Invasion of Land
Early vascular plants,
collectively known as rhyniophytes, first appeared in the late Early
Silurian and diversified considerably through the Late Silurian.
These plants had evolved an internal system of tubular cells through
which water traveled up the body of the plant, allowing them to
maintain cell functions higher above the land surface than would
otherwise have been possible. Simple, small plants such as Cooksonia
lacked leaves, roots, seeds and the capability to grow in diameter,
but they were the dominant early terrestrial plants.
By the Middle Silurian, very early terrestrial communities had developed
around the tiny, vertically growing plants that acted as primary
producers. True herbivores
were absent from these early land ecosystems. Dead plants were recycled
by fungi and bacteria. Early arthropods—possibly
related to millipedes—also fed on dead and decaying plant
matter rather than on living plant tissues. These arthropod detritivores
may also have digested the bacteria and fungi that lived in the
soil. Other arthropods, possibly related to centipedes, preyed upon
the detritivores. The Mid–Late Silurian terrestrial biota
was probably confined to relatively wet areas.
The brackish and freshwater habitats near land were invaded by a
variety of animal species, including eurypterids,
xiphosurids, scorpions, and jawless
fishes. This increase is probably linked to the expansion of
vascular plants into aquatic and terrestrial habitats, which would
have increased the amount of food available. Most aquatic animals
were relatively small (less than 20 centimeters), but some eurypterids
exceeded a meter in length and are, in fact, the largest known arthropods.
Feeding dynamics in Late Silurian estuaries appear to have been
relatively simple. Most animals probably fed on benthic
(bottom) detritus and/or algae. Eurypterids and
aquatic scorpions were the dominant large predators.