The K–Pg mass extinction took place at the end of Cretaceous Era and the onset of the Paleogene System. At the end of the Cretaceous, just before the K–Pg event, between 40% and 75% of marine life forms and terrestrial species disappear from fossilized records. Included in K-Pg mass extinction were well-known groups as flightless dinosaurs, related reptile species and many species of marine plankton.
The K-Pg (Cretaceous–Paleogene) mass extinction, also termed as the K-T (Cretaceous–Tertiary) extinction, is documented as a mass extinction of nearly three-quarters of fauna and flora on Earth, which includes all non-avian dinosaurs (Macleod, 2012). This event transpired over a geologically short period of 66 million years ago. This event marked the close of the Cretaceous period and the complete Mesozoic Era, beginning the Cenozoic Era that continues till date.
The mass extinction affected all terrestrial organisms big or small, including plants, insects, lizards, dinosaurs, mammals, flying reptiles (See Appendix for fossilized Pterosaur), and birds. In water bodies, the K–Pg mass extinction wiped out the giant marine lizards, plesiosaurs, many varieties of fish, varieties of sharks, most mollusks (ammonites especially) and many species of plankton (Rene et al., 2013). However, the devastation caused by this mass extinction made way for evolutionary opportunities. After the mass extinction, many species underwent remarkable adaptations to form abundant variance of new forms and species that began to inhabit the disrupted and collapsed ecological niches (Macleod, 2012). Mammals in particular were the most successful in diversification in the Paleogene period, adapting new forms from horses to whales and bats to primates. Birds, fish and reptiles like lizards also underwent adaptive changes.
The K–Pg mass extinction is widely accepted as a global event. Highly distributed groups for example Mosasaurs and ammonites disappeared from all over the world. The extinctions happened simultaneously on land and in the sea. The fact that the extinction was rapid suggests that the extinction resulted from a severe global environmental catastrophe.
Species dependent on photosynthesis deteriorated or became extinct as the atmosphere blocked the sun and reduced the amount of solar energy reaching the Earth. This widespread extinction of plants and phytoplankton gave a major reshuffle to the dominance of all groups. Photosynthesizing organisms, like phytoplankton and land plants, constituted the base of all the food chains in the late Cretaceous just as they do today (Rene et al., 2013). Fossil evidence suggests that herbivores died out when plants they depended on for nutrition became limited. As consequence, predators at the top of the food chain, for example Tyrannosaurus Rex, also disappeared.
The extinction was characteristically selective. Some living groups were comparatively unaffected, some were devastated, and others were completely wiped out. A large group of reptilians including alligators, turtles, and salamanders survived. Many species of mammals, birds, and smaller lizards suffered huge frequency of extinction (Fortey, 1999). Coccolithophorids and mollusks (See Appendix A), and those organisms whose nutrition derived from these shell building species, became extinct or suffered from heavy losses in population (Alvarez et al., 1980). Non-flying dinosaurs and pterosaurs completely disappeared (Elewa, 2008).
Omnivores, carrion-eaters and insectivores survived the K-Pg extinction, presumably because of increased availability of food sources. Scientists put forward hypothesis where such organisms survived collapse of plant-dependent food chains because they fed on detritus (non-living organic material). In aquatic communities especially in streams (See Appendix A), only small number animal groups became extinct because these relied more on detritus that washed in from land, saving them from extinction. Extinction was more prominent among animals living in oceanic water column than animals living on or in the ocean floor as animals in water columns were almost completely dependent on phytoplankton while animals on or in the ocean floor feed on detritus or could switch to detritus nutrition.
Post K–Pg mass extinction, biodiversity required a great amount of time to recover, although vacant ecological niches existed in abundance.
A Terodactyl Fossil
Alvarez LW, Alvarez W, Asaro F, Michel H.V. (1980). Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208: 1095-1108
MacLeod, N (2012) Extinction: K–Pg Mass Extinction Published Online: DOI: 10.1002/9780470015902.a0001656.pub3
Fortey, R (1999). Life: A Natural History of the First Four Billion Years of Life on Earth. Vintage. pp. 238–260.ISBN 978-0-375-70261-7.
Elewa, AMT (2008). Mass Extinction. pp 129-131 K-Pg mass extinction
Renne, Paul R.; Deino, Alan L.; Hilgen, Frederik J.; Kuiper, Klaudia F.; Mark, Darren F.; Mitchell III, William S.; Morgan, Leah E.; Mundil, Roland and Smit, Jan (2013). "Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary". Science 339 (6120): 684 687. Bibcode: 2013Sci339..684R. doi: 10.1126/science.1230492.
[Untitled photograph of Mollusk Fossil]. Retrieved on 26 September, 2013, from http://utahscience.oremjr.alpine.k12.ut.us/sciber02/elem/4th/fossils/html/intro.htm
[Untitled photograph of A Terodactyl Fossil]. Retrieved on 26 September, 2013, from http://www.drbateman.net/gcse2003/gcsesums/modsum9/mod9.htm
[Untitled photograph of a fossil specimen]. Retrieved on 26 September, 2013, from http://nahantmarsh.org/event/muddy-boots-nature-club-15/