Evolution What the Fossils Say and Why it Matters

232 Evolution? The Fossils Say YES!

Four on the Floor

In my days it was believed that the place for a fish was in the water. A perfectly sound

idea, too. If we wanted fish, for one reason or another, we knew where to find it. And

not up a tree.

For many of us, fish are still associated quite definitely with water. Speaking for

myself, they always will be, though certain fish seem to feel differently about it.

Indeed, we hear so much these days about the climbing perch, the walking goby, and

the galloping eel that a word in season appears to be needed.

Times change, of course—and I only wish I could say for the better. I know all that,

but you will never convince me that a fish that is out on a limb, or strolling around in

vacant lots, or hiking across the country, is getting a sane, normal view of life. I would

go so far as to venture that such a fish is not a fish in his right mind.

—Will Cuppy, How to Become Extinct

As humorist Will Cuppy noted (in the preceding quote), we think of fish as belonging in
the water and have trouble with the idea that they could crawl out on land. For years,
evolutionary biologists and paleontologists have argued and speculated about the forces
that led fishes to finally crawl out on land. The transition seems like a remarkable one.
Water-living fishes needed to develop some way of breathing air, supporting themselves
on land without the buoyancy of water, preventing their skins from drying out, see-
ing and hearing on land, and making many other physiological adjustments. Paleon-
tologists used to speculate endlessly about why they made this apparently difficult trek.
Some argued that it was to escape drying pools, and others suggested it was to escape
predation in the crowded Devonian waters, or to take advantage of new food sources
on land (since insects, spiders, scorpions, millipedes, and other arthropods had already
colonized the land over 100 million years before vertebrates did). Unfortunately, much
of this speculation was predicated on false assumptions and inadequate specimens, and
most of it is irrelevant now.
It turns out that crawling up on the land is no big deal. Many different teleost fishes
(with flimsy ray fins, not the robust lobe fins) do it all the time. A variety of tide-pool fishes,
such as gobies and sculpins, spend a lot of time out of water when the tide goes out, hunt-
ing vulnerable prey. In the southeastern United States, the walking catfish (fig. 10.4A) is a
legendary pest for its ability to wriggle from one pool of water to another, using only its ray
fins for propulsion. Eels, too, are capable of wriggling across the ground for some distance
in search of new pools of water. The climbing perch of Africa and Southeast Asia, Anabas
testudineus, travels in search of water when its ponds dry up. It walks supported by the
spiny edges of the gill plates and propelled by the fins and tail and can climb low trees. The
most specialized and most amphibious of all the “land fish” is the mudskipper (fig. 10.4B),
which is completely adapted to living on the land-water boundary. It even has its eyes up
on periscopes so it can look above water while it is swimming. The mudskipper haunts the
mudflats in mangrove swamps, catching prey in the mud. It uses its ability to wriggle on
land or swim in water to escape predators coming from either direction and can also climb
up the exposed roots of the mangrove trees.