paleontology8: The rise of mammals (v1.0)

In previous essays I talked about the rise of reptiles and subsequently dinosaurs, birds, and marine reptiles. But a new class of animals evolved called mammals. How did mammals evolve and when?

Reptiles are split into 4 groups by the number and position of temporal fenestrae (openings in the sides of the skull behind the eyes). Mammals evolved from synapsid vertebrates (one low fenestrae). In contrast, Dinosaurs, snakes, lizards, and crocodiles have two fenestrae and are called diapsid vertebrates. After the mass extinction 250 million years ago, a subset of synapsids called therapsids survived (sometimes called mammal like reptiles). Therapsids are characterized by having differentiated teeth and stronger jaws. These include canines, molars, and incisors. Mammals evolved from therapsids. The oldest known mammal is from 220 million years ago (in the Triassic period). So, they arose at around the same time as dinosaurs. They were small while dinosaurs ruled. Mammals are warm blooded, have mammary glands, a neocortex in the brain, fur, or hair, and the three middle ear bones. 

There are three types of mammals: Monotremes, marsupials and placentals.

1.     Monotremes are egg laying primitive mammals and an example is platypus and echidnas and today found in Australian Continent. 

2.     Marsupials carry their young in a pouch and include Kangaroos, Koalas, Tasmanian Devil, Wombats, Bandicoots, Wallabies, and opossum. Today they are found in Australian Continent and in South America. 

3.     Placentals are a large majority of mammals. The fetus is carried in the uterus, and it gives birth. 

The rest of this essay talks about Placentals. The oldest known placental mammal is from 160 million years ago (Jurassic Period). The emergence of placental mammals was a major evolutionary change. They were a great evolutionary success, especially after the extinction of the dinosaurs 66 million years ago (start of Cenozoic Era which stretches from 66 million years to today) when there was an explosion of placental mammals as they filled the void left behind by the dinosaurs, and the marine reptiles. Placental mammals spread across both the terrestrial and aquatic ecosystem. There are over 4000 placental mammal species today.

Placental mammals are divided into four major groups.

1.     Afrothere include certain shrews, moles, aardvark, elephants, and manatees.

2.     Xenarthra includes armadillo, anteater, and sloth.

3.     Breatharian includes certain shrews, lemurs, monkeys, gibbons, great apes (hominids), rabbits, hares, rodents, beavers, gophers, rats, mice, hamsters, squirrels, porcupines, and chipmunks.

4.     Laurasiatherian includes bats, horses, zebras, donkeys, rhinos, dogs, bears, big cats, seals, pandas, skunks, weasels, pigs, camels, giraffe, deer, sheep, cattle, whales, dolphins, and porpoise.

Marine placental mammals are an interesting group and include porpoises, dolphins, and whales. They adapted from land to the sea. The Blue Whale is the largest animal known ever to have existed. Let us look at the evolution of whales. Which land mammal is the ancestor of whales and when? This happened over the last 55 million years, so it is recent!! (oldest known primitive whale is 53 million years ago).

Some primitive whale fossils were found from 35 to 40 million years ago in southeastern US. This fossil was related to artiodactyls (includes pigs, hippos, and deer) which are hoofed land-dwelling mammals. Whales were initially presumed to have evolved from Mesonychids (an extinct mammal) and Mesonychids and Artiodactyls were presumed to have a common ancestor. But later evidence showed that whales belonged to Artiodactyls themselves and their detailed evolution was mapped out!

This essay outlines the rise of mammals 220 million years ago and their explosion after the mass extinction event 66 million years ago. They spread across both land and sea.