Now let's take a look at the inhabitants of the Triassic Period.
Three
categories of organisms can be distinguished in the Triassic record: survivors
from the Permian–Triassic extinction event, new groups which flourished briefly, and other new groups
which went on to dominate the Mesozoic
Era.
To go back to the beginning, after the
extinction event just before the Triassic Period began, the Earth's biosphere
was impoverished. It was well into the middle of the Triassic before life
recovered its former diversity. Therapsids (including what would become
mammals) and archosaurs (including crocodilian reptiles) were the chief
terrestrial vertebrates of this period. A specialized subgroup of archosaurs,
called dinosaurs, first appeared late in the Triassic, but did not become
dominant until the succeeding Jurassic Period.
The first true mammals also evolved
during this period, as well as the first flying vertebrates, the pterosaurs,
who were a specialized subgroup of archosaurs.
In marine environments, new types of
corals appeared in the Early Triassic, forming small patches of reefs of modest
extent compared to the great reef systems of modern times. The shelled
ammonites (whose shell resembled that of the modern nautilus, but is not an
ancestor) recovered, diversifying from a single line that survived the
Permian-Triassic extinction.
The fish fauna was remarkably uniform,
with many families and genera exhibiting a global distribution in the wake of
the mass extinction event. There were also many types of marine reptiles. The
first of the lizard-like animals appeared in the Early Triassic seas and soon
diversified, and some developed to huge size during the Late Triassic.
On land, the surviving plants included
ginkos, ferns, and horsetails, among others. Seed plants came to dominate the
terrestrial flora. In the northern hemisphere, conifers and ferns flourished. A
seed fern genus would dominate Gondwana throughout the period.
Many groups of terrestrial fauna
appeared in the Triassic period or achieved a new level of evolutionary success
during it. They include lungfish, Temnospondyls (early amphibians that had
mostly been replaced by reptiles, they made a come-back in this period),
Rhynchosaurs (the primary large herbivores in many Triassic ecosystems),
Phytosaurs (looked like crocodiles, but unrelated), Aetosaurs (heavily armored
and mostly herbivorous), Rauisuchians (the keystone predators of most Triassic
terrestrial ecosystems), Theropods (dinosaurs but not the large kind that would
come later; most were 1-2 meters long), and Cynodonts (a large group that
includes true mammals, complete with hair and a large brain).
Some amphibians
were among those groups that survived the Permian-Triassic extinction event.
The first ancestors of frogs are known from the Early Triassic, but did not
become common until the Jurassic (which comes next).
Among reptiles,
the earliest turtles appeared during the Late Triassic Period.
During the
Triassic, archosaurs displaced therapsids as the dominant amniotes. This may
have contributed to the evolution
of mammals by forcing
the surviving therapsids and their mammalia-form successors to live as small, mainly
nocturnal insectivores. Nocturnal life may have forced the
mammaliaforms to develop fur and a higher metabolic
rate.
Though the end-Triassic extinction
event was not equally devastating in all terrestrial ecosystems, several
important clades of large reptiles disappeared, as did most of the amphibians,
groups of small reptiles, and others (except for the proto-mammals). Some of
the early, primitive dinosaurs also became extinct, but more adaptive ones
survived into the Jurassic. Surviving plants that went on to dominate the
Mesozoic Era included modern conifers.
The cause of the Late Triassic
extinction in uncertain. It was accompanied by huge volcanic eruptions that
occurred as the supercontinent Pangaea began to break apart about 202 to 191
million years ago, forming one of the largest known inland volcanic events
since the planet had first cooled and stabilized. Another possible but less
likely cause for the extinction event might be global cooling.
https://en.wikipedia.org/wiki/Triassic
