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20.3: Evolution and Classification of Mammals

Created by: CK-12

Lesson Objectives

  • Describe the therapsid ancestors of mammals.
  • Outline the evolution of monotreme, marsupial, and placental mammals.
  • Summarize the evolution of modern mammals.
  • Contrast traditional and phylogenetic classifications of mammals.

Vocabulary

  • therapsid

Introduction

Which mammalian trait evolved first? What was the first mammal like? When did the earliest mammal live? Detailed answers to these questions are still in dispute. However, scientists generally agree on the major events in the evolution of mammals. These are summarized in Table below. Refer back to the table as you read about the events in this lesson. *mya = millions of years ago

Era Period Epoch Major Events Start (mya)*
Cenozoic Neogene Holocene Rise of human civilization; spread and dominance of modern humans 0.01
- - Pleistocene Spread and then extinction of many large mammals; appearance of modern humans 1.8
- - Pliocene Appearance of many existing genera of mammals, including the genus Homo 5.3
- - Miocene Appearance of remaining modern mammal families; diversification of horses and mastodons; first apes 23.0
- Paleogene Oligocene Rapid evolution and diversification of placental mammals 33.9
- - Eocene Appearance of several modern mammal families; diversification of primitive whales 55.8
- - Paleocene Appearance of the first large mammals 65.5
Mesozoic Cretaceous - Emergence of monotreme, marsupial, and placental mammals; possible first appearance of four clades (superorders) of placental mammals (Afrotheria, Xenarthra, Laurasiatheria, Supraprimates) 145.5
- Jurassic - Spread of mammals, which remain small in size 199.6
- Triassic - Evolution of cynodonts to become smaller and more mammal-like; appearance of the first mammals 251.0
Paleozoic Permian - Evolution and spread of synapsids (pelycosaurs and therapsids) 299.0
- Carboniferous - Appearance of amniotes, the first fully terrestrial vertebrates 359.0

Mammalian Ancestors

Ancestors of mammals evolved close to 300 million years ago. They were amniotes called synapsids. Figure below shows how modern mammals evolved from synapsids. The stages of evolution from synapsids to mammals are described below.

Phylogeny of Mammalian Evolution. This diagram represents the evolution of mammals.

Pelycosaurs

Synapsids called pelycosaurs became the most common land vertebrates during the first half of the Permian Period. A pelycosaur genus called Dimetrodon is shown in Figure below. Dimetrodon had sprawling legs and walked like a lizard. It also had a fairly small brain. However, it had started to develop some of the traits of mammals. For example, it had teeth of different types.

Pelycosaur Synapsid: Dimetrodon. Dimetrodon was a pelycosaur. It lived about 275 million years ago.

Therapsids

Some pelycosaurs gave rise to a group of animals called therapsids. The earliest therapsids lived about 260 million years ago. At first, the therapsids looked a lot like Dimetrodon. But after a while, they could easily be mistaken for mammals. They evolved a number of mammalian traits, such as legs positioned under the body instead of along the sides. Therapsids became the most common and diverse land vertebrates during the second half of the Permian Period.

The Permian Period ended about 250 million years ago with a mass extinction. Most therapsids went extinct. Their niches were taken over by sauropsids. These were the amniotes that evolved into dinosaurs, reptiles, and birds. Not all therapsids went extinct, however. The few that remained no longer had to compete with many other therapsids. Some of them eventually evolved into mammals.

Cynodonts

The surviving therapsids were small animals. Some of the most successful were the cynodonts (see Figure below). They flourished worldwide during the first half of the Triassic Period. Some of them ate insects and were nocturnal, or active at night. Being nocturnal may have helped save them from extinction. Why? A nocturnal niche was one of the few niches that dinosaurs did not take over in the Triassic Period.

Cynodonts became more mammal-like as they continued to evolve. Some of their mammalian traits may have been adaptations to their nocturnal niche. For example:

  • The ability to regulate body temperature might have been selected for because it would allow nocturnal animals to remain active in the cool of the night.
  • A good sense of hearing might have been selected for because it would be more useful than good vision when hunting in the dark.

Probable Mammalian Ancestor: Cynodont. Cynodonts were mammal-like therapsids. They may have been ancestral to mammals. They were about the size of a rat.

By the end of the Triassic Period, cynodonts had become even smaller in size. They also had evolved many mammalian traits. For example, they had

  • Four different types of teeth.
  • A relatively large brain.
  • Three tiny bones in the middle ear.
  • A diaphragm for breathing.
  • Endothermy.
  • Lactation.
  • Hair.

Cynodonts probably gave rise to mammals about 200 million years ago. However, they are not considered to be mammals themselves. In fact, competition with early mammals may have led to their extinction. They went extinct sometime during the Jurassic or Cretaceous Period.

Evolution of Early Mammals

The earliest mammals evolved from cynodonts. But the evolution of mammals didn’t end there. Mammals continued to evolve. Monotreme mammals probably split off from other mammals first. They were followed by marsupials. Placental mammals probably evolved last.

Evolution of Monotremes

The first monotremes may have evolved about 150 million years ago. Early monotreme fossils have been found in Australia. An example is a genus called Steropodon, shown in Figure below. It may have been the ancestor of the platypus. Early monotremes retained some of the traits of their therapsid ancestors. For example, they laid eggs and had a cloaca. These traits are still found in modern monotremes.

Probable Monotreme Ancestor: Steropodon. Like the platypus, Steropodon probably had a bill.

Evolution of Marsupials

The first marsupials may have evolved about 130 million years ago. One of the earliest was the extinct genus Sinodelphys. A fossil of this mammal has been identified. It is a remarkable fossil find. It represents a nearly complete animal. Even tufts of hair and imprints of soft tissues were preserved. For additional information, see Highway of Life at http://www.lihatvideo.com/highway-of-life-part-2-of-2/FtJC3C6hxzU. Sinodelphys was about 15 centimeters (6 inches) long. Its limb structure suggests that it was a climbing animal. It could escape from predators by climbing into trees. It probably lived on a diet of insects and worms.

Evolution of Placental Mammals

The earliest placental mammals may have evolved about 110 million years ago. The ancestor of placental mammals may be the extinct genus Eomaia. Fossils of Eomaia have been found in what is now China. It was only about 10 centimeters (4 inches) long. It was a tree climber and probably ate insects and worms. Eomaia had several traits of placental mammals. Figure below shows an Eomaia fossil.

Probable Ancestor of Placental Mammals: Eomaia. Eomaia lived over 100 million years ago.

The placental mammal descendants of Eomaia were generally more successful than marsupials and monotremes. On most continents, placental mammals became the dominant mammals, while marsupials and monotremes died out. Marsupials remained the most common and diverse mammals in Australia. The reason for their success there is not yet resolved.

Evolution of Modern Mammals

The Cretaceous Period ended with another mass extinction. This occurred about 65 million years ago. All of the dinosaurs went extinct at that time. Did the extinction of the dinosaurs allow mammals to take over?

Traditional View

Scientists have long assumed that the extinction of the dinosaurs opened up many niches for mammals to exploit. Presumably, this led to an explosion of new species of mammals early in Cenozoic Era. Few mammalian fossils from the early Cenozoic have been found to support this theory. Even so, it was still widely accepted until recently.

View from the Mammalian Supertree

In 2007, an international team of scientists compared the DNA of almost all known species of living mammals. They used the data to create a supertree of mammalian evolution. The supertree shows that placental mammals started to diversify as early as 95 million years ago.

What explains the diversification of mammals long before the dinosaurs went extinct? What else was happening at that time? One change was a drop in Earth’s temperature. This may have favored endothermic mammals over ectothermic dinosaurs. Flowering plants were also spreading at that time. They may have provided new and plentiful foods for small mammals or their insect prey.

The supertree also shows that another major diversification of mammals occurred about 50 million years ago. Again, worldwide climate change may have been one reason. This time Earth’s temperature rose. The warmer temperature led to a greater diversity of plants. This would have meant more food for mammals or their prey.

Classification of Placental Mammals

Traditional classifications of mammals are based on similarities in structure and function. Increasingly, mammals are being classified on the basis of molecular similarities.

Traditional Classification

The most widely accepted traditional classification of mammals divides living placental mammals into 17 orders. These orders are shown in Table below. This classification of mammals was widely accepted for more than 50 years. Placental mammals are still commonly placed in these orders. However, this classification is not very useful for studies of mammalian evolution. That’s because it groups together some mammals that do not seem to be closely related by descent from a recent common ancestor.

Order Example Sample Trait
Insectivora

mole

small sharp teeth
Edentata

anteater

few or no teeth
Pholidota

pangolin

large plate-like scales
Chiroptera

bat

digits support membranous wings
Carnivora

coyote

long pointed canine teeth
Rodentia

mouse

incisor teeth grow continuously
Lagomorpha

rabbit

chisel-like incisor teeth
Perissodactyla

horse

odd-toed hooves
Artiodactyla

deer

even-toed hooves
Cetacea

whale

paddle-like forelimbs
Primates

monkey

five digits on hands and feet
Proboscidea

elephant

tusks
Hyracoidea

hyrax

rubbery pads on feet
Dermoptera

colugo

membrane of skin between legs for gliding
Pinnipedia

seal

feet with fins
Sirenia

manatee

paddle-like tail
Tubulidentata

aardvark

teeth without enamel

Phylogenetic Classification

The mammalian supertree classifies placental mammals phylogenetically. It groups together mammals that are closely related because they share a recent common ancestor. These groups are not necessarily the same as the traditional groups based on structure and function.

The supertree classification places placental mammals in four superorders. The four superorders and some of the mammals in them are:

  • Afrotheria—aardvarks, elephants, manatees.
  • Xenarthra—anteaters, sloths, armadillos.
  • Laurasiatheria—bats, whales, hoofed mammals, carnivores.
  • Supraprimates—primates, rabbits, rodents.

All four superorders appear to have become distinct from one another between 85 and 105 million years ago. The exact relationships among the superorders are still not clear. Revisions in this classification of mammals may occur as new data become available.

Lesson Summary

  • Amniotes called synapsids were the ancestors of mammals. Synapsids named pelycosaurs had some of the traits of mammals by 275 million years ago. Some of them evolved into therapsids, which became widespread during the Permian Period. The few therapsids that survived the Triassic takeover were small, arboreal insect eaters. They were also nocturnal. Being active at night may explain why they survived and evolved still more mammalian traits.
  • Monotremes evolved about 150 million years ago. Like modern monotremes, they had a cloaca and laid eggs. Marsupials evolved about 130 million years ago. They were very small and ate insects and worms. Placental mammals evolved about 110 million years ago. They were also small and climbed trees. Placental mammals became the dominant land mammals. Most marsupials and monotremes died out except in Australia.
  • Mammals used to be classified on the basis of similarities in structure and function into 17 different orders. Recently, DNA analyses have shown that the traditional orders include mammals that are not closely related. Phylogenetic classification, based on DNA data, groups placental mammals in four superorders. The superorders appear to have become distinct from each other 85–105 million years ago.

Lesson Review Questions

Recall

1. What were the synapsids? When were they most widespread?

2. Identify the therapsids. How were they related to mammals?

3. Describe cynodonts. What is their place in the evolution of mammals?

4. Outline the evolution of monotreme, marsupial, and placental mammals.

5. What is the mammalian supertree?

Apply Concepts

6. Assume that a new species of placental mammal has been discovered. Scientists have examined it closely and studied its DNA. It has wings similar to a bat that it uses for gliding. Its DNA is most similar to the DNA of rodents such as mice. How would you classify the new mammal? Explain your answer.

Think Critically

7. Explain why the extinction of most therapsids at the end of the Permian Period may have allowed mammals to evolve.

8. Relate the extinction of dinosaurs to the diversification of modern mammals.

9. Compare and contrast traditional and phylogenetic classifications of placental mammals. Explain which type of classification is more useful for understanding how mammals evolved.

Points to Consider

Some mammalian traits, such as different types of teeth, evolved in ancestors of mammals. Other traits, such as placental reproduction, evolved after the first mammals appeared. Mammals also evolved many behavioral traits.

  • How do mammals behave? What behaviors do you think characterize mammals?
  • How do you think these behaviors evolved?

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