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Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era

Mesozoic Era: (248-65 mya)

Triassic | Jurassic | Cretaceous

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Triassic Period (248-206 mya)

The Triassic period, which opens the Mesozoic era, follows a near-complete extinction of life. Recovery from the brink is predictably slow. While mountain building occurs in what is now the west coast of North America, Alaska, and Chile, the Pangaea supercontinent's wide-open interior is covered by low-growing vegetation that tolerates drier conditions, including some ferns. Where forests appear, they are dominated by conifers and cycads.

The Triassic is a critical period for the land vertebrates. Mammal-like reptiles called lystrosaurs, among the only surviving land vertebrates from the Permian, are joined by the dinosaurs and other "ruling" reptiles, frogs, and early crocodiles. During the period, mammal-like reptiles evolve traits that are closer and closer to those of mammals. By period's end, some are classified as the first mammals.

In marine habitats, extinction survivors include ammonoids, some brachiopods, and bivalves. Fully aquatic reptiles evolve, including the ichthyosaurs. Because of their sleek profile and swimming prowess, they are said to resemble dolphins. Turtles also debut. A mass extinction at the end of the period affects half of all marine invertebrate genera and, some scientists think, significant numbers of land vertebrates.

220 mya: Dinosaurs

Dinosaurs emerge following a mid-period extinction that probably wipes out most of the mammal-like reptiles. The earliest dinosaurs are small, bipedal predators. Key skeletal features in the pelvis and ankle result in near-vertical posture, with rear limbs positioned directly under the body. This distinguishes dinosaurs from their low-slung thecodont ancestors. Dinosaurs spread rapidly beginning late in the Triassic. By the end of the period, the largest known species is 27 feet long. The heyday of the dinosaurs' 150-million-year reign spans the late Jurassic through the Cretaceous.

220 mya: Mammals

The mammals' reptilian ancestors, the cynodonts, already exhibited several mammalian traits: specialized teeth (canines and incisors), a hard palate that enabled simultaneous eating and breathing, upright posture (earlier reptiles had low-slung posture), and (perhaps) warm-bloodedness. Among the features true mammals evolve that distinguish them from their forebears include a smaller body size, a larger brain cavity, molars that sliced rather than ripped food, a single lower jaw bone, and a middle ear with three bones, which vastly improved hearing. The earliest mammals are rodentlike, nocturnal, and solitary.

220 mya: Crocodiles

Crocodiles, which are members of the archosaurs, or "ruling" reptiles, evolve as terrestrial predators. Their hind legs are larger than their front legs, allowing them to raise their bodies off the ground and move quickly on land. A sheet of bone along their long snout separates their nasal passages from their mouths, a feature that is especially important for later, aquatic species. A heavily reinforced skull resists the intense forces of strong, rapid bites. These basic features, established in the Triassic period, are still exhibited by contemporary crocodile species.

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Divide and conquer (220 mya)

Several now-familiar amphibian groups debut in the Triassic, including frogs and salamanders. Frogs, the oldest of the modern amphibians, have unusually long and highly specialized legs from their very beginning. Early ones probably leap as their descendents do today. The spotlight shines brightest on the reptiles, though, long split from the amphibian lineage. Reptiles conquer land and even some aquatic environments.

Reptiles are divided among three primary groups, according to the number of openings in the skull behind the eye socket.

  • Among the emerging anapsids -- true reptiles that have no skull openings -- are the turtles. Turtles are the only members of this line to survive to the present.
  • The synapsids, so-called "mammal-like reptiles," possess a single pair of openings behind the eye sockets. Though the earliest synapsids are probably cold-blooded and walk with their bodies low to the ground, they ultimately give rise to the mammals.
  • The first diapsids, which have two pairs of openings, include the archosaurs, or "ruling" reptiles. Crocodiles, pterosaurs, and dinosaurs count among the early archosaurs, and lizards, snakes, and birds follow later.

Reptiles adapt to marine life, too. Euryapsid skulls have a single pair of openings, but between different bones than the synapsids. They include mosasaurs, which, at up to 30 feet, are the largest lizards ever, and plesiosaurs, some of which have extremely long necks and measure 40 feet in length. The ichthyosaurs, the most specialized of all aquatic reptiles, might actually be diapsids. With their sleek bodies, they resemble dolphins, except an ichthyosaur's tail fluke is vertical like that of a fish. With the exception of plesiosaurs, all aquatic reptiles go extinct before the end of the Cretaceous.

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215 mya: Flying reptiles

Pterosaurs are the first of three vertebrate groups to take to the skies (birds and bats are the other two). Though wingspans of the largest ones reach up to 40 feet, most pterosaurs are similar in size to contemporary pigeons. Pterosaurs propel their hollow-boned bodies through the skies using well-developed flying muscles. With an estimated 120 different species, flying reptiles colonize every continent over the next 150 million years.

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Did you know?

Unlike bird wings, pterosaur wings are not feathered. In fact, several fossils show that these creatures may have been covered with insulating fur. Their wing membranes are supported by only the fourth digits of their forelimbs. These "fingers" extend the full length of the wings. While bats also have membranous wings, theirs are supported by four fingers, attach to the rear limbs as well, and are proportionally much wider.

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Late Triassic extinction

Date:

206 mya

Intensity:

2

Affected:

About 50 percent of marine invertebrate genera, possibly land vertebrates go extinct

Hypotheses:

Global cooling, meteor impact, sea-level changes, oxygen depletion

Summary:

Following several million years' recovery, diversity has returned to the oceans during the Triassic. Fully adapted marine reptiles swim the seas, and some bivalves and echinoderms have now developed burrowing skills, a clever adaptation that protects them from predators. Life takes another hit, though, in what are thought to be successive extinctions at the end of the Triassic. While some scientists argue that amphibians and aquatic reptiles are severely affected by the events, others do not support this. Destruction of marine invertebrate life, however, is certain. Cephalopods and bivalves absorb major hits, as do sponges, gastropods, conodonts, and brachiopods. Global cooling, meteor impact, and sea-level changes are among the proposed causes.

-> Go to the Jurassic Period

Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era

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