Earth's Jurassic Period

The Jurassic Period is probably the most popular and publicly well-known of the geologic periods. It followed the Late Triassic Extinctions and lasted from around 199 million years ago to 145 million years ago. The period constitutes the middle period of the Mesozoic Era and was the one in which dinosaurs exerted their supremacy over the Earth.

Geologically, the Earth was changing drastically during the Jurassic. The supercontinent of Pangaea broke apart during the Jurassic Period into the three separate continents of North America, Eurasia (containing present-day Europe and Asia), and Gondwana (containing present-day Antarctica, Africa, and South America). With the increased amount of coastline and smaller distance from the ocean to the continental interior, the continents experienced a much wetter climate than during the Permian or Triassic Periods. The continental rift also gave birth to a young and comparatively small Atlantic Ocean in between North American and Eurasia. Generally the Earth's climate was warm and wet, two very potent catalysts for evolution to run wild.

The wet, hot conditions of the Jurassic Period allowed for forests to regain their proliferation across the continents. Conifers dominated the trees in these new forests, though tree ferns and smaller undergrowth ferns made a comeback after being banished to the fringes since the Carboniferous. The various canopy ecosystems supported a wide range of dinosaurs, with smaller herbivores grazing on the ground ferns, and larger ones grazing out on fern prairies and in the treetops. As of yet there were no flowering plants or palm trees, although primitive forms of palms known as cycads were present in the Jurassic forests.

During the Jurassic, marine reptiles asserted their authority over the top of the ocean food chains. These marine reptiles ranged in size from the dolphin-like ichthyosaurs to the titanic whale-sized liopleurodon, which is arguably the largest carnivore ever to exist. Sharks evolved into their modern groups during this period, continuing in their niche as fearsome predators and giant marine crocodiles up to fifty feet in length also existed during the Jurassic. In terms of invertebrates and smaller marine organisms, fish continued to evolve and form the backbone of intermediate food chains, and plankton, corals, and brachiopods continued to thrive. Filter-feeding became increasingly important in the oceans and gigantic fish such as leedsichthys filled the niche that many baleen whales occupy today. Cephalopods called ammonites (shelled squid-like creatures) were also common in the Jurassic seas.

The dinosaurs ruled every inch of the continents. The Jurassic Period was the point in which dinosaurs reached their largest size, the largest of which were the sauropods. These gigantic tetrapods were some of the most successful herbivores of the time and could reach enormous sizes. Diplodocus could reach lengths of ninety feet and brachiosaurus was as tall as a seven story building. Once these great herbivores reached a certain size, they had no natural predators. Small The majority of Jurassic plant-eaters fed on ferns and conifers, as most lacked specialized teeth to chew or grind up tougher vegetation.

With the giant herbivores came giant predators. Therapods, or large bipedal predatory dinosaurs reached lengths of thirty feet long and included the well-known allosaurus, ceratosaurus, and megalosaurus. Although the Jurassic is known as a period of giants, not all of the resident herbivores or carnivores were huge. Small, gazelle like herbivores like hypsilophodonts were preyed upon by small predators called ceolophysids and still other dinosaurs were omnivorous. A rich diversity played out in the Jurassic forests, plains, and seas that is comparable to many ecosystems today.

It is also during the Late Jurassic that the first birds began to evolve from small predatory dinosaurs. However, it wasn't until later periods that the birds would become the dominant creatures of the skies. Flying reptiles called pterosaurs ruled the air and evolved into a diverse menagerie of shapes, forms, and sizes. Previous groups of dominant tetrapods such as the large amphibians began to diminish, and most amphibians were of smaller, more modern types like frogs and newts. The mammals were consigned in the Mesozoic to eek out an existence in the shadow of the giants.

As the Jurassic drew to a close, the Earth was once again drastically changing. It was during the next period of the Mesozoic that dinosaurs would play out their final act.