Mesozoic

Mesozoic The term ‘Mesozoic’ (Greek: middle life) was first used by Charles Lyell in his pioneering book Principles of geology (1830) to distinguish the rocks formed between the end of the Palaeozoic era and the beginning of the Tertiary (Cenozoic). It was thus roughly equivalent to the old Secondary category of the Italian classifications, and it included the rocks of the Triassic, Jurassic, and Cretaceous periods. Mesozoic rocks occur on all the continents, the Cretaceous being the most widespread and with enormous areas of outcrop in the northern continents and in Australia.

The fossils of the Mesozoic lie between two major extinction events (‘catastrophes’, as some of Lyell's predecessors called them). At the end of the Permian period, about 90 per cent of the marine biota seem to have died off, perhaps suddenly as a result of low global sea level and a profound change in atmospheric composition. This took place about 250 million years ago. A further mass extinction occurred at the end of the Cretaceous period, some 185 Ma later when, many geologists believe, a giant meteorite impact may have been the critical cause.

The biostratigraphical division of the Mesozoic rocks is primarily based upon common marine fossils: ammonoid and belemnite molluscs. The living world of the Mesozoic witnessed the reign of the mammal-like and other reptiles, including dinosaurs, the first appearance of the mammals and birds, the rise of many extant marine invertebrate families and of conifers and flowering plants.

At the beginning of the era the continents were gathered into a single supercontinent, Pangaea, but while moving northwards across the Equator and rotating during Mesozoic times this supercontinent broke up into the major present-day continental masses. Climatic conditions over much of Pangaea were markedly arid to begin with, but in Jurassic and Cretaceous times the climate grew to be predominantly warm and continental to equable. Perhaps associated with the continental movements and with processes in the mantle beneath was the enormous extrusion of basalts in several continents, and prodigious volcanic activity in the ocean basins. Activity in the mantle may also have been responsible for the changeable nature of the Earth's magnetic polarity, which swung from reverse early in the Triassic to normal and back to reverse many times throughout the rest of the era.

D. L. Dineley

Bibliography

Hailwood, E. A. and Kidd, R. B. (eds) (1993) High resolution stratigraphy. Geological Society Special Publication No. 70.
Moulade, M. and and Nairn, A. E. M. (1978) Phanerozoic geology of the world. Mesozoic A. Academic Press, London.
Pomerol, C. (1982) The Cenozoic Era: Tertiary and Quaternary. (Trans. D. and and E. E. Humphries .) Ellis Horwood, Chichester.