Extinctions and the Amniotes Shifting Opportunities for Tetrapods

March Onto Land has traced the transformation of Earth's terrestrial environments into a greening world that invited many kinds of organisms to leave their aquatic haven for new, unexplored territory. Plants in the form of algae made the first critical step; they were followed by arthropods as the land became increasingly green with new kinds of vegetation. The spread of the arthropods—including scorpions, spiders, and insects—was soon followed by the early tetrapods, the first vertebrates to step out of the water.

Tetrapods had opportunities on land, where they found themselves at the top of an untapped food chain of small arthropods and plants. Amphibians were the first vertebrate conquerors of the land, despite their need to return to the water to reproduce. This lifestyle was workable during the warm and wet Carboniferous Period. From the amphibians arose another line of tetrapods, a group of animals that were even better adapted for life on the land. These creatures, the amniotes, developed a way to lay their eggs out of the water. This freed them from oceans, lakes, and streams and allowed them to more fully explore and adapt to Earth's vast terrestrial resources.

Of the many remarkable evolutionary stories of the Paleozoic Era, probably none is more important to humans than the appearance of the synapsids—mammal-like reptiles. In their ancestry are found the first traces of mammals and the beginning of a long line

of evolutionary changes that would produce a dazzling array of mammals, including the first primates and humans some 300 million years later.

The foundation for all terrestrial organisms, plants and animals alike, was laid down in the Paleozoic Era. The forward momentum of life, however, is sometimes inexplicably interrupted by natural events over which that life has no control. Such was the case 251 million years ago, when nearly all life became extinct. The worldwide extinctions at the end of the Permian Period hit the land and the sea with equal force. This geologic catastrophe was triggered by massive and prolonged volcanic activity in Siberia, a runaway greenhouse effect, and the plummeting of worldwide oxygen levels due to the disruption of Earth's natural global chemostat. The extinctions played out over a span of 500,000 years, severely challenging organisms of all kinds and fundamentally changing the history of life on Earth.

Fossil evidence shows that as much as 95 percent of all ocean species were erased during the end-Permian extinction. Gone from the oceans were most crinoids, brachiopods, reef-building bryo-zoans, nautiloids, and ammonoids: Nearly all marine invertebrate groups suffered huge losses and never regained the diversity that they once had. There, 75 percent of all terrestrial vertebrate taxa were wiped out. Among the animals eradicated were six families of archaic vertebrates that represented early amphibians, reptiles, and synapsids, including several families of formidable saurian predators. Whatever specializations evolution had fine-tuned in these creatures suddenly meant nothing. A small community of surviving species was left to shape the future of life.

By the Early Triassic Epoch, tetrapod communities had been devastated by extinction and were much less diverse. Gone were the sailbacked synapsids. Gone were the rhinoceros-sized, plant-eating pareiasaurs. All but a few lines of amphibians survived, however, as did a handful of hearty synapsid species. These synapsid survivors, including Lystrosaurus and the cynodont ancestors of mammals, continued to spread to far corners of the globe. The once-diverse

Permian oasis or hunting ground? A group of herbivorous Estemmenosuchus drink from a watering hole while the predatory Eotitanosuchus assesses his chances for making a kill.

anapsids, including ancestral turtles, merely stumbled into the Mesozoic Era after losing most of their numbers.

The earliest diapsids—then a line of small, swift, long-legged insect eaters—had lived in the shadows of more dominant creatures for much of the Permian Period. They, too, survived the end-Permian extinction. Changes to Earth's habitats following the Permian extinction included a shift from a warm and humid climate to one that was hotter and drier, as well as a shift in vegetation away from low-growing seed ferns to gymnosperms and taller conifers. By the end of the Triassic Period, it was clear that the diapsids were more able than other amniotes to adapt to these changes in climate and food supply, and a dramatic shift in faunal composition took place. Diapsids—dinosaurs and their kin—gradually replaced the once-dominant synapsids.

Because of the end-Permian mass extinction, the changeover from the Paleozoic Era to the Mesozoic Era also marked the end of many kinds of archaic life-forms and the beginning of new orders of life that are still represented by organisms living today. This is true for plants and invertebrates, but it is also significant to the evolution of vertebrates. By the end of the Mesozoic Era, all modern forms of vertebrates had firmly taken root. The Mesozoic Era is known today as the age of reptiles because life on land, in the sea, and in the air was ruled by the archosaurs. These included dinosaurs, marine reptiles, and flying reptiles. Sharing the world of the archosaurs and continuing to prosper and diversify in the shadows of giants were fishes, mammals, amphibians, and birds.

MYA EONS ERAS

PERIODS

Approximate Ages of Major Groups of Organisms

65.5 100 150 200 250 300 350 400 450 500 542

CENOZOIC

Proterozoic

NEOCENE

PALEOGENE

PERMIAN

CARBONIFEROUS

DEVONIAN

SILURIAN

CAMBRIAN

Precambrian

Dinosaurs Human

Invertebrates Fishes Land Plants Amphibians Reptiles Mammals & Birds Ancestors

CRETACEOUS

JURASSIC TRIASSIC

ORDOVICIAN

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