Obliterating the traditional perception of the origins and evolution of life on Earth, fossils grant us unique snapshots of what once lived on our ever-changing planet!
The origin of life on Earth is irrevocably trapped in deep time. The epic, fluid and countless beginnings, evolutions and extinctions are immeasurable to humankind; our chronology is fractured, the picture is incomplete. For while the diversity of life on Earth today is awe-inspiring, with animals living within the most extreme environments imaginable – environments we as humans brave every day in a effort to chart and understand where life begins and ends – it is but only a fraction of the total life Earth has seen inhabit it over geological time.
Driven by the harsh realities of an ever-changing environment, Armageddon-level extinction events and the perpetual, ever-present force of natural selection, wondrous creatures with five eyes, fierce predators with 12-inch fangs and massive creatures twice the size of a double-decker bus have long since ceased to exist.
They’re forgotten, buried by not just millions, but billions of years. Still, all is not lost. By exploiting Earth’s natural processes and modern technology over the last two hundred years, scientists and palaeontologists have begun to unravel Earth’s tree of life and, through the discovery and excavation of fossils – preserved remains and traces of past life in Earth’s crust – piece the jigsaw back together.
The fossilization of an animal can occur in a variety of ways (see ‘Types of fossilization’) but, in general, it occurs when a recently deceased creature is rapidly buried by sediment or subsumed in an oxygen-deficient liquid. This has the effect of preserving parts of the creature – usually the harder, solid parts like its skeleton – often in the original, living form within the Earth’s crust.
The softer parts of fossilized creatures tend not to survive due to the speed of decay and their replacement by minerals contained in their sediment or liquid casing, a process that can leave casings and impressions of the animal that once lived, but not its remains. Importantly, however, creature fossilization tends to be specific to the environmental conditions in which it lived -and these in themselves are indicative of certain time periods in Earth’s geological history.
For example, certain species of trilobite (an extinct marine arthropod) are only found in certain rock strata (layers of sedimentary and igneous rocks formed through mineral deposition over millions of years), which itself is identifiable by its materials and mineralogic composition. This allows palaeontologists to extrapolate the environmental conditions (hot, cold, dry, wet, and soon) that the animal lived and died in and, in partnership with radiometric dating, assign a date to the fossil and/or the period.
Interestingly, however, by studying the strata and the contained fossils over multiple layers, through a mixture of this form of palaeontology and phylogenetics (the study of evolutionary relatedness between organism groups), scientists can chart the evolution of animals over geological time scales. A good example of this process is the now known transition of certain species of dinosaur into birds. Here, by dating and analysing specimens such as archaeopteryx (a famous dinosaur/bird transition fossil) both by strata and by radiometric methods, as well as recording their molecular and morphological data, scientists can then chart its progress through strata layers to the present day. In addition, by following the fossil record in this way, palaeontologists can also attribute the geophysical/chemical changes to the rise, fall or transition of any one animal/plant group, reading the sediment’s composition and structural data. For example, the Cretaceous-Tertiary extinction event is identified in sedimentary strata by a sharp decline in species’ diversity- notably non-avian dinosaurs – and increased calcium deposits from dead plants and plankton.
Excavating any discovered fossil in order to date and analyze it is a challenging, time-consuming process, which requires special tools and equipment. These include picks and shovels, trowels, whisks, hammers, dental drills and even explosives. There is also an accepted academic method all professional palaeontologists follow when preparing, removing and transporting any discovered fossil. First, the fossil is partially freed from the sedimentary matrix it is encased in and labeled, photographed and reported. Next, the overlying rock (commonly referred to as the ‘overburden’) is removed using large tools up to a distance of 6-9 centimetres (2-3 inches) from the fossil, before it is once again photographed. Then, depending on the stability of the fossil, it is coated with a thin glue via brush or aerosol in order to strengthen its structure, before being wrapped in a series of paper, bubble wrap and Hessian cloth. Finally, it is transported to the laboratory.
Types of Fossilization
A form of fossilization caused by compression within sedimentary rock. This type of fossilization occurs mainly where fine sediment is deposited frequently, such as along rivers. Many fossilized plants are formed this way.
Referred to as amber, fossil resin is a natural polymer excreted by trees and plants. As it is sticky and soft when produced, small invertebrates such as insects and spiders are often trapped and sealed within resin, preserving their form.
Bioimmuration is a type of fossil that in its formation subsumes another organism, leaving an impression of it within the fossil. This type of fossilization usually occurs between sessile skeletal organisms, such as oysters.
A process in which mineral deposits form internal casts of organisms, permineralization works when a deceased animal dies and then is rapidly submerged with groundwater. The water fills the creature’s lungs and empty spaces, before draining away leaving a mineral cast.
When a shelled creature’s shell, bone or tissue maintains its original form but is replaced with a crystal such as aragonite and calcite – then it is said to be re-crystallized.
A type of fossilization process similar to permineralization, molds occur when an animal is completely dissolved or destroyed, leaving only an organism-shaped hole in the rock. Molds can turn into casts if they are then filled with minerals.
Fossil record examples
By examining discovered fossil record examples, it is possible to piece together a rough history of the development of life on Earth over a geological timescale:
The most recent period in Earth’s history, the Quaternary is characterised by major changes in climate, as well as the evolution and dispersement of modern humans. Due to the rapid changes in environment and climate (ie, ice ages), many larger mammal fossils have been discovered, including those of mammoths and sabre-toothed cats.
2. NEOGENE 23.03-2.588 Ma
Covering 23 million years, the Neogene period’s fossils show a marked development in mammals and birds, with many hominid remains excavated. The extinct hominid australopithecus afarensis – a common ancestor of the genus homo (that of modern humans) – is one of the most notable fossil finds, as exemplified in the specimens Lucy and Selam.
3. PALEOGENE 65.5-23.03 Ma
The first period of the Cenozoic era, the Paleogen is notable for the rise of mammals as the dominant animal group on Earth, driven by the Cretaceous-Tertiary extinction event that wiped out the dinosaurs. The most important fossil to be discovered from this period is darwinius, a lemur-like creature uncovered from a shale quarry in Messel, Germany.
4. CRETACEOUS 145.5-65.5 Ma
Fossils discovered from the cretaceous indicate an explosion of insect diversification, with the first ants and grasshoppers evolving, as well as the dominance of large dinosaurs such as the colossal tyrannosaurus rex. Mammals increased in diversity but remained small and largely marsupial.
5. JURASSIC 199.6-145.5 Ma
The period in Earth’s history when the supercontinent Pangaea broke up in to the northern Laurasia and southern Gondwana, the Jurassic saw an explosion in marine and terrestrial life. The fossil record points to dinosaurs thriving, such as megalosaurus, an increase in large predatory fish like ichthyosaurus, as well as the evolution of the first birds-shown famously by the archaeopteryx fossil find.
6. TRIASSIC 250-200 Ma
Beginning and ending with an extinction event, the Triassic period’s fossils show the evolution of the first dinosaurs such as Coelophysis, a small carnivorous biped animal. Fossil evidence also shows the development of modem corals and reefs.
7. PERMIAN 299-251 Ma
A period characterised by the diversification of early amniotes (egg-bearing invertebrates) in to mammals, turtles, lepidosaurs and archosaurs, the Permian has yielded many diverse fossils. Notable examples include reptile therapsids, dragonflies and, driven by late warmer climates, lycopod trees.
8. CARBONIFEROUS 359.2-299 Ma
A period of sign ificantglaciation, the Carboniferous saw the development of ferns and conifers, bivalve molluscs and a wide-variety of basal tetrapods such as labyrinthodontia. Notable fossilized finds include the seed ferns pecopteris and neuropteris.
9. DEVONIAN 1416-359.2 Ma
An incredibly important time for the development of life, the Devonian period has relinquished fossils demonstrating the evolution of the pectoral and pelvic fins of fish into legs. The first land-based creatures, tetrapods and arthopods, become entrenched and seed-bearing plants spread across dry lands. A notable find is the genus tiktaalik.
10. SILURIAN 443.7-416 Ma
With its base set at major extinction event at the end of the Ordovician, the Silurian fossils found differ markedly from those that pre-date the period. Notable life developments include the first bony fish, and organisms with moveable jaws.
11. ORDOVICIAN 488.3-443.7 Ma
Boasting the highest sea levels on the Palaezoic era, the Ordovician saw the proliferation of planktonics, brachiopods and cephalopods. Nautiloids, suspension feeders, are among the largest creatures from this period to be discovered.
12. CAMBRIAN 542-488.3 Ma
The first geological period of the Paleozoic era, the Cambrian is unique in its high proportion of sedimentary layers and, consequently, adpression fossils. The Burgess Shale Formation, a notable fossil field dating from the Cambrian, has revealed many fossils including the genus opabinia, a five-eyed ocean crawler.