Plants are vital to life on Earth.They capture solar enrgy by the process
of The 'forests' of the sea are huge groups of microscopic, single celled plants
known collectively as phytoplankton. About 70% of the surface of the Earth is
covered by water, so think how important they are. There are many forms, that use different
types of minerals to produce their skeletons. These groups are very important on helping
geologists to date rocks, as they are abundant, spread all over the oceans and
evolve quite quickly.
It is possible that some types of algae that colonised the land as
long ago as the Precambrian.True land plants appeared during the Silurian, by the time that dinosaurs
emerged they had diversified considerably. Many of the palnts would like quite
strange now, but many still have living relatives.
The lycophytes grew in swamps, and provided a lot of the material that has makes
up the world's coal beds. The had a criss-cross, scaly bark, like a pineapple/ Their
leaves stuck directly out from the trunk and were spiny, unlike modern leaves.
Some were tall trees but others were small bushes.
Plants that looked like gigantic horsetails grew alongside the lycophytes in
these marshy areas. They have much smaller modern relatives called Equisetum.
their leaves grew out in circles from joints in the stems. The leaves are often found
separately from the main plant, a problem with nearly all plants in the fossil
record.
Ferns are more familiar to us today. This group (Pteridophytes) is usually quite
small,but can grow as large as trees in equatorial zones. They too prefer damp,
boggy ground. They need the wet to help their sperm reach the eggs. Ferns have any spores
and are often the first plants to recolonise an area after a fire or flood.
There is a huge increase in the number of fern spores at the K-T boundary in North
America. This fern 'spike', as it is known, is taken as evidence that at
the K-T boundary many forests were destroyed suddenly at this time by fire, which
can be seen as further evidence that a meteor hit the Earth at this time.
The forests regenerated, but with different types of trees, which is viewed as
another piece of evidence for the impact. The forests in North America were hardest hit,
which ties in with the proposed impact site off the coast of Mexico.
There are two main groups of seed plants. They are the gymnosperms ('naked seed') and the
flowering plants ('angiosperms'). The gymosperms stretch back into the Upper Devonian.
Unlike previous plants, in which the spores formed tiny plants on the parent and
were vulnerable for a long time, seeds allow the next generation to lie dormant
for months. This meant that they could survive fire, drought and other natural disasters.
Seed ferns originated seeds and this group gave rise to groups which we can still
see today.
Conifers and cycads are gymnosperms. Conifers will be familiar to you as cone bearing pine trees.
Their leaves, like most early trees ar needlelike. They rely on the wind to disperse their seed bearing
cones. All gymnosperms depend mostly on wind dispersal. The cycads are stumpy trees that
look like palm trees. They still live in Central America and in some areas of the southern hemisphere.
Some were short and bushy, but others were up to 10m tall. Their pollen was sometimes
dispersed by beetles, as well as by the wind. This is an early example of insect pollination
that is associated with insects like bees today.
The flowering plants ('angiosperms') radiated quite suddenly in the Early Cretaceous. They radiated very quickly and
now comprise 95% of all land plants. It is still unclear which group of plants gave rise to the
flowering palnts, but it seems probable that one of the gymnosperm groups was responsible.
There are some Triassic fossils that may be angiosperms, but this is unconfirmed.
Two 'tricks' have helped the flowering plants dominate the modern flora. the first is the
surrounding of the seed with a coating of nutrients. The second is the use of bright colours,
ultra-violet signalling and scents to attract insects and birds to them to help with
pollination.
Bob Bakker, a flambuoyant American dinosaur palaeontologist, has proposed that
dinosaurs made conditions favourable for flowers. He thinks that flowering plants were
able to dominate the land plant niche becuse of a change in the feeding habits of herbiverous dinosaurs.
At the end of the Jurassic dinosaurs like Stegasaurus and Apatosaurus
died out. These dinosaurs reared up on their hind legs to browse the vegetation on
the tops of trees and left the gymnosperm saplings alone. In the Early Cretaceous new
groups of dinosaurs, the iganodonts and nodosaurs appeared. They had a different
feeding strategy. They grazed low down on the young plants. Gymnosperms don't regenerate
as quickly as angiosperms, so what Bakker thinks happened is that dinosaurs would
eat the young conifers and ferns in an area. The flowering plants could then take over the space
left, because the grow much faster. Many gymnosperms cannot tolerate shade and would have
been unable to grow in the shade of the angiosperms. This theory is not accepted by
everyone, but it is interesting.
The overall climate of the Mesozoic was much warmer and wetter than today.
Seasonality did not really exist and there were no icecaps.
TRIASSIC
The Triassic land was one giant supercontient called Pangea. The interior was
quite hot and arid so plant life was concentrated in the coastal areas. Although
all the land was one continent the plants did show a lot of variation. The northern part
of Pangea was lusher, with tree-fern and gingko forests. The floors of these
forests were covered in luxuriant fern growth. Around the Equator conditions were
less favourable and the forests down here were sparse, consisting of conifers and
cycads. In the southern area trees were relegated to a secondary role by giant seed
ferns which grew up to form the top storey of the forest, with smaller ferns forming
the undergrowth.
JURASSIC
The climate of the Jurassic was less harsh. Rainfall increased and temperatures fell.
These conditions promoted a greener world. The massive amounts of vegetation made it
possible for the huge sauropods to appear and survive, munching their way through an
abundance of plant material.
The main change in vegetation was the replacement of the giant seed ferns of the
southern continents by cycads and conifers. Some conifers adapted an almost cycad style
of growth, with most of their needles concentrated right at the top of the trunk.
CRETACEOUS
The beginning of the Cretaceous saw climate stay much the same as in the Jurassic.
At about 100 My the climate started to cool. Rainfall decreased again.
The flowering plants would have started to put splodges of colour amomgst the monotonous green of the
foliage. Insects would be moving around these plants, attracted by their 'wares'.
The drop in rainfall meant that the forests around the equator started to die back and open palins,
like the modern 'pampas' of South America would have developed. Modern grasses had
not yet appeared, so the horsetails and ferns would have formed the vegetation in these areas.
The higher latitudes were initially filled with forests of cycads and conifers. The flowering
plants started to take over these niches as the Cretaceous progressed. They began as
small, weedy plants but we all know how well weeds grow. They began to fill the
understorey of the conifer forests and eventually grew into the tall deciduous
trees of today, the oaks, birches and ashes.
OCEANIC PRIMARY PRODUCERS
LAND PLANTS
SEED PLANTS
FLOWERS TAKE OVER
CHANGING CLIMATE-CHANGING VEGETATION