Geology & Paleontology
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Environmental Niche - Extinction of the Dinosaurs
"The mass extinction of dinosaurs at the end of the Cretaceous Period seems to have cleared the way for mammals to expand into vacated niches".
The statement suggests that the extinction of the dinosaurs was a necessary precondition for the development of mammals and implies that mammals fill the same ecological niches as the dinosaurs. This account will discuss the concepts involved and consider the evidence concerning the mass extinction of dinosaurs.
Dinosaurs were a group of reptiles which originated in the middle Triassic period (206 to 248 million years ago) and became extinct at the end of the Cretaceous (65 million years ago) (Morely, P., p 24) They were the dominant land creatures for over 150 million years. Other reptile groups lived in the sea (ichthyosaurs) and had the capacity for flight (plesiosaurs).
The evidence for the existence of dinosaurs is found in the fossil record. Fossil remains (including fossilised bones, tracks and faeces) have been found for dinosaurs of all sizes from chicken-sized creatures to enormous creatures like the Brachiosaur (Morely, P., p 26). Dinosaurs were carnivores, herbivores and omnivores. This suggests that dinosaurs were found in all ecological niches for land vertebrates (Morely, P., p 26).
An organism's total requirements for all resources and physical conditions is called its ecological niche, determining where it can live and the population level it can sustain. An organism's niche reflects its life history, habitat and position in the food chain. (Pidwirny, M., online). Gause's Law of competitive exclusion (Gause: 1934) holds that no two species can occupy the same niche. If so, dinosaurs' occupation of existing niches impeded the development of mammalian species.
The fossil record supports this argument. Although the earliest mammals appeared in the mid-Triassic, there was limited mammal species diversity until the Tertiary (early Cenozoic). Diversity increased slowly but steadily from the beginning of the Paleogene era (65 Ma), becoming ever greater throughout the Neogene era (Morely, P., p 24). Reptile species diversity increased over the same periods but has not expanded rapidly (unlike mammals) nor approached the dinosaur era levels.
There is a "normal" level of extinction, an intrinsic part of the processes of evolution and adaptation. At any time, individual species may become extinct in a locality but survive elsewhere, or may evolve into new ones. Whole species may become extinct and new species arise, in a process that can be seen as a normal level of "turnover of species" (Morely, P., p 25).
When the fossil record shows a geographically widespread reduction in diversity of species taking place over a relatively short time, this is known as mass extinction. The identification of distinct geological periods reflects the changing varieties of fossil species in particular layers of rock. There have been five major mass extinctions, the last of which happened near the end of the late Cretaceous (K-T boundary). Whole groups of organisms became extinct (including dinosaurs, plesiosaurs, pterosaurs and 70% of marine species (Morely, P., p 25) or suffered a massive reduction in diversity. On the evidence of the K-T boundary mass extinctions, it appears that larger bodied species - such as the dinosaur - were most vulnerable, possibly because they tended to have smaller populations, lower rates of population increase and greater specialisation (Morely, P., p 25).
An explanation for the K-T boundary mass extinctions may be found in the high concentration of iridium in a K-T boundary clay layer. Iridium is a metallic element, rare on Earth but relatively common in some meteorites. It was postulated that the clay was fallout from the impact of a fast-moving (more than 10 km s-1) and large (ca. 10km across) meteorite in the Chicxulub Crater, in the Gulf of Mexico (Morely, P., p 29). Such an impact would have impeded photosynthesis, with adverse effects on the food chain - including the dinosaur herbivores, omnivores and carnivores. The argument is supported by other geological findings, such as the presence in the clay layer of splash shaped droplets and of diamonds and other mineral structures formed by pressure.
Morley, P., ed. (1998) S103 Block 10, Earth and life through time, Milton Keynes, The Open University.
Odum, H.T. (1983) Systems Ecology: An Introduction. New York, Wiley & Sons.
Pidwirny, M., Dr (2004) "Concept of Ecological Niche" available from http://www.physicalgeography.net/fundamentals/9g.html [accessed 25 April 2005]
Gause, G. F. (1934). "The struggle for existence". Baltimore, Williams & Wilkins, referenced by Wikipedia available from http://en.wikipedia.org/wiki/Competitive_exclusion_principle [accessed 25 April 2005]