mostly on shellfish, fish, and other marine reptiles; they looked similar to, and probably had some of the same habits of, modern dolphins, whales, and sharks; they lived in the oceans from the Early Triassic to Middle Cretaceous periods, probably out-competed by the mosasaurs of the Middle Cretaceous period.
Plesiosaurs: Medium to large, long-to short-necked reptiles, with bulbous bodies; their four legs were modified into paddles; they probably ate mostly fish; they lived mostly in marine environments, but some also lived in freshwater lakes; they lived from the Early Triassic to the end of the Cretaceous period and are often sighted as the model for what the Loch Ness monster is presumed to look like.
Placodonts: Large marine reptiles that had a long trunk and tail, with feet that were probably webbed; their teeth were using for crushing, and they probably ate clams and other shelled invertebrates from the ocean floor.
Nothosaurs: Small to moderate-sized marine reptile with long necks and sharp, conical teeth for spearing fish; their legs were modified flippers, rather than the paddle-shaped legs of the more advanced eurapsids; they lived from the Early to Late Triassic period.
Other Marine Creatures
Sea urchins: The few pencil urchins that survived the Permian period extinction are also the ancestors of all modern urchins; the Triassic period was also the time of the first burrowing urchins.
Corals: First relatives to the modern corals evolved during the Triassic period.
Crabs and lobsters (crustaceans): First close relatives of modern crabs and lobsters evolved during the Triassic period.
Ammonoids (chamber-shelled organisms): Ammonoids rapidly diversified during the Triassic period.
Bony fishes: Found in salt, brackish, and fresh water, and could often move back and forth among the three; they are divided into two groups based on their structure: the ray-finned (for example, the Triassic period’s Perleidus) and lobe-finned (for example, the Triassic period’s Diplurus).
Sharks: During the Triassic, the intermediate form between primitive and modern sharks evolved; the earliest sharks evolved during the Paleozoic era, middle Devonian period, about 130 million years before; one of the modern survivors of this group is the Port Jackson shark.
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
