Reptile-like mammal ancestors (synapsids) dominated the Permian Period; however, the Great Permian extinction wiped out the dominant synapsid carnivores and the archosaurs were dominant. Tiny eutherian mammals evolved superior sensory systems and intelligence in the world of archosaurs. Little eutherian insectivores survived the End Cretaceous extinction, somehow became placental, and diversified to become the placental mammal orders of the Cenozoic Era.
The mammals descended from one branch of the reptiles, the synapsids. In the early Permian, they were similar to the ancestors of the archosaurs, with jaws constructed from five bones, peg teeth but larger canines, a sprawling gait, no hair, and no internal temperature regulation. By the end of the Permian, mammal legs had shifted from a sprawling position to a semi sprawling position. Other evolved characteristics included differentiated teeth, secondary palate, minimal thermal regulation, possible beginning of hair, and progress toward a mammalian jaw.
The cynodont Thrinaxodon survived the great Permian extinction. In the world of the archosaurs, conodonts became small and evolved many mammalian traits as they adapted to hunting at night and steering clear of dangerous archosaurs, including birds. The tritheolodonts (Banner above) evolved by the end of the Triassic. They were just on the cusp of classification as a mammal.
The class Mammalia evolved in the Early Jurassic and diverged into several subclasses during the Jurassic and Cretaceous, which included the ancestors of monotreme, marsupial, and placental mammals. Although controversial, the fossil record indicates that three nonplacental ancestors of modern placental mammals survived the End Cretaceous extinction and became the placental ancestors of the three major groups of placental mammals on northern continents: (1) carnivores (Ferae), (2) horses and cows (Euungulata), and (3) rodents and primates (Euarchontaglires).
There have been five great extinctions in the Phanerozoic Eon. Sixty six million years ago, the End Cretaceous extinction destroyed the dinosaurs and opened the way for the little mammal insectivores of the Cretaceous to take over the world during the Age of Mammals. The catastrophe had two causes on opposite sides of the world, an asteroid impact in what is now Mexico and a flood magma event in India. This section focuses on the asteroid.
Initially, most scientists concluded that the asteroid killed the dinosaurs, but there was another candidate. Peter Vogt had proposed in 1972 that a massive flood volcanic event in India, which formed the Deccan Traps, emitted sulfur dioxide, which polluted the atmosphere and killed the dinosaurs (Figure 11‑35). Hot magma plumes periodically well up from the inner earth and release massive quantities of lava to earth’s surface, along with toxic gases.
Now that the geologists and geophysicists have precisely dated the geologic events, if not their precise linkages, paleontologists, geologists, and atmospheric scientists are examining the precise effects of each and their combined effects in the geologic and fossil record. Analyzing the effects of different parameters helps scientists determine the precise effects of each event and their effects on different types of animals.
After the End Cretaceous extinction eliminated the dinosaurs, mammals rapidly diversified and occupied the ecological niches that the End Cretaceous Extinction opened. The Boreoeutheria (placental mammals) evolved on the northern continents, which were disconnected from Africa and South America. The nonplacental marsupials evolved in South America and Australia (possums, kangaroos). The placental Xenartha (sloths, armadillos, and anteaters) also evolved in South America, and the placental Afrotheria (elephants) evolved in Africa.
At the beginning of the sixth day, Moses described the evolution of protomammals on Earth and a subsequent miracle that modified mammals and gave them their present form.
Mammaliaformes (trithelodont) – Morganucodon (205 Ma). Credit: Funkmonk. Used here per CC BY-SA 3.0.