Evolution__3rd_Edition

540 PART 5 / Macroevolution
Some mammals coevolved with
grass
There is a good fossil record from
fish to amphibians
“molecules versus fossils” controversy, the difference in dates could reflect incompleteness
of the fossil record, inaccuracy of the molecular clock, or a delay between the
origin and the proliferation of the taxon. However, the controversy about the time of
angiosperm origin is currently unresolved.
The proliferation of the angiosperms in the Cretaceous and Tertiary is often
explained in terms of coevolution with insect pollinators. We look at that hypothesis in
Section 22.3.4 (p. 622). Later on, around 60 million years ago, the fossil record shows
the origin and global proliferation of grasses. The proliferation of grasses has been
explained by coevolution with mammals. Mammals proliferated at the same time, and
included forms with specialized teeth for grazing. Grass is well adapted to thrive where
mammalian grazers are present, because grass regrows from the base rather than the tip
of its stem. The spread of grass may in turn have helped to set the stage for the future
evolution of humans. Human evolution has often, if uncertainly, been associated with
a shift from arboreal to savannah grassland habitats.
18.6 Vertebrate evolution
18.6.1 Colonization of the land
The earliest vertebrate fossils are fish and date back to Cambrian, or even (in some
recently described fossils from China) late Precambrian, times. Fish proliferated in the
Ordovician fossil record, but we can pick up the story where we began with plants: the
move on to land. The fossil evidence points to the late Devonian, around 360 million
years ago, as the time when terrestrial vertebrates originated.
The terrestrial plants probably prepared the way. Terrestrial plants proliferated during
the Devonian, at the water’s edge. The presence of plants, and their roots growing
down into the water, and the arthropod life associated with them, combined to create a
new habitat at the water’s edge. Fish would have evolved to exploit the resources there.
The fossil record documents in excellent detail the evolutionary transition from fish to
terrestrial amphibians (Figure 18.9). The amphibians were the first of the tetrapod
groups to evolve. (The tetrapods are the group of four-legged vertebrates: amphibians,
reptiles, birds and mammals. “Tetrapods” and “terrestrial vertebrates” refer to roughly
the same group of animals.) We can notice a few features of the story.
Modern fish (or, to be more exact, bony fish) divide into two main groups: rayfinned
fish and lobe-finned fish. Most fish are ray-finned, but modern tetrapods are
descended from lobe-finned fish ancestors. Modern lungfish and the coelocanth are
lobe-finned fish. Within the lobe-finned fish, the lungfish, rather than the coelocanth,
is thought to be the closest relative of tetrapods. Morphological evidence had been
ambiguous, and in the 1980s an authoritative cladistic analysis suggested that coelocanths
were closer to tetrapods than were lungfish (Rosen et al. 1981). However, molecular
evidence in the 1990s pointed to the opposite conclusion. For now, the molecular
evidence is generally accepted.
Between the lungfish and amphibians, a series of fossil forms range from the completely
fish-like Eusthenopteron, through aquatic (Acanthostega), and partly terrestrial
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