SILURIAN
PERIOD
A significant evolutionary milestone during the Silurian was
the appearance of jawed and bony fish. Life also began to appear on land in the
form of small, moss-like, vascular plants which grew beside lakes, streams, and
coastlines, and also in the form of small terrestrial arthropods. However,
terrestrial life would not greatly diversify and affect the landscape until the
Devonian.
The first fossil records of vascular plants, that is, land
plants with tissues that carry food, appeared in the second half of the
Silurian period. The earliest known representatives of this group are Cooksonia
(mostly from the northern hemisphere) and Baragwanathia (from Australia). Most
of the sediments containing Cooksonia are marine in nature. Preferred habitats
were likely along rivers and streams. Baragwanathia, appears to be almost as
old dating to the Early Ludlow (420 million years) and has branching stems and
needle-like leaves of 10-20 cm. The plant shows a high degree of development in
relation to its age. As mentioned, fossils of this plant are only found in
Australia.
The much-branched Psilophyton was a primitive Silurian land
plant with xylem and phloem but no differentiation in root, stem or leaf. It
reproduced by spores, had stomata on every surface, and probably
photosynthesized in every tissue exposed to light. Rhyniophyta and primitive
lycopods were other land plants that first appear during this period. Neither
mosses nor the earliest vascular plants had deep roots. Silurian rocks often
have a brownish tint, possibly a result of extensive erosion of the early
soils.
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