The Jurassic Period was the second segment of the Mesozoic Era. It occurred from 199.6 to 145.5 million years ago, following the Triassic Period and preceding the Cretaceous Period.
During the Jurassic Period, the supercontinent Pangaea split apart. The northern half, known as Laurentia, was splitting into landmasses that would eventually form North America and Eurasia, opening basins for the central Atlantic and the Gulf of Mexico. The southern half, Gondwana, was drifting into an eastern segment that would form Antarctica, Madagascar, India and Australia, and a western portion that would form Africa and South America. This rifting, along with generally warmer global temperatures, allowed for diversification and dominance of the reptiles known as dinosaurs.
By the Mesozoic Era, living things had evolved the capability of living on the land rather than being confined to the oceans. By the beginning of the Jurassic, plant life had evolved from Bryophytes, the low-growing mosses and liverworts that lacked vascular tissue and were confined to swampy moist areas.
Ferns and gingkoes, complete with roots and vascular tissue to move water and nutrients and a spore system of reproduction, were the dominant plants of the early Jurassic. During the Jurassic, a new method of plant reproduction evolved. Gymnosperms, cone-bearing plants such as conifers, allowed for wind distribution of pollen. This bisexual reproduction allowed for greater genetic combination and by the end of the Jurassic, the gymnosperms were widespread. True flowering plants did not evolve until the Cretaceous.
Age of the dinosaurs
As Steven Spielberg’s 1993 film "Jurassic Park" asserts, reptiles were the dominant animal life forms during the Jurassic Period. Reptiles had overcome the evolutionary hurdles of support and reproduction that limited the amphibians. Reptiles had strong ossified skeletons supported by advanced muscular systems for body support and locomotion. Some of the largest animals ever to live were dinosaurs of the Jurassic Period. Reptiles were also capable of laying amniotic eggs, which kept the developing young moist and nourished during gestation. This allowed for the first fully terrestrial animal life cycles.
Sauropods, the “lizard hipped” dinosaurs, were herbivorous quadrupeds with long necks balanced by heavy tails. Many, such as Brachiosaurus, were huge. Some genera obtained lengths greater than 100 feet and weights over 100 tons, making them the largest land animals ever to walk the earth. Their skulls were relatively small, with nostrils carried high near their eyes. Such small skulls meant that they had very small brains as well. Despite the small brains, this group was very successful during the Jurassic Period and had a wide geographic distribution. Sauropod fossils have been found on every continent except Antarctica. Other well known dinosaurs of the Jurassic include the plated Stegosaurus and the flying Pterosaurs.
Carnosaurus means “meat-eating dinosaur.” With such large herbivorous prey animals, it makes sense that large predators were also common. Allosaurus was one of the most common Carnosaurs in North America; numerous intact skeletons have been found in the fossil beds of Utah. Allosaurus was superficially similar to the later evolving Tyrannosaurus rex, although cladistic analysis shows them to be only distantly related. Allosaurus was a bit smaller with a longer jaw and heavier forelimbs. They relied on the stronger hind limbs for a running gait, but it is unclear how fast they could move.
It is unlikely to have been common for an Allosaurus to take on a healthy large adult herbivore like a Brachiosaurus or even a Stegosaurus. They were likely opportunistic, consuming young, sick, aged or injured prey. They were probably able to grasp such prey with their heavily muscled forelimbs, tearing it to pieces with large claws and then swallowing the pieces whole.
Dinosaurs may have been the dominant land animals, but they were not alone. Early mammals were mostly very small herbivores or insectivores and were not in competition with the larger reptiles. Adelobasileus, a shrew-like animal, had the differentiated ear and jaw bones of a mammal and dates from the late Triassic.
In August 2011, scientists in China announced discovery of Juramaia. This tiny animal of the mid-Jurassic has caused excitement among scientists because it is clearly a eutherian, an ancestor of placental mammals, indicating that mammals evolved much earlier than previously thought.
Marine life of the Jurassic Period was also highly diversified. The largest marine carnivores were the Plesiosaurs. These carnivorous marine reptiles typically had broad bodies and long necks with four flipper shaped limbs. Ichthyosaurus was a more fish-shaped reptile that was most common in the early Jurassic. Because some fossils have been found with smaller individuals that appear to have been inside the larger ones it is hypothesized that these animals may have been among the first to have internal gestation and bear live young. Cephalopod ancestors of modern squid and finned relatives of modern sharks and rays were also common. Among the most beautiful fossils of marine life were left by the spiral shells of the Ammonites.
Editor's note; This article was updated on April 17, 2013, to correct descriptions of life during the Mesozoic Era and of the Juramaia.