Cretaceous-Tertiary extinction event: Encyclopedia II - Cretaceous-Tertiary extinction event - Casualties of the extinction

Cretaceous-Tertiary extinction event - Casualties of the extinction

A wide range of organisms became extinct at the end of the Cretaceous period. The most conspicuous, of course, were the dinosaurs. While there is evidence that dinosaur diversity declined in the Late Cretaceous of North America, many species are known from the Hell Creek and Lance Formations of the Late Cretaceous. These include six or seven families of theropods and a similar number of ornithischians. Among the Dinosauria, the only survivors were the birds, but birds suffered heavy losses. A number of diverse groups became extinct, including the Enantiornithes and Hesperornithiformes; the last of the pterosaurs also went extinct. A number of mammal groups also became extinct. In the sea, many species of phytoplankton were wiped out. The great sea reptiles of the Cretaceous, the mosasaurs and plesiosaurs, also fell victim to extinction. Among mollusks, the ammonites, a diverse group of coiled cephalopods, were exterminated, as were the specialized rudist and inoceramid clams. Freshwater mussels and snails also suffered heavy losses in North America. Much less is known about how the K-T event affected the rest of the world. It should be emphasized that the survival of a group does not mean that the group was unaffected: a species may be 99% annihilated by an asteroid strike, yet still manage to survive.

Darkness from an impact-generated dust cloud (Alvarez et al. 1980) may have been supplemented by associated gases.

Darkness resulted in loss of photosynthesis both on land and in the oceans. On land preferential survival may be closely tied to animals that were not in food chains directly dependent on plants. Dinosaurs (both herbivores and carnivores) were in plant-eating food chains.

Mammals of the Late Cretaceous were not herbivores. Many mammals fed on insects, larvae, worms, snails etc., which in turn fed on dead plant matter. During the crisis when green plants disappeared, mammals may have survived, because they lived in “detritus-based” food chains. Soon after the K/T extinction the mammals radiated into plant-eating lifestyles, and were soon followed by other mammals that became carnivores.

In stream communities few groups of animals became extinct. Stream communities tend to be less reliant on food from living plants and are more dependent on detritus that washes in from land. The stream communities may also have been buffered from extinction by their reliance on detritus-based food chains. (See Sheehan and Fastovsky, Geology, v. 20, p. 556-560.) Similar, but more complex patterns have been found in the oceans. For example, animals living in the water column are almost entirely dependent on primary production from living phytoplankton. Many animals living on or in the ocean floor feed on detritus, or at least can switch to detritus feeding. Extinction was more severe among those animals living in the water column than among animals living on or in the sea floor.

Adapted from the Wikipedia article "Casualties of the extinction", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki