12: Phanerozoic Eon - Paleozoic Era

Introduction

The Phanerozoic Eon is the most recent, from 541 million years ago to today; it means “visible life” because an abundance of fossils marks the Phanerozoic rock record. Phanerozoic organisms had evolved hard body parts like claws, scales, shells, and bones that made it more likely they would be preserved as fossils. Rocks older than the first period of this eon, the Cambrian, are called Precambrian and rarely contain fossils because these organisms had soft body parts. Rocks of the Phanerozoic are younger and more common, and contain the majority of visible fossils; the abundance of preserved fossils in rocks from this Eon makes them the most informative storytellers of Earth’s history. The Phanerozoic is subdivided into three Eras, the Paleozoic (“ancient life”), Mesozoic (“middle life”), and Cenozoic (“recent life”). This chapter focuses on the Paleozoic Era.

Here is the the simplified geologic time scale used in the Geologic Time chapter of this book. The Paleozoic Era is highlighted toward the bottom.

Marine organisms dominated life in the early Paleozoic Era, but by the middle of the Era, plants and animals evolved to live and reproduce on land. Fish evolved jaws, and fins evolved into jointed limbs. The development of lungs allowed animals to emerge from the sea and become the first air-breathing tetrapods (four-legged animals) such as amphibians. From amphibians evolved reptiles with the amniotic egg. From reptiles evolved an early ancestor to birds and mammals, and their scales became feathers and fur. Near the end of the Paleozoic Era, the Carboniferous Period had some of the most extensive forests in Earth’s history. Their fossilized remains became the coal that powered the Industrial Revolution.

During the Paleozoic Era, sea levels rose and fell during four transgressive-regressive sequences. Most of North America was a shallow, tropical ocean, as evidenced by the abundant marine sedimentary rocks such as limestones containing corals, ooids, and other fossils covering much of the continent. Unconformities document falling sea levels when rock sequences were lost to erosion.

By the late Paleozoic, the supercontinent Pangea was close to fully assembled. This resulted from the collisions and accretions of island arcs and continents, as well as the closures of ocean basins. In North America, these tectonic events happened along the east coast and are known as the Taconian, Acadian, Caledonian, and Alleghanian orogenies. The Appalachian Mountains are the erosional remnants of these mountain-building events. The Panthalassa ocean basin surrounded Pangea. Subsequent plate movement formed the huge Tethys Sea bay, dividing Pangea into smaller supercontinents, Laurasia and Gondwana. Laurasia comprised the North American craton, Laurentia, and Eurasia. Gondwana comprised the remaining continents of South America, Africa, India, Australia, and Antarctica. The east coast of North America had a very high mountain range from continental crust collisions and the creation of Pangea. The west coast of North America had smaller and isolated volcanic highlands from subduction and the associated island arc accretion. During the Mesozoic Era, the size of the mountains on either side of North America would flip, with the west coast being a more tectonically active plate boundary and the east coast changing into a passive margin after the breakup of Pangea.

The video below, "From the Cambrian Explosion to the Great Dying", offers a concise and thorough summary of the Era.