An Introduction to the Mesozoic Palaeobotany Auður Agla Óladóttir
The Mesozoic age was a time of great changes, not only in animals but also in the terrestrial vegetation. The Mesozoic flora was the vegetation eaten by the dinosaurs, other reptiles and mammal herbivores during this Era. New genus and new plants, e.g. gymnosperms, first appeared in their current recognizable forms. Ferns had already appeared in Palaeozoic but their diversity and spread increased in the Mesozoic. Conifers, cycads and the living fossil ginkgo dominated and made up the forests. Angiosperms and flowers also appeared in Mesozoic and began to diversify and take over from the other plants. This article expounds the main characteristic features of the Mesozoic terrestrial vegetation. It follows the evolution of the flora in chronological order and points out the most dominating plants, and the appearance of new genera.
Before the Mesozoic Era, in the Cambrian others went through reduction of Era, many land plants had made their numbers. Lycopod trees for example, that appearance and put their mark on the dominated the coal-forming swamps in environment on Earth. The land plants do the Carboniferous Period, horsetails, that not appear to have undergone such a is a single genus of homosporpus vascular dramatic mass extinction like the plants and cordaite trees, that looked very terrestrial animals at the end of the similar to conifers but became extinct Palaeozoic Era. Ferns, carboniferous probably in the Triassic Period. Among pteridosperms (seed ferns), lycopsides plants that increased but were still and horsetails had already appeared but abundant in the end of Permian, and when come to the Mesozoic, the flora and beginning of Triassic, are ferns and seed fauna changed in many different ways. ferns. Most trees that were bigger than Some groups increased in diversity but
Figure 1 The numbers of species of terrestrial vegetation from Silurian to Tertiary. gymnosperms appear in the Carboniferous period but diversify quickly during the Triassic period. Angiosperms appeared early in Cretaceous and increased dramatically, in the number of angiosperms through the mid-Cretaceous. ferns belong to three groups of plants; all extinction by the end of the Triassic. characterized by exposed seeds and had Glossopteris was long considered a fern already appeared in the Permian; but later assigned to the gymnosperms. bennettitales and cycads, followed by Some authorities consider glossopteris to conifers and ginkgos. Late in the have been large trees with a substantial Mesozoic the rise of angiosperms began trunk and close to the ancestral and became extremely important in all of angiosperm. Earth’s floras and also for insects and bigger animals. GYMNOSPERMS The first appearance of seed plants, or spermatophytes, stretches back FERNS AND SEED FERNS into Devonian. Seed plants have some Ferns were a major component of Late major advantages over all other plants Palaeozoic vegetation, but underwent a and are therefore very important. Most dramatic decline at the end of the Permian important is that they are independent Period. The “Age of Ferns” in the late from water as a intermediary of the Carboniferous Period had passed. Only transport of the sperm to the egg. The three families persisted into the Mesozoic seed allows the next generation to lie Era and they have survived to the present dormant for months and in that way seed day albeit with a relatively restricted can survive droughts, fire and other distribution. Nevertheless, ferns were still natural catastrophes. widespread in the Triassic and Jurassic Seed plants are divided into two forests. groups, gymnosperms and angiosperms. The remains of seed ferns are The phylum Gymnosperm first appeared common fossils in rocks of Carboniferous in the Permian Era and included the age. They are generally characterized as tallest and the oldest trees and many having been slender trees or, in some common trees like pine, spruce, fir, cases, woody, climbing vines. They were hemlock and cedar. The flora may have generally large, up to 5 metres tall and looked similar to the flora today but their large fronds were so fernlike that absence of flowering plant made it rather these plants were long regarded as ferns homonymous. but now they are known to be an artificial When the Jurassic Era began, the group of primitive seed-bearing plants seed-fern floras of the Triassic declined in which however were reduced in importance. The heirs were the abundance and apparently failed to gymnosperms. One ancient group of survive into the Jurassic Period. In fact gymnosperms that were among the most sources are not unanimous about when prominent plants were cycads, unique and seed ferns became extinct, but it is certain palm like plants with a crown of large that they had disappeared from fossil compound leaves and a stout trunk, record before the end of the Cretaceous sparcely branched or unbranched. They Era. appeared at least 250 million years ago Glossopteris is an extinct group of during the Permian Period, with possible seed plants that arose during the Permian ancestors in the Upper Carboniferous, and on the great southern continent of were so numerous and dominating in the Gondwanaland and became a dominant Mesozoic Era, along with the part of the southern flora but dwindled to superficially similar bennettitales, that Figure 2 At the end of Paleozoic cordaites disappear but conifers appear and diversify rapidly in the Triassic Most consider that the origin of cycads suggests that they originated from the pteridosperms. this period is often called the “Age of cycadeoids, because their growth habit Cycads and Dinosaurs”. During the and leaf structure is similar to that of the Triassic and Jurassic, at the time of their cycads. Studies of seed- and pollen- greatest diversity, cycads made up about producing structures of bennettitales have 20% of the flora of the world. Today demonstrated, however, that they are there are 140 species in 10 genera. The remarkably different to those of cycads. reproduction is slow, each individual of Their reproductive structure, that was cycads is unisexual and they have, until bisexual in some species, suggests a recently, been thought to be pollinated by closer relationship with angiosperms and wind but new researches suggest that they therefore it is still uncertain where have possibly been pollinated by beetles. bennettitales fit phylogenetically. An extinct group of gymnosperms Even though the earliest conifer in is bennettitales, an enigmatic group of the fossil record was discovered from the Mesozoic gymnosperms that disappeared upper Carboniferous locality, the from the fossil record during Cretaceous. evolution and radiation of the conifers Bennettitales are sometimes called occurred ~ 248-206 million years ago in the Triassic Era. It is suggested that they 1950´s in temple gardens of China where most probably inhabited the drier these sacred trees were carefully tended environments of upper areas, from where and have been saved from extinction for they subsequently radiated. the foreseeable future. Now Ginkgo Characteristics that distinguish extant biloba is valued in many parts of the Coniferales include a pyramidal world as an attractive, fungus- and insect- arborescent growth form; small simple resistant ornamental tree. It looks more leaves that are often needle-like in like a hardwood tree than a conifer and appearance and the rooting systems are the unusual fan-shaped leaves are easily simple in structure and consist of a recognized with parallel veins and the branching tap-root system. Nine families outer margin either split or entire. It is a that radiated at the Mesozoic time still slow-growing but tall and beautiful tree, have widespread global coverage today. and unlike other gymnosperms it is Five distinct vegetation biomes deciduous. In autumn the leaves turn a have been identified for the biographical beautiful golden colour before falling. It distribution of global vegetation during is also known as the maidenhair tree the early Jurassic. The higher latitudes of because of the resemblance of its bilobed both hemispheres were characterized by leaves to those of the maidenhair fern. G. relatively low species diversity, but biloba may be the oldest living seed dominance of ginkgos and macrophyllus plant, and it is regarded by some as one of conifers, together with ferns. Closer to the the wonders of the world. Each individual equator, in the lower latitudes where the is either male or female, bearing small climate was warmer, the biomes were reproductive organs of one sex or another. more rich of cycads, bennettitales and microphyllus conifers. By the early Jurassic, global floras, for the first time, contained a significant component of vegetation recognizably similar to the present day´s.
The living fossil -Ginkgo biloba Ginkgophytes, as already mentioned, are gymnosperms phylum with active cambial growth and fan- shaped leaves. They first appeared in the Permian and increased in the Triassic. Ginkgo biloba is the only living member of the whole Phylum Ginkgophyta. It was “rediscovered” in
Fi gure 4 Ginkgo: (a) habit of G.Biloba; (b) fossil (upper) and modern (lower) leaves; (c) fossil male reproductive structure; (d) fossil Figure 3 pollen; (e) female reproductive structure (fossil seed). Leaf of Ginkgo Biloba Figure 5 Suggested biomes for the Early Jurassic with representatives of the most abundant and/or dominant fossil plant taxa shown.
ANGIOSPERMS identifiable fossils of angiosperms are Perhaps the most important evolution for 135 million years old, from the early the terrestrial life in the Cretaceous Cretaceous, 300 million years later than Period and in fact probably the whole Era the first vascular plant and 220 million was the appearance of the angiosperms. years later than the first seed plant. Angiosperms include all flowering plants Various hypotheses have been suggested and also the important hardwood trees to account for their relatively late such as oak, ivy and maple. These are the appearance in the fossil record, including dominant plants in the world today, a bias in the fossil evidence, a particular accounting for between 300 and 400 combination of environmental conditions families and 250.000-300.000 species but and/or biotic interactions that led to their in evolutionary terms, flowering plants later evolution. Two biotic interactions are relatively recent. The oldest, clearly that are especially interesting are the coevolution of flowering plants with low- browsing dinosaurs and with pollinating Reconstruction of the biogeo-graphical insect groups. distribution of vegetation during the late The unique characteristics of the Cretaceous indicates that six global angiosperms include flowers, closed biomes can be recognized during this carpels and double fertilizing leading to period, with all but the highest latitudes endosperm formation. Extremely rapid dominated by angiosperms. diversification of angiosperms led to replacement of the declining cycads, bennettitales and ginkgophytes. Although the gymnosperm floras were still abundant, the rapid diversification and radiation of the angiosperms led to their increasing domination throughout the world during the 35 million years of the upper Cretaceous. A number of factors seem to have been important in the early and continuing success of the angiosperms. One important factor is the evolution of the precise system of pollination and specialized mechanisms of seed dispersal that became characteristic of the more advanced flowering plants allowing them Figure 6 Angiosperms Evidence from fossil leaf to exist as widely scattered individuals in assemblages (summed genus and species many kinds of habitat. They evolved a set diversity) indicating the major expansion of the of features that attracted a wide variety of angiosperms from -140 Ma and a dramatic pollinators. The use of bright colours, increase in the absolute number (summed ultra-violet signalling and scents attracted diversity) of angiosperms through the mid- Cretaceous (ca. 10 Ma) Data taken from 147 insects and birds to them to help with late Jurassic to Palaeocene macrofossil floras. pollination and in this way they ensured a high degree of cross-pollination and Beside the aesthetic appeal of the evolutionary development. flowering plants they are also important The step between gymnosperms in other ways. Barely a day goes by in and angiosperms is still not very well which our lives are not affected by understood and has been a matter of great flowering plants. Nearly all our food and interest to paleobotanists for a long time. spices comes from flowering plants; The most striking results from recent grains, nuts, fruits vegetables, coffee, tea, phylogenetic analyses, based on fossils, wine, tequila, beer and chocolate, the morphological and molecular data, cotton in our clothes, linen, dyes and also support earlier ideas that bennettitales and opium, cocaine, tobacco and medicines gnetophytes are the seed plants most such as aspirin. closely related to angiosperms.
Figure 7 Suggested biomes for the late Cretaceous with representatives of the most abundant and/or dominant fossil plant taxa shown.
SUMMARY for animals but many of the gymnosperms were large plants and trees In the Mesozoic Era the terrestrial and therefore they were more prominent vegetation evolved greatly. At the in the flora. In the Cretaceous the beginning of the Era, plants from the angiosperms began to appear and they Carboniferous Period were very common, increased their diversity and quantity in a especially ferns, seed ferns, lycopsides very short time. These coloured flowers and horsetails. The gymnosperms and fruit bearing trees dramatically appeared also in Carboniferous Period changed the appearance of the forests. At but did not become dominant until the end of the Era, flowers and large trees Triassic. In the last part of Triassic and were spread all over the earth and Jurassic, gymnosperms increased in provided a most encouraging diversity and many new species were environment for both insects and animals. formed. The most dominating plants were The biomes were diverse and grew well. ginkgo, cycads, bennettitales and Under these circumstances, dinosaurs, conifers. Ferns and seed ferns still grew birds, insects and small mammals in the forests and were important as food flourished in the forests.
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