Australia: The Land Where Time Began

A biography of the Australian continent 

The Mesozoic Era - Triassic, Jurassic, and Cretaceous

For most of the Mesozoic, that covered the period from about 251 Ma to about 65 Ma, all the land masses of the Earth were joined together into one supercontinent of Pangaea, stretching on one side of the Earth from the north Pole to the South Pole. This meant that there was only one huge ocean, Panthalassa, which occupied more than 70 % of the Earth's equator. Because the ocean was undivided by multiple intervening landmasses the marine circulation was much different from today, the water heated at the equator formed equatorial currents that move the warm water along the coast of the one enormous landmass to the Arctic and Antarctic. With no cold circumpolar currents to keep the warm water from the tropics away from the poles ice caps were unable to form. The result was that the poles, even during the winter when they spent several months in darkness, the warm tropical water acted like a huge heater keeping the temperatures from going too low for abundant plant life to survive, possibly in a dormant state, until the sun returned.

A band of extremely arid conditions extended for 50o north and south of the equator on the Pangaea landmass, thus separating the abundant life at the polar regions, and therefore speciation would allow the populations north and south of the arid zone to move in different directions. Gene flow must have been severely limited, if not completely stopped, at least for most species, by such an enormous expanse of aridity.

During most of the Mesozoic the flora was dominated by podocarp conifers, cycads and ferns, that included ginkgos (maidenhair trees), as well as mosses and liverworts, though angiosperms were yet to evolve.

An overview of life in the oceans at this time has been presented in the book Dinosaurs in Australia2. In the oceans this was a time when organisms such as bivalve molluscs, sea urchins, star fish and advanced colonial corals underwent a rapid diversification. For cephalopods it was also a time of major radiation, the appearance of the earliest ammonoids and belemnoids occurred at this time, these forms becoming dominant elements many of the pelagic marine invertebrate faunas. 

In the marine assemblages from this time were primitive bony fish and modern sharks, marine reptiles, such as ichthyosaurs, sauropterygians, the group that includes plesiosaurs. On the land the era began with a vertebrate fauna that included as dominant components, from the Early to Middle Triassic, temnospondyl amphibians (labyrinthodonts), that were crocodile-like, therapsids (mammal-like reptiles) and early archosauromorphs (ancestral dinosaurs and crocodiles). Forms such as the earliest true mammals, turtles, sphenodontids (Tuatara), crocodiles, pterosaurs, and dinosaurs.

Sources & Further reading
  1. Mary E. White, The Nature of Hidden Worlds, Reed, 1993
  2. Kear, B.P. & Hailton-Bruce, R.J., 2011, Dinosaurs in Australia, Mesozoic life from the southern continent, CSIRO Publishing.
Author: M. H. Monroe
Last updated 06/11/2011 



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