Origin and Evolution of Seed Plants, Biology tutorial

Fossil Record of Seed Plants:

Nowadays, the seed plants are some of the most significant organisms on earth. Life on land as we are familiar with it is shaped largely by the actions of seed plants. This large and significant group appeared early in the evolution of the vascular plants and all through the Late Paleozoic shared dominance of the land flora having ferns, Lycophyta and sphenopsids. As the starting of the Mesozoic, though, most trees and forests have comprised of the seed plants.

a) The Late Devonian Seed Plants:

The oldest acknowledged seed plant is Elkinsia polymorpha, a 'seed fern' from the Late Devonian (Famennian) of West Virginia. However, the fossils comprise only of small seed-bearing shoots, such fragments are quite well-preserved. The other fossil from about this time is Archaeosperma, as well known only from the fragments. 

The first seed plants generated their seeds all along their branches devoid of specialized structures, like flowers or cones, dissimilar most living seed plants. The seeds were generated singly or in pairs and were enclosed by loose cupules. This small cup-like structure was lobed in the initial seeds, producing a somewhat sheltered chamber at one end of the seed. Within such cupules, the seed was surrounded by a more tightly apprised tissue termed as the integument. The integument is a layer of tissue found in all the seeds; it is generated by the parent plant and builds up into the seed coat. As the integument evolved to surround the seed more tightly, an opening was left at one end, termed as the microphylls that permitted pollen to enter and give sperm to fertilize the egg cell. Both the integuments and cupules are believed to be the outcome of reduced and fused branches or leaves. 

In present seed plants, a small pollen chamber seems just within the micropyle. In modern cycads and conifers, this chamber shows sticky fluids to aid in the pollen capture, and as the fluid dries, it pulls the pollen within the micropyle. This structure is conserved in detail in a number of recently discovered per mineralized Devonian seeds. Apart from preserving the pollen drop, minerals substituted the original tissues slowly, in such a way that fine detail of the cell walls can be studied -- a few Permian seeds even encompass preserved embryos.

b) The Late Paleozoic Seed Plants:

By the end of the Devonian, a diversity of early seed plants together termed as 'lyginopterids' appeared. These comprise Sphenopteris, a plant having fern-like leaves; however which bore cupules and seeds. It is not apparent whether Sphenopteris is a single group of closely associated plants or several by similar leaves. 

The Carboniferous age saw a raise in the number and types of seed plants. In the coal swamps of North America grew pteridosperms such as Medullosa, a seed plant which looks like modern tree-ferns, however which bore seeds. Cordaites as well grew in such swamps, and in a number of other habitats comprising ocean-edge environments identical to that of the modern mangrove. Though, the cordaites are believed to be closer relatives of the modern conifers. Both the cordaites and medullosans were small trees if compared to the great scale-trees which dominated such Late Paleozoic coal swamps. Seed plants were therefore overshadowed in their early evolution by plants which didn't produce seeds.

The Fossil Record of Angiosperms:

Most of the plants from past decomposed devoid of leaving a trace of their existence. In fact, the fossil record of plant species might be just 1% complete and that at least 90% of the species that ever existed are vanished. However, the fossil record of plants does give a base for some general ideas regarding where flowering plants came from and how they might have developed.

The very first fossils of vascular plants are more than 420 million years old, and the first seeds appeared as long as 360 million years ago. Though, fossils of plant fragments that perhaps came from angiosperms are not recognized before the early Cretaceous period, around 135 million years ago. Unluckily, most of the oldest of such fossils are so fragmented and incomplete that paleobotanist is not sure that they are angiosperms at all. However, one specific fossil stands out as it comprises of all the portions of a flower joined to a reasonable intact plant. This flowering plant is from a 120 million year old fossil dump near Koonwarra, Australia. Paleobotanists assumed that this plant shows the ancestral kind of flower. If this is true, then the characteristics shared by the Koonwarra angiosperm and certain modern angiosperms might exhibit which living plants are closest to the ancestral origin of the group.

The Koonwarran Angiosperm:

The fossil of the world's first known flower was discovered in the year 1986. The Koonwarra angiosperm had some characteristics which are typical of numerous modern angiosperms. For instance, it had small flowers with no petals, a spike-like inflorescence, single carpel ovaries having short stigmas and no styles and imperfect flowers having several bracts at their bases. Such characteristics take place in present-day members of the Lizard's tail family (that is, Saururaceae), chloranthus family (that is, Chloranthaceae) and the pepper family (that is, Piperaceae) all of which are dicotyledonous plants (dicots).

The Koonwarra angiosperm represents how the ancestor of flowering plants might have looked: a small, rhizome-bearing herb which had secondary growth, small reproductive organs and simple, imperfect flowers having complexes of bracts at their bases. Families of living plants which share some characteristics having the Koonwarra angiosperm are assumed to be primitive members of the monocots and dicots. Moreover, the appearance of this plant close to the apparent starting of the evolution of angiosperms and its similarity to dicot and monocot propose that the Koonwarra angiosperm evolved prior to the divergence among monocots and dicots. This means that the monocots and dicots separated into two evolutionary lineages less than 120 millions ago, most likely from an ancestor identical to the Koonwarra angiosperm.

The Origin of Angiosperms:

However the Koonwarra angiosperm is the oldest acknowledged flower, it is perhaps not the oldest flowering plant. No one knows just how long ago the first angiosperm lived, however fossil pollen from the early Cretaceous period, possibly 10 million years older than the Koonwarra angiosperm, might have come from the angiosperms. 

When did Angiosperm evolve?

Supposing gradual evolution, the unexpected appearance of a diversity of angiosperms in the Cretaceous period recommends that the evolution of flowering plants start much prior, possibly as much as 100 million years before the oldest known angiosperm fossil. If so, then the starting of the flowering plants might be found among cycadeoids or other extinct group which might have shared ancestors by the angiosperms.

Of special significance in describing when angiosperms evolved is finding out when the carpel arose. One theory is that the carpel builds up from the cupule of the seed fern such as caytonia According to this theory, cupule tissue surrounding the seeds fused to make a closed carpel. Seed ferns were well-known in the carboniferous period; however few persisted into the Mesozoic era. This signifies that the carpel, or a pre-carpel, might have originated as early as 200 million years ago. Though, some other theories exist.

Where did Angiosperms evolve?

Pre-Cretaceous angiosperms were well adapted to cool, dry climates. Such plants were as well perhaps small, having tough leaves, seed coats and vessels in their secondary xylem. The majority of them was probably deciduous and therefore avoided seasonal drying. Such theories represent guesswork based on the fossils of more recent Cretaceous angiosperms, and if right, they propose that the most probable places for angiosperms to have evolved were in the semi-arid central areas of the western Gondwanaland.

Angiosperms apparently start to invade the lowland basins from the Jurassic, Triassic, and Cretaceous periods by the early Tertiary period, less than 6 million years ago. The more recent invasion of angiosperms into such lowland regions can be described through climatic and geologic modifications at the end of the Mesozoic period.

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