The Handy Dinosaur Answer Book. Patricia Barnes-Svarney

      What is the Jurassic period and how did it get its name?

      The Jurassic period follows the Triassic on the geological time scale. Though the dinosaurs had their origins and approximately 25 million years of evolution in the Triassic period, it was not until the Jurassic that this group really blossomed. This was the time when the giant, herbivorous sauropods like Apatosaurus roamed the land; when plated dinosaurs like Stegosaurus first appeared; and when large carnivorous species like Allosaurus preyed on the other dinosaurs. It was also when Archaeopteryx—a creature that many paleontologists consider to be one of the first ancestors of birds—flew through the air.

      The name Jurassic comes from the Jura mountain range, a chain of mountains that straddle the border between France and Switzerland. It was there that the first Jurassic period sedimentary rock and accompanying fossils were found. The Jurassic is the second of three periods (the first is the Triassic and the last is the Cretaceous) making up the Mesozoic era.

      What are the divisions of the Jurassic period?

      The Jurassic period is normally divided into three main divisions, or epochs: Early, Middle, and Late; more informally, the period is labeled with lower case letters, or the early, middle, and late Jurassic. In addition, scientists often use the terms Lower, Middle, and Upper to describe the divisions of the Jurassic.

      Each of these main epochs is further broken up into subdivisions. To make things more confusing, the smaller ages often have different names (and often dates), depending on whether you are using European, North American, or Australian and New Zealand nomenclature. Presented here is a general list of North American Jurassic period divisions. These dates are not absolute, and may vary slightly from source to source.

      Jurassic Period

Epoch	Age	(approximate) Millions of Years Ago
Early	Navajo	195–178
	Kayenta	202–195
Middle	Twin Creek	170–163
	Gypsum Springs	178–170
Late	Morrison	156–141
	Sundance	163–156

      How long did the Jurassic period last?

      The Jurassic period lasted from approximately 200 to 145 million years ago, a time period of approximately 55 million years. The exact dates are debated, of course, and there are some variations of the dates in the literature, but the time frame is close.

      What event occurred at the division between the Triassic and Jurassic periods, and why was this important to the dinosaurs?

      There was apparently a major extinction around 200 million years ago between the Triassic and Jurassic periods. This event (or events that lasted 10,000 years) led to the almost complete disappearance of many marine groups, such as some of the ammonoids; as well as the complete disappearance of some reptiles, including some types of archosaurs, phytosaurs, aetosaurs, and rauisuchians. Though many scientists speculate that this extinction was caused by an asteroid impact, the suspected resulting crater, Manicouagan in British Columbia, Canada, has been dated at 10 million years too early. There are thus heated debates as to the causes of this extinction event.

      Some scientists think that this end-of-the-Triassic-period extinction event opened up more ecological niches (how a species fits into its environment) into which the dinosaurs dispersed, allowing them to flourish and become dominant. However, others feel that the dinosaurs were already on their way to dominance due to the major extinction event at the end of the Permian period. The real sequence of these events may never be known, but in any case, the dinosaurs did start to become dominant during the early Jurassic period.

      There is another theory that tries to explain the extinction event between the Triassic and Jurassic periods: There were very large lava flows for approximately 600,000 years close to the division between the Triassic and Jurassic periods—one of the largest such events known to have occurred on our planet. The side effects of these flows, such as the emission of carbon dioxide and sulfur aerosols, may have contributed to the mass extinctions at this time by changing the atmosphere’s composition and/or climate.

      Another suggestion is that an impacting asteroid or comet may have actually caused the lava flows, or worked in conjunction with the flows, to create an even harsher environment. The resulting environmental changes—from climate to vegetation—could have led to the mass extinction event between the Triassic and Jurassic periods.

      What is the Morrison formation and where is it found?

      The Morrison formation is a layer of sedimentary rocks famous for the number and diversity of their Jurassic period dinosaur fossils. This formation, named for Morrison, Colorado, is found throughout a large region of western North America. The number of Jurassic dinosaur fossils found in Utah’s Morrison formation include Allosaurus, Apatosaurus, Barosaurus, Brachiosaurus, Camarasaurus, Camptosaurus, Ceratosaurus, Diplodocus, Dryosaurus, Dystrophaeus, Marshosaurus, Stegosaurus, Stokesosaurus, and Torvosaurus.