Chapter 2: EVOLUTION OF ANIMALS AND THE AGE OF REPTILES
Registered UCI students: view the slide show for this chapter or download it: http://darwin.bio.uci.edu:80/~sustain/protected/chap2slides.ppt
The purpose of
Conservation Biology is to find ways of maintaining the high levels of
biological diversity that are seen in today's world. But first, in order to
appreciate that diversity, we need to understand how it has evolved, and what a
complicated series of geological, climatic and biological events have led up to
the present day situation. Therefore, we will review briefly the history of
life on earth. Visit the virtual
The chart shows the life forms that have been present on the earth since Cambrian times, and whose fossils are therefore found in different layers of rocks. Each time a new layer of rock is deposited (usually by sedimentation) it is on top of the older layers, so the chart reflects this order.
The history of global biological diversity is best seen in the marine animals since the ocean is where life started, and marine animals are the best represented in the fossil record. The chart in the slide show shows the number of families of marine organisms on the earth at different times in biological history.
Multicellular animals first appeared about 600 million years ago in the early Paleozoic and there was a rapid rise in number of families during the Cambrian and Ordovician. Diversity remained relatively constant (perhaps even declining) up until about 200 million years ago and then it rose again to its current all-time high of close to 800 families.
Four eras are recognized, and are characterized by typical life forms:
1. Precambrian: The origin of life.
2. Paleozoic ("Ancient Life"): The origin of plants, most invertebrate types, the first vertebrates (back-boned animals, including fishes, amphibians, and reptiles).
3. Mesozoic (the "Age of Reptiles"): The origin of flowering plants, dinosaurs, birds, and mammals.
4. Cenozoic (the "Age of Mammals"): The diversification of flowering plants, insects, birds and mammals, and the appearance of humans.
The eras are divided into periods. Biological diversity was dramatically depleted by five mass extinction episodes at the ends of the Ordovician, Devonian, Permian, Triassic and Cretaceous Periods. At each of these times a large fraction of existing species was wiped out, leaving the survivors to repopulate the biological world. The most famous of these was the extinction at the end of the Cretaceous because this ended the age of dinosaurs and made possible the evolution and dominance of mammals. But it was not the most devastating of the mass extinctions. Now we will review the history of life in more detail.
The origin of life: the Earth was formed about 5 billion years ago. A series of giant meteorites ("the late heavy bombardment") essentially sterilized the planet about 3.8 billion years ago. Rocks 3.5 billion years old contain microfossils of primitive one-celled organisms without a nucleus ("prokaryotes") resembling bacteria and blue-green algae, and carbon isotope ratios characteristic of biological materials, representing the earliest clear signs of life. The first cells with a nucleus ("eukaryotes") appeared 2 billion years ago, and the first organisms made up of many cells (multicellular algae) appeared about 1.8 billion years ago.
In addition to prokaryotes and eukaryotes, a third major group of organisms, called Archaea, consisting of about 500 species but making up about 30% of the biomass on Earth, was not discovered until 1977. They live in the most extreme environments on Earth - the hottest, coldest, and highest-pressure environments, so they are sometimes called "extremophiles". Most of their known biomass is in the Antarctic.
After 3 billion years of very little change, all of a sudden during the Cambrian period, there was a frenzy of evolutionary innovation that generated just about all of the existing major types (phyla and classes) of marine invertebrates plus many others that no longer exist. Accurate dating methods used in 1993 (involving measurements of uranium and lead isotopes) indicate that the explosion happened during a very short period - from 533 to 525 million years ago. This is sometimes called the Cambrian explosion. What caused it is not known - one of the most plausible suggestions is that it followed shortly after marine animals evolved protective shells and cuticles that allowed them to exploit new ecological niches. Another possibility is that more complex body organization became possible after the atmospheric oxygen reached a certain threshold.
Many of the important fossils telling the story of the Cambrian explosion were found in a fossil bed in the Canadian Rockies called Burgess Shale, where both soft- and hard-bodied animals were buried in an underwater mudslide and preserved in water so deep and oxygen-free that they did not decompose. They are described in a book by Stephen J. Gould called "Wonderful Life". View pictures of the actual fossils.
Two different views on the role of chance and selection in evolution:
Wonderful Life: The Burgess Shale and the Nature of History by Stephen Jay Gould (1990). W.W. Norton & Company.
The Crucible of Creation:
The Burgess Shale and the Rise of Animals by Simon Conway Morris (1998).
The Cambrian explosion probably generated over 100 major animal groups with fundamentally different body plans, but only about 30 survived to this day. Examples of those that survived are arthropods (insects, spiders, crustaceans), echinoderms (sea urchins and starfish), mollusks (snails, clams), and chordates, the latter represented in the Cambrian by worm-like animals with a rod-like backbone but eventually giving rise to the vertebrates. Although large numbers of new families evolved in the later periods of earth history, these later innovations did not include major reorganizations of the body plan, but rather variations on themes that were established in the Cambrian.
We will look at the kinds of animals and plants that were present on the earth at different periods, starting with a jump to the Carboniferous because at that time, both plants and animals had moved out on to the land and were beginning to diversify.
During the Carboniferous the climate was hot and humid, and there were extensive swampy forests dominated by giant tree ferns and conifers, club mosses, and horsetails. The decomposed remains of these plants gave rise to the major coal and oil reserves of today. There were no flowering plants and no grasses.
Unlike modern fish that breathe through gills, some of the Carboniferous fish breathed air through two lungs, and they had developed lobe-like fins. The fascinating search for a surviving relative, the Coelacanth, has been recounted in Samantha' Weinberg's "A Fish Caught in Time": click on the picture. These fish live in the deep ocean, and since their discovery in 1938 more than 200 more of them have been captured. The other surviving relative of these Carboniferous fish is the lungfish.
The Carboniferous fish gave rise to modern bony fishes. In these the fins have lost their lobes, and one lung has been lost and the other converted to a swim bladder. They also gave rise to the amphibians, in which both lungs have been retained and the limbs strengthened and specialized for walking. These amphibians, like modern frogs and salamanders, hatched from eggs and spent their larval period in the water as tadpoles. They then went through a change of body organization called metamorphosis, in which they lost their tail and developed legs, and moved out on to the land as adults. They were the first land-dwelling vertebrates, and some of them were massive animals two or three feet long. In addition to the primitive amphibians there were some very primitive reptiles that were totally terrestrial, like today's lizards. In these animals the egg gave rise directly to a miniature version of the adult, which could survive on the land.
The arthropods of the Carboniferous had also moved onto the land and had in fact taken to the air. Many of the ancient insects were gigantic compared to present day ones, among them mayflies with a 14-inch wing span, and giant six-winged insects, in which the first pair of wings had been already reduced to nubs but not eliminated as they are in all present-day insects. These insects had long sucking mouthparts with which they sucked the juices out of large primitive pinecones. Feeding on these insects were dragonflies with a 30-inch wingspan!
The land was increasing in altitude in the Permian and the climate was cooler and dryer than it was in the Carboniferous. The landscape was also developing more topography, with low hills covered with primitive vegetation such as ferns. Small streams and lakes were starting to appear.
Reptiles such as the large sail-backed carnivore Dimetrodon, were common at this time. Some of these early reptiles had elaborate sails on the back. The purpose of these structures is not known although they are thought to have been involved in temperature regulation; that is, to radiate heat when the animal was too hot and to absorb it when the animal was too cold.
The most successful land animals of the middle Permian were many kinds of mammal-like reptiles, which were quite varied and included both herbivores and carnivores. Most of them were large, heavy and slow moving. None of them survived, so we can't tell if they were warm-blooded or if they had mammary glands, like true mammals, but they did have a jaw articulation like that found in mammals and they had several types of teeth, which is a mammalian feature.
At the end of the Permian, 245
million years ago, huge numbers of animals became extinct. About 96% of all
marine animal species and 52% of the families disappeared in the greatest mass extinction
the world has ever known. Recent data shows that 8 of 27 orders of insects, 21
of 27 families of reptiles, 6 of 9 families of amphibians and most of the
terrestrial plant life disappeared at this time. This happened over a
remarkably short period - about one million years. The most popular theory is
that it was caused by flood volcanism in
Reptiles and mammal-like reptiles dominated the landscape in the Triassic. Late Triassic was the time when the first true mammals, descendants of the mammal-like reptiles, appeared. The first ones were small, like Megazostrodon, resembling a modern shrew.
At the end of the Triassic many of the amphibians, reptiles and most of the mammal-like reptiles disappeared and were replaced by the dinosaurs and crocodiles in addition to some early turtles, lizards and frogs.
The Jurassic and Cretaceous together were the age of giant reptiles. For 150 million years, the dominant vertebrates on the land were the dinosaurs. (By comparison, humans have been on the earth for only about 3 million years).
The dinosaurs are composed of two distinct orders:
The Ornithischians (bird-hip dinosaurs) included the duck-billed dinosaurs or Hadrosaurs, Stegosaurs with their plate-like armor along the back, Ankylosaurs which were heavily armored and flattened, and the rhinoceros-like horned dinosaurs or Ceratopsians (example - Triceratops).
(lizard-hips) included the Sauropods
- gigantic herbivorous dinosaurs with extremely long necks and tails, such as
Brontosaurus and Brachiosaurus - the largest terrestrial animals that ever lived;
- carnivorous dinosaurs with enormous skulls, powerful teeth and ridiculously
small front legs, Tyrannosaurus
being the best known example but Giganotosaurus was bigger. Fossils of a new species of
theropod, about the size of a dog, were recently discovered on
The dominant animals of the oceans were the ichthyosaurs, some of which were as large as medium sized whales, the long-necked plesiosaurs and some marine crocodiles. The dinosaurs also took to the air and evolved some spectacular and huge flying reptiles called pterosaurs. One of them, Quetzalcoatlus, was the largest flying vertebrate the world has ever known, with a 40-foot wingspan!
Some cretaceous dinosaur fossils
The predominant land plants in the Jurassic were the Cycads, primitive palm-like seed plants. A few species of cycads still exist in tropical and sub-tropical regions today, and although they are used as ornamental plants some of them are facing possible extinction. Early Cretaceous saw the evolution of flowering plants (angiosperms). The flowers provided a new food source for pollinators, mainly insects, with profound effects on the evolution of those forms.
The end of the Cretaceous saw the
culmination of dinosaur evolution. Dinosaurs were more varied and adapted for
more different modes of life than any other group, before or since. There were
at least 100 species of dinosaurs. Some of them even lived in the
(picture by NASA). At the end of the Cretaceous, 65 million years ago, not only did the dinosaurs disappear completely, but so did flying reptiles (pterosaurs), and marine reptiles (ichthyosaurs and plesiosaurs). In fact, between 60 and 80% of all animal species, including many marine forms, disappeared. Most turtles, crocodiles and primitive birds also disappeared but some survived to give rise to modern forms.
There have been numerous theories to account for the extinction of dinosaurs. But during the 1980's strong evidence was obtained to support the idea, originally proposed by Luis Alvarez, that a global catastrophe, caused by the impact of an asteroid, comet or meteorite, was responsible. Enormous amounts of debris would have been thrown into the atmosphere, making the Earth so cold and dark that cold-blooded animals like dinosaurs were unable to survive.
Supporting the impact theory, about
150 Impact Craters
have now been discovered on the earth. One of the most spectacular is the
Barringer Crater in the
The impact responsible for the extinction of the dinosaurs would have produced a crater at least 100 miles across. The entire planet shows a 3 mm-thick layer of rock at the appropriate level (i.e. exactly at the boundary between Cretaceous and Tertiary), containing several kinds of evidence for the impact:
a high concentration of the element iridium, which is rare in terrestrial rocks but common in meteorites
"shocked" quartz grains showing parallel, colored markings which are an indicator of violent impact
unusual amino acids, thought to be extraterrestrial in origin
Over the past few years geologists
have discovered a 200 mile-diameter submarine crater (the Chicxulub
crater), of exactly the right age but buried under 2 km of sediment, off
the coast of the Yucatan peninsula in Central America. There are actually two
layers, at least in western
Geologists have found that the rock in the Chicxulub crater is unusually rich in sulfur. They conclude that the impact could have produced a global sulfur dioxide fog that could have caused darkness and acid rain for over a decade.
An impact crater in northwestern Australia has been dated to the end of the Permian period, suggesting a possible connection to the mass extinctions that occurred at that time.
For a long time people found it hard to believe in such spectacular collisions. But impact craters are very common on many planets and the moon. And in 1994 a comet that had already broken into 13 pieces crashed into the planet Jupiter. Although the crash was not visible from earth, we were able to see the effects of the impact soon after it happened. This made it much easier to believe that comets can crash into planets. However, not everybody believes that this kind of catastrophe finished off the dinosaurs.
A meteorite impact was observed in Greenland in December 1997. A flash of light seen from fishing trawlers turned night into day, seismic signals were detected in Norway, and satellite images showed a cloud that suddenly formed and then blew away.
If you are interested in whether the Earth may be hit again by an asteroid or comet, NASA has a web site on impact hazards for you, including a list of the predicted close approaches!
J. Bryant (email@example.com), School
of Biological Sciences,