5 Taphonomy and Fossilization

Dr. Richa Rohatgi

Contents:

Introduction

Preservation of Fossils

Observations made from studies of fossils

Rocks and fossils

a. Igneous rocks.

b. Sedimentary rocks and their fossils.

c. Metamorphic rocks.

Introduction to Taphonomy

Taphonomic processes

Taphonomic grades

Methods used in Dating Fossils

Summary

Learning Objectives:

1.  To build a formative base on study of fossils

2.  To learn about the process of fossilization.

Introduction

This module deals with the study, interpretation and progression of decaying of organisms while becoming fossils under the branch of palaeo-anthropology. The word taphonomy is derived from Greek word ‘taphos’ meaning burial and ‘nomos’ meaning law. Paleontologists study fossils to determine age of rock beds and reconstruct ancient environments. Fossilization is a rare phenomenon. Chemical decomposition, erosion, scavengers, pressure and temperature changes are several processes that decrease the odds of fossilization occurring. The possession of hard parts, rapid deep burial, and protection from bacteria are conducive for fossilization. Fossil could be remains of a part of an organism, such as teeth, bones, or shells or complete skeleton itself. In the simplest form, “fossils are remains of traces of organisms that lived during ancient geological times and were buried in rocks that accumulated in the earth’s outer portion or crust.” This definition is complete in itself if we consider two augmentations in mind. First, the term “ancient geologic times” refers to all earth’s history from pre-historic times. Second, the word “rock” means any considerable deposit on earth’s crust that makes up from gravel beds, clay to ground ice, natural paraffin beds and asphalt. In other words, fossils are the remnants of prehistoric animals and plants that was preserved in rock or other materials. The remnant embedded in rock is called a fossil. As we talk about fossils being so rarely found, we need to understand the importance of preserving them for better learning of them. There can be numerous ways by which fossils can be preserved.

Picture 1: This wood has been replaced by silica, preserving cells, growth rings, wood rays, and distortion produced by disease. Colours are due to iron minerals and are not those of original wood (U.S. National Museum).

Picture 2: Mene rhobeus (Volta), a deep-bodied fish related to the modern pompano. Hundreds of petrified bones make uo this fossil, which shows the shape and structure of the fish and its strange hind, or pelvic, fins. They have moved far forward and have degenerated into bony spines (U.S. National Museum).

Picture 3: Impressions of a spice bush leaf (Lindera), showing its shape and its veins; the colour is due to mineral matter. Oligocene, Republic, Washington (U.S National Museum).

Preservation of Fossils

As we have understood that fossils can be anything from footprints to petrified bones to shells to frozen bodies to woods or desiccated skins. Their preservation is equally important to study the generative degradation over a time period. There have been different methods of preserving fossils which are as follows:

i.  Freezing: It is an ideal method to preserve dead bodies/ bones/ woods/ plants, etc. as it causes minimal change in dead remains. These remains are ideal but are rarely found.

ii.  Drying or desiccation: This is the second best method in quality preservation of dead remains. Cave-dwelling animals are mostly found fossilized in such a manner.

iii. Wax and Asphalt: The next in sequence is natural paraffin wax. It is in fact as good as ice preservation. Asphalt, however, preserves only the hard parts such as bones, teeth, horns and shells of insects.

iv. Simple burial: Plant remains often lie for long periods without much change. Postglacial peat contains cones, stems, pollen grains that accumulated in bogs. Logs buried in German lignite about 40,000,000 years old are discoloured and slightly decayed, but the texture and grain of their wood show little alterations.

v. Carbonization: It is a process of incomplete decay which gets rid of the volatile substances but leaves carbon behind. In extreme case, carbonisation causes plants and animals to become shiny black films thinner than tissue paper.

vi. Petrification: This method can take place in two probable ways. First is perminerilization when fat and other organic substances decay while water containing dissolved mineral matter soaks into every cavity and pore of hard and limy structures. This is the most common way of preserving marine and aquatic fossils, as were the bones and teeth of varied vertebrates. Second is replacement method that takes place when water dissolves the original hard parts and replaces them with mineral matter. This happens very slowly that new mineral matter duplicates microscopic structures of shells, corals, bones or wood. Sometimes, replacement may happen at a speed that no trace of original structure remains.

vii. Molds and Casts: Sometimes shells, stumps and other remains often lie in sediment until it becomes firm. Later the dead objects decay or dissolve leaving behind a cavity known as a natural mold. This may be filled with plaster of paris, wax, or some other compound to be collected as a cast or squeezed and studied.

viii.  Tracks, Trails and Burrows: The most famous fossil tracks are those left by dinosaurs that walked upon moist mud banks or plastic swamp bottoms which later solidified. Tracks of other reptiles as well as trails and footprints of some amphibians, birds, elephants and some stone age men have also been found.

ix. Coprolite: The ancient feces of vertebrates preserved by petrifaction or as molds or casts is called coprolites. Some show grooves made by the spiral valve found in sharks’ intestines.

Observations made from studies of fossils

Many petrified trees preserve woody cell walls, stems, roots and cones found in chert or limy coal balls. Palm like cycads still bear organs that served the purpose of flowers and plants that falsely resemble ferns contain seeds in pods at the tips of their leaves. Fossil spores are abundant in cannel coal, and some reveal nuclei that once were the centres of life in their gelatinous cells. A cactus some 50,000,000 years old bears both flowers and fruits. Carbonised marine invertebrates of vastly greater age show internal organs as well as bristles, scales, and other structures. Fossil vertebrates are often called perfect if they contain all important bones and if most of their teeth remain in jaws. Dinosaurs buried in fine-grained sandstone have impressions of skin around their bones.

Picture 4.4: Disintegration of biosphere zones and type of fossils most likely found in them.

Even bones devoid of any flesh may also reveal great deal of knowledge about soft anatomy. The key parts of any body are its muscles, for they give both shape and determine its movements. Muscles, in turn, are fastened to bones, leaving marks that indicate sizes, shapes, and functions of these varied organs. Brains and nerves determine intelligence and behaviour and their principal features can be determined from cavities and channels in skulls. Enlarged areas in brains means some animals hunted by sight though others relied on smell. The habits of invertebrates are sometimes shown by their shapes, relationships and positions in which they are found.

Fossils also tell a great deal about the surroundings and the conditions under which they lived. Trees grew on land but seaweeds inhabited salt water and so did corals, oysters, and squid like creatures. But snails, mussels, and fish belonging to groups now found only in fresh water may be assigned to that environment. Animals such as horses, camels lived on dry lands while animals like hippopotamus made their homes in swamps. Palms grew in warm regions while spruce thrives in colder regions. Ferns require a great deal of moisture but grasses and most cacti get along with much less.

Rocks and fossils

Rocks which vary greatly in hardness, also differs in origin. Origins, in turn, determine our chances of finding fossils in rocks.

a. Igneous rocks. Igneous means fiery; though rocks of this class did not really burn, all once were really hot. They include lavas that came to the surface in eruptions as well as related deposits that cooled and hardened underground. Most lavas were fluid when they came from cracks and volcanoes and flowed out upon the surface. Subterranean masses were much stiffer; before cooling they are called magna, a Greek word for ‘dough’. Upon cooling they form granite and similar rocks, which lack the bubbles and traces of flowage that are characteristic of lava. Fossils are very rare in lavas as nothing can live in great heat or far below the surfaces.

Lavas often are blown to pieces and shot into air, where they cool rapidly. Falling to the ground, they form deposits called agglomerate if the fragments are coarse, and tuff if they are very fine. Both bridge the gap between igneous and sedimentary rocks, since their particles settle upon the earth’s surface although they once were hot.

Agglomerate covers many fossil trees, some of which were dead logs at the time of burial. Redwood logs are excellently preserved and are found in light coloured tuff a few miles from Calistoga, California. Ash mixed with mud settled in lakes are found near Colorado, forming light grey shale. It contains enormous numbers of leaves, as well as eleven hundred species of butterflies, crickets, grass-hoppers, flies, beetles, and other insects.

b. Sedimentary rocks and their fossils. Though fossils are exceptional in igneous rocks, they are commonly found in sedimentary rock deposits, as sedimentary rocks form at temperatures and pressures that do not destroy fossil remains. Sediments literally means ‘something that settles’; sedimentary rocks consists of dusts, sand, mud, and other materials that settled under water or land. However, not all sedimentary rocks contain fossils. Fine-grained limestones may be equally barren, since they consists of material that once was dissolved in water. Shells and corals are rare in most marine sandstones- coral because they could not live amid sand; shells because they were destroyed by acids in beds of sand or by water seeping through the rock after it solidified.

Coarse sandstones may enclose bones of large dinosaurs, while fine grained beds containing clay and mica are often abound in tracks and burrows. Other fine-grained sandstones that settled on land are rich in fossil mammals.

c. Metamorphic rocks. These are typified by slate, true marble and contorted crystalline rocks often called granite, though the proper terms are gneiss and schist. Some began as sediments; others as igneous. All have been changed by heat, by steam from buried magmas or by pressure that bent and squeezed rocks into mountains. The process often went so far that we no longer can tell whether a given deposit began as magma, lava or sediment.

Intensely metamorphosed rocks contain no fossils. But stromatolites are abundant in some slightly changed marbles, and shells of various kinds that have been found in slate, which is mildly metamorphosed shale. Such fossils were flattened and squeezed sidewise or stretched as the rocks were forced into mountain ranges. Though such remains can be recognised, their shapes may be quite different from those creatures when alive.

Introduction to Taphonomy

Taphonomy is the study of the process of fossilization. Taphonomy is derived from greek word “taphos” meaning ‘death’ and “nomos” meaning ‘law’. Taphonomy is the study of the transition (in all its details) of animal remains from the biosphere into the lithosphere. In other words, taphonomy is the study of post-mortem processes through the action of environmental factors of the living organisms until its discovery as fossils. For any organism to be found in the museum display it has to pass through three stages namely: Necrology stage, Biostratinomy stage and Diagenesis stage. Therefore, taphonomy is a way to determine predisposition in the fossil remains and their preservation methods. It is documented that the relative abundance of species in a fossil assemblage may not be an accurate reflection of the relative abundances in the original assemblage of living populations.

Picture 4.5: Pictorial diagram of Taphonomy

Taphonomic processes

The taphonomic processes can be classified into three major categories of alteration and destruction namely, physical, chemical and biological processes. In the physical processes, mechanical breakdown of organic material via water and/or wind action storms are taken into consideration. However, in chemical processes, any alteration of a material’s mineralogy as well as any leaching of material by the surrounding water or air is taken into consideration. And finally, biological processes, such as sponge or algal borings, can help to alter and eventually destroy potential fossil materials. At any given situation all these three types may act in consortium. It is however, a difficult task to look for intensity of interactions of these processes and their effects on a fossil assemblage. Fossils can be concentrated either by physical processes such as storms and currents or by aggradations, where living organisms pile up on one another such as those found in oyster beds or coral reefs.

Taphonomic grades

The high tidal energy currents of rivers or beach waters may grind and abrade bones or shells so that they are unidentifiable after only a short period of time. Therefore, some sedimentary environments are better than others when it comes to preserving fossils. The quiet waters of swamps and lagoons, on the other hand, may permit the preservation of the delicate features of many hard parts. The taphonomic grade can vary from sample to sample. Taphonomic grades can be recognized and analyzed, and their variations can be used to interpret the ancient environment of deposition.

The challenge of the paleo-ecological approach is to reconstruct from the fossil register (in T1) the factors and conditions when the death of the individual (in T0) occurred. Our (palaeo) biological (including the taxonomy) and (paleo) ecological knowledge of the fossilised individuals, as well as that of the biocoenoses and the bionomy, are of fundamental importance in order to identify all the factors. In fact, by definition, the taphonomic transition is biospherical. There remains a major difficulty: how to define the moment at which the dead individual entered the fossil register, which could be interpreted as being different from its entrance into the lithosphere. When individual remains reach the historic layer of a sediment, they can be considered as being within the “lithosphere”, but we can interpret them by observing the thanatocenose remains located near and on the sediment surface. The taphonomic approach is close to that of forensic medicine as requires an integration of the complementary methods of the Life Sciences and of the Earth Sciences. This is the originality of taphonomy. The research of criteria and their analysis have to be made with the same rigor as the police to identify and analyse the causes of the death and environmental conditions in T0.

Methods used in Dating Fossils

There are two main methods of dating fossils namely, relative dating and absolute dating.

i. Relative dating is used to determine the fossils approximate age by comparing it to similar rocks and fossils of known ages.

ii. While, in Absolute dating, a precise age of a fossil is ascertained by using radiometric dating to measure the decay of isotopes, either within the fossil or more often the rocks associated with it.

iii. Another method of dating fossils is magnetism in rocks. This method can be used to estimate the age of a fossil site by using the orientation of the earth’s magnetic field, which has changed through time, to determine ages for fossils and rocks.

Although seemingly stable Earth (and Earth’s surface) has gone through dramatic changes over a period of 4.6 billion years. Many mountains have been built and many eroded, while many new continents and oceans have emerged. Earth’s surface that was once covered with glaciers is now rapidly melting due to increase in global warming. These changes are slow but never ceasing and happening at a slower pace, even today. Due to these changes, organisms have evolved, and remnants of some have been preserved as fossils.

A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved. Fossil’s age must be determined so as to be compared with those of other fossils from corresponding period. Simply by comparing primate species, experts can examine the changes that have taken place and how species have been evolved over a span of time. The age of each fossil primate needs to be known so that fossils of same age found at different parts of the world can be compared with.

Picture 7: The picture below depicts the principle of faunal succession to understand the relative age of rocks and fossils.”

“Fossils occur for a distinct, limited interval of time. In the figure 7 , the distinct age range for each fossil species is indicated by the grey arrows underlying the picture of each fossil. The position of the lower arrowhead indicates the first occurrence of the fossil and the upper arrowhead indicates its last occurrence – when it went extinct. Using the overlapping age ranges of multiple fossils, it is possible to determine the relative age of the fossil species (i.e., the relative interval of time during which that fossil species occurred). For example, there is a specific interval of time, indicated by the red box, during which both the blue ammonite and orange ammonite co-existed. If both the blue and orange ammonites are found together, the rock must have been deposited during the time interval indicated by the red box, which represents the time during which both fossil species co-existed. In this figure, the unknown fossil, a red sponge, occurs with five other fossils in fossil assemblage B. Fossil assemblage B includes the index fossils the orange ammonite and the blue ammonite, meaning that assemblage B must have been deposited during the interval of time indicated by the red box. Because, the unknown fossil, the red sponge, was found with the fossils in fossil assemblage B it also must have existed during the interval of time indicated by the red box. Paleo-anthropologists can measure the paleo-magnetism of rocks at a site to reveal its record of ancient magnetic reversals. Every reversal looks the same in the rock record, so other lines of evidence are needed to correlate the site to the Geomagnetic Polarity Time-Scale (GPTS). Information such as index fossils or radiometric dates can be used to correlate a particular paleomagnetic reversal to a known reversal in the GPTS. Once one reversal of the fossils or rocks has been related to the GPTS, the numerical age of the entire sequence can be determined.”

Summary

Preservation and collection of fossils have been the hardest part of the Paleo-anthropologists. Most investigations require the knowledge, skill and expertise of the expert/ geologist/ arccheologist/ anthropologist to study the remains and their fossil records. It is evident that soft tissues and bones always do not end up into becoming fossils whereas it is most often (if not always) the hard bones, teeth and shells that end up becoming fossilised. It is mostly due to destruction from outer forces of the environment, sand predators and sometimes due to biological decay and decomposition.

Using a variety of methods, Paleo-anthropologists are able to determine the age of geological materials to answer the question: “how old is the fossil?” Relative dating methods are used to describe a sequence of events. These methods rely on the principles of stratigraphy to place events recorded in rocks from oldest to youngest. Absolute dating methods determine how much time has passed since the rocks formed by measuring the radioactive decay of isotopes or the effects of radiation on the crystal structure of minerals. Paleo-magnetism measures the ancient orientation of the Earth’s magnetic field to help determine the age of rocks.

The simple sedimentological model is a surprisingly powerful predictor of post-mortem bias and ecological composition of fossil assemblages, suggesting that fossil-rich and fossil-poor strata are qualitatively different, both as repositories of palaeontological information and as settings for biotic interactions. Moreover, the apparent primary importance of rates of sedimentation in skeletal accumulation—despite emphasis usually placed on rates of hardpart input—suggests a new approach to inferring the detailed dynamics of sediment deposition and erosion in the formation of stratigraphical sequences.

References

1. Fenton, Carroll Lane, et al. The fossil book: a record of prehistoric life. Courier Corporation, 1989.
2. Brehrensmeyer, A. K. & Kidwell, S. M. Paleobiology 11, 105−119 (1985).
3. http://www.uno.edu/cos/earth-environmental-sciences/ees-docs/ClassResources/Lab6_Fossilization.pdf
4. https://en.wikipedia.org/wiki/Taphonomy
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