A fossil is defined as any remains or trace of a biological entity from the past. It is a very broad concept, which goes beyond the petrified bones of dinosaurs that we see in many museums. Fossils are also, for example, the shells and shells of prehistoric invertebrates, the preserved sap of an ancient tree —and the organisms it contains—, the preserved excrements or coprolites, and even the tracks of an animal passing through a mudflat, and that are preserved in the rock that formed later.
The fossil is the unit of available taphonomic information about living beings of the past, and provides information at many levels: anatomical, physiological or metabolic . It is also possible to infer data about behavior, ecology, climate and biogeography , and of course, fossils are a strong support for evolutionary information . The branch of paleontology that studies fossilization processes is called taphonomy —not to be confused with taxonomy, which studies the principles of classification of living beings—.
The immense variety of fossils that exist can be classified according to different criteria: according to their chemical composition, according to the type of substrate in which they are found, depending on whether they are organic remains or traces and marks; according to their diagenetic history, etc.
One of the most robust methods of classifying fossils is based on how they were formed , which directly influences the final result. And following this criterion, we have five main types of fossils.
charring
Before living things had the ability to break down the toughest plant structures, those that make up wood, there was nothing that could break down dead plants. They simply accumulated in the ground and were buried with their structure intact.
In an oxygen-deprived environment, tissues soften, flatten, collapse, and solidify, and the organic matter that makes them up turns to carbon .
When this process happens with microscopic algae, they accumulate in the substrate, forming shale, or exceptionally, they are preserved in the form of oil .
molds and positives
Fossil casts form when the organism acts or dies on a relatively malleable surface, which is rapidly entombed by new sediment. The marks left by the living being —or its own corpse— form a relief in the material, which is later filled with new material. Often the carcass, if it exists, decomposes, and eventually, when the sediment turns into rock, only the cast showing the surface of the body or the mark it left behind remains.
In this type of phenomenon, it is also common for the sediment that fills the mold to acquire its shape, thus forming a rock positive that coincides with the shape that the animal had in life.
Frequently, the body of the organism itself favors the foundation of the new material that will fill the fossil; when this happens, it is called an autogenous fossil .
Conservation of hard parts
Although they are by no means the most common fossils, they are probably the best known. If the conditions in which the organism dies are right, some hard parts, such as skeletons or shells, are preserved .
Sometimes, these hard parts are already mineralized, and do not undergo significant changes; other times, due to the chemical conditions of the environment, the minerals that make up these shells or skeletons are replaced by others.
This form of fossilization can coincide with the formation of molds, so it is not uncommon to find a shell of a mollusk and its mold in the opposite rock.
Cellular permineralization or petrifaction
Petrifaction or permineralization is surely the most complex form of fossilization, and it occurs under very specific conditions.
Initially, there is an infiltration of mineral-laden water into the tissues, which precipitates into the interior of the cells and into the spaces between them. The infiltrated mineral can be siliceous, forming chalcedony or microcrystalline quartz or calcareous type and more exceptionally, pyrites, limonites, etc. can also infiltrate.
A special type of permineralization would be cryopreservation. Soft tissues, which are hydrated, are quickly frozen and it is considered that there is a permeation of microcrystalline ice. The best example would be the frozen mammoths of the Pleistocene.
This type of fossil preserves very well the appearance of the soft parts of the organism. But there is still another kind of fossil that preserves organisms intact, almost unchanged.
amberization; a fossil within another fossil
Resins are polymers excreted by certain plants, as an evolutionary adaptation, which gives them protection against the attack of fungi, insects or other organisms. With relative frequency, the animals are trapped in the drops of resin or amber , which polymerizes and fossilizes, keeping the animal inside.
Although the most common to find in fossil amber are insects and arachnids, spores, seeds, bark fragments and other plant remains are also frequent. On rare occasions, pieces of larger animals can be found. Perhaps one of the most exceptional amber fossils is that of a fully feathered dinosaur tail , probably coelurosaur, from the mid-Cretaceous, described in 2016.
Fossils preserved in amber often preserve traces of the original materials and provide a wealth of scientific information.
References:
Fernández-López, SR 2000. Topics in Taphonomy. (p.167). Paleontology Department, Complutense University of Madrid.
Schopf, JM 1975. Modes of fossil preservation. Review of Palaeobotany and Palynology, 20(1), 27-53. DOI: 10.1016/0034-6667(75)90005-6
Xing, L. et al. 2016. A Feathered Dinosaur Tail with Primitive Plumage Trapped in Mid-Cretaceous Amber. Current Biology, 26(24), 3352-3360. DOI: 10.1016/j.cub.2016.10.008
