Paleobotany: An Overview

Paleobotany is a specialised branch of paleontology that focuses on the study and identification of plant remains found in geological contexts. This field aims to reconstruct past environments, understand the evolutionary history of plants, and gain insights into the broader evolution of life. Paleobotany encompasses the examination of terrestrial plant fossils as well as prehistoric marine photoautotrophs, such as photosynthetic algae, seaweeds, and kelp. A closely related discipline, palynology, studies fossilised and modern spores and pollen.

Understanding paleobotany is crucial for reconstructing ancient ecosystems and climates, a field known as paleoecology and paleoclimatology, respectively. It is also fundamental to the study of plant evolution. Additionally, paleobotany has become increasingly important in archaeology, particularly through the use of phytoliths for relative dating and in paleoethnobotany.

The fossil record reveals that true vascular plants first appeared during the Silurian Period of the Paleozoic era. Fragmentary fossils, including spores and cuticles, from the Ordovician Period found in Oman, are thought to originate from early liverworts or moss-like plants. An important early land plant fossil locality is the Rhynie Chert, located near the village of Rhynie in Scotland. This Early Devonian sinter deposit, primarily composed of silica, is notable for its preservation of various plant clades, from mosses and lycopods to more enigmatic forms. The Rhynie Chert also contains numerous fossil animals, such as arthropods and arachnids, offering a unique glimpse into early terrestrial life.

By the Late Devonian, plant macrofossils became more common, including tree trunks, fronds, and roots. The earliest tree known is Archaeopteris, which had simple, fern-like leaves arranged spirally on branches and a trunk resembling conifers. During the Carboniferous Period, widespread coal swamps across North America and Europe yielded a wealth of fossils, including towering arborescent lycopods, conifers, seed ferns, and numerous smaller herbaceous plants.

Angiosperms, or flowering plants, emerged during the Mesozoic era. The earliest evidence of flowering plants is found in Early Cretaceous deposits, approximately 130 million years ago. Plant fossils can range from ancient impressions millions of years old to charcoal remnants just a few hundred years old. Prehistoric plants are those that existed before recorded history, prior to around 3500 BC.

Types of plant fossils

Plant fossils come in various types. One common type is the compression fossil, where a leaf or other plant part is pressed between layers of sediment and preserved as a carbonaceous film. Fossil pollen and spores from ancient lake beds, as well as charcoal, are also common. Coal from Carboniferous swamps, though less common, is economically significant. Among the most remarkable plant fossils are petrified wood.

Plant fossils differ from animal fossils due to the distinct architecture of plants. Animals have specific, fixed body parts that persist through life, even in species that undergo metamorphosis. In contrast, plants continually produce new branches, leaves, and other parts throughout their lives, with parts often falling off without harming the plant. Consequently, plant fossils are frequently fragmentary, including leaves, branches, or pollen.

Because a leaf, stem, spore, or seed may be found without any physical connection to the original plant, paleobotanists use form taxa to name and classify such fossils. As more information becomes available, these form taxa may be merged with the true identity of the plant. For example, in the 1960s, fossil leaves previously classified as Archaeopteris (ancient ferns) were found attached to fossil wood from the tree Callixylon. The plant is now recognised as a Devonian tree with fern-like leaves and gymnosperm-like wood. Some form taxa remain in use even after the plant's true identity is known, primarily for convenience in identifying fossil parts, especially when a fossil might originate from multiple plant types. Leaves classified under the form taxon Sphenopteris may come from both ferns and seed plants; it is often impossible to determine the exact group from isolated fossils.

General fossil groupings

Some plants have remained relatively unchanged throughout geological time. Early ferns appeared by the Mississippian, and conifers by the Pennsylvanian. Some prehistoric plants still exist today as living fossils, such as Ginkgo biloba and Sciadopitys verticillata. Other plants have undergone significant changes or have become extinct. Examples of prehistoric plants include:

Araucaria

Archaeopteris

Calamites

Glossopteris

Hymenaea

Nelumbo

Protosalvinia

Palaeoraphe

Trochodendron

Dillhoffia

Peltandra