Plant Science Portal 
Angiosperms (Flowering Plants)
 Plant Evolution Tour: Part XIII of XV

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 (flowering 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 revolution began.

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 Ginkgoales, Cycadales, Coniferales 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 species. However, 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 seeds)
Welwitschia mirabilis
Welwitschia mirabilis. Photo by David Eickhoff, reproduced under a Creative Commons licence.
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 dicotyledons. The leaves of the angiosperms are thought to be megaphyllous in origin.
Amborella trichopoda
Amborella trichopoda. Photo by Pennsylvania State University, reproduced under a Creative Commons licence.
The first Australian angiosperm-like plant recorded is Williamsonia, 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 like cycads.

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 internally.

Williamsoniella 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.

What is 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 (2002), many of the oldest angiosperm families (which includes the families Chloranthanaceae, Piperaceae, Plantanaceae, Magnoliaceae, Degeneriaceae, and Winteraceae) had both herbaceous and arborescent (tree-like) species.

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.
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