JURASSIC PERIOD www.johnsibbick.com
By the beginning of the Jurassic, the supercontinent Pangaea
had begun rifting into two landmasses, Laurasia to the north and Gondwana to the
south. This created more coastlines and shifted the continental climate from
dry to humid, and many of the arid deserts of the Triassic were replaced by
lush rainforests. On land, the fauna transitioned from the Triassic fauna,
dominated by both dinosauromorph and crocodylomorph archosaurs, to one
dominated by dinosaurs alone. The first birds also appeared during the
Jurassic, having evolved from a branch of theropod dinosaurs. Other major
events include the appearance of the earliest lizards, and the evolution of
therian mammals, including primitive placentals. Crocodylians made the
transition from a terrestrial to an aquatic mode of life. The oceans were
inhabited by marine reptiles such as ichthyosaurs and plesiosaurs, while
pterosaurs were the dominant flying vertebrates.
The arid, continental conditions characteristic of the
Triassic steadily eased during the Jurassic period, especially at higher
latitudes; the warm, humid climate allowed lush jungles to cover much of the
landscape. Gymnosperms were relatively diverse during the Jurassic period. The
Conifers in particular dominated the flora, as during the Triassic; they were
the most diverse group and constituted the majority of large trees.
Extant conifer families that flourished during the Jurassic
included the Araucariaceae, Cephalotaxaceae, Pinaceae, Podocarpaceae, Taxaceae
and Taxodiaceae. The extinct Mesozoic conifer family Cheirolepidiaceae
dominated low latitude vegetation, as did the shrubby Bennettitales. Cycads
were also common, as were ginkgos and Dicksoniaceous tree ferns in the forest.
Smaller ferns were probably the dominant undergrowth. Caytoniaceous seed ferns
were another group of important plants during this time and are thought to have
been shrub to small-tree sized. Ginkgo plants were particularly common in the
mid- to high northern latitudes. In the Southern Hemisphere, podocarps were
especially successful, while Ginkgos and Czekanowskiales were rare.
In the oceans, modern coralline algae appeared for the first
time.
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