Biology Now, 2e

254 ■ CHAPTER 14 The History of Life

BIODIVERSITY

some combination of these. An evolutionary

tree maps the relationships between ancestral

groups and their descendants, and it clusters

the most closely related groups on neighboring

branches.

In an evolutionary tree, organisms are

de picted as if they were leaves at the tips of

the tree branches. A given ancestor and all its

descendants make up a clade, or branch, on the

evolutionary tree. Archaeopteryx and all subse-

quent animals that evolved from it are consid-

ered a clade (Figure 14.6).

A node marks the moment in time when

an ancestral group split, or diverged, into two

on two legs and had hollow, thin-walled bones

(as birds do). Theropods were a diverse group of

dinosaurs (Figure 14.5). Most were carnivores,

including insectivores, but a few were herbi-

vores or omnivores. There were theropods that

could swim and eat fish. Some theropod species

boasted enlarged scales, and of course, many

theropods had feathers.

Xu and other scientists map out lineages

using a diagram called an evolutionary

tree, a model of evolutionary relationships

among groups of organisms based on simi-

larities and differences in their DNA, physi-

cal features, biochemical characteristics, or

Spinosaurus

aegyptiacus

Spinosaurus Tyrannosaurus Velociraptor

Archaeopteryx Eosinopteryx Aurornis Xiaotingia

Deinonychus

Figure 14.5

Dinosaurs large and small

Theropods ranged in size from tiny, like a chicken, to huge, like the group’s most famous member, Tyra n n o s a u ru s rex.

Q1: In what ways were theropods the same as modern birds? Give at least two similarities.

Q2: In what ways did theropods differ from modern birds? Give at least two differences.

Q3: Birds are often referred to as “living dinosaurs.” Is this accurate? Why or why not?