Ward's World Activity Guides

more than 3.5 billion years. The record of these simple bacteria in the form of stromatolites (masses of layered sediments trapped on mucilaginous bacterial mats) is extensive through- out the Precambrian and later. Eukaryotes with nucleated cells and organelles evolved perhaps 2.5 billion years ago, in association with elevated levels of atmospheric oxygen; their organelles may represent symbiotic bacteria. Unicellular and multicellular algae appeared by approximately 2 billion years ago (Fig. 7). Single-celled animal-like protists (formerly, protozoans) are questionably represented by fossils about 800 million years old. Animal-like trace fossils, including simple burrows, may extend back at least 600 million years, whereas strings of fecal pellets preserved in ancient sediments indicate the presence of animals with guts. Moreover, extraordinary assemblages of large "leaflike" and radially symmetrical organisms occur as imprints in fine- grained marine sandstones worldwide about 580–550 million years ago—the Ediacaran Period of the Neoproterozoic Era. The exact affinities of these Ediacaran organisms ("vendobionts" or "vendozoans") have been the subject of debate, with some asserting that they represent early animals such as jellyfish, sea pens (colonial marine cnidarians that resemble a quill pen), and perhaps annelid worms; others have argued that the Ediacaran organisms represent a different type of life form, possibly simple, baglike mouthless organisms that died out without leaving descendants. Regardless of the affinities of the Edia- caran organisms, the extraordinary discovery in the late 1990s of fossil animal embryos in China provides ample evidence for simple animals, such as sponges and relatives of jellyfish, nearly 600 million years ago. The most dramatic recorded event in the history of animal life occurred about 543–530 million years ago (Early Cambrian Period)—the so-called Cambrian Explosion (Fig. 8). Within less than 10 million years, nearly all of the major body plans of life, and perhaps others that are now extinct, appeared in the oceans. These include brachiopods, mollusks, arthropods (trilobites, crustaceans, and many others), echinoderms (simple early relatives of starfish), and the chordates. A further phase of diversification in the following Ordovician Period gave rise to a much larger number of class- and order-level groupings. For example, some 20 different classes of echinoderms existed at this time, including the ancestors of crinoids, starfish, and sea urchins that have persisted to the present day. Vertebrates (animals with backbones) are now known from fossils to range back nearly as far as most invertebrates. Recent discoveries from China indicate that early vertebrates, simple jawless fishes, had evolved at least 530 million years ago. Con- odont animals represent an early, eel-like vertebrate group that flourished for more than 300 million years before going extinct in the Triassic Period. Armored jawless fishes (ostracoderms) and the first jawed fishes appeared early in the Ordovician Period, with jaws evolving from gill bars. The Devonian Period (417–365 million years ago) is often termed the "Age of Fishes" because of the rapid diversification of a large number of jawed fish groups—placoderms (which diversified and then went extinct within the period), sharks, bony ray-finned fishes, and lobe-finned fishes, including lungfish and crossopterygians. Predator-prey interactions in the marine ecosystem also intensified in two pulses. The middle Paleozoic showed rapid diversification of jawed fish predators and animals with the ability to crush and bore into shells. Potential prey organisms show a variety of probable responses, including increased shell thickness and spinosity. Paleontology (continued) + ward ' s science Fig. 8: Major evolutionary events of both marine and terrestrial life. (Credit: Adapted from an original drawing by S. M. Stanley)

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