Reptile Evolution
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
Say Thanks to the Authors
Click http://www.ck12.org/saythanks
(No sign in required)
To access a customizable version of this book, as well as other
interactive content, visit www.ck12.org
CK-12 Foundation is a non-profit organization with a mission to
reduce the cost of textbook materials for the K-12 market both in
the U.S. and worldwide. Using an open-source, collaborative, and
web-based compilation model, CK-12 pioneers and promotes the
creation and distribution of high-quality, adaptive online textbooks
that can be mixed, modified and printed (i.e., the FlexBook®
textbooks).
Copyright © 2015 CK-12 Foundation, www.ck12.org
The names “CK-12” and “CK12” and associated logos and the
terms “FlexBook®” and “FlexBook Platform®” (collectively
“CK-12 Marks”) are trademarks and service marks of CK-12
Foundation and are protected by federal, state, and international
laws.
Any form of reproduction of this book in any format or medium,
in whole or in sections must include the referral attribution link
http://www.ck12.org/saythanks (placed in a visible location) in
addition to the following terms.
Except as otherwise noted, all CK-12 Content (including CK-12
Curriculum Material) is made available to Users in accordance
with the Creative Commons Attribution-Non-Commercial 3.0
Unported (CC BY-NC 3.0) License (http://creativecommons.org/
licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated
herein by this reference.
Complete terms can be found at http://www.ck12.org/about/
terms-of-use.
Printed: December 22, 2015
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
•
•
•
•
Chapter 1. Reptile Evolution
1
Reptile Evolution
Summarize how reptiles evolved.
Describe synapsids and sauropsids.
Explain the Triassic takeover.
Explain why the Mesozoic Era is called the Age of the Dinosaurs.
So what exactly is a dinosaur?
Dinosaurs were not lizards. Rather, they were a separate group of reptiles with a distinct upright posture not found
in lizards. Dinosaurs can be described as large, powerful reptiles. And many were very big. But dinosaurs were
more than that. They were a varied group of animals with over 1,000 non-avian species.
Evolution of Reptiles
The earliest amniotes evolved about 350 million years ago. They resembled small lizards, but they were not yet
reptiles. Their amniotic eggs allowed them to move away from bodies of water and become larger. They soon
became the most important land vertebrates.
Synapsids and Sauropsids
By about 320 million years ago, early amniotes had diverged into two groups, called synapsids and sauropsids.
Synapsids were amniotes that eventually gave rise to mammals. Sauropsids were amniotes that evolved into
reptiles, dinosaurs, and birds. The two groups of amniotes differed in their skulls. The earliest known reptile,
pictured in Figure 1.1, dates back about 315 million years.
At first, synapsids were more successful than sauropsids. They became the most common vertebrates on land.
However, during the Permian mass extinction 245 million years ago, most synapsids went extinct. Their niches were
taken over by sauropsids, which had been relatively unimportant until then. This is called the Triassic takeover.
1
www.ck12.org
FIGURE 1.1
Earliest Reptile: Hylonomus. The earliest
known reptile is given the genus name
Hylonomus. It was about 20 to 30 centimeters (8 to 12 inches) long, lived in
swamps, and ate insects and other small
invertebrates.
Rise and Fall of the Dinosaurs
By the middle of the Triassic about 225 million years ago, sauropsids had evolved into dinosaurs. Dinosaurs became
increasingly important throughout the rest of the Mesozoic Era, as they radiated to fill most terrestrial niches. This
is why the Mesozoic Era is called the Age of the Dinosaurs. During the next mass extinction, which occurred at
the end of the Mesozoic Era, all of the dinosaurs went extinct. Many other reptiles survived, however, and they
eventually gave rise to modern reptiles.
Evolution of Modern Reptile Orders
Figure 1.2 shows a traditional phylogenetic tree of living reptiles. Based on this tree, some of the earliest reptiles
to diverge were ancestors of turtles. The first turtle-like reptiles are thought to have evolved about 250 million years
ago. Ancestral crocodilians evolved at least 220 million years ago. Tuataras may have diverged from squamates
(snakes and lizards) not long after that. Finally, lizards and snakes went their separate ways about 150 million years
ago.
FIGURE 1.2
Traditional Reptile Phylogenetic Tree.
This phylogenetic tree is based on physical traits of living and fossil reptiles. Trees
based on DNA comparisons may differ
from the traditional tree and from each
other, depending on the DNA sequences
used.
Reptile evolution is currently an
area of intense research and constant
revision.
2
www.ck12.org
Chapter 1. Reptile Evolution
Summary
• The earliest amniotes appeared about 350 million years ago, and the earliest reptiles evolved from a sauropsida
ancestor by about 315 million years ago.
• Dinosaurs evolved around 225 million years ago and dominated animal life on land until 65 million years ago,
when they all went extinct.
• Other reptiles survived and evolved into the classes of reptiles that exist today.
Explore More
Use this resource to answer the questions that follow.
• Evolution of Amphibians Reptiles at http://www.michellecaldwell.com/linkfiles/scaredcricket/articles/oth
er/evolution.html .
1.
2.
3.
4.
5.
Describe Seymouria.
Describe the adaptations that led to reptiles.
What allowed reptiles to become more abundant?
List two advantages of laying eggs on land.
List three advantages reptiles have over amphibians.
Review
1. Identify amniotes called synapsids and sauropsids.
2. Give a brief overview of reptile evolution.
3. Explain why reptiles were able to replace amphibians as the dominant land vertebrates.
References
1. Nobu Tamura. Hylonomus illustration: earliest reptile . CC BY 2.5
2. Christopher Auyeung. Reptilian phylogenetic tree . CC BY-NC 3.0
3