Biology Now, 2e

The First Bird ■ 255

separate lineages. A node represents the most

recent common ancestor of two lineages

in question—that is, the most immediate

ancestor that both lineages share. For over

100 years, researchers considered Archaeop-

teryx the most recent common ancestor of

both birds and dinosaurs and thus placed it

at the root of the avian clade—the f irst bird

(Figure 14.6, top).

The first bird, that is, until Xu stumbled

across a fossil that threw the field into contro-

versy. In 2008, Xu visited the Shandong Tianyu

Museum of Nature, a dinosaur museum in east-

ern China. There, he happened upon a unique

fossil that had been collected by a farmer in

Liaoning and sold through a dealer to the

museum. The fossil, entombed in yellowish

rock, shows a small, birdlike dinosaur seemingly

craning its neck forward and spreading its short

wings (Figure 14.7). “I saw it and said, ‘Oh, this

is an important species,’” recalls Xu. He asked

the museum to let him study it.

As Xu examined the fossil, which he later

named Xiaotingia zhengi, he wondered where it

belonged on the dinosaur-bird evolutionary tree.

The root shows the

common ancestor

of all organisms on

the tree.

Each node marks

the divergence of

an ancestor into

two lineages.

A given ancestor and

all its descendants

make up a clade.

Theropods

Theropods

Deinonychosaurs

Archaeopteryx

Deinonychosaurs

Birds

Troodontids

Dromaeosaurids

Modern birds and

their ancestors

Troodontids

Archaeopteryx and

Xiaotingia

Modern birds and

their ancestors

Dromaeosaurids

Traditional dinosaur-bird tree

Xu’s dinosaur-bird tree

Figure 14.6

The evolutionary origins of birds

(Top) The traditional evolutionary tree, showing

Archaeopteryx as an early bird that split off

from the deinonychosaurs (birdlike, carnivorous

dinosaur). (Bottom) The evolutionary tree proposed

by Xu after the discovery of Xiaotingia, with

Archaeopteryx as a deinonychosaur rather than an

early bird.

Q1: In the traditional tree, identify the node

showing the common ancestor for early

birds and dinosaurs.

Q2: What do both the traditional tree and

Xu’s tree suggest about troodontids and

dromaeosaurids?

Q3: In both trees, identify the node for

the common ancestor of Archaeopteryx

and other birds. In what way are the nodes

different in the two trees?

5 cm

Forelimbs

Hind limb

Tail

Head

Figure 14.7

Xiaotingia zhengi : a controversial new leaf on the

dinosaur-bird evolutionary tree

To find out, he analyzed shared derived traits

of the fossil and similar early birds. Shared

derived traits are unique features common

to all members of a group that originated in

the group’s most recent common ancestor and

then were passed down in the group (but not in

groups that are not direct descendants of that

ancestor). In this case, the original ancestor