Triassic Period (248-206 mya)
The Triassic period, which opens the Mesozoic era, follows anear-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 bylow-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 vertebratesfrom the Permian, are joined by the dinosaurs and other "ruling" reptiles, frogs,and early crocodiles. During the period, mammal-like reptiles evolve traits thatare closer and closer to those of mammals. By period's end, some are classifiedas 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, theyare said to resemble dolphins. Turtles also debut. A mass extinction at the end of theperiod affects half of all marine invertebrate genera and, some scientiststhink, significant numbers of land vertebrates.
220 mya: Dinosaurs
Dinosaurs emerge following a mid-period extinction that probablywipes out most of the mammal-like reptiles. The earliest dinosaurs are small, bipedalpredators. Key skeletal features in the pelvis and ankle result in near-vertical posture,with rear limbs positioned directly under the body. This distinguishes dinosaurs fromtheir low-slung thecodont ancestors. Dinosaurs spread rapidly beginning latein 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 throughthe Cretaceous.
220 mya: Mammals
The mammals' reptilian ancestors, the cynodonts, already exhibitedseveral mammalian traits: specialized teeth (canines and incisors), a hard palate thatenabled simultaneous eating and breathing, upright posture (earlier reptiles hadlow-slung posture), and (perhaps) warm-bloodedness. Among the features true mammalsevolve that distinguish them from their forebears include a smaller body size, a largerbrain cavity, molars that sliced rather than ripped food, a single lower jaw bone, anda middle ear with three bones, which vastly improved hearing. The earliest mammalsare 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 theirfront legs, allowing them to raise their bodies off the ground and move quickly onland. A sheet of bone along their long snout separates their nasal passages from theirmouths, a feature that is especially important for later, aquatic species. A heavilyreinforced skull resists the intense forces of strong, rapid bites. These basicfeatures, established in the Triassic period, are still exhibited by contemporarycrocodile species.
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| Read more 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, haveunusually long and highly specialized legs from their very beginning. Early onesprobably leap as their descendents do today. The spotlight shines brightest on thereptiles, though, long split from the amphibian lineage. Reptiles conquer land andeven some aquatic environments. Reptiles are divided among three primary groups, according to thenumber of openings in the skull behind the eye socket.
Reptiles adapt to marine life, too. Euryapsid skulls have a singlepair of openings, but between different bones than the synapsids. They includemosasaurs, 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 theirsleek bodies, they resemble dolphins, except an ichthyosaur's tail fluke is verticallike that of a fish. With the exception of plesiosaurs, all aquatic reptiles goextinct 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 theskies (birds and bats are the other two). Though wingspans of the largest ones reachup to 40 feet, most pterosaurs are similar in size to contemporary pigeons. Pterosaurspropel their hollow-boned bodies through the skies using well-developed flying muscles.With an estimated 120 different species, flying reptiles colonize every continent overthe next 150 million years.
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Late Triassic extinction | ||||
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Date: | | 206 mya | ||
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Intensity: | | 2 | ||
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Affected: | | About 50 percent of marineinvertebrate genera, possibly land vertebrates go extinct | ||
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Hypotheses: | | Global cooling, meteor impact,sea-level changes, oxygen depletion | ||
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Summary: | | Following several millionyears' recovery, diversity has returned to the oceans during the Triassic.Fully adapted marine reptiles swim the seas, and some bivalvesand echinoderms have now developed burrowing skills, a cleveradaptation 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 severelyaffected by the events, others do not support this. Destruction of marineinvertebrate life, however, is certain. Cephalopods and bivalvesabsorb major hits, as do sponges, gastropods, conodonts,and brachiopods. Global cooling, meteor impact, and sea-level changesare among the proposed causes. | ||
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