This article provides an overview on diversities of life forms through ages.
Introduction:
Plants and animals of extremely diverse and varied morphological characteristics and structural organization are encountered on the surface of the Earth.
Arising out of a very simple form of life an intricate balance has been maintained for over 4,000 million years or 4 billion year or 4 Ga [one million year (m.y.) = 106 years; 1 Ga (a giga annum) or billion year (b.y.) is = 109 years] as slowly more complex plants and animals have evolved.
In a normal course of life cycle plants and animals die and get decomposed. In some cases during the course of evolution, they escape from final destruction and leave the sign of their existence as fossils in rocks. Such fossil remains are thoroughly studied and a definite idea is formed about the course of evolution of a particular group of plants or animals.
Concept of Evolution:
The term ‘evolution’ is derived from Latin ‘evolvere’ which means “to unroll or unfold”. The idea of evolution is very ancient. Its roots are found in the writings of old scriptures. But with the advent of modern science, the early idea of origin of life moved from the realm of philosophy and imagination to that of biological science.
Today it is this concept that unites all fields of biology. Biological evolution is usually considered a series of processes involving descent of organisms with modification marked by successive adaptations to environmental conditions governed by competition and natural selection acting on variation.
It explains the origin of species, their modifications, and to some extent, the reasonable explanation of the many peculiarities of structure and function found in organisms. The fact of evolution is now well documented, but the mechanisms of evolution, the methods by which new characters appear, are not as yet clearly understood.
Before describing the fossils of various geological time periods of different plant and animal groups and discussing the probable course of evolution, I will give a brief account of origin of cellular organization, multicellular forms and sexuality.
It has been ascertained from geological evidences that the Earth was formed approximately 4.6 Ga ago. Somewhere between 4.6 Ga, when the Earth was formed, to about 3.8 Ga (age of the oldest fossil) life began. In addition to microfossils, the earliest evidences of organisms come from two different data sources : Carbon isotope ratios and geological formations called stromatolites.
There are two theories explaining the accumulation of organic matter in the prebiotic environment. According to the terrestrial accretion theory (Miller 1953) when gases such as hydrogen, carbon dioxide, methane and ammonia are heated with water and energized by electrical discharge half of the carbon present in methane will be converted into amino acids, sugars and other organic molecules including purines and pyrimidine’s.
Amino acids and nucleotides can associate to form polymers including polypeptides (proteins) and polynucleotides (RNA and DNA). These formed the first building block of the cell. Earlier in 1920s Oparin and Haldane independently suggested that in aqueous solutions under a low-oxygen atmosphere with energy supplied by lightning or UV a variety of organic molecules will be synthesized.
An alternative theory is that the first organic material on Earth came from other planets and entered the Earth’s atmosphere via meteoritic input. A large variety of organic compounds would have been present on Precambrian Earth in the so-called primordial soup.
Primordial Soup:
It was J.B.S. Haldane who came up with the concept of the primordial soup or broth. He indicated that in various bodies of water the reactions will be going on under the large concentration of substances leading to the formation of primordial broth.
The whole process can be explained in the following manner:
There was no life 4.7 b.y. ago. During this time there was a reducing atmosphere which consisted of N2, H2, CO2, CH4, water vapour and devoid of O2. Deriving energy from lightning, radioactive decay of various elements and intense dry heat of volcanic eruptions CH4, NH3, water and other molecules interacted with one another and a whole array of simple organic substances like aldehydes, alcohols, organic acids, amino acids, purines, pyrimidine’s, pentose and hexose sugars, fatty acids, glycerol were formed.
Once the Earth’s atmosphere cooled, water vapours condensed and there was rain. With all rain oceans were formed. There was little degradation due to absence of enzymes and 02. Organic molecules accumulated in water bodies and primordial soup or prebiotic soup was formed. Polymerization of simple organic molecules required their concentration, a source of energy and removal or evaporation of water.
The simple organic molecules reacted among themselves to form newer and larger molecules. Simple sugars formed polysaccharides, fatty acids and glycerols reacted to form fats, amino acids formed polypeptides, while nucleotides formed nucleic acids.
Critical characteristic of the macromolecules from which life evolved must have the ability to replicate itself. One macromolecule capable of directing the synthesis of new copies of itself would have been capable of reproduction and further evolution. Of the two major classes of informational macromolecules (proteins and nucleic acids) in present day cells only the nucleic acids are capable of directing their own self-replication.
A crucial finding in the molecular evolution is that RNA is capable of catalyzing a number of chemical reactions including polymerization of nucleotides. RNA molecules called ribozymes are thus uniquely capable of both to serve as a template for and to catalyze its own replication.
While ribozyme has not been found in nature that can replicate itself, ribozyme has been synthesized in the laboratory that can catalyze the assembly of short oligonucleotides into exact complements of themselves. RNA is thus believed, by most workers to have been the initial genetic system and an early stage of chemical evolution is thought to have been based on self-replicating RNA molecules—a period of evolution called RNA world.
Another significant argument for the idea that RNA came first is that all the major elements of the translation machinery mRNA, tRNA and ribosomes are either pure RNA or RNA-protein complex. The transition from the RNA to the DNA world began when ribozymes acquired amino acids (which were abundant in the primitive Earth) as cofactors, thereby increasing their range, specificity and efficiency as enzymes. As amino acids were linked to ribozyme, the ribozyme was gradually transformed into a protein enzyme through a series of hybrid intermediates.
The genetic code is thought to have originated in a gradual switchover from ribozymes with amino acid cofactors to protein enzymes with nucleotide cofactors. This is a process in which tRNAs emerged naturally from the oligonucleotides that endowed the amino acid cofactors with specificity. Pro-to-mRNAs followed, coding for polypeptides of increasing length, eventually by DNA as a template coding for mRNA. DNA was selected because of its much lower mutation rate.
These key events eventually resulted into larger genomes coding for more complicated organisms to emerge. The lower mutation rate of DNA allowed longer messages to be transmitted reliably. Due to the absence of a complementary strand there is no proof reading when RNA is replicated. As a result per-base error rate is high. This high error-rate limits RNA to being the genetic material only in small genomes.
The first cell is presumed to have arisen by the enclosure of self-replicating RNA in a membrane composed of phospholipids. The common ancestor of all life probably used RNA as its genetic material and was most likely a progenote—an organism whose genes were not arranged into a genome.
The progenote gave rise to three major lineages of life. These are: the prokaryotes (bacteria), archaebacteria (the thermophilic, methanogenic and halophilic) and eukaryotes. The process of translation is similar in these lineages, but the organization of the genome and transcription (mRNA from DNA template) is very different in prokaryotes than in eukaryotes and archaebacteria. Scientists interpret this to mean that the progenote (common ancestor) was RNA based, it gave rise to two lineages that independently formed a DNA genome and have independently evolved mechanisms to transcribe DNA into RNA.
The first formed life (the earliest cells) derived energy by anaerobic fermentation of abiotically produced organic molecules. So, simple fermentative heterotrophs (which require few enzymes for energy generation) could have lived on the organic compounds formed abiotically.
It is also proposed that early life-forms might have been hydrogen bacteria, those that obtain energy from the oxidation of hydrogen gas. Both bacterial and archaeal hydrogen user are known. Gradually the metabolic pathways fixing CO2 and elemental N2 evolved.
Fossil stromatolites in 3.5 billion years of rocks show that photoautotrophic prokaryotes evolved early in the history of life. Early photosynthesis (PS) was anoxygenic. H2S, H2 or organic compounds were used as hydrogen donor and CO2 and organic compounds are carbon donors. Purple and green-sulfur bacteria were the first photosynthetic organisms and carried out anoxygenic photosynthesis (it proceeds in an anoxic environment without the evolution of O2).
The first photosystem to evolve (PS I) uses light to convert CO2 and H2S to glucose. This process releases sulfur as a waste product. Later, a second photosystem (PS II) evolved, probably from a duplication of the first photosystem. Organisms with PS II use both photosystems in conjunction to convert CO2 and water into glucose. This process releases oxygen as a waste product. Cyanobacteria were the first to carry out oxygenic photosynthesis.
This particular group of organisms evolved from two anoxygenic photosynthetic group, the firmicutes and the chlorobi. Their photosystems I and II are thought to have descended from the photosynthetic apparatus of each of these two groups of photosynthetic bacteria. Fossil evidences establish that unicellular coccoid cyanobacteria evolved about 3.5 b.y. ago.
These organisms gradually evolved into multicellular filamentous forms with differentiated cells like akinete and heterocyst and there was gradual increase of larger genome size. Oxygenic photosynthesis released O2 into the anaerobic environment of earth and gradually the atmosphere become aerobic.
The production of O2 would have had a major impact on many different processes in the biosphere. First, it caused oxidation of reduced inorganic compounds such as iron sulfides and iron oxides were deposited in geological strata called iron formations. It also led to evolution of aerobic, respiratory bacteria that have a more efficient mode of metabolism.
It provided the key condition for the evolution of plants and animals which had obtained their mitochondria from proteobacteria and chloroplasts from cyanobacteria through endosymblosis. Another consequence of oxygenic photosynthesis was the development of an ozone layer. This protected the land plants and animals from UV radiation.
Heterotrophic bacteria had two choices, retreat away from the growing oxygen environment or also evolve adaptations to neutralize oxygen. In this direction the aerobic respiration (chemical burning of carbohydrate to obtain energy for living cells) evolved, resulted in the formation of H2O and CO2. Aerobic respiration evolved in autotrophic and heterotrophic bacteria.
Origin of Eukaryotic Cells:
It has generally been agreed upon from DNA evidence that prokaryotic cells gave rise to eukaryotic cells between 2.5 b.y. and 1.0 b.y. Eukaryotic cells evolved from two cells by a process of gradual evolution in which the organelles of the eukaryotic cells became progressively more complex. Origin of eukaryotic organelles like chloroplast and mitochondria was explained by endosymbiotic theory.
Other basic structures of eukaryotic cells, including a system of membranes (a nuclear membrane, endoplasmic reticulum, golgi complex, lysosomes), a cytoskeleton and a mitotic type of cell division proposed to have evolved by a gradual process of evolution, rather than in a single step like the process of endosymbiosis.
The endoplasmic reticulum and nuclear membranes, might have been derived from a portion of the cell’s outer plasma membrane that became internalized and then modified into a different type of membrane.
Fossil record of first eukaryotic cell is ambiguous because it is difficult to ascertain from fossil record whether a cell is eukaryotic or prokaryotic. Organic chemical compounds which indicated origin of eukaryotic cell membrane was 2.5-1.6 b.y. old. Oldest photosynthetic eukaryote Gypanica (considered as a colonial eukaryote) comes from fossil which is 2.1 b.y. old but was frequently found from 1.3 b.y. onwards. Fossils of eukaryotes that resemble living brown algae have been found in sedimentary rocks that are 1.7 b.y. old.
By about 1 b.y. there was a rapid rise in diversity of eukaryotes which diversified more rapidly than prokaryotes. This is due to the import of new trophic structure and origin of sex. From 3.8 to about 1.5-1.0 b.y. evolution occurred intracellular and biochemically. From 2.1 to 1.5-1.0 b.y. various organisms evolved with different morphology.
For 1.5 billion years photosynthetic organisms remained in the sea. This is due to absence of protective O3 layer. The land was bathed with UV. Once O3 layer was formed it was possible for the living organisms to venture onto the land.
Origin of Multicellular Forms:
Around 750-650 m.y. there was boom in multicellular life which was followed by increase in diversity. In simple multicellular forms the cells have to coordinate as a functional unit. A cell that continued to divide would increase its representation in the colony but would lower colony performance : genomic conflict within such a colony is possible. Development in multicellular organisms usually starts from a single cell to prevent genomic conflict within multicellular or colonial forms. Maximal relatedness between the cells of an individual is assured.
A transition to multicellularity with different specialized cell types require two things. First, few cells with identical genes to develop into cell types with different functions. Genes expressed in some cell types must be shut off in others. This requires the evolution of mechanisms for gene regulation.
The second requirement is some form of cellular memory or epigenetic inheritance that maintains the differentiated state through their cell divisions. In eukaryotes the functional state is often inherited through DNA methylation or proteins bound to DNA. If a stretch of DNA containing a gene is methylated or bound to a protein, the gene cannot be expressed. This epigenetic system may have evolved from defense system found in bacteria.
Evolution of Reproductive and Non-Reproductive Units—Germline and Soma:
Many multicellular organisms are organized into a reproductive and a nonreproductive part. In the older Volvocales, there are colonies of 500-50,000 cells. Most cells are somatic, a few cells called gonidia reproduce asexually in the absence of hormones. In the presence of hormones (specific chemical stimulus) they differentiate into sexual colonies with definite sex organs.
It is assumed that division of labour enhances individual tissues because it allows more efficient conversion of resources into reproductive capacity. A mutation in a somatic cell that converted it into a reproductive cell would be selected against for descendant colonies changing this mutation that would have more than optimal number of gonidium cells.
Origin of Sex:
First evidence of sex was around 1.2 b.y. Sex is merger of genetic material from two individuals to make a new individual. Sex may have originated by the fusion of haploid cells. Starved cells tried to eat each other of their own species.
In time of extreme stress some individuals may not have digested the ingested cell and formed a new diploid individual with it. Origin of sexual reproduction may have fuelled the mesoproterozoic boom at about 1.0 b.y. with strong increase in diversity of prokaryotes and eukaryotes. Sexual reproduction contrasts sharply with asexual reproduction.
In sexual reproduction the genetic instruction from two lines of descent fuse to create a new individual. However, sexual reproduction is less efficient (in terms of production of offspring) and very complicated. The intimate association of reproduction and sex is a derived state and a product of long evolution after the origin of sex.
There are variations in sexual process in different organisms. In prokaryotes (like E. Coli) a process called conjugation takes place where DNA from a donor cell is transferred into a recipient cell that results into a recombinant genome. However, in bacterial sex, there is no meiosis and no segregation.
In an alga like Chlamydomonas eugametos there are two genetically mating types called ‘+’ and ‘-‘. Zygote is formed only when gametes from ‘+’ and mating types fuse. When the zygote germinates, it undergoes meiosis and cell division producing four recombinant haploid offspring.
In Aspergillus nidulans (member of ascomycetes) sexual ascospores are found without recombination. The species is self-fertile and a single individual can form sexual spores. In complex organisms (plants or animals) sexual reproduction requires sophisticated cell division like mitosis, meiosis. There must be fusion of two haploid cells (syngamy) into a new diploid cell. Mammals are exclusively sexual because of genetic imprinting (the silencing of certain genes in the germ line of parents prior to gamete production, accompanied by methylating the DNA sequences).
With few exceptions, sexual fusions are asymmetric when two gametes can be distinguished. The most common situation is anisogamy where fusing gametes have different sizes and shapes. Isogamy is the ancestral state while anisogamy is the derived state. Most advanced form is oogamy.
Advantages of sex are as follows:
(i) Neutralization of negative mutation-recombination during sexual reproduction enables selective elimination of negative deleterious mutations and repairs genetic lineage with mutational damage.
(ii) Recombination accelerates the rate of evolution—recombination can rapidly bring together in the same genome two beneficial mutations that originated in different organisms.
(iii) Sex creates rare genetic combinations that flourish when common types suffer.
(iv) DNA repair—during chromosome duplication many complex process of DNA repair occur.
The Fossil Records of Different Geological Strata:
Any evidence of prehistoric life is fossil. The word fossil is derived from a Greek verb ‘fodere’ means ‘to dig’. They are preserved in different ways and help us to understand conditions of the past. Say for examples, a dinosaur foot print preserved in rock as a trace fossil; shells of mollusks often found as molds or casts; a leaf’s shape is often preserved as impression or compression (with some carbonaceous matters); fossilized wood is preserved as petrifaction. Plant or animal remains may be buried in whole or in part in environments where the process of decay is prevented. Acidity, lack of oxygen and water rich in mineral can prevent microbes from breaking down the tissues.
Such remains must be permanently entombed in rocks if they are to survive for any length of time. They must, therefore, be covered by sediments, like sands, silts or mud, soon after they fall. As more and more layers of sediments accumulate, the grains and mud are pressed together into a solid rock which seal in the plant and animal remains and fossilize them. I will present here the plant and animal types of various geological ages.
A. The Precambrian:
The Precambrian includes approximately 90% of the geologic time which covers approximately 4200 m.y. of the earth history (Table 1). The Precambrian world was almost certainly as diverse and complex a place as today’s world.
Most information are from Cratons i.e. the large portions of continents which have not been deformed since Precambrian. The exposed cratons are called Precambrian shields viz. Canadian shield. The Precambrian is divided into three epochs or eons namely, Hadean, Archaean or Archaeozoic and Proterozoic (Stewart & Rothwell, 1993):
Precambrian Life Forms are Not Well Known because:
1. Rocks are poorly exposed.
2. Many rocks have been eroded or metamorphosed.
3. Most are deeply buried beneath younger rocks.
4. Fossils seldom found.
(a) Hadean (4.7-3.8 b.y. old)—No Fossil Record:
Major times of this epoch was involved in the origin of the Earth and solar system, ultimately led to the differentiation of the Earth to form crust, mantle and core. More precisely, the events that took place in the Hadean are: Origin of primary and secondary atmosphere; condensation of water vapour, occurrence of rain; runoff leading to lakes, rivers and oceans; origin of continental crust.
The oceans were originally freshwater which may have been acidic due of sulphurous gases. Subsequently, there was slow accumulation of salts due to weathering. The most of the early continental crust developed secondarily involving partial melting and weathering. The continental crust was probably present prior to 4.0 b.y. ago and the oldest dated Hadean rocks are 3.9 b.y. old located in Canada (e.g. Canadian shield).
(b) Archaean or Archaeozoic (3.8-2.5 b.y. old):
The earliest known life forms (Prokaryotes) are known from Archaean rocks. They are microscopic rod shaped bacteria (Eobacterium isolatum) and spherical coccoid bacteria (Archaeospheroides barbartonensis) recovered from Onverwacht series and Fig Tree series of South Africa (3.7-3.2 b.y. old) (Barghoorn and Schopf, 1966; Schopf and Barghoorn, 1967; Knoll and Barghoorn, 1977). Stromatolites contain fossils of cyanobacteria (blue-green algae).
Structures resembling the colony of Chroococcalean types has also been recovered from carbonate sediments (3.5-3.4 b.y. old) of Tower Formation and Apex Basalt of the Warrawoona group in Archaean rocks of West Australia (Schopf and Packer, 1987).
The earliest bacterial cells that formed and existed in anoxic (oxygen free environment) conditions were probably chemosynthetic in nature. They were obviously heterotrophs which consumed simple organic compounds. Eventually some amount of oxygen was released in the atmosphere by the cyanobacteria which might have been used up by minerals through oxidation (discussed earlier).
Evidence for a Lack of Free Oxygen in the Earth’s Early Atmosphere:
(i) Urananite and pyrite are readily oxidized today, but are found unoxidized in early Precambrian sediments.
(ii) There are no early Precambrian iron oxides (no red beds).
(iii) Banded iron formations appear in stratigraphic record in mid or late Precambrian (2.5-1.8 b.y.).
(iv) Evidence from mid or late Precambrian soils shows oxygen was only about 2% of modern atmospheric levels.
(v) Chemical building blocks of life (amino acids, DNA) could not have formed in the presence of oxygen.
(vi) The simplest living organisms have an anaerobic metabolism. These organisms and archaeobacteria (such as botulism) are killed by oxygen, some or all archaeobacteria inhabit unusual conditions.
(c) Proterozoic (2.5-0.57 b.y. old):
Several autotrophic chlorophyllous algae (possibly blue green algae-presently referred to as cyanobacteria) were reported from Gunflint rock of Canada which dated back to 2.0 b.y. (Tyler and Barghoorn, 1954; Barghoorn and Tyler, 1965). The best known blue green alga was Animikiea (similar to modern cyanobacteria Oscillatoria).
The existence of free oxygen was established by the occurrence of chemosynthetic iron forming bacteria known as Gunflintia in the iron banded Gunflint rock. The putative eubacterial spores namely, Huroniospora and Eostrion were reported from Gunflint rock. The ozone layer screened the lethal UV rays and created a situation conducive to the growth of other autotrophic primitive (Cyanobacteria), advanced (Green algae) algae, etc. in free floating condition.
The more diversified prokaryotes and eukaryotes were observed from several sediments like Duck Creek Dolomite of Western Australia (1.3 b.y.), Bitter Spring formation of Australia (900-800 m.y.) and the Svanbergfjellet shale of Spitsbergen Island (800-700 m.y.) (Knoll et al. 1988; Schopf, 1968; Schopf and Blacic, 1971; Knoll 1985; Butterfield et al. 1988).
Several cyanobacteria like microorganisms were also described. Several metazoans (multicellular) Ichnofossils or trace fossils were discovered from Australia and the Svanber. Oldest metazoan body fossils in the name of Ediacara fauna were recovered from Ediacara Hills, South Australia (700-590 m.y.) as impressions and molds of animals.
About 26 species of soft-bodied jellyfish-like animals were identified belonging to 18 genera under about 4 phyla consisting of 67% Cnidaria, 25% Annelids (worms) and 5% Arthropods. The most common animals were Mawsonites (similar to jelly fish), Dickensonia costata (Segmented worm), Tribrachidium heraldicum (Echinoderm), etc.
Stromatolites:
A rounded, multilayered, sedimentary structure up to about 1 meter across, formed by microbial mats in which a rock-like layer of either sand or precipitated minerals are present. They are biologically produced structures formed by colonies of photosynthesizing cyanobacteria. Fossil stromatolites constitute the earliest and most pervasive direct record of life on Earth and have been found in rocks dating back at least 2-8 b.y.
Although many stromatolites are fossils, there are a number of locations on the modern day Earth where stromatolites are still forming. Modern stromatolites were first discovered in 1956 throughout Western Australia in both marine and non- marine environments. While formation by colonies of cyanobacteria is probably the primary mechanism for formation of stromatolites in the deep time of the Archaean and half way through the Proterozoic it is unlikely to be the only mechanism.
Excluding some exceedingly rare Precambrian fossils such as the Ediacaran fauna, stromatolites are the only fossils encoding the first 7/8th of the history of life on Earth. They encode the role that ancient microorganisms played in the evolution of life on Earth and in shaping Earth’s environment.
The fossil record of stromatolites is astonishingly extensive, spanning 4 b.y. of geological history with the forming organisms possibly having occupied every conceivable environment that ever existed. Recent research indicates that the other prokaryotic and the most genetically diverse domain of life, the Archaeans, evolved alongside and possibly swapped genes with the Eubacteria. Molecular fossils indicate that primitive eukaryotic microorganisms appeared more than 3.5 b.y. ago.
Thus, before the end of the Archaean time some 2.5 b.y., all three domains of life (eubacteria, archaea and eukaryotes) existed and were likely already quite diverse. Some were autotrophs, some chemotrophs and some heterotrophs and collectively they had a multiplicity of metabolic processes to derive their energy.
Just as the microorganisms were extremely diverse in deep time so was there a corresponding extreme diversity of biogenic and chemical mechanisms that are plausible for the formation of laminar carbonate and other structures that we call stromatolites. Nonetheless, the cyanobacteria are conjectured to have been the predominant form of life on early Earth for more than 2 b.y. and were likely responsible for O2 in the atmosphere. Cyanobacteia are capable of directly precipitating CaCO3 with minimal incorporation of sediment within the structure.
The cyanobacteria could repeatedly recolonize the growing hard sedimentary platform, forming layer after layer in a cyclic repetitive process. The resulting successive layering can assume a myriad of shapes dependent upon microorganisms and environment and if left undisturbed by forces of nature could form huge domes and flat laminar structure that grew upward toward the life-sustaining rays of sun.
B. The Palaeozoic Era:
At the onset of the Palaeozoic Era the Earth’s climate was found to change rapidly resulting into the formation of diversified life forms. Some of which invaded the land escaping from aquatic habitat and initially was amphibian in nature. For example, while acquiring land habit from water, the basal part of the bryophytes developed in such a way that it served the purpose of anchorage and absorption whereas the erect part performed photosynthetic function and some part of the branching system became flattened to form rudimentary leaves and finally simple reproductive organs at maturity. The Cambrian and the Ordovician periods were characterized by a diverse group of algae and bryophytes in the warm ocean and inland seas.
The members of red algae were prevalent in the warm seas of lower Palaeozoic along with the cyanophytes. One significant fossil genus of Rhodophyta from the Cambrian is Solenopora. It forms irregular nodular masses of limestone. Vertical sections of Solenspora show slightly radiating filaments of cells.
The arrangement of the filaments are similar with extant multicellular coralline red algae. Lithothamnium and Lithoporella are two fossil coralline red algae which have vegetative and reproductive structures very similar to the extant genera. Their biochemical similarities and coexistence with cyanophytes in the Late Precambrian establishes their phylogenetic relationship with cyanophytes.
More than 100 genera of fossil dinoflagellates have been found in the Mesozoic and Tertiary periods. Fossil hystrichospores and dinoflagellates occur most commonly in marine sediments. Marine species of chrysophyceae form plates of calcium carbonate called coccoliths.
As the coccolithophorids die the calcareous plates are deposited on the sea floor which form chalk beds and are of Cretaceous age. Diatoms are unique in having a bipartite silicified cell wall and this cell wall of dead diatom cells accumulate to form diatomaceous earth. About 70 fossil genera of diatoms have been recorded. The centric diatoms have been traced back to early Jurassic while the pennate diatoms are supposed to have originated from centrics in the late cretaceous.
The fossil record of the green algae (Chlorophyta) is of great importance because it is established that ancestors of green land plant belong to this group. Members of chlorophyta were extant in the late Precambrian. Well preserved algal remains were found in Late Proterozoic shale of 700-800 m.y. of age. Morphologically they resembled extant members of Cladophorales and Chaetophorales. Chlorophycean members secreting lime and having morphologies similar to Codiaceae and Dasycladaceae have been found from Cambrian. Palaeoporella from the Late Cambrian has articulated organization of the thallus. Crosssections show intertwined branching tubules and the pattern is similar to Halimeda. Members of the order Charales (Chlorophyta, Charophyceae) are significantly different from other green algae in the structure of their vegetative and reproductive parts.
The group is well represented in fossils, especially by female sex organ (nucule), which when calcified is called gyrogonites. The earliest fossils of charales occur in the Upper Silurian (420 m.y. ago). The genera present today date back to the Mesozoic era (130- 200 m.y. ago) and seem to be very conservative in their morphology. Extensive study of the geological distribution and evolution of these structures show that the fossilized nucules of the Devonian period had an irregular number of vertical corticating filaments.
By the middle Devonian the number of filaments stabilised at five. Subsequently, the filaments became sinistrally spiral and a pore developed at the apex of the nucule. This basic type appeared in the Carboniferous with numerous variations in size, shape and ornamentations. Vegetative structure resembling Nitella was described from Rhynie Chert and named Palaeonitella. Fragments of vegetative parts of Chara was obtained from the Tertiary deposit.
Three important fossil genera assigned to Chlorophyta are Botryococcus, Courvoisiella and Parka. Courvoisiella from Upper Devonian resembled members of Siphonales and cellulose was found in its cell wall (biochemical evidence to establish relation with Chlorophyta). Affinities of Parka and Pachytheca, belonging to Upper Silurian-Middle Devonian period, were established with the members of Chlorophyta which resemble Coleochaete in thallus structure and development. Biochemical studies of Parka support its inclusion in green algae and thin cutinized thallus indicate adaptation to land habitat.
Adaptation of Parka to land habitat is also supported by the finding that the spore walls have an inner laminated region and an outer region similar to bryophyte spores. There are important similarities between Parka and Coleochaete—sporangia of Parka are homologised with the oogonia of Coleochaete, sporopollenin has been found in the zygote walls of Coleochaete and in the reproductive organ of Parka.
Such revelations of significant characteristics among the fossil members of green algae are important. It is established that the green land plants evolved from ancestors belonging to the Chlorophyta. This evolutionary conquest of land probably occurred sometimes between the Late Cambrian and Early Silurian.
On one hand there are significant structural similarities between Coleochaete and Parka (fossil green alga with indication of land adaptation) and on the other hand molecular phylogenetic study establishes that Coleochaetales as the clade most closely related to land plants. Role of green algae in the evolution of land plant in general and that of Coleochaete in particular is thus established.
Further, cytological evidence, biochemical similarities, similar vegetative morphologies and tendency to retain the zygote in the oogonium until after germination indicate origin of green land plant from a Coleochaete like green algae.
The oldest unequivocal fossil bryophyte is a small ribbon like thalloid liverwort Pallavicinites devonicus. The fossils are compressions found in rocks of Upper Devonian age. Specimens obtained by maceration show a surprising amount of cellular detail, including cells of a dichotomizing apex.
The characteristics studied established its affinities with the Acrogynous-Jungermanniales. There is striking similarity between extant Palavicinia and Pallavicinites devonicus. Beautifully preserved specimens of anacrogynous liverworts were discovered from the Carboniferous.
One is a leafy form Hepaticites kidstoni which is similar to extant anacrogynous Treubia. Thalites nillsi is similar to Metzgeria and has a dorsiventral dichotomizing thallus. Metzgerites from the Triassic confirms that the anacrogynous liverworts have a more or less continuous fossil record initiated in the Devonian.
From the fossil record available it is inferred that acrogynous Jungermanniales (leafy forms) appeared much before the Tertiary. Specimens resembling leafy liverworts are put in the form genus Jungermannites. Specimens showing unequivocal relationship with the Marchantiales is Hepaticites cyathodoides from the Middle Triassic. Pores with airchambers, scales on ventral surface are the characters that assign them to Marchantiales.
The genus Ricciopsis found in the Jurassic-Triassic rocks of Sweden bears a striking similarly with extant Riccia. Marchantia like fossil is the Marchantites sezannensis from Eocene. The specimen showed antheridial gametophore. Naiadita lanceolata from the Triassic is placed in the order Sphaerocarpales. Most aspects of the life-history of this fossil plant is well studied except the antheridial parts.
The gametophore is leafy with unistratose, elliptic to lanceolate leaves transversely attached and spirally arranged on the erect terete stem. Rhizoids are attached at the base. Archegonia are lateral and supplant the position of a leaf. The sporogonium is spherical with unistratose jacket of thin walled cells with spores and elaters.
It has affinity with extant Riella due to the presence of unicellular rhizoids, sessile archegonia with one cell layer thick venter and leaf cells of similar shape. Fossil bryophytes with leaf cells like Sphagnum have been found from the Permian of Russia. However, these leaves have midribs. Of the three genera one is Protosphagnum. The oldest known fossil of a true moss is Muscites polytrichaceus from the Carboniferous.
There are several fossil mosses resemble the extant forms. Intia resemble a modern Mnium or Bryum. Two major groups of bryophytes (mosses and hepatics) were well differentiated long before the end of the Palaeozoic. Hepatics appeared before the Musci. With the absence of a fossil evidence linking mosses and hepatics it is concluded that bryophytes are polyphyletic in origin. This origin is dated back to the Upper Devonian and it is assumed that evolution of liverworts occurred earlier with the mosses originating later.
Chlorophyta and Green Land Plant:
The green algae and embryophytes together form a monophyletic group called the Viridiplantae, which consists of two lineages the Streptophyta and the Chlorophyta. Streptophyta consists of all embryophytes, that is the bryophytes and vascccular plants and a distinct group of green algae belonging to the class Chanphyleae which includes the orders—Zygnematates, Klebsormidiales, Zygematales. Coleochaetules and charales. Based on biochemical ultrastructural and molecular characters the chanphyleae is considered to be the green algat lineage most closely related to land plants.
The characters are:
(i) Open mitosis,
(ii) Presence of Multilayered Structure with the flagellum apparatus,
(iii) A persistent mitotic spindle with the formation of phragmoplast during cytollinesis,
(iv) Presence of Cn/Zn superoxide dismitase, class-I aldolases, glycolate oxidase containing peroxisomes,
(v) SSU rDNA, rbc L and other gene sequences reneal close relationship to embryophytes.
The sharing of so many conservative characters of charophyceans with land plants strongly suggest that embryophytes arose from charopliycean ancestors. The molecular analyses to ascertain which order with charophyceae—charales or coleochaetules is more closely aligned to the land plants give conflicting information.
Several analyses of 18 S rDNA sequences yielded a topology in which charales was placed with strong support as sister to the other four orders and land plants. In contrast, rbc L data and some 18 S rDNA data indicated that charales plus coleochactales or charales alone were sister to land plants.
The genus Coleochacte is considered as a model organism in the study of land plant ancestor.
The Characters Indicative of Close Relation between Coleochaete and Land Plant are as follows:
(i) Apical bitlasellate male gamete is consistent with the sperm of land plant,
(ii) The non-motile female gamate in Coleochaete and oogamnes reproduction in embryophytes is comparable,
(iii) The zygotes are retained on maternal plant in Coleochaete and corticated by a layer of sterile gametophytic cells. In one species the zygote received nourishment from placental transfer cells with wall ingrowths. This pattern is similar to the archegonial venter cells of lower embryophytes,
(iv) Sporopol- lenin in the inner wall of the zygote of Coleochaete and liguin in the thallus, functioning as antimicrobial agent, are additional features that are used to suggest a relationship between this charophyceann algae and early land plants.
About 410 m.y. ago, during the Silurian period the land plants evolved. By this time the early photoautotrophic plants added oxygen which was 20% of present level in the atmosphere through photosynthesis. As already mentioned Ozone (a product of oxygen) prevented UV rays from reaching the earth’s surface.
To acquire terrestrial habitats after leaving their aquatic habitat the plants needed to be self-supporting and they have to be able to withstand the drying effect of the air leading to a series of adaptation. Cooksonia was the foremost successful land invader (Middle to Upper Silurian). It was from such a humble beginning that all the major groups of land plants originated.
In the following geological period, the Devonian (410-345 m.y. old), more and increasingly complex plants appeared with significant modifications. The important structural modifications are formation of vascular tissue (for conduction), epidermal cuticle (to check dessication of water) and stomata (for gaseous exchange).
All these features were visualized in the three early vascular plant groups (viz. Rhyniopsida, Zosterophyllopsida and Trimerophytopsida). Another important adaptation to land life is the very tough wall that evolved around the spores. This provided an excellent protection against desiccation.
About 450 m.y. ago, at the end of the Ordovician and in the beginning of the Silurian, the land was desolate and empty. The oldest indications for the existence of real land plants have been found in cores from boreholes in Oman. They contained tetrads enveloped by remains of the spore sac in which they have been formed.
The first fossils of macroscopic land plants have been found in the Middle Silurian of Ireland (Wenlock strata, Tipperary) which are about 425 m.y. old. The best known plant from that time is called Cooksonia. It is named after Isabel Cookson who occupied himself with intensive collecting and describing plant fossils.
This plant is only a few centimeter in height and very simple in structure. It consists of a stem which bifurcated several times and ended in small spheres (sporangia) in which the spores were formed. Horizontal growing stems are connected with the soil by root hairs that functioned as roots.
It was a true vascular plant with tracheids-resembling internal conducting cells and equipped for life in an aerial, terrestrial environment. Few species of Cooksonia are known — C. pestoni, C. caledonica, C. cambrenis and C. hemispherica. They differ in only minor ways, chiefly in the forms and structure of the spore-case.
When these Cooksonia species are analyzed phylogenetically, they emerge as a highly paraphyletic group (a group of organisms with similar general organization but may not be related to one another). Rhynia is slightly younger but similar in appearance to Cooksonia. Baragawanathia is much larger and more complex with spirally arranged leaves. This has been described as a Lycophyte (a vascular plant). Presence of such a complex plant in Silurian indicates that land plants must have emerged in the Ordovician.
The Devonian period marked a major shift in plant evolution and terrestrial ecosystems. Early Devonian plants such as the Rhyniophytes, Zosterophyllophytes and Lycophytes have features such as vascular tissue, stomata, a cuticle to protect against drying, rhizoids and sporangia at the tip of short lateral branches instead of terminal as in Cooksonia.
These forms were small, non-rooted or shallowly rooted, lacked woody tissue and hence were unable to grow beyond the height of small bushes. These plants reproduced by means of spores, which require a moist habitat. They were therefore confined to moist lowland habitats thus having little effect on their physical environment.
The first shrub and tree like plants such as Progymnosperms and lycopsids had evolved by the Middle Devonian. By the Late Devonian the first real trees such as Archaeopteris (ancient fern) had appeared. Trees have special vascular systems to allow the water circulation and nutrient flow against the pull of gravity.
During Devonian the first wood appeared in the genus Rhynia which had xylem. At the end of the Devonian seed-bearing gymnospermous plants appeared for the first time, breaking free of the dependence on moisture that limits spore-bearing Pteridophytes. Along with these developments came the development of advanced root systems and the production of soil, increased weathering and huge ecological feedback.
Rhyniophytes are a basal radiation of land plants such as Aglaophyton or Horneophyton. Trimerophytes include plants such as Psilophyton. Lycophytes arrived in the Middle Devonian. They originally appeared as low lying herbaceous forms such as Asteroxylon.
Tree-sized lycopods, Lepidosigillaria appeared by the end of Middle Devonian. Progymnosperms such as Tetraxylopteris arose in the Frasnian. By the Famennian Archaeopteris forests are common. At the end of the Devonian Archaeopteris is found together with early gymnosperms such as Elkinsia and Moresnetia.
Rhynie in the Grampian region of Scotland became famous as one of the most important palaeobotanical localities in the world. In 1912 the Scotish geologist William Mackie discovered an occurrence of Lower Devonian plant-bearing cherts near this small village. Recent radiometric datings of these rocks give an age of this chert as 396 million years (Pragian). Kidston and Lang gave the first descriptions of these oldest known vascular land plants whose anatomy is well preserved.
Most species appeared to belong to a group that is completely extinct. In addition, they described a number of lower plants. The preservation of plant material and the associated early terrestrial animals remains is often superb. The large diversity of life-forms varying from unicellular organisms to vascular land plants and terrestrial arthropods as well as their exquisite preservation and the fact that these fossils often occur in life position make Rhynie a unique locality.
The Rhynie Chert plants have had a profound influence on the evolution of early vascular plants. Some well preserved gametophytes of early vascular plants have been identified from the Rhynie chert. Lyonophyton rhyniensis and Sciadophyton are the two well documented gametophytes.
It has been suggested that Lyonophyton may be the gametophyte of Aglaophyton major and Sciadophyton the gametophyte of Taexiocrada langii based on the nature of conducting tubes and epidermal characters.
The following Plants have been Reported from the Rhynie Chert:
Cyanobacteria:
Archaeothrix contexta, Archaeothrix oscillatoriformis, Kidstoniella fritschii, Langiella scourfeldii, Rhyniella vermiformis, Rhyniococcus uniformis.
Fungi:
Allomyces sp., Glomites rhyniensis, Krispiromyces discoides, Milleromyces rhyniensis, Palaeoblastocladia milleri, Palaeomyces agglomerate, Palaeomyces asteroxyli, Palaeomyces gordonii, Palaeomyces horneae, Palaeomyces simpsonii, several Chytridiomycetes, Ascomycetes, various other undescribed fungi.
Lichens:
Winfrenatia reticulate
Nematophytes:
Nematophyton taitii, Nematoplexus rhyniensis
Algae:
Mackiella rotundata, Palaeonitella cranii, Rhynchertia punctata
Tracheophytes (Sporophytes):
Aglaophyton major, Asteroxylon mackiei, Horneophyton lignieri, Nothia aphylla, Rhynia Gwynne-vaughanii, Trichopherophyton teuchansii, Ventarura lyonii.
Tracheophytes (Gametophytes):
Langiophyton mackiei, Lyonophyton rhyniensis, Kidstonophyton discoides, still undescribed female gametophyte of Aglaophyton major, still undescribed male gametophyte of Horneophyton lignieri.
The existence of such plants made possible the subsequent emergence of a variety of arborescent (tree sized) plants that flourished in the swamp of carboniferous period (345-325 m.y.) in northern hemisphere. The most dominant plants were Lepidodendron (giant club moss), Calamites (giant horsetail), etc. which attained a height of 30-50 metres.
During the optimum period of diversification a new plant group i.e. Gymnosperms (naked seeded plants) got evolved. Those plants mostly comprising of Pteridosperms — the primitive seed bearing plants, Cordaites — the progenitors of the modern conifers, became more successful land plants because of their selective advantage of seed formation.
The embryos of the seeds were well protected and had the potentiality to overcome the adverse condition. One of the best known seed fern is Glossopteris which was restricted to Gondwanaland continents during the Carboniferous and Permian.
Adaptation in Land Habitat:
The earliest evidence for the appearance of land plants in the form of fossilized spores comes from the Ordovician period (510-439 m.y.) a time when global climate was mild and extensive shallow seas surrounded the low-lying continental masses. The seashore would have been enormously important in the colonization of land.
In this zone algae would have been exposed to freshwater running off the land and would have colonized the freshwater habitat before making the move to terrestrial existence. They would be exposed to an alternating wet and desiccating environment.
The earliest photosynthetic organisms on land would have resembled modern algae, cyanobacteria, lichens followed by bryophytes that evolved from green algae. Their lack of vascular tissue for transport of water and nutrients limits their size.
The Transition to Land:
The ability to exist on land is the result of numerous complea interactions that involved the interplay between structural and physiological adaptations in the plants themselves, symbiotic interactions at several levels and physical and chemical changes in the environment—(i) Anchorage and water uptake—The ancestral aquatic alga was suspended in water and water was taken up by diffusion and osmosis.
But the terrestrial habitat was hostile and desiecating environment. Extant land plants overcome these obstacles in two different ways. Byophytes are poikilohydric and they rely directly on environment for water. When environment dries so does the plant. It remains dormant white dry but recover rapidly when wetted. Water content had to reach equilibrium with that of environment. The vascular plants are homoiohydric.
These plants keep thin water content constant and do not equilbriate with the environment. The land plants control water loss by stomata or by structural modification and replace lost water by conducting tissue. They have specialise subterranean and aerial absorbing tissue. (ii) Structural support and water transport—In the terrestrial vascular plants, the vascular times (xylem and phloem) are involved in both support and conduction.
In early land plants, the central strand primarily functioned in conduction and the plant stood erect as a result of turgor pressure in the parenchymatous cells of the main axis. During the course of evolution of land plants vascular frame took over the role of providing support and the plants grew larger. Tracheary elements in the xylem and fibres in the phloem have secondary cell walls impregnated with lignin, a polyphenolic polymer which provides, structural support and flexural stiffness to the plant organ. (iii) Protection against desiceation and radiation: During the course of evolution of land plants developed a method to retain water in a desiccating environment.
Many of the plants had a non-cellular outer envelope referred to as cuticle or cutricl-like layer which was very effective as a protective layer against excessive transpiration. The presence of sporofrollenin in the spore wall represents a similar adaptation to prevent desiccation of reproductive propagules. The inticle was also effective in the attenuation of uv-radiation including uv-B. (iv) Gas exchange : In land plants CO2 enters the plant through specialized openings termed stomata. (v) Reproduction on land : A primitive land plant requires several adaptations in order to reproduce on land.
It should have the ability to move gametes from one gametophyte to another in order to effect fertilization and should be able to disperse spores to colonize new habitats. Land plant sporophytes produce spores or seeds that resist desiccation and can be transported great distances by biotic and abiotic vectors.
As the plants were conquering land, there was a great evolutionary radiation of multicellular animals in the earliest Cambrian oceans. The first animals dating from just before the Cambrian were found in rocks near Adelaide, Australia. They are called Ediacarian fauna. Cambrian explosion of animals was very rapid.
In contrast, the colonization of land by vegetation was a slow process as the plants had to brave desiccation, extremes of temperature and harsh UV radiation. Among the animal kingdom, the deep Cambrian rocks contained the invertebrate fossils like sponges, jelly fishes, worms, shell fishes, star fishes and crustaceans.
One of the major developments of the Cambrian period was the advent of hard parts in the form of protective shells and plates. Early Palaeozoic waters were invaded by the crablike Trilobites and the large scorpion like Eurypterids. The most dominating elements in the Silurian waters were the Eurypterids as much as 3.5 meter tall.
The other common Palaeozoic elements were Nautiloids (related to modern Nautilus), Shellfish Brachiopods or Lamp shells, Carboniferous Graptolites. Hence the Paleozoic era is often called, “The Age of Invertebrates”. No land animals are known from Cambrian and Ordovician periods. Extraordinarily well preserved Cambrian Burgess shale fauna of Canada provides a window into the past to view the spectacular diversity of the Middle Cambrian.
Several groups of arthropods, sponges, onycophorans, crinoids, molluscs are found in this shale. In the Ordovician period invertebrate marine life diversified. The number of families increased from 160 (505 mya ago) to 530 (438 mya). During this period orogenies in several marginal mobile belts raised fold mountains.
The habitat for the existing species were altered and there was an increase in number. This is probably due to the entry of mud and nutrient into the nutrient-poor carbonate shelf environment. There are definite evidences of geological and biological co-revolutions in this period.
After studying the geographic distribution of 6,570 Ordovician marine fossils of trilobites, brachiopods and molluscs it was inferred that increase in species diversity was closest to growing mountain ranges. No vertebrate animal existed during the Ordovician. Fishes were present but there were small jawless forms (Agnatha).
The Agnatha fossils occur in association with trilobite fossils which indicate that they were marine. The absence of invertebrates in fresh water Ordovician sediments indicate that the conditions were sterile. The Cambrian fauna declined slowly during this time. By the Devonian period vertebrates had moved onto the land.
The Devonian period is known as “The Age of Fishes” where bony fishes arose. The crawler Crossopterygians (lobed-finned air breathing fishes) gave rise to the first land-dweller among vertebrates, the Amphibian. Ichthyostega, an amphibian, is among the first known land vertebrates.
The end of the Palaeozoic era witnessed a decline in Amphibians and the initiation of Reptilian diversification. Reptiles were with scales to decrease water loss and a shelled egg permitting young one to be hatched on land. Among the earliest well preserved reptiles is Hylonomus. Permian extinction was the largest extinction and the Palaeozoic fauna decreased from 300 families to 50. The modern fauna (including fish, bivalves, gastropods, crabs) was not affected by the Permian extinction and increased to 600 marine families.
C. The Mesozoic Era:
Most of the Palaeozoic flora failed to survive and was largely replaced by the naked seeded Gymnosperms. The gymnospermous plants mostly cycads reached a climax in the Mesozoic era (225-65 m.y.) which constituted the world’s dominant vegetation co-existent with the giant reptiles known as Dinosaurs.
Still later, at the end of the Mesozoic era (i.e. in Creataceous period) the closed seeded plants i.e. Angiosperms evolved which subsequently become dominant in Cenozoic era replacing most of the pteridophytes and gymnosperms. The Mesozoic era was “The age of Gymnosperms” as evidenced by the diverse assemblage of cycads, ginkgos (maidin hair tree) and primitive conifers.
In this era, air breathing insects (most dominant was Meganeuron, a giant dragon fly) and vertebrates, notably the widely distributed reptiles (Dinosaurs), held the centre of the stage and the large shelled ammonoids thrived in the seas. Dinosaurs evolved from reptiles and they had an upright stance and this allowed continued locomotion.
They evolved to be warm blooded. The end of Cretaceous is marked by a minor mass extinction and all lineages of dinosaurs and marine ammonoids perished giving way to the birds and mammals.
D. The Cenozoic Era:
In the Cenozoic era (65 m.y. to present day) the dominance of gymnosperms steadily declined in number and distribution, and in the meanwhile, the angiosperms got evolved and occupied most of the land mass surface.
In present times, gymnosperms are represented by only 730 species while angiosperms are distributed globally by having over 300,000 species. It has now been accepted that the angiosperms evolved from gymnospermic ancestors, and is the highest evolved and dominant terrestrial plant life-form. Angiosperms grow in a greater range of environments (tropical, temperate, alpine, coastal etc.) and a variety of habitats (aquatic, terrestrial, epiphytic, etc.) displaying an astonishing array of morphological, anatomical and physiological variations. Angiosperms include the world’s major food yielding plants and other plants of great economic importance.
As a group, the angiosperms have typically been viewed as being monophyletic (i.e. a group consisting of all descendants derived from a single ancestor). Several characters including the closed carpel, the eight-celled female gametophyte and double fertilization are considered defining features of angiosperms and support the monophyletic groupings.
Recently, phylogenetic analyses using nuclear, mitochondrial and plastid gene sequences have aided in clarifying relationships between the various angiosperm families. The monocots and eudicots are each considered as monophyletic.
The oldest indisputable angiosperm fossil is from the Valanginian of the Cretaceous period (130 m.y.). Although this pollen is monosulcate (magnoliid) it is followed by the appearance of colpate pollen from the Barreanian-Aptian boundary. By the upper Cretaceous many of the extant angiosperm families and subclasses had already differentiated.
The angiosperms underwent a major diversification during Mid-Cretaceous. There is an abundance of angiosperm fossils that have been found at this stratigraphic level which has been associated with a decrease in gymnosperm. Hence the Cenozoic era is often known as “the age of angiosperms”.
It has been argued that early angiosperms may have evolved in montane habitat (upland theory) far away from depositional environments necessary for fossilization. A microfossil of a plant named Archaefructus was found from the Upper Jurassic of China. This angiosperm fossil has been named Archaefructus liaoningensis Sun, Dilcher, Zheng et Zhou, gen. Nov. and placed in the division- Magnoliophyta, class-Magnoliopsida, subclass-Archaemagnoliidae.
The plant has a series of conduplicate carpels born spirally on an elongate axis but strangely, the axis is subtended only by a leaf-like structure (not floral whorls). In summary, this plant combined features of two magnoliid groups: the woody magnoliales and the herbaceous group called paleoherbs.
More recent data indicates that the Archaefructus was actually from Cretaceous, not Jurassic strata. Archaefructus is a clear indicator that large reproductive axes of angiosperms existed early in angiosperm evolution. This suggest that small angiosperm flowers and fruits of early angiosperm is derived and reduced to small sizes from an ancestor with large flowers.
The characteristics of Archaefructus indicate that it has seed- fern ancestry. Some lineages of Mesozoic seed ferns are the ancestors of the Mesozoic radiation of the angiosperms. By Mid-Cretaceous and into the lower Upper Cretaceous, a tremendous increase in angiosperm diversity appears in fossil record.
Nearly all of these fossils represent lines of evolution progressing toward extant taxonomic clades of angiosperms at the family or generic level. The evolution of modern taxonomic groups of angiosperms thus seems to have transpired relatively quickly during the lower cretaceous.
An example of a fossil woody Archaeanthus (means ancient flower) have been found that are 98 million year old. This has magnolidae features like — (i) woody plants with simple evergreen leaves, (ii) flowers large, showy, bisexual, (iii) perianth parts many, free, spirally arranged.
The geographic origin of the angiosperms has been under considerable debate. In the late nineteenth and early twentieth centuries, the dominant view had been that the angiosperms originated high in the northern latitudes and even in the polar regions. This view has been disputed for a more tropical “center of origin” from which they migrated towards the polar regions.
The fossil evidence suggests that angiosperms did not establish in the Arctic until the Late Cretaceous, since angiosperm fossils from the Early Cretaceous are found only at low latitudes. Although the precise location of the angiosperm center of origin is uncertain, it is predicted to be in south-eastern Asia. Takhtajan (1969) has suggested that the cradle of angiosperm occurs somewhere between Assam and Fiji due to high abundance of ‘primitive’ angiosperm families Magnoliaceae and Winteraceae found in that region of Pacific Basin. But there is lack of fossil evidence.
However, determination of center of origin must depend on the distribution of extant taxa in a phylogenetic series. The angiosperms owe their success to the evolution of flower. The flower’s pollen and necter encourage pollinating animals to visit, increasing the odds of fertilization by ensuring that pollen is transferred efficiently from flower to flower.
After fertlisation fruit is formed with seeds. Seeds are dispersed to various destinations. Xylem vessels of angiosperms allow very rapid movement of water through the plant. This means that flowering plants can keep their stomata open through much of the day, achieving higher photosynthetic rates than gymnosperms. This spare photosynthetic activity can support development of fruit.
The rapid diversification of angiosperm taxa began in the Albian, in the Mid- Cretaceous and has continued to this day. At that time, there is an almost exponential increase in angiosperm diversity and there does not appear to have been any major extinction of groups in between.
However, there is no definite indication from the fossil record about the exact origin of these diversified forms. Although there is rapid diversification beginning in the Albian, it is not until the Cenozoic that angiosperms began to take on important ecological roles. Studies of a preserved Maastrichtian (Late Cretaceous) landscape from Wyoming suggest that the high diversity of angiosperm species was confined to small populations and that the vegetation was still largely dominated by ferns and cycads.
One of the biggest questions about early angiosperms, besides their origin is the nature of their growth habit. Were the first angiosperms woody trees and shrubs or were they small herbs? The two competing hypotheses for angiosperm origin point very different pictures about the biology of the earliest flowering plants.
The Paleoherb hypothesis suggests that the basal lineages were herbs with rapid life-cycles, while the Magnoliid hypothesis suggests that the basal lineages were small trees with slower life-cycles. Cladistic analyses favour an early angiosperm with morphology similar to living members of the Magnoliales and Laurales.
These groups are small medium sized trees with long broad leaves and large flowers with indeterminate numbers of perianth parts. The carpels are imperfectly fused and make a physical intermediate between a folded leaf and fused pistil. This hypothesis is supported by molecular studies.
Molecular Evidence of Angiosperm Origin:
According to the “anthophyte hypothesis” woody Gnetales are sister to angiosperms and was the most popular concept up until fairly recently. With the advent of molecular data, it became apparent that there was conflict between this idea and one in which gymnosperms were monophyletic and in which this entire clade was sister to angiosperms. Doyle attributed this conflict between the morphology and molecular data on the “inability of molecular data to restore relationships at this level”.
His major source of doubt is the high number of morphological characters that contradict the molecular tree topologies. A number of “synapomorphies” between angiosperms and Gnetales must be reevaluated, for they may represent parallelisms.
There are now many molecular data sets that all point to the same tree topology, i.e. gymnosperms are monophyletic and sister group to angiosperms. This relationship is borne by genes derived from nuclear, plastid and mitochondrial genomes.
Among animals, the warm-blooded vertebrates (birds and mammals) which were evolved at the end of Mesozoic era, reached a climax in the Cenozoic era and now constitute the world’s dominant fauna. Once the dinosaurs were out mammals diversified. Morgonucudon a contemporary of dinosaurs is an example of one of the first mammals. Insects have continued to thrive and differentiated in a variety of species which is now the most dominant life forms on the earth comprising of over 800,000 species.
The human species arrives on the scene in the closing stages of this era. Fruit forming plants (Angiosperms) exerted an influence on the evolution of birds and mammals. Birds, which feed on fruits, seeds, flowers, evolved rapidly in co-association with angiosperms.
The emergence of herbivorous mammals coincided with the widespread distribution of grasses and herbs over the plains. In turn, the herbivores furnished the setting for the evolution of carnivorous animals, thus maintaining an intricate ecological balance by making interdependence between plants and animals.
The study of fossils has an applied significance in understanding the biostratigraphical sequence which provides to trace the plant as well as animal evolution through ages. In this context there are two aspects to be considered: (1) Correlation of the data showing quantitative and qualitative value to retrieved fossil taxa, and (2) use of microfossils for comparison to get the empirical value in terms of appearance, duration of dominance and then gradual disappearance through migration or extermination due to climatic and other factors establishing a biostratigraphical scale.
For example, in the coal bearing strata of the Middle Carboniferous of West Europe, seven successive vegetational sequences were established using the presence of plant fossils. It has been demonstrated that climatic episodes were directly or indirectly connected with the change in floral and faunal composition creating sharp boundaries between: Devonian and Carboniferous, Lower and Middle Carboniferous, Middle and Upper Carboniferous, Carboniferous and Permian, Permian and Triassic, Middle and Upper Jurassic, Creataceous and Early Tertiary.
To sum up, stratigraphic zones during a given time interval witnessed evolution, migration or extinction of both plant and animal groups. It is thus implied that the fossil assemblages in these zones correspond a definite span of time and throw light on the evolutionary trend of the plant and / or animal groups. Thus an analysis of fossils of a given stratigraphic zone provides the needed data to interpret its changing floral or faunal composition and consequent evolutionary sequence (Table 1).
