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Flora of Earlier Geologic Periods

The “Rhyniophyte Flora” and the “Coal Age Flora”

The rhyniophytes which has completely extinct comprises of the earliest vascular land plants. It is only known from its fossils and helps in understanding the origin plants. It existed 450 million years ago and once formed a dominants species of the Devonian flora. Rhyniophyte is a group of simple, small and stick-like vascular plants. Some studies have established that simple rhyniophytoid plants such as cooksonia inhabited salty marshes.

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As a result, it is widely thought that early rhyniophytes grew under freshwater environments. Late Silurian rhyniophytes were accompanied by a number of new clades in the Early Devonian, such as trimerophytes (Siegenian) and zesterophylls(Gedinnian) all which had small heights and were vegetation types. All the Devonian plants required water for fertilization and were small in size. Similarly, the small rhizoids types of these rhyniophytes show habitats with virtually continuous moisture. During the middle Paleozoic, rhyniophytes seems to have habited similar environments, even though rhyniophytes were probably drought resistant than the flora of wet meadows that exists today.

Coal age flora existed some 450 million years ago, a period known as Pennsylvanian period. When the flora was nearing its culmination, it had inhabited the swampy rain forests that occupied most of the land located on the northern hemisphere. These forests had existed in different parts of the world for millions of years, and as a result, transforming into massive accumulation of substances called coal which currently is the main source of the worlds deposits of anthracite and bituminous. Most of Paleozoic clubmosses culminated towards the end of the carboniferous age, although some of the smaller ones endured much longer. This group consists of the present selaginellas and ground pines, which mostly inhabited temperate zones and tropics.

The “Gymnosperm Flora”

The other type of this flora that existed around this time was the early gymnosperms, also referred to as Cordaites a name that originates from the paleontologist Corda. Although they have since extinct, they belong to a line which owes to the origin of such confers such as the Australia kaun tree, Araucaria and Agathis which essentially exists in the presented and situated at the southern hemisphere.

Gymnosperms seems to have left a very continuous and full record and had existed for a very long spread of time. Indeed, its history is usually reported to be comprehensive than any other group of plants. This is partly because angiosperms have always been plentiful in the flora of every period that has contributed towards plants knowledge and it still remains plentiful in the present period. Even though angiosperm may be considered relatively modern, during early Paleozoic period, gymnosperms represented seed-plants.

During the Paleozoic, Gymnosperms was comprised of two groups, Cordiatales and Cycadofilicales. They constituted the seed-plant flora besides being members of the Paaleozoic flora. There have been no certain traces of either of the two groups into the Mesozoic, although some few forms may be traced at later times.

In essence the two groups are restricted to the Paleozoic, and their information has been derived mainly from Coal-measures materials. Even though the two groups existed ages ago, and particularly during the Middle Devonian, is apparent that the Cycadofilicales are the more ancient. They remain the most ancient seed plants whose knowledge is available. The seeds of gymnospermous were naked and hence believed to be the first seed plant.

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Angiosperms which are a seed bearing plant had dominated the world before the evolution of the flowering plants. Various morphological transformations are needed in the process of the development of the seed plant. Apparently, seed plants are heterosporous meaning that two types of spores that includes megaspores and microspores are brought into being by the sporophyte. As a result, it is thought that heterosporous must have been the precursors of the seed plants. Angiosperms are considered to be the source of the origin of flower. There are some studies that have shown that the first flowering plants evolved from some genetic programs which were found in gymnosperms before evolving into flowering plants.

Some studies have shown that angiosperm originated from a seed bearing fern which belonged to Devonian plant group.

The “Angiosperm Flora”

The angiosperm flowering plant started off during the late Devinian in Gondwanaland at least 360 million years ago. Genetic analysis which results from molecular study of plant DNA has contributed to the hypothesis of origin of Devonian angiosperm. Some studies have shown that angiosperm originated from a seed bearing fern which belonged to Devonian plant group. There is an indication shows that earlier angiosperms family covered a wide area of southern hemisphere supercontinent more than 300 million years ago. It inhabited regions such as South Africa, Antarctica, Africa, and smaller terranes.

Angiosperms flora which was thought to have originated from mild uplands during pre-Cretaceous period was spread fast when the sea reached its maximum degree during Cenomanian. In addition, the angiosperms invaded the lowlands as a result of favorable and equitable climate. This is essentially the reason for the widespread of the three major tropical angiosperms as a result of the Cretaceou. The existence of angiosperm at middle latitudes can be explained by changes that occurred in Mesozoic paleogeography.

It is towards the end of Carboniferous or beginning of Permian that diversification which took place before the angiosperm transpired. It is around this time that numerous angiosperm groups evolved

It is towards the end of Carboniferous or beginning of Permian that diversification which took place before the angiosperm transpired. It is around this time that numerous angiosperm groups evolved. Gonophyl theory by Melville tries to explain the origin of the male sex organ (androecium) and the female sex organ (gynoecium) belonging to angiosperm flower. This theory maintains that flowerers originated from fern leaves containing either microsporangia or ovules.

Even though Melville faced discrepancy in his endeavors, he maintained that the great deviation between Magnoliaceae and Ranunculaceae’s floral vascular systems involves a prehistoric parting of these stocks. Melville was however not able to establish the most primitive forerunner of the angiosperm.

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There is a divergence between Melville theory and the commonly accepted principle of Cretaceous Origin. As such, gonophyll theory finds itself overlooked rather than refuted. Hurley came up with a chart concerning the fossil records of Gondwanaland which portrays adequate time for monophylesis.Gangamopteris (seed fern) is the propbable Proteaceae origin. It inhabited some parts of Eastern Australia during Devonian more than 360 millions of years ago. The angiosperm flora which had a vast coverage had its traces found in Cretaceous fossils of the Northern Hemisphere. As such; it can be explained without involving polyphylesis.

Works cited:

Cummings, Robert and Larry, Friesen. Botany 121 Plant, 2010. Web.

Maynard, John. The theory of evolution. London: Cambridge University Press, 1993. Print.

Raven, Peter and Ray, Evert and Susan, Eichhorn. Biology of Plants. New York: W.H. Freeman, 2004. Print.

Steward, Wilson and Rothwell, Gar. Paleobotany and Evolution of Plants. London: Cambridge University Press, 1993. Print.

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