Evolution of endothermy: Similarly, nocturnal habits developed endothermy. The air temperature started falling after sun set. Thus the small endotherm mammals maintain their body temperatures above their surroundings.