In early Devonian-age rocks, approximately 363- 409 million years old, fossils of simple vascular and nonvascular plants can be seen. Ferns, lycopods, horsetails, and early gymnosperms became prominent during the Carboniferous period (approximately 290-363 million years ago).
The gymnosperms were the dominant flora during the Age of Dinosaurs, the Mesozoic era (65-245million years ago). More than 130 million years ago, from the Jurassic period to early in the Cretaceous period, the first flowering plants, or angiosperms (phylum Anthophyta), arose. Over the following 40 million years, angiosperms became the world’s dominant plants.
The angiosperms show high species diversity, and they occupy almost every habitat on earth, from deserts to high mountain peaks and from freshwater ecosystems to marine estuaries. Angiosperms range in size from eucalyptus trees well over 100 meters (328 feet) tall with trunks nearly 20 meters (66 feet) in circumference to duckweed, simple floating plants barely 1 millimeter (0.003 inch) long.
Some of the defining characteristics of angio- sperms involve their physical appearance or morphology and internal anatomy: the presence of flowers and fruits containing seeds, stamens with two pairs of pollen sacs, a microgametophyte (the male, haploid stage of the life cycle contained in the pollen) with three nuclei, a megagametophyte (the female, haploid stage of the life cycle enclosed in the ovary) with eight nuclei, companion cells, and sieve tubes in the phloem (vascular tissue important in the transport of organic molecules).
Some of these characteristics involve life-cycle features, such as double fertilization, that are distinct from almost all other members of the plant kingdom. (Double fertilization is also known in the genera Ephedra and Gnetum, members of the gymnosperms.)
Because angiosperms possess so many unique features, plant taxonomists have long believed that angiosperms originated from a single common ancestor. Because the first flowers and pollen grains appear in fossils from the early Cretaceous period, up to about 130 million years ago, it is probable that angiosperms actually arose more than 130 million years ago.
As the findings of paleobotanists (botanists who study plants in the fossil record) have been combined with more recent knowledge from evolutionary genetics and biochemistry, a clearer picture of angiosperm evolution has emerged.
Because gymnosperms (the other large group of seed plants) have long been considered ancestral to the angiosperms, researchers have attempted to develop models for the evolution of the ovule-bearing structures of flowering plants from the similar, naked ovule-bearing structures of gymnosperms.
Some lines of evidence indicate that groups of extinct cycad-like gymnosperms known as the Bennettitales and the gnetophytes, amodern division of the gymnospermswhich showup in the fossil record about 225 million years ago, are the seed plantsmost closely related to angiosperms.
All three groups, the Bennettitales, the gnetophytes, and the angiosperms, share, or shared, superficially similar flowerlike reproductive structures. The strobili, or cones, of some gnetophytes closely resemble flowers, and the xylem (vascular tissue specialized for transporting water) of some gnetophytes is similar to the xylem found in angiosperms.
Other lines of evidence suggest that a group of plants called the seed ferns, or pteridosperms, might represent the ancestors of the angiosperms. The seed ferns, which predate the angiosperms by many millions of years, had seed-bearing cupules and specialized organs that produced pollen.Many plant taxonomists believe that the seed-bearing cupules in some groups of seed ferns could have evolved into the carpels of flowers.
Most paleobotanists assume that the first flowerswere small, simple, and green in color and by modern standards were rather unattractive. Their petals and sepals were probably not clearly differentiated.
In November of 1998, Ge Sun and David Dilcher and their colleagues published their discovery of the oldest angiosperm fossil to date, estimated to be at least 122 million years old and possibly as old as 145million years. Either age qualifies it as the oldest.
The fossils were discovered in China, and the fruits show the characteristic enclosed ovule (a carpel) that is distinctive to angiosperms. It was given the scientific name Archaefructus liaoningensis. Given its great age, this find implies that angiosperms may have arisen as early as the Jurassic period, more than 145 million years ago.
Other early flowers produced pollen with a single aperture, or opening, a trait that the monocot branch of the angiosperms shares with cycads and ginkgos. Plant taxonomists believe that pollen with a single opening is an ancestral feature that some plants have kept as they evolved. The pollen of eudicots,with its three apertures, is thought to be a derived feature (that is, a later evolutionary development).
Recent studies of angiosperm evolution, using data fromdeoxyribonucleic acid (DNA) sequences, have led to the proposal that an obscure shrub from the South Pacific island of New Caledonia, called Amborella trichopoda, represents what is left of the ancestral sister group (a related organism that branched off before the evolution of another group of organisms) to all the angiosperms.
As a sister group to all the angiosperms, it is considered to be themost primitive (in an evolutionary sense) of the angiosperms and therefore should resemble what the ancestor to the angiosperms was like. It does possess some of the expected primitive traits for a primitive angiosperm, such as small, greenish-yellow flowers and a lack of vessels for conducting water from the ground to the leaves.
Approximately 97 percent of angiosperm species are classified as either Monocotyledones (monocots), with approximately 65,000 species, or Eudicotyledones (eudicots), with about 165,000 species. The monocots include such familiar plants as the grasses, lilies, irises, orchids, cattails, and palms. The more diverse eudicots include most of the familiar trees and shrubs (other than the conifers) and many of the herbaceous plants.
The remaining 3 percent of angiosperms are called the magnoliids, a group of plants considered to have primitive features, among them pollen with a single aperture. Many magnoliids also feature oil cells with ether-containing oils providing the characteristic scents of laurel and pepper, for example. The magnoliids are typically divided into the woody magnoliids and paleoherbs.
Woody magnoliids have large, often showy, bisexual flowers with multiple free parts.Magnolia trees and tulip trees (both in Magnoliaceae, or the magnolia family) are examples of this group. The paleoherbs have small, often unisexual flowers and usually just a few flower parts. Modern paleoherbs include the pepper family (Piperaceae), the birth-wort family (Aristolochiaceae), and the water lily family (Nymphaeaceae).
Recent studies of angiosperm evolution, using data from DNA sequences, have also sharpened the understanding of some of the relationships among monocots, eudicots, and magnoliids. If these groups are displayed as an evolutionary tree (or phylogenetic tree), the magnoliids are polyphyletic (that is, they do not share a single common ancestor).
The magnoliids branch off near the base of the tree on several different branches. The monocots are monophyletic (that is, they share a single common ancestor) and form a separate branch from among the magnoliid branches. The eudicots branch off last and represent the most diverse and evolutionarily complex group.
As hotly debated, perhaps, as exactly which group of plants were ancestral to the angiosperms is the question of where the angiosperms first evolved. Some botanists believe that angiosperms first developed in theNorthernHemisphere; others look at the Southern Hemisphere.
At the time angiosperms are proposed to have evolved, the continents were not arranged the way they are now. At that time, all of the world’s major landmasses were grouped into a supercontinent called Pangaea.
The southern part of this continent is referred to as Gondwanaland, and the northern part is called Laurasia. Based on what is known about late Cretaceous angiosperms and their habitats, some scientists suggest that the westernmost, semiarid regions of Gondwanaland may be the place where angiosperms first evolved.
As Pangaea broke up, the separate continents moved in different positions, resulting in new configurations. India collided with Asia, raising the Himalaya Mountains and the Tibetan Plateau. Antarctica slipped over the South Pole, and Australia became isolated. These plate movements created new climatic regimes, opening up new niches that were rapidly exploited by the angiosperms.
Diversification and Spread
Regardless of their geographic origins, by about ninety million years ago the flowering plants were well on their way to dominating the world’s flora. The early angiosperms were well adapted to drought and cold.
Adaptations that conferred resistance to these conditions included strong leaves, efficient water-conducting cells, and tough, resistant seed coats. Some woody flowering plants evolved the ability to lose their leaves, called the deciduous habit.
This characteristic allows the shutdown of metabolism during adverse environmental conditions, such as during seasonal droughts or winter weather. Because of greater climate instability during the past fifty million years or so compared to earlier times, the above-mentioned traits were important in allowing the flowering plants to adapt to different and often harsher climatic conditions.
A major innovation that likely led to some of the great diversity seen in angiosperms was pollination by insects or other animals. As plants adapted to the various available pollinators, the pollinators also adapted to the plants, sometimes in very specific ways. Many pollination systems include specialized colors or markings, flower shapes, and flower scents.
This process of evolving "together" is called coevolution. Coevolution has also occurred between plants and their predators. Evolution of chemical compounds to deter herbivory have, in turn, led to adaptations in many animal groups to circumvent the toxicity of the chemical compounds.