The Cretaceous Period (135-66.4 million years ago) saw a cooling of the
climate. The gymnosperms which had dominated began to decline, and make way
for a new group of plants called the angiosperms
plants). The cretaceous was the first era which included 'modern' flora, and
it was in the Rift Valley - a gap between Africa and South America where the
Gondwana was finally drifting away from Laurasia as
all of the continents started to move significantly. Due to the change in
the climate and the shifting of continents, there were mass extinctions as
flora which did not have the genetic potential to adjust to the changes was
wiped out. Furthermore, the seas rose and covered vast tracts of land where
the already stressed forests of conifers, cycads and ferns were growing -
killing them and leaving the land bare when the water retracted later on.
Much of Australia was flooded, and the continent was broken up into four
islands in the early Cretaceous.
The macrofossil record of the
Cretaceous is somewhat incomplete for the later parts of the era and pollen
provides evidence of the presence of angiosperms. While the
and ferns were under immense stress, the
angiosperms found the change in climate advantageous and so began to
increase in number and diversity.
The earliest fossil evidence for
the presence of angiosperms (flowering plants) comes from West Gondwana.
There are three types of flowering plant; Mongolioid, Lauralian, and
Lilioid, which date back to 127-120 Ma. Some of the most significant ancient
fossils of flowers have been found near Melbourne, Australia and in
Portugal. It has been suggested that the first flowers were similar in
structure and bisexual like modern-day Magnolia
other fossilised flowers dating from the same period which were unisexual,
small and contained minimal flower parts.
There are several features
used to separate angiosperms from other plant types (Willis
& McElwain 2002
); these are:
- an enclosed ovary (carpel or carpels)
- presence of flowers
- a xylem and phloem with specialised conducting cells
- a bi-layered seed coat covering the ovules
- pollen with columellae composing the exocarp
- double fertilisation (two sperms are released into the ovaries
from each pollen grain, one fertilising the egg and the other
leading to the creation of endosperm which is the energy source for
Welwitschia mirabilis. Photo by
, reproduced under a
The angiosperms are divided into three groups:
- Eudicotyledons which germinate with two seed
leaves and can form wood or be herbaceous.
- Monocotyledons which germinate with one seed
leaf, can't form wood and have parallel leaf venation.
- Magnoliids which are similar to the eudicots.
Traditionally the eudicots and magnoliids have been classified together as
. The leaves of the angiosperms are
thought to be megaphyllous in origin.
Amborella trichopoda. Photo by
Pennsylvania State University
, reproduced under a
The first Australian angiosperm-like plant recorded is
, which had cones with a bulbous receptacle, surrounded
by bracts forming the "flower". Williamsonia
was a bennettitalean
species; meaning that it belonged to the group of Bennettales which were
Bennettales were distributed globally between the
Triassic and late Cretaceous (248-140 Ma) and were often mistaken for cycads
by palaeobotanists, but are now well recognised as being different because
they had lateral subsidiary cells and epidermal cells distinct from the
Cycads. The Bennettales were precursors for the angiosperms, and provides a
link between the gymnosperms and angiosperms. One of the most ancient extant
species of angiosperm is a shrub called Amborella trichopoda. This may be
the precursor to all of the angiosperms. This is a native shrub to New
Caledonia and appears to have no xylem vessels for transporting water
is another Bennettalean genus
which is credited as being one of the first angiosperms. The other group of
plants which may have been early precursors to the flowering plants are the
Gnetales which appeared in the fossil record after the Cretaceous (c.140 Ma)
and still exist today (an example is Welwitschia mirabilis; pictured).
Genetic analysis have indicated a close relationship with the angiosperms
and the Gnetales.
It was from early plants that the angiosperms
quickly evolved. There was rapid evolution and diversification, and because
many of the Gondwanan continents were still joined at this time, these early
flowering plants were distributed to all continents. By 100 Ma, the
angiosperms had diversified considerably and were widespread.
uncertain is what the earliest angiosperms were like. Were they trees,
shrubs or herbs?
The fossil evidence doesn't provide much of an
answer in this regard. According to
Willis & McElwain
, many of the oldest angiosperm families (which includes the
families Chloranthanaceae, Piperaceae, Plantanaceae, Magnoliaceae,
Degeneriaceae, and Winteraceae) had both herbaceous and arborescent
Theories state that they were either all
initially rhizomatous and herbaceous; either all shrubs and trees, or were
herbaceous shrubs which is the current thinking on this issue.
There is also some dispute as to whether the
monocotyledons or dicotyledons evolved first. The most popular theory on
this states that the dicotyledons evolved first, and from them the
monocotyledons evolved, but not long after the evolution of the angiosperms.