|Evolution of Plants|
The general trend of earth’s plant diversification involves four major plant groups that rose to dominance from about the Middle Silurian period to present time. The first major group providing land vegetation comprised the seedless vascular plants, represented by the phyla Rhyniophyta, Zosterophyllophyta, and Trimerophytophyta. The second major group appearing in the late Devonian period was made up of the ferns (Pterophyta).
The third group, the seed plants (sometimes called the Coal Age plants), appeared at least 380 million years ago (mya). This third group includes the gymnosperms (Gymnospermophyta), which dominated land flora for most of the Mesozoic era until 100 mya.
The last group, the flowering angiosperms (Anthophyta), appeared in the fossil record 130 mya. The fossil record also shows that this group of plants was abundant in most parts of the world within 30 million to 40 million years. Thus, the angiosperms have dominated land vegetation for close to 100 million years.
The Paleozoic Era
The Proterozoic and Archean eons have restricted fossil records and predate the appearance of land plants. Seedless, vascular land plants appeared in themiddle of the Silurian period (437-407 mya) and are represented by the rhyniophytes or rhyniophytoids and possibly the Lycophyta (lycophytes or club mosses). From the primitive rhyniophytes and lycophytes, land vegetation rapidly diversified during the Devonian period (407-360 mya).
Pre-fern ancestors and maybe true ferns (Pterophyta) were developed by themid-Devonian. By the Late Devonian the horsetails (Sphenophyta) and gymnosperms (Gymnospermophyta) were present. By the end of the period, all major divisions of vascular plants had appeared except the angiosperms.
Development of vascular plant structures during the Devonian allowed for greater geographical diversity of plants. One such structure was flattened, planated leaves, which increased photosynthetic efficiency.
|The Paleozoic Era|
Ancestors of the conifers and cycads appeared in the Carboniferous period (360-287 mya), but their documentation is poor in the fossil record. During the early Carboniferous in the high and middle latitudes, vegetation shows a dominance of club mosses and progymnosperms (Progymnospermophyta).
In the lower latitudes of North America and Europe, a greater diversity of clubmosses and progymnosperms are found, along with a greater diversity of vegetation. Seed ferns (lagenostomaleans, calamopityaleans) are present, along with true ferns and horsetails (Archaeocalamites).
Late Carboniferous vegetation in the high latitudes was greatly affected by the start of the Permo-Carboniferous Ice Age. In the northern middle latitudes, the fossil record reveals a dominance of horsetails and primitive seed ferns (pteridosperms) but few other plants.
In northern low latitudes, landmasses of North America, Europe, and China were covered by shallow seas or swamps and, because they were close to the equator, experienced tropical to subtropical climatic conditions.
The first tropical rain forests appeared there, known as the Coal Measure Forests or the Age of Coal. Vast amounts of peat were laid down as a result of favorable conditions of year round growth and the giant club mosses’ adaptation to the wetland tropical environments.
In drier areas surrounding the lowlands, forests of horsetails (calamites, sphenophylls), seed ferns (medullosans, callistophytes, lagenostomaleans), cordaites, and diverse ferns (including marattialean tree ferns) existed in great abundance.
The Permian period (287-250 mya) marks a major transition of the conifers, cycads, glossopterids, gigantopterids, and the peltasperms from a poor fossil record in the Carboniferous to significantly abundant land vegetation. The two most prevalent plant assemblages of the Permian were the horsetails, peltasperms, cycadophytes, and conifers.
The second most prevalent were the gigantopterids, peltasperms, and conifers. These two plant assemblages are considered the typical paleo-equatorial lowland vegetation of the Permian. Other plants, such as the tree ferns and giant club mosses, were present in the Permian but not abundant.
As a result of the Permian-Triassic extinction event, tropical swamp forests disappeared, with the extinction of the club mosses; the cordaites and glossopterids disappeared from higher latitudes; and 96 percent of all plant and animal species became extinct.
The Mesozoic Era
|The Mesozoic Era|
After the mass extinction, the bennettites moved into vacant lowland niches. They may be significant because of the similarity of their reproductive organs to the reproductive organs of the angiosperms.
Late Triassic flora in the equatorial latitudes are represented by a wide range of ferns, horsetails, pteriosperms, cycads, bennettites, leptostrobaleans, ginkgos, and conifers. The plant assemblages in themiddle latitudes are similar but not as species-rich. This lack of plant variation in low and middle latitudes reflects a global frost-free climate.
In the Jurassic period (208-144 mya), land vegetation similar to modern vegetation began to appear, and the ferns of this age can be assigned tomodern families: Dipteridaceae, Matoniaceae, Gleicheniaceae, and Cyatheaceae.
Conifers of this age can also be assigned tomodern families: Podocarpaceae, Araucariaceae (Norfolk pines), Pinaceae (pines), and Taxaceae (yews). These conifers created substantial coal deposits in the Mesozoic.
During the Early to Middle Jurassic, diverse vegetation grew in the equatorial latitudes of western North America, Europe, Central Asia, and the Far East and comprised the horsetails, pteridosperms, cycads, bennettites, leptostrobaleans, ginkgos, ferns, and conifers.
Warm, moist conditions also existed in the northern middle latitudes (Siberia and northwest Canada), supporting Ginkgoalean forests and leptostrobaleans. Desert conditions existed in central and eastern North America and North Africa, and the presence of bennettites, cycads, peltasperms and cheirolepidiacean conifers there are plant indicators of drier conditions.
The southern latitudes had similar vegetation to the equatorial latitudes, but owing to drier conditions, cheirolepidiacean conifers were abundant, ginkgos scarce. This southern vegetation spread into very high latitudes, including Antarctica, because of the lack of polar ice.
In the Cretaceous period (144-66.4 mya), arid, subdesert conditions existed in South America, Central and North Africa, and central Asia. Thus, the land vegetation was dominated by cheirolepidiacean conifers andmatoniacean ferns.
The northern middle latitudes of Europe and North America had a more diverse vegetation comprising bennettites, cycads, ferns, peltasperms, and cheirolepidiacean conifers,with the southern middle latitudes dominated by bennettites and cheirolepidiaceans.
A major change in land vegetation took place in the late Cretaceous with the appearance and proliferation of flowering seed plants, the angiosperms. The presence of the angiosperms marked the end of the typical gymnosperm-dominated Mesozoic flora and a definite decline in the leptostrobaleans, bennettites, ginkgos, and cycads.
During the late Cretaceous in South America, centralAfrica, and India, arid conditions prevailed, resulting in tropical vegetation dominated by palms. The southern middle latitudes were also affected by desert conditions, and the plants that fringed these desert areaswere horsetails, ferns, conifers (araucarias, podocarps), and angiosperms, specifically Nothofagus (southern beech).
The high-latitude areas were devoid of polar ice; owing to the warmer conditions, angiosperms were able to thrive. The most diverse flora was found in North America, with the presence of evergreens, angiosperms, and conifers, especially the redwood, Sequoia.
The Cretaceous-Tertiary (K/T) mass extinction event occurred at about 66.4 mya. This event has been hypothesized to be a meteoritic impact;whatever the cause, at this time an event took place that suddenly induced global climatic change and initiated the extinction of many species, notably the dinosaurs.
The K/T had a greater effect on plants with many families than it did on plants with very few families. Those that did become extinct, such as the bennettites and caytonias, had been in decline.
The greatest shock to land vegetation occurred in the middle latitudes of North America. The pollen and spore record just above the K/T boundary in the fossil record shows a dominance of ferns and evergreens. Subsequent plant colonization in North America shows a dominance of deciduous plants.
The Cenozoic Era
|The Cenozoic Era|
Notable in this period was the polar Arcto Tertiary forest flora found in northwest Canada at paleolatitudes of 75-80 degrees, north. Mild, moist summers alternated with continuous winter darkness, with temperatures ranging from 0 to 25 degrees Celsius.
These climatic conditions supported deciduous vegetation that included Platanaceae (sycamore), Judlandaceae (walnut), Betulaceae (birch), Menispermaceae, Cercidophyllaceae, Ulmaceae (elm), Fagaceae (beech), Magnoliaceae; and gymnosperms such as Taxodiaceae (redwood), Cypressaceae (cypress), Pinaceae (pine), andGinkgoaceae (gingko). This flora spread across North America to Europe via a land bridge between the continents.
About eleven million years ago, during the Miocene epoch, a marked change in vegetation occurred, with the appearance of grasses and their subsequent spread to grassy plains and prairies. The appearance of this wide spread flora supported the development and evolution of herbivorous mammals.
The Quaternary period (1.8 mya to present) began with continental glaciation in northwest Europe, Siberia, and North America. This glaciation affected land vegetation, with plants migrating north and south in response to glacial and interglacial fluctuations. Pollen grains and spores document the presence of Aceraceae (maple), hazel, and Fraxinus (ash) during interglacial periods.
Final migrations of plant species at the close of the last ice age (about eleven thousand years ago), formed the modern geographical distribution of land plants. Some areas, such as mountain slopes or islands, have unusual distribution of plant species as a result of their isolation from the global plant migrations.