In this essay we will discuss about the fossil records of different geological strata.
Essay # 1. Fossil Records of Pre-Cambrian Era:
The Pre-Cambrian includes approximately 90% of the geologic time which covers approximately 4,200 million years (m.y.) of the earth history (Table 1.2). The Pre-Cambrian 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 Pre-Cambrian. The exposed cratons are called Pre-Cambrian shields viz. Canadian shield. The Pre-Cambrian divides into three Epochs or Eons, viz. Hadean, Archaean or Archaeozoic and Proterozoic.
(a) Hadean (4700-3,800 m.y. old):
There is no fossil record in this Epoch. A major times of this epoch was involved on the origin of the earth and solar system, ultimately led to the differentiation of the earth to form crust, mantle and core.
The events that took place in the Hadean are: origin of primary and secondary atmosphere; condensation of water vapour, formation of rain; run off leads to lakes, rivers and oceans; origin of continental crust. The oldest dated Hadean rock is 3,960 million years old, located in Canada (e.g. Cariadian shield).
(b) Archaen or Archaeozoic (3,800-2,500 m.y. old):
The earliest known life forms (Prokaryotes) from Archaean rocks are known. They are microscopic rod-shaped bacteria (Eobacterium isolatum) and spherical coccoid bacteria (Archaeospheroides barbartonensis) recovered from Onverwacht series (Fig. 1.89A) and Fig Tree series of South Africa (3,700-3,200 m.y. old).
A plenty of stromatolites containing the colony of chroococcalean cyanobacteria or blue-green algae has been recovered from the carbonate sediments (3,500-3,400 m.y. old) of Towers Formation and Apex Basalt of the Warrawoona group in Archaean rocks. The earliest bacterial cells had to form and exist in anoxic (oxygen-free environment) conditions probably chemosynthetic in nature, produced CO2, or H2S.
They were obviously heterotrophs which consumed simple organic compounds. Eventually, some amount of oxygen was released by the cyanobacteria which might have been used up by minerals through oxidation.
Evidence for a lack of free oxygen in the earth’s early atmosphere:
(i) Urananite and pyrite are readily oxidised today, but are found unoxidised in early Precambrian sediments.
(ii) There are no early Precambrian iron oxides (no red beds).
(iii) Banded iron formations appear in strati- graphic record in mid or late Precambrian (2,500-1,800 m.y.).
(iv) Evidence from mid or late Precambrian soils shows that oxygen was only about 2% of modern levels.
(v) Chemical building blocks of life (amino acids, DNA) could not have formed in presence of oxygen.
(vi) The simplest living organisms have an anaerobic metabolism. They are killed by oxygen which include some bacteria (such as botulism), some or all Archaebacteria which inhabit unusual conditions.
(c) Proterozoic (2,500-570 m.y. old):
The first autotrophic chlorophyllous alga (possibly blue green alga) was reported from Gunflint rock of Canada which dated back to 2,000 m.y. The best-known alga was Animikiea (Fig. 1.89B) (similar to modern blue-green alga, Oscillatoria) which was able to perform photosynthesis to liberate oxygen, eventually giving rise to ozone.
It is believed that the stratospheric ozonosphere was formed through this process. The existence of free oxygen was established by the occurrence of chemosynthetic bacterium known as Gunflintia (Fig. 1.89C) in the iron banded Gunflint rock. The putative eubacterial spores, namely Huroniospora (Fig. 1.89D) and Eostrion (Fig. 1.89E) were reported from Gunflint rock.
The ozone layer screened the lethal UV rays forming a situation conducive to the growth of other autotrophic primitive (Cyanobacteria) as well as 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 California (1,300 m.y.), Bitter Spring Formation of Australia (900-800 m.y.) and Svanbergfiellet shale of Spitsbergen island (800-700 m.y.).
Several cyanobacteria were described from Proterozoic age which include Palaeolyngbaea (Fig. 1.89F) (comparable to modern Lyngbaea), Cephalophytarion (Fig. 1.89G) (comparable to extant Microcoleus) and putative eukaryotes like Glenobotrydion (Fig. 1.89H) and Caryospheroides (Fig. 1.89I).
Stromatolities, the oldest known fossils, are rock-like structures built by photosynthesised cyanobacteria of more than 3,000 m.y. old. Those cyanobacteria thrived in warm aquatic environment and built reefs much the same way as coral does today. They were likely responsible for the creation of much of the planet’s free breathable oxygen, which allowed the development of oxygen-breathing organisms.
They were dominant life forms on the earth for over 2,000 million years. Stromatolites were thought to have been extinct until 40 years ago. However, with the discovery of modern stromatolites in Shark Bay, Australia, and elsewhere, it is now known that some living cyanobacteria are still growing in rocks on top of each other at a rate of 1 mm a year.
Essay # 2. Fossil Records of the Palaeozoic Era:
At the onset of the Proterozoic Epoch the earth’s climate was found to change rapidly resulting into the formation of diversified life forms (Fig. 1.90). Some of which invaded the land escaping from aquatic habit and initially being amphibian in nature.
For example, while acquiring land habit from water, the basal part of the Bryophytes (moss-like plants) 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 flattered to form rudimentary leaves and, finally, simple reproductive organs at maturity.
The Cambrian and the Ordovician periods were characterised by a diverse group of algae and bryophytes in the warm ocean and island seas. About 410 m.y. ago, during the Silurian period, a most dramatic event took place in the history of plant life i.e., evolution of land plants. By this time the early plants added oxygen which was 20% of modern level in the atmosphere through photosynthesis. Ozone (a product of oxygen) prevented UV rays from reaching the earth’s surface.
To acquire terrestrial habit leaving their aquatic habitat, the plants needed to be self-supporting and they had 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 some modifications, through the formation of vascular tissue (for conduction), epidermal cuticle (to check desiccation of water) and stomata (for gaseous exchange), which were visualised in the three early vascular plant groups, viz. Rhyniopsida (primitive land plants). Zosterophyllopsida (ancestor of microphylls) and Trimerophytopsida (ancestor of megaphylls).
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). Calamities (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.
Essay # 3. Fossil Records of Mesozoic Era:
Most of the Palaeozoic flora failed to survive and was largely replaced by the naked seeded gymnosperms. The gymnospermous plants mostly cycads, cycadeoids, conifers, ginkgos along with ferns reached their climax in the Jurassic period which constituted the world’s dominant vegetation co-existent with the giant reptiles known as Dinosaurs. Glossopterids were totally disappeared and sphenopsids and lycopsids were less conspicuous.
Some angiosperm- like leaves and tectate pollen were reported from the Jurassic period. Still later, in the end of Mesozoic era (i.e., in Cretaceous period) the closed seeded plants i.e., angiosperms got evolved which subsequently became dominant in Cenozoic Era replacing cycads, cycadeoids, conifers and ginkgos.
There was a rapid diversification of angiosperms in the Cretaceous period where ferns and conifers still continued and cycadeoids got extinct. The evidences of both monocot and dicot plants were established by the occurrence of leaves, flowers with closed carpels, perianths and stamens, tectate collumellate pollen, etc. The Mesozoic Era is called “The age of gymnosperms’ because of the diverse assemblage of cycads, ginkgos (maiden hair tree) and primitive conifers.
Essay # 4. Fossil Records of Cenozoic Era:
In the Cenozoic Era (65 m.y. to present day), the dominance of gymnosperms steadily declined in number and distribution, as in the meanwhile, the angiosperms diversified and occupied most of the land mass surface. Now the 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 now is considered to be 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, which include the world’s major food- yielding plants and other plants of great economic importance. Hence the Cenozoic Era is often known as “The age of angiosperms”.
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 herbivours 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 retrieve 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 others establishing a biostratigraphical scale.
For example, in the coal-bearing strata of the Middle Carboniferous of West Europe, seven successive vegetaional sequences were established using 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 to the plant groups.
Thus an analysis of fossils of a given stratigraphic zone provides the needed data to interpret its changing floral composition and consequent evolutionary sequence (Table 1.2):