Evolution__3rd_Edition

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Figure 18.9
The origin of tetrapods.
A good series of fossil forms
exist, connecting fish and
the earliest tetrapods. Redrawn,
by permission of the publisher,
from Zimmer (1998).
Tetrapody preceded life on land
Coelocanth
CHAPTER 18 / The History of Life 541
400 385 370 355 340 325 310 295 Myr
Devonian Carboniferous
Amphibians
Tulerpeton
Lungfish
Amniotes
Ray-finned fish
Eusthenopteron
Panderichthys
Elginerpeton
Ventastega
Metaxygnathus
Acanthostega
Ichthyostega
Hynerpeton
Greerpeton
(Ichthyostega) tetrapods, to amphibians. The fossil evidence showing the gradual transition
is noteworthy in itself, because few major evolutionary transitions are so well
documented. The evidence also has some important details. One is that the tetrapod
condition seems to have evolved first in fully aquatic vertebrates. Acanthostega had four
good legs, homologous with the four limbs of a cat and a lizard, but it also had gills and
a tail for swimming. The fossil evidence therefore suggests that the tretrapod limb originally
evolved as a paddle, for swimming. Its subsequent use for walking on land is an
instance of preadaptation (Section 10.4.2, p. 264).
In modern tetrapods, the foot always has five digits (or if the number differs from
five, it can be seen to be derived from a five-digit condition). However, the tetrapods of
the Devonian include forms with different numbers of digits, such as seven or nine.