Evolution, 4th Edition

448 CHAPTER 17

fin to limb. Hoping to fill in more of this evolutionary sequence, Neil Shubin and

colleagues explored promising Devonian deposits in northern Canada and found

just what they sought: a rich fossil deposit of a new “tetrapodomorph” which they

named Tiktaalik roseae [23a, 85]. Like ichthyostegids, Tiktaalik had a flat, mobile

head and elongate snout and lacked gill cover bones; it also had overlapping ribs,

which would have provided the support that the body of a partly terrestrial ani-

mal requires (FIGURE 17.16B). Most important, the pectoral (shoulder) girdle and

fins of Tiktaalik are intermediate between those of the sarcopterygian and tetrapod

conditions. The forelimb and wrist bones are clearly homologous to those of early

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(B)

(A)

WRIST

(C)

ELBOW

Ulnare

Intermedium

Radius

Humerus

Ulna

Eusthenopteron (osteolepiform)

Panderichthys

Tiktaalik (“tetrapodomorph”)

Acanthostega (tetrapod)

Ichthyostega (tetrapod)

Stem

sarcopterygian

Time

FIGURE 17.16 The lineage from stem sarcopterygian fishes to early tetra-

pods, such as Ichthyostega, shows the vertebrate transition to life on land.

The recently discovered intermediate Tiktaalik and the tetrapods have

a flatter skull than the fish Eusthenopteron, but its structure (as seen from

above, at left) is very similar, except that the gill cover bones at the rear have

been lost. Among the drawings of forelimbs at far left, note the intermediate

structure of the forelimb of Tiktaalik. (B) An articulated skeleton of Tiktaalik.

(C) Drawing of the pectoral fin, or forelimb, of Tiktaalik, showing positions of

joints and the homologues of the limb bones of tetrapods. Tetrapod digits

probably evolved from the many small bones (radials) at the end of the fin.

(A after [1, 85]; C after [85].)

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