rear appendage on the opposite side. Such an adaptation would have eased the
transition from sea to land, making the coelacanth the most direct ancestor of
higher terrestrial animals.
A possible link between fish and terrestrial vertebrates were the Devon-
ian crossopterygians and lungfish, another living fossil still in existence today.
The crossopterygians were lobe finned, meaning that the bones in their fins
were attached to the skeleton and arranged into primitive elements of a walk-
ing limb.They breathed by taking air into primitive nostrils and lungs as well
as by using gills. This placed them into the direct line of evolution from fish
to land-living vertebrates that gave rise to amphibians and reptiles (Fig. 94).
The sharks were highly successful from the Devonian to the present. An
ancient freshwater shark called Xenacanthushad a back fin that stretched from
head to tail,allowing it to slither through the water like an aquatic snake. Closely
related to the sharks are the rays, with flattened bodies, pectoral fins enlarged into
wings up to 20 feet across, and a tail reduced to a thin, whiplike appendage.The
rays literally fly through the sea as they scoop up plankton into their mouths.
Figure 94 The rough
evolution from
crossopterygians (top)to
the amphibious fish
(middle)to the
amphibians (bottom).DEVONIAN FISH