23 Palaeobotany

N.N. Dogra and O.P. Thakur

Objectives

 

After reading this module the user would be able to know about the plant fossils, their modes of preservation and evolution of plant life through the geological past.

Introduction

Palaeobotany is the fusion of Plants and Earth sciences i.e., Botany and Geology that deals with the study of plant life of geologic past, and is mainly concerned with the fossil records preserved in the sedimentary rocks. These rocks are superimposed upon one another in sequence. Usually the older sediments remain at the bottom and the younger ones toward the top. These stratified sedimentary layers vary greatly in thickness, mineral composition, fossil content and architecture. Composition of fossil flora from one particular time interval of geological time scale vary from another in their assemblage, floral diversity and level of complexity due to the evolution of plant life through the history of planet earth.

 

Fossils are the relics of the past and are the evidences of prehistoric life of plants (studied under the Palaeobotany) or animals (studied under the Palaeontology). These are considered as direct or indirect evidences of past life. Direct evidences of life are in the form of body fossils which provide information about their morphology, their structures, ornamentation, internal cellular and subcellular structures and chronological information of the sediments (strata) wherein they are preserved. Indirect Fossil Evidences include organic signatures of past life, imprints, tracks and trails made by organisms, commonly known as ichnofossils or trace fossils and impressions.

 

Types of Fossilization

 

 

Compressions— compressions generally retain organic matter, usually more or less coalified, peat, lignite, and coal are essentially compressions of thick accumulations of plant debris.

 

Impressions— only the imprints of plant remains are preserved in the sedimentary rocks. Impression fossils are devoid of any organic matter. If the sediments wherein plants are buried upon are shales,clays or very fine-grained sandstones, impressions may faithfully replicate remarkable details of original external features, regardless of subsequent consolidation of the sediments.

 

Casts/Moulds— a cast results when sediment is deposited into cavities left by the decay of plant parts. A mould is essentially a cavity left in the sediment by the decayed plant tissue. Moulds are generally unfilled, or may be partially filled with sediment. Casts and molds commonly lack organic matter, but a resistant structure may be preserved as a compression on the outside of the cast or the inside of a mould. Casts and moulds may be found together with the cast filling the mould.

Petrifaction/Permineralizations— permineralization occurs when the plant tissues are infiltrated with minerals in cell lumens and intercellular spaces, thus preserving internal structures and anatomical details of the plant parts in three dimensions. This process is also known as petrification.

 

Trace fossils— any indirect evidence of past life records behavior and ecological constraints e.g., tracks, trails, burrows, coprolites (fossilized fecal matters), gastroliths (gizzard stones), which may help in understanding the size, living conditions, or habitat of biogenic entities. Trace fossils provide important information for the reconstruction of palaeoecology, palaeoenvironment and depositional history of any area.

 

Unaltered remains/Ambers— unaltered remains entombed in the amber are recovered from different parts of the world. They are preserved in resin/wax which is exuded from coniferous plants and fell on the forest floor often containing insects and plant parts. This type of preservation in the amber contains unoxidised life as such without any decay of the organism/part of life forms in the resinous material.

 

Record of plant fossils through the geological time scale usually comprise plant macrofossils (megafossils) which are large enough and can be studied with the naked eye and plant microfossils, which on the other hand, require the help of microscope for their study. Plant macrofossils usually include wood, leaves, seeds, or roots which are preserved in the sedimentary rocks as plant fossils. Microfossils are very small in size and commonly include pollen, spores, algal cysts, acritarchs, diatoms, radiolarians, nannofossils, and other microscopic plant bodies. Due to the wide and varied applications of plant microfossils in stratigraphy, biochronology and hydrocarbon source rock  evaluation, they are studied under a specialized branch of Palaeobotany which is known as palynology.

 

Palynology

 

Palynology is the study of organic walled microfossils preserved in the sedimentary rocks including pollen, spores, acritarchs, dinoflagellates, nannoplankton, diatoms, chitinozoans, scolecodonts, micro-algae, etc. recorded from the terrestrial, marine and various transitional environments. These palynomorphs have great significance in assigning ages to the sedimentary rocks because they are considered as very good index fossils or markers in biostratigraphy (in the classification of rocks). Palynofossils do possess an added advantage that due to their small size they are present in large amount/number in the rock/sediments samples and also they are documented almost in the entire range of Geological Time Scale i.e., from Precambrian to the Holocene.

 

Applications of Palynology

 

Palynology is one of the subdivisions of Palaeoboatany and it can be applied in the following fields of geology:

 

Biostratigraphy: Palynology is largely used in biostratigraphic studies for demarcation, zonation and correlation of rocks at local, regional, continental and intercontinental scale. In biostratigraphic characterization of strata, palynology helps in formulation of palynochronological sequences, thereby help in assigning age to each interval of a stratigraphic sequence through geologic time.

 

Palaeoecology: Palynology can be used to reconstruct palaeovegetational scenario in any part of the world and to understand palaeogeographic conditions in addition to understanding past environmental and palaeoclimatic conditions.

 

Petroleum Exploration: The main source material for hydrocarbon generation is organic debris that includes fossil spores and pollen, phytoplankton, marine and terrestrial algae as well as lipid-rich land plant remains. Direct application of these dispersed organic matters (also known as sedimentary organic matters) lies in identifying hydrocarbon source rock evaluation and potential of the sequences while estimating their types, quantity and maturity in the sedimentary rocks. There are number of  organic matter maturation scales for measuring the degree of thermal alteration index (TAI values) for delineating hydrocarbon source potential of sedimentary sequences.

 

Evolution of Plant Life through Geological Time

 

Studies on the evolution of plant life through ages reveal that early plants were small, unicellular or filamentous composed mostly of soft body tissues, with simple or no branching pattern. The oldest record of life on earth, as old as 3.2 billion years, comprise of sporomorphs of nostocacean cyanobacteria (cyanophytes) and cellulose microspheres of initial plant life on earth. Early life has evolved in the Precambrian comprising blue-green algae. There were no land plants with vascular tissues until the mid-Silurian time of Phanerozoic Eon. Remains of true vascular plants are first found in the rocks of Silurian Period of the Palaeozoic Era. Early Devonian plants did not have roots or leaves like the plants most common today, and many had no vascular tissue at all. They probably spread largely by vegetative growth, and did not grow much more than a few centimeters tall.

 

Rhynie Chert, an early land plant fossil locality of Scotland is an Early Devonian deposit composed primarily of silica having preservation of mosses and lycopods along with many other fossils. Plant-derived macrofossils became abundant in the Late Devonian and include tree trunks, fronds, and roots. The earliest tree was having simple, fern-like leaves spirally arranged on branches atop a conifer-like trunk. First seed-forming plants appeared by the end of the Devonian. This rapid appearance of so many plant groups and growth forms has been called the “Devonian Explosion”.

 

Widespread coal swamp deposits during the Carboniferous Period contain a wealth of fossils containing arborescent lycopods up to 30 meters tall, abundant seed plants, such as conifers and seed ferns, and countless smaller, herbaceous plants. The main Early Carboniferous plants were the Equisetales (Horse-tails). The Equisetales included the common giant form Calamites, with a trunk diameter of 30 to 60 cm and a height of up to 20 meters. The plant fossil assemblage prior to Late Carboniferous Period is known as Rhacopteris flora which is followed by Gondwana flora comprising Glossopteris flora in the Permian, followed by Dicroidium flora in the Triassic, and Ptillophylum flora during Jurassic to Cretaceous. This was followed by emergence and diversification of flowering plants on the earth from Middle Cretaceous in the Mesozoic. Cenozoic Era has recorded the explosion of  flowering plants and in the later part of the Cenozoic Era almost entire plant biodiversity of modern time had prevailed.

 

 

 

 

Selected Questions:

 

  1. Define Palaeobotany.
  2. Discuss the interrelationship of Palaeobotany with Botany and Geology.
  3. Comment on the evolution of plant life through ages.
  4. Discuss various processes of fossilization of plant life.
  5. Define Palynology and comment on the application of Palynology in the disciplines of botany and geology.