Competitive Exams Botany Notes: Paleobotany

Paleobotany, also spelled as palaeobotany (from the Greek words paleon = old and “botany” study of plants), is the branch of paleontology or paleobiology dealing with the recovery and identification of plant remains from geological contexts, and their use for the biological reconstruction of past environments, and the evolution of both the plant kingdom and life in general. A synonym is paleophytology. Paleobotany includes the study of terrestrial plant fossils, as well as the study of prehistoric marine photoautotrophs, such as photosynthetic algae, seaweeds or kelp. A closely-related field is palynology, which is the study of fossilized and extant spores and pollen.

Paleobotany is important in the reconstruction of ancient ecological systems and climate, known as paleoecology and paleoclimatology respectively; and is fundamental to the study of green plant development and evolution. Paleobotany has also become important to the field of archaeology, primarily for the use of phytoliths in relative dating and in paleoethnobotany.

Overview of the Paleobotanical Record

Macroscopic remains of true vascular plants are first found in the fossil record during the Silurian Period of the Paleozoic era. Some dispersed, fragmentary fossils of disputed affinity, primarily spores and cuticles, have been found in rocks from the Ordovician Period in Oman, and are thought to derive from liverwort-or moss-grade fossil plants (Wellman et al. 2003).

An important early land plant fossil locality is the Rhynie Chert, an Early Devonian sinter (hot spring) deposit composed primarily of silica found outside the town of Rhynie in Scotland. The Rhynie Chert is exceptional due to its preservation of several different clades of plants, from mosses and lycopods to more unusual, problematic forms. Many fossil animals, including arthropods and arachnids, are also found in the Rhynie Chert, and it offers a unique window on the history of early terrestrial life.

Plant-derived macrofossils become abundant in the Late Devonian and include tree trunks, fronds, and roots. The earliest tree is Archaeopteris, which bears simple, fern-like leaves spirally arranged on branches atop a conifer-like trunk (Meyer-Berthaud et al. 1999). Widespread coal swamp deposits across North America and Europe 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.

Angiosperms (flowering plants) evolved during the Mesozoic, and flowering plant pollen and leaves first appear during the Early Cretaceous, approximately 130 million years ago.

Palynology

Palynology is the science that studies contemporary and fossil palynomorphs, including pollen, spores, dinoflagellate cysts, acritarchs, chitinozoans and scolecodonts, together with particulate organic matter (POM) and kerogen found in sedimentary rocks and sediments. Palynology does not include diatoms, foraminiferans or other organisms with silicaceous or calcareous exoskeletons.

Palynology is an interdisciplinary science and is a branch of earth science (geology or geological science) and biological science (biology), particularly plant science (botany). Stratigraphical palynology is a branch of micropalaeontology and paleobotany which studies fossil palynomorphs from the Precambrian to the Holocene.

Methods of study

Palynomorphs are broadly defined as organic-walled microfossils between 5 and 500 micrometres in size. They are extracted from rocks and sediment cores both physically, by wet sieving, often after ultrasonic treatment, and chemically, by using chemical digestion to remove the non-organic fraction.

Chemical Preparation

Chemical digestion follows a number of steps. Initially the only chemical treatment used by researchers was treatment with KOH to remove humic substances; defloculation was accomplished through surface treatment or ultra-sonic treatment, although sonification may cause the pollen exine to rupture. The use of hydrofluoric acid (HF) to digest silicate minerals was introduced by Assarson and Granlund in 1924, greatly reducing the amount of time required to scan slides for palynomorphs.

Palynological studies using peats presented a particular challenge because of the presence of well preserved organic material including fine rootlets, moss leaflets and organic litter. This was the last major challenge in the chemical preparation of materials for palynological study. Acetolysis was developed by Gunnar Erdtman and his brother to remove these fine cellulose materials by dissolving them. In acetolysis the material is treated with acetic anhydride and sulfuric acid, dissolving cellulistic materials and providing better visibility for palynomorphs.

Some steps of the chemical treatments require special care for safety reason, in particular the use of HF which diffuses very fast through the skin and could cause severe chemical burns. Other treatment include kerosene flotation for chitinous materials.