Vertebrate Evolution

From Origins to Dinosaurs and Beyond

Donald R. Prothero

• Publisher: CRC Press
• Year: 2022
• ISBN:
  9781000515718
• Edition: 1
• Topic: Sciences physiques
• Subtopic: Géologie et sciences de la Terre

The first vertebrate animals appear in the fossil record over 520 million years ago. These lineages diversified and eventually crept ashore leading to further evolutionary divergence and the appearance of the familiar charismatic vertebrates of today. From the tiniest fishes, diminutive salamanders, and miniaturized lizards to gargantuan dinosaurs, enormous brontotheres, and immense whales, vertebrates have captured the imagination of the lay public as well as the most erudite academics. They are the among the best studied organisms. This book employs beautifully rendered illustrations of these diverse lineages along with informative text to document a rich evolutionary history. The prolific and best-selling author reveals much of the latest findings regarding the phylogenetic history of vertebrates without overwhelming the reader with pedantry and excessive jargon. Simultaneously, comprehensive and authoritative while being approachable and lucid, this book should appeal to both the scholar, the student, and the fossil enthusiast.

Key Features

• Provides an up-to-date account of evolution of vertebrates

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• Includes numerous beautiful color reconstructions of prehistoric vertebrates

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• Describes extinct vertebrates and their evolutionary history

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• Discusses and illustrates the first vertebrates, as well as familiar lineages of fishes, amphibians, reptiles, birds, and mammals

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• Reviews mass extinctions and other important events in the diversification of vertebrates

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Related Titles

Bard, J. Evolution: The Origins and Mechanisms of Diversity (ISBN 9780367357016)

Böhmer, C., et al. Atlas of Terrestrial Mammal Limbs (ISBN 9781138705906)

Diogo, R., et al. Muscles of Chordates: Development, Homologies, and Evolution (ISBN 9781138571167)

Schweitzer, M. H., et al. Dinosaurs: How We Know What We Know (ISBN 9780367563813)

1 INTRODUCTION FINDING, DATING, AND CLASSIFYING FOSSILS

DOI: 10.1201/9781003128205-1

Being a paleontologist is like being a coroner except that all the witnesses are dead and all the evidence has been left out in the rain for 65 million years.

—Mike Brett-Surman, 1994

HOW DO YOU FIND FOSSILS?

Many kids grow up fascinated with dinosaurs and other prehistoric creatures. Some even start digging holes in their backyards or driveways looking for dinosaur bones. Most give up and become discouraged, because fossil bones are extremely rare, and found only in a few places on earth.

If you wanted to find fossils, where would you look? Why are certain rocks and certain places on earth good for finding fossils, while others have none at all? First, nearly all fossils are primarily found in one kind of rock, known as sedimentary rocks. These are rocks that are made from the loose grains of sand, gravel, or mud, or other particles that weather out of the hard bedrock and are deposited in rivers or flood-plains or in the bottom of the ocean. When animals and plants die, their hard parts (bones, shells, wood) can be buried (Figure 1.1). If the conditions are right, these hard parts will be deeply buried and covered by loose sediments. Over time, the sands are cemented together by minerals in the groundwater to become sandstone, or the soft mud grains are squeezed and compressed until they become a hard splintery rock called shale.

Figure 1.1 How an animal, such as a dinosaur, becomes a fossil. As soon as the animal dies, its bones must survive being destroyed by scavengers, and it needs to be quickly buried in sand or mud. It then turns to stone and becomes a fossil as it is buried, as the groundwater seeps through it and deposits minerals that replace the actual bone. Finally, it must survive millions of years of heating and compression in the earth’s crust, and then the rocks containing it must be uplifted and eroded and exposed in a dry badlands area over the last 200 years, where, if it’s lucky, a collector might find it before it erodes away and is destroyed.

These sedimentary rocks might then be deeply buried in the earth’s crust. Millions of years later, these ancient rocks might be uplifted to the surface by immense tectonic forces, and crumpled upward by the collision of continents to form a mountain belt. Or they might be tilted on their side and erosion will expose the ancient sediment. There they might be brought to the surface by erosion, and rain and frost and wind will break down the fossils as well as the rock surrounding them. This has been happening for millions of years, and most fossils that were once buried over millions of years have already been exposed by erosion and destroyed when no human was around to collect them. Only in the past 200 years have humans (especially paleontologists) been actively looking for and collecting and preserving fossils before they are lost forever. Fossil collectors have only visited small parts of the earth more than a few times. Even today, large areas of unexplored land remain in remote regions, and most fossils there are lost before any human sees them in time to save them.

In addition to sands and gravels and muds, another common kind of sedimentary rock is known as limestone, and it is literally made of fossils—mostly the broken fragments of shells of sea creatures that lived millions of years ago. So, if you happen to be collecting in an area where limestones are common in the bedrock, fossils are everywhere. However, most may be highly fragmentary and not worth collecting.

There are two other classes of rock. Igneous rocks are formed by the cooling of magma, or molten rock, that comes up from the hot deep interior of the earth. This can happen when a volcano explodes and scatters volcanic ash across the landscape (as happened with Mt. St. Helens in 1980), or when lava flows out of an erupting volcano (as happens on Kilauea on the Big Island of Hawaii nearly every year). The magma might remain underground without ever erupting from a volcano, but instead cool in a deep magma chamber until it is a hard crystalline rock like granite. Either way, igneous rocks almost never preserve fossils. If the soft tissues of an animal or plant encounter hot magma, it usually incinerates or vaporizes without leaving a trace. Only in a few cases do volcanic ashes blown from long distance bury a creature and actually preserve it in some way.

The third class of rocks is known as metamorphic rocks. They are formed when igneous or sedimentary rocks descend deep into the earth’s crust and are put under immense pressures and extremely hot temperatures. These conditions transform the original rock into a new rock with new minerals and a new fabric. Any remains of plants or animals are destroyed in this process, so there are no fossils to be found in metamorphic rocks (unless the rocks are just barely metamorphosed).

DATING FOSSILS

How old is your fossil? This is a question that is fundamental not only to identifying it, but also to knowing where to look. If you’re looking in beds of the wrong age, you won’t find the right kinds of fossils—or maybe no fossils at all.

There are two fundamental ways in which geologists and paleontologists determine the age of rocks and geologic events (Figure 1.2). The first method is by relative dating or relative age. In other words, geological event A is younger or older in relation to geologic event B. The primary way geologists do this is by using the principle of superposition first proposed by the Danish scholar Nicholas Steno in 1669. In any layered sequence of rocks (usually layered sedimentary rocks, although it applies to layered lava flows as well), the oldest rocks are at the bottom of the stack, and each layer above it is progressively younger (Figure 1.2[A]). In other words, the stack of rocks goes from older at the bottom to younger at the top. You can’t stack one layer on top of another if the lower layer isn’t already there first. A good analogy is the stack of papers on a messy desk or table. If they just keep accumulating through time without being turned over or sorted out, then the oldest papers will be at the bottom of the stack and the most recent ones will be at the top. Thus, if you are looking at the impressive pile of layers in the Grand Canyon, the oldest ones are always at the bottom and each layer above it is younger. They are like the pages in a book, with the first page at the bottom of the stack and the last at the top.

Figure 1.2 Steno’s laws are used to determine the relative age of one rock body compared to another. (A) The principle of superposition says that the rocks near the top of a stack of layered sediments or lava flows are younger than those at the bottom of the stack. Thus, the top layer is the youngest and the bottom layer is the oldest. (B) The principle of original continuity says that rocks that match from one outcrop to another once connected, and have since been carved away by erosion. (C) The principle of original horizontality is based on the fact that rocks form in horizontal layers, so if you find them tilted or folded or faulted, then the deformation is younger than the rocks it deforms. (D) The principle of cross-cutting relationship says that when rock body (such as a dike of molten lava) or a fault cuts through another rock, then the material that cuts through is older than whatever it cuts.

Another useful concept is the principle of cross-cutting relationships (Figure 1.2[D]). If a molten igneous rock intrudes into another rock (such an intrusion is usually called a “dike”), then the rock that does the intruding must be younger that the rocks that it cuts through. You can’t cut through something if it isn’t already there. Likewise, if a fault cuts through rocks, it must be younger than the rocks it cuts. The principles of relative dating not only go back to 1669, but also were in wide use when modern geology was born in about 1800–1830, and the geologic times-cale was born. The various names for the eras and periods and epochs of the geologic timescale are relative ages.

The other fundamental way to date rocks is known as numerical dating (formerly but incorrectly called “absolute dating” in older books). In other words, the date is given in number of years, or thousands of years or millions of years. Numerical dating is a young technique, only developed in the early twentieth century, and the most popular method, potassium-argon dating, has only been around since the 1950s.

Numerical dating is done by measuring the ticks of the radioactive “clock” in certain minerals. As minerals crystallize out of a magma, they trap radioactive elements such as uranium-238, uranium-235, rubidium-87, or potassium-40. These radioactive elements are naturally unstable, and spontaneously decay into different elements. As this decay proceeds over millions of years, the unstable radioactive parent atoms decay into a stable known daughter atom, such as lead-206, lead-207, strontium-87, and argon-40 (respectively, for each of the elements listed previously). The rate of this decay is known precisely for each of these elements; thus, by measuring the ratio of parent atoms to daughter atoms in a crystal of feldspar or mica or zircon, you can obtain the numerical date since that crystal formed.

Because this process only occurs in crystals that form from a molten rock, you can only date igneous rocks directly. What about sedimentary rocks, which contain the fossils? You cannot directly date them by radioactive minerals. Instead, we need to find places where igneous rocks (such as lava flows or volcanic ash deposits) are interbedded with fossiliferous sedimentary rocks. If a bed has Oligocene fossils (“Oligocene” is a relative age term), and the bottom of the bed has an ash dated 34 million years old, and the top of the bed has a lava flow dated 23 million years old, then we bracket the age of the Oligocene between 23 and 34 million years old. The entire geologic timescale (Figure 1.3) was constructed this way by finding fossiliferous sequences with fossil that gave well-determined relative ages and then using any and all available igneous rocks that are in right position to tell us the age.

Figure 1.3 The modern geologic timescale.

There is one other radiometric system, known as radiocarbon dating, or carbon-14 dating. Unlike the other methods, you can date the fossil bones or shells o...