Evolution, 4th Edition

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_20.04.ai Date 12-27-2016

(A) (B)

(C)

Chicken

WR

mp

mp

r

r r

nt

lbw

Turtle

Ancestral amniote Odontochelys Modern turtles

dsc dsc

hu hu

5 cm

WR

lbw

CR

mp

2 cm

FIGURE 20.4 How turtles’ ribs came to lie above the shoulder

blade (scapula) instead of below. (A) Cross sections through

three embryonic stages of a typical amniote (chicken, left) and

a modern turtle (right). Only the left half of the body is shown.

Part of the neural tube (nt) provides orientation. Arrows show a

groove, with the Wolffian ridge (WR) below, at the upper part of

the developing body wall (lbw). Cells destined to become ribs

(r) and a muscle plate (mp) invade the body wall, due to signals

from the Wolffian ridge. The muscle plate will give rise to the

pectoral girdle. In the chicken, rib cells grow down into the body

wall, beneath the muscle plate, and form ribs underneath the

muscle plate and developing pectoral girdle. But the turtle (right)

develops a novel carapacial ridge (CR), which emits signals that

arrest the extension of ribs into the body wall (see lower right

diagram), so that the ribs develop above the girdle. (B) A phylo-

genetic hypothesis of how the arrangement of ribs and pec-

toral girdle evolved from ancestral amniotes to modern turtles,

via a stage represented by the extinct turtle Odontochelys, in

which the anterior ribs were deflected toward the rear. After the

changes described in part (A) evolved, the ribs became directed

forward and over the girdle. The scapula and carapacial ridge are

shown in red. (C) Dorsal view of the Triassic turtle Odontochelys,

which lacked a carapace. The line drawing distinguishes bones.

The scapula (dsc) and humerus (hu), highlighted in red, are clearly

visible, lying in front of the ribs (highlighted in blue), which are

directed toward the rear. (A after [57]; B from [80]; C from [60].)

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