Gilboa - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 84

Gilboa

B ACKGROUND : COLONIZATION OF
THE LAND
The consequences of plants and animals
leaving the marine realm to colonize the
land were far-reaching, not least for the
evolution of Man, who is part of
the Earth's terrestrial biota. Few of the
metazoan phyla which emerged from the
great Cambrian explosion produced
terrestrial forms, but those which did have
became very successful in terms of
diversity. The Arthropoda includes some
terrestrial crustaceans (e.g. woodlice), but
of much greater importance are the
terrestrial chelicerates (spiders, scorpions,
mites, and their allies) and the insects,
which make up 70% of all animals alive
today. From fish arose the tetrapods:
amphibians, reptiles, birds and mammals.
Mollusks, too, in the form of slugs and
snails, have been remarkably successful on
land, as any gardener will testify. In order
to live successfully on land, plants
developed features such as stiff trunks and
reproductive devices which gave rise to the
familiar trees and flowers we see on land
today.
Terrestrialization was thus a major
episode in the evolution of life on Earth. It
was neither an instantaneous event nor
restricted to a particular geological period;
indeed some organisms, such as crabs, may
be considered to be terrestrializing today.
Nevertheless, when the physical conditions
on the land surface became sufficiently

favorable to life in the Silurian Period,
then the invasion began in earnest.
A number of physical barriers have to be
overcome before an organism can make
the jump from life in the sea to land.
Water is necessary for all biological
processes but its supply is variable on land
compared to the sea. Plants and animals
adopt four main strategies to cope with
under- (or over-) supply of water. Some,
like microbes, ostracodes, and algae, live
in permanent water on land, between
soil particles and in ponds, so they
are effectively aquatic. Others, like
amphibians, millipedes, slugs, and
woodlice, live in moist habitats and only
venture out into dry air for short periods.
Poikilohydric organisms can tolerate
desiccation and rehydrate when necessary;
examples are bryophytes (mosses and
liverworts), and 'resting' stages such as
plant spores, fairy shrimp eggs, and
tardigrade ('water-bear') tuns. This group
was probably the first onto land in the
Paleozoic Era. The most successful
terrestrial organisms of all are the
homoiohydric forms, which maintain
permanent internal hydration mainly
by having a waterproof cuticle or skin.
These are the familiar land plants
(tracheophytes), tetrapods, and the
arthropods.
Another physiological barrier to be
overcome when crossing the threshold
from water to land is breathing or, more