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General Effects
During a mass extinction, many types of animals, plants or both appear to die off all at the same time. The causes are unknown, although scientists continually study and revise theories. When these species die out, new species fill the ecological niches left. They take over the territory of the extinct species and gain access to whatever resources are left. This allows a few species to diversify into many species. The positive effect of a mass extinction is speciation -- new species develop and branch out from the species that survived.
K-T Extinction
The K-T extinction refers to the mass extinction at the end of the Cretaceous Period. This was when the dinosaurs died off. The survivors were the small, rodent-like mammal, the first birds, some amphibians and reptiles like crocodiles, which still exist in the same form today. The end of the dinosaurs left room for mammals and birds to develop, when before they were scrounging for resources left by the much larger dinosaurs.
Pre-Dinosaur Extinctions
The Cambrian era from 543 million to 510 million years ago experienced four mass extinctions. Life only existed in the ocean at this point, but these extinctions gave room for the ancestral species of clams, snails, sea urchins and octopi to develop, not necessarily in that order.
The Ordovician extinction occurred 440 million years ago and was the second most devastating mass extinction. Many types of invertebrates like coral and clams, as well as most of the water plants that built reefs, died off. This made room for the first sharks and bony fish.
After the Ordovician extinction was the Devonian period. The extinction that ended this era decimated reef-building organisms so badly that reefs were uncommon for millions of years to follow. This left room for sharks, clams and other marine life to diversify, and many land species like coniferous plants and insects flourished afterward during the following Permian period.
Permian Extinction
The Permian extinction happened roughly 248 millions years ago. This was the most devastating mass extinction, affecting 90 percent to 95 percent of marine life, and 70 percent of land animals. It killed off the pelycosaurs like Dimetrodon, euryptarids (a type of arthropod) and trilobites. The ancestors or modern sharks, sea urchins, corals and bony fish were devastated. This left a low diversity of surviving species, but made room for dinosaurs and the first mammals.
Neogene Extinction
The most recent mass extinction occurred around 11,000 years ago, rather than millions of years. The Neogene extinction only affected large mammals and killed off saber-tooth cats, wooly mammoths, dire wolves and ground sloths, among other species. Humans were around by this time, and are implicated as a cause of this mass extinction. Regardless of cause, the extinction of these giant mammals left room for humans to migrate, and for modern animals to develop in their stead.