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Triassic, Jurassic and
Cretaceous woodlands
Ancient trees: A cycad plant (above)
and the limb of a rare, prehistoric
Wollemi pine tree (below)
The world’s forests slowly recovered from the Permian extinctions.
In the hot and dry Triassic Period (248-206 mya), there were primitive
conifers, ferns and gingkos. One common variety of primitive conifers
was the Araucariae. The world became more humid during the Jurassic
Period (206-144 mya), helping the growth of conifer, fern and cycad forests
in the tropics, while conifers dominated the forests toward the polar
regions. Flowering plants began to evolve in the late Jurassic but began
to dominate the landscape during the Cretaceous Period (144-65 mya). At
the end of the Cretaceous Period there was another mass extinction of
species, brought on by a range of factors, including a cooling of the planet,
and exacerbated by what is thought to be the impact of a massive asteroid.
NN
IVER
S
British novelist John Fowles
Did you know?
Plant fossils
Herbivores: An
artist’s impression
of the dinosaur
Abydosaurus
mcintoshi feeding
on conifer trees
A
h
The first trees
C
E
T
Evolution did not intend
trees to grow singly. Far more
than ourselves they are
social creatures, and no
more natural as isolated
specimens than man
is as a marooned
sailor or hermit
SS M A
A
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10t
Ancient forests
he forests we see around the world today are not
the same forests inhabited by our prehistoric
ancestors. Many of the plant and animal
inhabitants of forests have evolved as a result
of the changing landforms and climates
that shape a forest. However other species
have not changed significantly over
millions of years.
In the Silurian Age, 443 to 416 million years ago (mya),
there were no forests as we now know them. Most plant
life was short and restricted to narrow bands near the
edge of water. In the Devonian Period (416-354 mya) larger
plants began to evolve. Archaeopteris (below left) is the first
known modern tree, growing up to 30m with a trunk diameter
of 1m. Unlike previous plants, it developed a deep root system
and thick trunk, enabling it to survive winds and drought. It
had broad fern-like leaves that created shade and its shed leaves
would have covered the ground with organic matter, allowing other
plants and animals to thrive in the shade of the trees, creating a new
biome. It reproduced from two different kinds of spores — large egg
and small sperm spores. The fossilised remains of Archaeopteris have
been found all around the world, showing that
it was capable of surviving in many different
environments, from tropical to
sub-polar. Fossils also show there
was usually a single dominant
species of Archaeopteris in
a given forest area. It
became extinct in the
late Devonian and
the forest biome
temporarily
disappeared.
Relation: A 135 millionyear-old fossil alongside
a living conifer
metasequoia branch
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Series 11
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Evidence of what forests looked like in the past comes from plant fossils.
When prehistoric plants died they either left impressions in mud, or were covered by
layers of silt, which then hardened into stone. Sometimes the plant simply rotted away
leaving other silts or chemicals to fill the cavity and harden into stone, or in some cases
the plant itself was transformed by chemical reactions into stone. Some plant fossils are
little more than leaf patterns in the middle of sedimentary rocks, while others are complete
trees or plants preserved in stone (see Petrified forests).
n Although common across the world in Triassic and
Jurassic forests, Araucarias died out in the northern
hemisphere and became native only to areas of the
southern hemisphere. Surviving species include
the Norfolk Island pine (Araucaria heterophylla),
the monkey puzzle tree (A. araucana) and also the
Wollemi pine (Wollemia nobilis).
n The evolution of larger trees with deeper roots
made the land more hospitable to animal life. Without
trees there would have been nothing for herbivorous
land-dwelling animals to eat and few places for them
to be protected from the glare of the sun.
Living fossils
There are several species of plants that are considered
living fossils because they are found in the fossil records
from millions of years ago and many only exist in small
remnant populations. Ginkgo trees (left), native to China,
are known from the fossil record dating back to the
Permian Period. There is an 840-year-old ginkgo tree in
Seoul. Australia also has a living fossil, the Wollemi pine,
which has fossil ancestors dating back to the Jurassic era.
It was thought to be extinct until it was found in
the Wollemi National Park in 1994.
Petrified forests
Carboniferous
ferneries
to Permian
deserts
In the Carboniferous Period
(354-290 mya) Earth became
hotter and more humid, allowing
new kinds of plants to evolve to
replace the forests made extinct
at the end of the Devonian.
These were not forests that
we would recognise, they
were populated mostly by
giant ferns, horsetails and
huge lycopsids. Lycopsids were
gigantic, weird-looking plants
as tall as 50m. An ice age at
the end of the Carboniferous
shrank the forests. When the ice
sheets receded in the Permian
Period (290-248 mya), the
world became hotter and drier,
with sparse forests populated by
small stands of ferns and primitive
conifers. A major extinction event
wiped out 95 per cent of all land
vegetation. Carboniferous is from
carbo, the Latin word for coal, and
the period was named after the fact
that coal comes from the fossilised
remains of the forests of this time.
Cenezoic forests
In the Cenezoic Era (65 mya to the present), the plants quickly recovered
and thick forests spread across the globe. There were even lush forests
in what is today Antarctica and across what is today the desert of Central
Australia. These began to thin out when the climate became cooler and
drier about 43 million years ago and Antarctica’s thick forests were
covered with an ice sheet. The separation of the continents resulted
in isolated southern continents with distinctive forests. About
25 million years ago grasslands started to spread, eventually
dominating large areas of the planet. By about two million years
ago, this encouraged some of the great ape species to leave the
trees and the forests and to forage in the grasslands. At the
start of the Pleistocene, the planet entered an ice age.
Several alternating periods of warmer weather and ice
ages made significant changes to the forests. At the
end of the last ice age, about 10,000 years ago, the
forests of the Taiga spread northward, creating
what is still the world’s largest forest.
AP Photo
Prehistoric vegetarians
The evolution of forests on land made possible animal life on land.
There is plenty of evidence for prehistoric creatures munching on
trees from primeval forests. The earliest forests would have provided
rotting leaves for microbes to break down and primitive species
of insects to make homes in. The earliest herbivores are believed
to have evolved about 320 million years ago but these merely tore
leaves off plants and swallowed them whole, allowing their gut to
process the leaves. About 290 million years ago, the first herbivores
capable of efficiently digesting plant matter began to appear. The
earliest known was a synapsid ­— a reptilian ancestor of mammals
known as Suminia getmanovi — which had sharp teeth capable of
shredding the leaves to process them better. It was also well adapted
to climbing to the tops of trees to reach the tender leaves. When
dinosaurs roamed Earth between 215 and 65 million years
ago, most evolved as plant eaters.
Unique:
mammal
bones have
been found in
fossilised tree
resin from
more than
18 million
years ago
Around the world there are various places where prehistoric forests have left remnants
known as petrified forests. There are petrified forests in places such as Lake Macquarie in
NSW, Bridgewater in Victoria and Chinchilla in Queensland. On the island of Lesbos in Greece,
there is also a stone forest. In the US the most famous example is the Petrified Forest
National Park in Arizona. Petrified means literally turned to stone or rock — from the Greek
“petros” meaning stone — which happens when the wood is fossilised (see Plant fossils).
The stone in petrified forests still resembles wood but is actually stone.
Enduring: Petrified remains of trees in South Australia’s Canunda National Park
Amber
n The monkey puzzle tree (above) was first seen by
Europeans in 1780 and initially called the Chile pine. In
about 1850 someone remarked that it would “puzzle
a monkey to climb it’’ after which it became known as
the monkey-puzzler or the monkey puzzle tree.
n Petrified wood is the state gem of Washington
in the US. It is often made into jewellery and other
decorative objects.
n Because the cellulose in plants was hard to digest,
some species of dinosaurs swallowed stones to help
grind up the material. Known as gastroliths, these
have been found with the remains of dinosaurs.
n Mosquitoes have been found trapped in amber
dating from the time of the dinosaurs, causing
some to speculate that they may hold the blood of
dinosaurs, making it possible to clone the beasts.
Author Michael Crichton used this as the basis for his
novel Jurassic Park.
Sources and further study
The forests of prehistoric times have left
interesting traces in the form of amber. This
is fossilised tree sap that has become altered
over time, by heat and pressure that causes the
chemical composition to change, making it more
stable and longer-lasting. Often the amber has
been found with animals or plant matter trapped
inside. The oldest known deposits of amber come
from the Carboniferous Period 320 million years
ago. But the most significant deposits are those
from the shores of the Baltic Sea dating
back 60 million years.
Fossil, by Paul Taylor (Penguin/DK)
Prehistoric Life, by William Lindsay (Penguin/DK)
The Complete Guide To Prehistoric Life, by Tim Haines
and Paul Chambers (BBC Books)
First Life (BBC/Roadshow DVD)
The Complete Walking With series (BBC/Roadshow DVD)
The First Forests devoniantimes.org/opportunity/
forests.html
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