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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.
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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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Late Triassic extinction
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Date:
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206 mya
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Intensity:
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2
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Affected:
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About 50 percent of marine
invertebrate genera, possibly land vertebrates go extinct
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Hypotheses:
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Global cooling, meteor impact,
sea-level changes, oxygen depletion
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Summary:
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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.
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