THE HiSToRy oF LiFE 449
tetrapods and reveal a critical feature: the limb could be flexed at the elbow and
wrist (FIGURE 17.16C). All the anatomical details of girdle, limbs, and ribs show
that Tiktaalik could hold its body off the ground—it could do push-ups. Its skull
also has several intermediate features, including a less mobile braincase joint [24].
Its mode of breathing was intermediate between that of lungfishes and that of ter-
restrial tetrapods. These features are more pronounced in recently discovered fos-
sils of Ventastega, a very early tetrapod that is intermediate between Tiktaalik and
Ichthyostega. Its limbs and girdles resemble those of Ichthyostega, and its skull is like
that of Tiktaalik, but it is solid, without the braincase joint [2]. Tiktaalik, Ventastega,
and their relatives are transitional forms that make the distinction between fishes
and the earliest tetrapods difficult to draw.Paleozoic life on land
Imagine that you have traveled back through time to the Carboniferous, about 325
Mya, and go for a walk. Needless to say, there are no paths; you have to make your
way through the ferns and horsetails, probably in one of the widespread swamps,
where you may see gigantic dragonflies or millipedes (FIGURE 17.17). You are a
few million years too late to see a Tiktaalik, but you see its relatives, some up to
1–2 m long, crawling about and occasionally catching a large insect or crusta-
cean. You are pleasantly surprised not to be harassed by mosquitoes or other bit-
ing insects, and then you realize that you don’t see any butterflies, or ants, or bees;
in fact, there aren’t any flowers. And it is strangely quiet: no birdsong—not even
any cricket song, because there aren’t any crickets. No birds are in the sky because
there are no birds to fly [17].
But some changes soon followed. During the Carboniferous (359–299
Mya), land masses became aggregated into the supercontinent Gondwana in
the Southern Hemisphere and into several smaller continents in the Northern
Hemisphere. Widespread tropical climates favored the development of exten-
sive swamp forests dominated by horsetails, ferns, and lycophyte trees, which
were preserved as the coal beds that we mine today. The seed plants diversi-
fied in the late Paleozoic. Unlike earlier plants that depended on water for their
swimming sperm to fertilize ovules, some seed plants had
wind-dispersed pollen. The evolution of the seed provided
the embryo with protection against desiccation as well as
a store of nutrients that enabled the young plant to grow
rapidly and overcome adverse conditions. Bear in mind that
none of these plants had flowers.
The first winged insects evolved during the Carboniferous, and
they rapidly diversified into many orders, including primi-
tive dragonflies, orthopteroids (roaches, grasshoppers, and
relatives), and hemipteroids (leafhoppers and their relatives).
Some Carboniferous insects and other arthropods were
gigantic (see Fig ure 17.17). In the Permian (299–252 Mya), the
first insect groups with complete metamorphosis (distinct lar-
val and pupal stages) evolved, including beetles, hymenopter-
ans (wasps and their relatives), primitive flies (Diptera), and
the ancestors of the Lepidoptera (moths and butterflies). The
DNA-based phylogeny of living insects suggests that insects
evolved wings in the Devonian, before the earliest winged
fossils. But the sequence of fossil appearances corresponds to
the phylogeny, in which groups without complete metamor-
phosis are the basal branches and the orders with complete
metamorphosis form a derived clade (FIGURE 17.18).Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_1717.ai Date 11-02-2016FIGURE 17.17 Giant arthropods of the Carboniferous included
Arthropleura, a millipede up to 7 feet (2.3 m) long, and the
griffenfly Meganeura, related to dragonflies, with a wingspan of 2
feet (25 cm). The human is for scale only; no human has ever seen
these arthropods alive.17_EVOL4E_CH17.indd 449 3/22/17 1:37 PM