Perhaps nudged over the evolutionary cliff by a giant boloid striking the earth, the incredible and fascinating group of animals called dinosaurs became extinct some 65 million years ago (except for their feathered descendants). In their place evolved an enormous variety of land creatures, especially the mammals, which in their way were every bit as remarkable as their Mesozoic cousins.
The Age of Mammals, the Cenozoic Era, has never had its Jurassic Park, but it was an amazing time in earth’s history, populated by a wonderful assortment of bizarre animals. The rapid evolution of thousands of species of mammals brought forth gigantic hornless rhinos, sabertooth cats, mastodonts and mammoths, and many other creaturesincluding our own ancestors.
Their story is part of a larger story of a world emerging from the greenhouse conditions of the Mesozoic, warming up dramatically about 55 million years ago, and then cooling rapidly so that 33 million years ago the glacial ice returned. The earth’s vegetation went through equally dramatic changes, from tropical jungles in Montana and forests at the poles, to grasslands and savannas across the entire world. Life in the sea also underwent striking evolution reflecting global climate change, including the emergence of such creatures as giant sharks, seals, sea lions, dolphins, and whales.
After the Dinosaurs is a book for everyone who has an abiding fascination with the remarkable life of the past.
About the Author
Recipient of the 2013 James Shea Award of the National Association of Geology Teachers for outstanding writing and editing in the geosciences.
Donald R. Prothero is Emeritus Professor of Geology at Occidental College and Lecturer in Geobiology at the California Institute of Technology. He has published 32 books, including Reality Check: How Science Deniers Threaten Our Future (IU Press, 2013); Rhinoceros Giants: The Paleobiology of Indricotheres ; Earth: Portrait of a Planet ; The Evolution of Earth ; Evolution: What the Fossils Say and Why It Matters ; Catastrophes! ; and After the Dinosaurs: The Age of Mammals (IU Press, 2006).
Read an Excerpt
Fossil hunting is by far the most fascinating of all sports. It has some danger, enough to give it zest and probably about as much as in the average modern engineered big-game hunt, and the danger is wholly to the hunter. It has uncertainty and excitement and all the thrills of gambling with none of its vicious features. The hunter never knows what his bag may be, perhaps nothing, perhaps a creature never before seen by human eyes. It requires knowledge, skill, and some degree of hardihood. And its results are so much more important, more worthwhile, and more enduring that those of any other sport! The fossil hunter does not kill, he resurrects. And the result of this sport is to add to the sum of human pleasure and to the treasure of human knowledge.
George G. Simpson, Attending Marvels, 1934
The sun blazed down on the two men as they slowly walked up and down the ravines of the badlands. They walked stooped over with their eyes glued to the ground. The temperature was over 104°F (40°C), and there was no shade anywhere in the desolate landscape. They had been working like this all day and yet had only a few fossil jaws and teeth to show for their time and effort. Wide, floppy hats and loose, light-colored clothing kept off the sun, but they dared not wear dark glasses, despite the glare from the ground. To find the fossils they were seeking, they needed to detect subtle differences in the color and surface texture of the rocks on the ground, and dark glasses made this difficult. Many of the things they picked up were shiny black pebbles or concretions that resembled fossils. Frequently, they found chunks of fossil bone, which were clearly identifiable by their spongy texture in cross-section. Most of these pieces of bone were too broken to be identified. Others were scraps of fossil turtle shell, which had little scientific value. Occasionally they got lucky and found an isolated mammal tooth or two. These were worth saving, since the pattern of the tooth crowns of most mammals is distinctive. Fossil teeth are sometimes easy to spot, for instance, when the tooth enamel is black and shiny and stands out on the baked tan muds.
The men were hoping to find remains of the largest animals of the Eocene, the elephant-sized brontotheres, which were distantly related to horses and rhinos but had two blunt battering-ram horns on their noses (fig. 1.1). If the men were really lucky, they might find two or more brontothere teeth together, or a partial jaw with three or more teeth in it. Even a complete jaw and skull of a common animal, however, is not as valuable as a single tooth of a rare animal, which may be known only from a few scraps. Every isolated tooth of a rare fossil gets immediate attention when it is brought back to a museum. Sometimes it is described and published before anything else in the collection.
The two scientists were in luck today. One stooped down and noticed a small pile of bone fragments (fig. 1.2). In the midst of the pile, the skull and lower jaw of a fossil mammal protruded from the ground, lying on its side. Although the skull and jaw were nearly complete, they did not cause a lot of excitement. They belonged to a common fossil mammal, an oreodont (discussed in chapter 5), which must have roamed this area in herds of thousands over 30 million years ago (fig. 1.3). Oreodonts have no living descendants; they are distantly related to camels, yet they looked nothing like today's ships of the desert. Although there were already hundreds of unstudied oreodont specimens back in the museum, this oreodont skull was worth collecting because it was so complete.
The collectors carefully dug a trench around the specimen until it rested on a pedestal of rock. Since the specimen was fragile, they made a cast of plaster bandages around the skull. Once the cast had dried, they carefully pried it up and turned it over. The skull had come out in one piece without breaking! After a few more strips of plaster bandage had been wrapped around the exposed surface, it was ready to carry back to the truck.
A complete oreodont skull was a good day's work but nothing to write home about. As the men were working their way back to the truck, however, one of them spotted another ridge of fossil bone protruding from the ground. Although only a few inches were exposed, the thickness and curvature suggested that it was the back of a large jaw (fig. 1.4). A few minutes of careful excavation of the exposed part revealed that the specimen was indeed a very large one and that it continued into the hillside. The two men returned to the truck and carefully drove it up to the ravine as close as four-wheel drive could reach. First, they used the heavy-duty truck jack to lift a huge slab of sandstone from over the specimen and slide the slab off the cliff. Then they used picks and brooms to carefully dig a trench around the specimen, exposing it on all sides. When they were done, they could see that they had a complete set of the lower jaws of a fossil brontothere (fig. 1.1). The jaws were almost two feet long and in excellent condition, but still fragile. With all the surrounding rock, the specimen weighed several hundred pounds, so it could not be moved easily. To protect it for transport, the two scientists mixed up a small tub of plaster of Paris and tore burlap bags into strips. After dipping the strips into the plaster, they smoothed them over the fossil, overlapping each strip so that a solid bandage was formed. After about half an hour, the plaster jacket was finished and drying quickly in the hot sun. Next came the hard part. The jacket surrounded the specimen on nearly all sides, but it was still attached to the ground. More digging isolated the plaster jacket on a higher pedestal of rock. Carefully, the two scientists dug the pedestal from underneath the specimen. At last, they wedged the pickaxe underneath the cast, prying it from the ground and flipping it over. The underside of the jaw was revealed in almost perfect condition, with very few broken pieces. After carefully trimming the ragged edge of the jacket, they covered the exposed side with more plaster and burlap. This brontothere was ready to be transported to the museum for study.
Not all fossils are so large or glamorous. In some areas, the fossils are so small that they cannot be seen from more than a foot away. The only way to collect them is to crawl on your hands and knees, with your eyes six inches from the ground. If the ground is rich in small teeth and bones, it is more efficient to use a large crew of students or volunteers. The greater the number of trained eyes covering the ground, the better. In such deposits, a few teeth are considered an excellent find since the fossils are badly crushed and seldom yield a complete skull. However, these tiny, isolated teeth are important because, for most mammals, teeth are our only record of their early evolution.
If fossil hunting sounds like grueling, backbreaking work, it is. Most fossil hunting bears little resemblance to the glamorous misconceptions we see in the movies. Scientist who study fossils, paleontologists, must put up not only with difficult conditions but also with days and weeks of looking without finding anything. To persevere in the face of such disappointment and discouragement, paleontologists must really love their work. However, one excellent find in a field season is often enough to make thousands of hours of toiling in the sun worthwhile.
Many a youngster has dug large holes in the backyard, unsuccessfully looking for the dinosaurs from the children's books. How do paleontologists know where to dig? First of all, they must know where to look. Fossils are nearly always found in sedimentary rocks, which are formed from sand or mud or fossil shells. Only a small fraction of the earth's sedimentary rocks carry fossils, so it helps to look in rocks that are known to be fossil bearing. Rock strata that were laid down in the ocean rarely produce fossils of land animals. Only sandstones and mudstones that were originally sands and muds on a river floodplain or in a lake will yield fossils of land mammals or dinosaurs. The rocks also have to be of the correct age. If they are more than 65 million years old, they will not produce many mammals, but they might produce dinosaurs. If the rocks are younger than 65 million years, however, no dinosaurs will be found, since they all became extinct at that time. Paleontologists must take all these factors is into account when they study the geology of an area, or learn of a fossil locality from some other collector.
Once you're in the right place, you have to know how to look. Slowly scanning the ground a few inches at a time is a suitable pace, even if it takes tremendous patience. Finally, you have to know what to look for. Paleontologists develop a mental filter, known as a "search image," that screens out all the nonfossils and fossil-like objects they see. Only the genuine glint of enamel or spongy texture of bone catches the eye among all the objects on the desert floor. Once paleontologists have spotted bone or enamel, they must also have the training to recognize and identify what they've found. If it's really worthwhile, it deserves special treatment. To develop this kind of skill generally requires years of education and many more years of practice in the field, collecting and identifying hundreds of specimens. Since most finds are fragmentary, paleontologists must know the skeleton of each animal so well that any piece is instantly recognized. Only a few of the handful of paleontologists employed today have all these skills so well developed that they are master collectors. Good fossil collectors are a rare breed these days, but what they have found is extremely impressive considering their small numbers. From their years of collecting, we have fossils in museums that tell us the story of the evolution of dinosaurs, elephants, horses, rhinos, and many other important fossil animal groups.
These methods are standard for collecting fossil vertebrates (animals with backbones, like fish, reptiles, amphibians, birds, and mammals), which are generally rare and difficult to collect. By contrast, invertebrates (animals without backbones) are generally much more common — at least those with hard shells or skeletons, like clams, snails, sea urchins, and corals. Obviously, soft-bodied animals without skeletons, like worms and jellyfish, seldom fossilize. In many places, fossils occur as dense shell beds with thousands to millions of shells packed in close together. Here, collecting is much easier, and the collector need worry only about damaging the fragile shells as they are collected, and about keeping good records of everything that is collected. More often, however, marine shales and sandstones have relatively few fossils, so collecting in these locations is the same kind of backbreaking work I have just described, hiking over miles of landscape, looking for the rare shell.
Yet another set of conditions applies to microfossils, the skeletons of tiny organisms usually less than a millimeter in size (fig. 1.5). Most microfossils are the shells of single-celled organisms, such as the amoeba-like foraminifera and radiolaria that float in the plankton and settle on the sea bottom. Other microfossils come from plant-like single-celled organisms (such as diatoms). Still others are from multicellular animals that happen to be microscopic in size, such as the tiny snail-like pteropods that float in the plankton, or the minute crustaceans known as ostracodes, which litter the sea bottom with trillions of their tiny kidney-bean-shaped shells that hinge over their backs. In any case, microfossils are usually not rare. Some oceanic sediments are composed of nothing but microfossils, so even a sample of a few grams yields thousands of shells. In most marine sediments around the world, microfossils are abundant, so the experienced micropaleontologist need scoop only a few grams of sample into a bag, take it back to the lab, and look through the microscope. Better still, microfossils are so abundant that they can even be recovered from samples drilled from deep underground in the search for oil. For years, oil companies hired micropaleontologists because they could use the tiny microfossils found throughout the deep drill holes to determine how old the sediment was, or how deep the water once was at the site of deposition. In addition, many microfossils are sensitive to the oceanic conditions in which they lived. They often track changes not only in water depth but also in oceanic temperature and chemistry. As we shall see later in this book, the study of microfossils and the chemicals trapped in their skeletons is the key to understanding how ancient oceans and climates have evolved over time.
Paleontologists work in a world with a time frame completely different from ordinary everyday history. From various methods, we now know that the earth is about 4.6 billion years old, a staggering number in human terms. It is such an immense amount of time that some sort of analogy is necessary to make it comprehensible. Suppose we were to compress all 4.6 billion years of earth history into a single calendar year. On this scale, each of the 365 "calendar days" equals 12 million years, and each minute of the "calendar" is 8561 years long! The earth forms on New Year's Day in this calendar. The first recognizable life — consisting of tiny, single-celled bacteria and blue-green cyanobacteria — does not appear until February 21. Complex, multicellular life, such as jellyfish, trilobites, and corals, does not appear until November 12. The first amphibians crawl out on land on November 28. The first tiny mammals and the first bird, Archaeopteryx, appear during the peak of the Age of Dinosaurs, the Jurassic Period, on December 17. The final extinction of the dinosaurs and the beginning of the Age of Mammals occur on the day after Christmas. The first ape-like primates that are members of our own family, the hominids, do not appear until eight hours before New Year's Eve. Neanderthal Man, the classic Stone Age "caveman," appears ten minutes before New Year's Eve, as the countdown begins at parties everywhere. Recorded history begins less than one minute before New Year's Eve, as the conductor raises his baton to start Auld Lang Syne. Within a second before midnight, Charles Darwin's On the Origin of Species is published, and the American Civil War is fought. Virtually all of human history, especially the last few millennia, is drowned out by the drunks who blow their noisemakers a fraction of a second too early!
On the scale of geologic time, human affairs appear pretty insignificant. Geologists are accustomed to dealing with such large amounts of time and routinely deal with thousands and millions of years. For most geologic problems, events of less than thousands of years in duration cannot even be distinguished in the layers of sedimentary rocks. When dealing with events that occurred hundreds of millions or billions of years ago, even a million years here or there is negligible. A sense of "deep time" (as John McPhee labeled it) is important to all of us, not just to the geologists. Most geologists, however, find it practical to deal with time not in millions of years but in relative time terms. Just as historians use "Elizabethan" or "Edwardian" to refer to periods in English history, so geologists use "Cambrian" and "Cretaceous" to refer to distinct episodes in earth history.
For the purposes of this book, most of these time terms will not be necessary. The last 65 million years, known as the Age of Mammals in popular parlance, is formally known as the Cenozoic Era. The Cenozoic is divided into a number of epochs (fig. 1.6), beginning with the Paleocene approximately 65 million years ago and running to the present. The Paleocene, which lasted from 65 to 55 million years ago, is followed by the Eocene (55–34 million years ago), the Oligocene (34–23 million years ago), the Miocene (23–5 million years ago), the Pliocene (5–1.8 million years ago), and the Pleistocene, or ice ages (1.8 million years to 10,000 years ago). The period since the last retreat of the glaciers, which includes the present interglacial warming, is called the Holocene, or Recent (10,000 years ago to present). Although these terms may seem intimidating at first, using them is much easier than trying to talk about the age of an event in terms of millions of years.
How did we establish these divisions, and where did these terms come from? Since the late 1600s, geologists have been able to establish the relative ages of fossils and rocks (i.e., this fossil is younger than or older than that fossil) by the principle of superposition. First proposed by the Danish physician Nicolaus Steno in 1669, this principle states that in any layered sequence of rocks (layered sediments or lava flows), the oldest rocks are at the bottom of the stack, and the rocks get progressively younger as you move up the pile. Clearly, the rocks at the top of the stack could not have accumulated unless there were already rocks on the bottom of the stack to build upon. A good analogy is a stack of papers on a messy desk. Those at the top were put there recently, whereas those at the bottom of the stack may have been laid there months ago and have been gradually buried by more recent activity.
Excerpted from "After the Dinosaurs"
Copyright © 2006 Donald R. Prothero.
Excerpted by permission of Indiana University Press.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents
2. The End of the Dinosaurs?
3. Brave New World: The Paleocene
4. Dawn of the Recent: The Eocene
5. The Icehouse Cometh: The Oligocene
6. The Savanna Story: The Miocene
7. The World in Transition: The Pliocene
8. Ice Time: The Pleistocene
9. Our Interglacial: The Holocene
Most Helpful Customer Reviews
If you are interested in mammal evolution, this book is for you. easy to read yet informative, it gives the reader a very clear picture of what has happened to mammal evolution over the last sixty-five million years.