The Origin and Evolution of Seed Plants

March 2, 2023 Agric4Profits

Fossil Record of Seed Plants: Today, the seed plants are some of the most important organisms on earth. Life on land as we know it is shaped largely by the activities of seed plants. This large and important group appeared early in the evolution of vascular plants, and throughout the Late Paleozoic shared dominance of the land flora with ferns, lycophytes, and sphenopsids.

Since the beginning of the Mesozoic, however, most trees and forests have consisted of seed plants.

The Late Devonian Seed Plants

The oldest known seed plant is Elkinsiapolymorpha, a “seed fern” from Late Devonian (Famennian) of West Virginia. Though the fossils consist only of small seed-bearing shoots, these fragments are quite well-preserved. This has allowed us to learn details about the evolutionary development of the seed. Another such fossil from about this time is Archaeosperma, also known only from fragments.

The earliest seed plants produced their seeds along their branches without specialized structures, such as cones or flowers, unlike most living seed plants. The seeds were produced singly or in pairs, and were surrounded by loose cupules.

This small cup-like structure was lobed in the earliest seeds, producing a somewhat sheltered chamber at one end of the seed. Within these cupules, the seed was enclosed by a more tightly appressed tissue called the integument.

The integument is a layer of tissue found in all seeds; it is produced by the parent plant, and develops into the seed coat. As the integument evolved to enclose the seed more tightly, an opening was left at one end, called the micropyle, which permitted pollen to enter and provide sperm to fertilize the egg cell. Both the integuments and cupules are believed to be the result of reduced and fused branches or leaves.

In later seed plants, a small pollen chamber appears just inside the micropyle. In modern cycads and conifers, this chamber exudes sticky fluids to aid in pollen capture, and as the fluid dries, it pulls the pollen inside the micropyle.

This structure is preserved in detail in a number of recently discovered per mineralized Devonian seeds. Besides preserving the pollen drop, minerals replaced the original tissues gradually, such that fine detail of the cell walls can be studied — a few Permian seeds even have preserved embryos. Presented below are some seeds of Trigonocarpusfrom the Carboniferous age.

The Late Paleozoic Seed Plants

By the end of the Devonian, a variety of early seed plants collectively known as “lyginopterids” appeared. These include Sphenopteris, a plant with fern-like leaves, but which bore seeds and cupules. It is not clear whether Sphenopteris is a single group of closely related plants, or several with similar leaves.

The Carboniferous age saw an increase in the number and kinds of seed plants. In the coal swamps of North America grew pteridosperms like Medullosa, a seed plant that resembles modern tree-ferns, but which bore seeds. Cordaites also grew in these swamps, and in a number of other habitats including ocean-edge environments similar to that of the modern mangrove.

However, the cordaites are believed to be closer relatives of modern conifers. Both the medullosans and cordaites were small trees when compared to the great scale-trees which dominated these Late Paleozoic coal swamps. Seed plants were thus overshadowed in their early evolution by plants which did not produce seeds.

By the Westphalian (Late Carboniferous), the Voltziales first show up. These are believed to be the closest relatives of modern conifers, and in fact some paleobotanists classify them as conifers. By the Permian, the seed plants were beginning to produce large trees, and by the Triassic, all major groups of seedplants had appeared, except for the flowering plants. (Gastaldo,1986, Stewart et al, 1993)

Having learnt about the evolution of the gymnosperm, let us take a look at that of the angiosperms.

The Fossil Record of Angiosperms

Most plants from the past decomposed without leaving a trace of their existence. Indeed, the fossil record of plant species may be only 1% complete and that at least 90% of the species that ever existed are extinct.

Nevertheless, the fossil record of plants does provide a basis for some general ideas about where flowering plants came from and how they might have evolved.

The first fossils of vascular plants are more than 420 million years old, and the first seeds appeared as long as 360 million years ago. However, fossils of plant fragments that probably came from angiosperms are not known before the early Cretaceous period, about 135 million years ago. Unfortunately, most of the oldest of these fossils are so fragmented and incomplete that paleobotanist are not certain that they are angiosperms at all. Nevertheless, one particular fossil stands out because it consists of all the parts of a flower attached to a reasonable intact plant.

This flowering plant is from a 120million year old fossil deposit near Koonwarra, Australia. Paleobotanists believe that this plant represents the ancestral type of flower. If this is true, then the features shared by the Koonwarra angiosperm and certain modern angiosperms may show which living plants are closest to the ancestral origin of the group. (Randi etal, 1998)

The Koonwarran Angiosperm

The fossil of the world’s earliest known flower was discovered in 1986. The Koonwarra angiosperm had several features that are typical of many modern angiosperms. For example, it had small flowers without petals, a spikelike inflorescence, single carpel ovaries with short stigmas and no styles, and imperfect flowers with several bracts at their bases.

These features occur in present-day members of the Lizard’s tail family (Saururaceae), the pepper family (Piperaceae), and the chloranthus family (Chloranthaceae), all of which are dicotyledonous plants (dicots)

The Koonwarra angiosperm shows how the ancestor of flowering plants may have looked: a small, rhizome-bearing herb that had secondary growth, small reproductive organs, and simple, imperfect flowers with complexes of bracts at their bases.

Families of living plants that share several features with the Koonwarra angiosperm are believed to be primitive members of the dicots and monocots.

Furthermore, the appearance of this plant near the apparent beginning of the evolution of angiosperms and its similarities to dicot and monocot suggest that the Koonwarra angiosperm evolved before the divergence between monocots and dicots.

This implies that the monocots and dicots separated into two evolutionary lineages less than 120millions ago, probably from an ancestor similar to the Koonwarra angiosperm.

The Koonwarra angiosperm is the earliest intact fossil of a flower. It has features that occur in several modern families of flowering plants. The early evolution of certain floral features is known from fragments of angiosperms, beginning at about the time of the Koonwarra angiosperm and continuing through the Cretaceous period and the Tertiary period. (Randi et al, 1998)

The Origin of Angiosperms

Although the Koonwarra angiosperm is the oldest known flower, it is probably not the oldest flowering plant. Nobody knows just how long ago the first angiosperm lived, but fossil pollen from the early Cretaceous period, perhaps 10million years older than the Koonwarra angiosperm, may have come from angiosperms.

When did angiosperm evolve?

Assuming gradual evolution, the sudden appearance of a diversity of angiosperms in the Cretaceous period suggests that the evolution of flowering plants began much earlier, perhaps as much as 100million years before the oldest known angiosperm fossil.

If so, then the beginning of the flowering plants may be found among cycadeoids or other extinct group that may have shared ancestors with the angiosperms.

Of special importance in explaining when angiosperms evolved is determining the when the carpel arose. One hypothesis is that the carpel developed from the cupule of the seed fern like caytonia According to this hypothesis, cupule tissue surrounding the seeds fused to form a closed carpel.

Seed ferns were prominent in the carboniferous period, but few persisted into the Mesozoic era. This means that the carpel, or a precarpel, may have originated as early as 200million years ago. However, several other hypotheses exist.

Cycadeoids were once considered to be the ancestors of angiosperms, partly because the microsporangia and ovules of cycadeoids occur in the same one. Such an arrangement simulates perfect flowers, that is, flowers with both stamens and carpel on the same receptacle, which are the most common type of flower in angiosperms.

More recently, botanists have used the methods of cladistics to show how angiosperms may have descended rather directly from seed-fern ancestors in a line parallel to cycadeoids. If this were the case, then angiosperms could have originated in the Triassic period at about the same time the cycadeoids first appeared in the fossil record.

Where did angiosperms evolve?

Pre-Cretaceous angiosperms were well adapted to cool, dry climates. These plants were also probably small, with tough leaves, seed coats, and vessels in their secondary xylem. Most of them were probably deciduous and thus avoided seasonal drying.

These hypotheses represent guesswork based on the fossils of more recent Cretaceous angiosperms, and if correct, they suggest that the most likely places for angiosperms to have evolved were in the semi-arid central regions of western Gondwanaland.

Unfortunately, the drier conditions in these upland regions, unlike the wet conditions along shorelines and in lowland basins, did not allowed much chance for plants to be preserved in the fossil record.

Angiosperms apparently began to invade the lowland basins from the Triassic, Jurassic, and Cretaceous periods by the early Tertiary period, less than 6million years ago. The more recent invasion of angiosperms into these lowland areas can be explained by climatic and geologic changes at the end of the Mesozoic era.

How did Angiosperms Evolve?

Insects played a prominent role in how the angiosperms evolved into the largest and most diverse group of plants. Early in seed-plant evolution, insects became pollen carriers as the searched for food. In turn, plants evolved floral nectar and odors for attracting insects to carry pollen.

The earliest, unequivocal angiosperm nectaries are from the late Cretaceous period, but they probably evolved even earlier. Some of these insects include beetles, bees, butterflies and moths. (Randi etal, 1998).

Why Angiosperms were Successful

Angiosperms first appeared in the fossil record about 135 million years ago. By about 90 million years ago, angiosperms had probably begun to outnumber gymnosperms. What led to the success of this new kind of plant? Several factors were probably involved.

These factors may have included:

In many angiosperms, seeds germinate and produce mature plants, which in turn produce new seeds, all in one growing season. In the case of gymnosperms, it often takes 10 or more years to reach maturity and produce seeds

Fruits of flowering plants protect seeds and aid in their dispersal.

Angiosperms have a more efficient vascular system and are more likely to be associated with mycorrhizae than gymnosperms are.

Angiosperms also may gain an advantage by using animal pollination rather than less efficient wind pollination used by gymnosperms. However, wind pollination is used by many successful angiosperms, such as grasses and many deciduous trees.

Angiosperms occupy more niches, such as in aquatic, epiphytic, and parasitic environment. Hence, they are more diverse than gymnosperms

In conclusion, seedlike structures first appeared about 360 million years ago in progymnosperms. Pollen arose more than 150 million years after the origin of seeds. Seed plants diverged and flourished throughout the carboniferous period; and the Jurassic period was dominated by cycads, cycadeoids and early members of the Pinophyta.

The angiosperms which are the most dominant plants on earth, are also called flowering plant. Fossils of carpels and other parts of flower are known from Cretaceous deposits that are at least 135 million years old, but the first complete fossil flower is about 120 million years old.

The oldest known seed plant is Elkinsapolymorpha, a seed fern from the late . Devonian West Virginia. The seed plants produced their seeds along their branches without specialized structures.

The integument is a layer of tissue found in all seeds; it is produced by the parent plant, and develops into the seedcoat. By the end of the Devonian, a variety of early seed plants collectively known as Lyginopterids appeared.

The Carboniferous period saw an increase in the number and kinds of seed plants. The angiosperms are the most dominant plants on earth. The angiosperms have flowers that include seeds in a Carpel.

The main force behind the rapid evolutionary radiation of angiosperms may have been pollination by insects and the availability of habitats left open by the disappearance of many gymnosperms. The first flowers were probably pollinated by beetles; later angiosperms attracted butterflies and bees.