Jurassic Period
The Jurassic Period, spanning from 202 to 145 million years ago, is recognized as the middle segment of the Mesozoic Era and is often referred to as the "age of dinosaurs" due to the dominance of these creatures on Earth during this time. Named after the Jura Mountains in Europe, the period is characterized by the gradual separation of the supercontinent Pangaea, leading to unique evolutionary paths for various plant and animal species according to their isolated environments. The Jurassic is divided into three epochs—Early, Middle, and Late—each marked by distinct fossil records shaped by geological processes.
During this period, dinosaurs reached remarkable sizes and diversity, with notable species such as Apatosaurus and Stegosaurus. The era also saw the emergence of early mammals and various marine reptiles like Ichthyosaurus and plesiosaurs. The Jurassic landscape was rich in flora, featuring ferns, cycads, and conifers which supported the diverse fauna. Major geological events, including oceanic expansions and contractions, contributed to the sedimentary rock formations that preserved fossils, revealing insights into the life forms of the time. The study of Jurassic fossils has evolved significantly, enhancing our understanding of these ancient creatures and their environments.
Jurassic Period
Introduction
The Jurassic is the middle period of the Mesozoic era. Named after the Jura Mountains in Europe, where large geological deposits from that time are prominent, the Jurassic is sometimes known as the “age of dinosaurs,” in reference to the creatures that dominated life of the planet during the period. As the landmasses that had once made up the supercontinent of Pangaea continued to separate throughout the period, the plants and animals of the different regions began to differentiate along individual evolutionary paths according to the particular environments they inhabited.
Time Defined
The Jurassic is one of three geological time periods that make up the Mesozoic era. Beginning 202 million years ago and ending 145 million years ago, the Jurassic falls between the Triassic and Cretaceous. It is divided into three epochs—the Early, Middle, and Late—which collectively comprise 11 shorter ages. These divisions are based on the nature of the fossil record in different areas and their relative locations in the rock strata. Numerical dating of fossils from this period relies on measuring the decay of radioactive elements within the geological formations in which they are found, as well as their positions relative to other fossils with known dates.
Research History
Dinosaurs from the Jurassic period were the first to be discovered and studied scientifically. As the demands and technological capabilities of the Industrial Revolution drove geologists and miners to dig deeper and go farther afield, numerous new fossils were discovered, including the remains of the Jurassic carnivore Megalosaurus, which were found in England in the 1820s.
As industrialization in Europe progressed, particularly in Germany, mining efforts continued to be the primary source of new fossil discoveries. The United States, however, experienced the “Bone Wars” of the famous fossil hunters and bitter rivals Edward Drinker Cope and Othniel Charles Marsh in the late nineteenth century, as they sought and found fossils from new species in western geological deposits from the Late Jurassic, including Apatosaurus, Stegosaurus, Camarasaurus, Diplodocus, Allosaurus, Camptosaurus, and Ceratosaurus.
Since the 1960s, paleontologists such as Robert Bakker and John Ostrom have used progressively more sophisticated means to study newly discovered fossils, and their research has revised the view of what dinosaurs from the Jurassic and other periods were like. Once believed to have been cold-blooded, scaly-skinned, slow-moving reptiles that were forced to spend their lives buoyed up by water, dinosaurs are now increasingly understood to have been warm-blooded, active, remarkably well-developed creatures, many of which bore feathers like their present-day descendants—birds.
Major Events
The Jurassic is noted for the many expansions and contractions of the oceans, particularly the Tethys Seaway, which began forming in the Triassic. As these expansions and contractions occurred, many regions of the surface land became the sea floor as new lakes and sea areas formed. When the waters retreated, they became land again. Many of the sedimentary rock deposits of Europe, Africa, and North America were formed from this action. As Pangaea continued to separate during the period, the movement of continent-sized blocks of the earth's crust formed mountain ranges and triggered increased occurrences of volcanic and seismic activity. In addition, dinosaurs grew to their greatest sizes during this period. At the same time, tiny mammals began to develop.
Life in the Period
The Jurassic is often referred to as the age of dinosaurs, as it was in this period that these well-known creatures reached their greatest size and diversity. The fossil record is replete with Jurassic dinosaurs, more so than the earlier Triassic, in which they first developed, and the later Cretaceous. The reason for this may be attributed, at least in part, to the ebb and flow of oceanic changes. These changes occurred over a great span of time; nevertheless, they laid down the vast sediment beds in which Jurassic fossils are preserved. The effects these oceans had on rainfall and weather patterns contributed to flash floods, sedimentation, and other related effects inland that provided conditions for the preservation of fossils.
These geological conditions also brought about increased diversity of creatures, as species separated by inland seas, or other new formations began to evolve in different ways. The creatures of the Jurassic varied in size and type, ranging from giant sauropods such as Apatosaurus to the protobird Archaepteryx. Jurassic seas were home to Ichthyosaurus and Ophthalmosaurus, the pliosaurs and plesiosaurs, Kronosaurus, Liopleurodon, and many different kinds of fish, including sharks. Dimorphodon, Rhamphorynchus, Pterodactylus, and their ilk commanded the Jurassic skies, while on the ground, true mammals, such as the tiny Ctenacodon, began to populate the earth among the giant dinosaurs. Along with the animals of the time, an abundance of plants helped to support these many living creatures. Ferns, ginkgoes, bennettitales and cycads flourished during this period. Many different conifers were also present, including cypresses, pines, yews, and a close relative of living redwoods.
Bibliography
Bakker, Robert. Dinosaur Heresies: New Theories Unlocking the Mystery of the Dinosaurs and Their Extinction. Kensington, 1986.
Desmond, Adrian J. Hot-Blooded Dinosaurs: A Revolution in Paleontology. Doubleday, 1981.
Holmes, Thom. Dawn of the Dinosaur Age: The Late Triassic and Early Jurassic Epochs. Chelsea House, 2008.
Illustrated World of Science Encyclopedia, Vol. 5. Creative World Publications, 1971.
Novacek, M. Dinosaurs of the Flaming Cliffs. Doubleday, 1996.
Paul, Gregory S. Princeton Field Guide to Dinosaurs. Princeton UP, 2010.
Schoepfer, Shane D., et al. "The Triassic–Jurassic Transition–A Review of Environmental Change at the Dawn of Modern Life." Earth-Science Reviews, vol. 232, 2022, p. 104099.