Presentation on theme: "Fossils and their preservation. Why are people fascinated with fossils ? A few reasons 1.Fossils provide our only tangible link with lifeforms of the."— Presentation transcript:
Fossils and their preservation
Why are people fascinated with fossils ? A few reasons 1.Fossils provide our only tangible link with lifeforms of the distant past and are our only direct means of envisioning and understanding the appearance and nature of Earth’s biosphere long before we (individually, and as a species) existed. 2.They remind us that all species (even the greatest and most successful organisms) are ephemeral. Consequently, they make us realize how insignificant we really are in the big picture. 3.Fossils are beautiful objects in their own right and are highly sought after by collectors of various types for various reasons. Usually the fossils that are preserved to the greatest extent (i.e. those that are least altered from their original condition) are those that carry the greatest value (aesthetic and monetary), though these are not necessarily of the highest value to science.
What exactly are fossils? “Fossil” (Latin prefix “Foss” refers to digging/excavation) in its most general sense refers to any preserved evidence of ancient biological organisms in the rock record that is obtained through digging/extraction from the host rock. Three general types: 1) Body fossils: These represent the actual physical remains of ancient organisms as preserved in the rock record. 2) Trace fossils: These represent the activities of ancient organisms as preserved in the rock record. Usually these reflect interactions between the organisms and the sediment in/on which they lived, crawled, fed etc. 3) Chemical fossils: Organisms may secrete chemical compounds (biomarkers), preservable in the rock record, which are unique to particular group and therefore may be used as evidence of the existence of that group in the corresponding ancient environment.
Why do fossils look the way they do ? Obviously, a fossil’s appearance will be primarily dictated by the shape and structure of the living organism which produced it. In body fossils this will relate most particularly to the characteristics of the preservable (hard) parts. For example, this coiled shell has a chambered interior that was used as a buoyancy control device by the animal that produced it. Fossil nautiloidModern Nautilus The appearance of a fossil, however, is also governed by processes which occurred after its maker died. It is important to realize that only a tiny proportion of the total number of living organism to have lived on the Earth have become fossils. Even organisms which have ‘high preservation potential’ are only scarcely represented in the fossil record compared to the total number of individuals of these species which existed in ancient ecosystems. Thus, fossils are typically also a representation of particular post-mortem conditions which were conducive to preservation.
Information loss in the fossil record After an organism dies, its tissues are commonly subjected to destructive processes occurring through several means, and at several scales. The average person can see these processes in action! For instance, take a close look at road kill next summer! At the largest scale, scavenging organisms descend upon the carcass to eat the choicest parts of the soft tissues. The carcass may remain largely intact or may be dismembered and scattered over a large area. Small scavengers (e.g. insect larvae; maggots) Large scavengers
Smaller scavengers (e.g. insect larvae) focus on removing the remainder of the soft tissues. At the smallest scale, bacteria break down the dead organic matter at the molecular level. Once denuded of all soft tissues, physical and chemical weathering processes will begin to break down the remaining hard, mineralized tissues (e.g. shells, bone, teeth). As chemical weathering is less of a factor in dryer climates, bones may remain intact for considerable lengths of time (decades, centuries?) in deserts. Corpse bacteria Physical and chemical Weathering, and erosion
Such remains (particularly cattle skulls) are commonly encountered in the American southwest, and have become icons of this area, strongly influencing native and non-native art of the region. Various skulls collected form the New Mexico desert were featured in several paintings by Georgia O’Keeffe. Cow’s Skull with Calico Roses (1932)Georgia O’Keeffe (1887-1986)
These, however, are not fossils! Their age (<500 years) and lack of burial (they haven’t been dug up!) generally rules them out as such. Although there is no absolute limit in terms of the age of remains considered to be fossils, it is generally assumed that the process of fossilization (usually accompanied by lithification of the sediment) requires, at minimum, thousands of years to complete. One might argue, however, that fossils can be formed on much shorter timescales through burial of organisms in volcanic ash (e.g. Pompeii) and through immersion in sticky hydrocarbons (e.g. the LaBrea Tar pits), and freezing (e.g. Siberian Wooly Mammoths). These modes of preservation are quite uncommon and therefore may be regarded to constitute “grey areas” at the periphery of what is normally considered to constitute fossilization.
As a general rule, hard parts have a greater chance of preservation in the fossil record than do soft tissues. This is because hard parts are more physically more robust, more chemically stable and are less prone to destruction via decay.
Major pre-burial processes affecting fossil preservation include: Soft Tissue Decay: Destruction of soft parts (e.g. muscle, skin, nerve tissue)- this also occurs in soft tissues (e.g. blood vessels) within hardparts such as bone. These processes will occur rapidly wherever sufficient oxygen is present for oxidizing aerobic bacteria to catalyze the decay process. Disarticulation: Dissociation of hard parts (e.g. limb bones). Fragmentation: Breakage and dissociation of fragments thus formed. Dissolution: Breakdown of hardparts via dissolution of minerals in hardparts. Abrasion: Erosion of hard tissues due to “sandblasting” effects of suspended sediment particles. Note: Some of these processes affect others. For example, decay of organic components of hard parts commonly produces acids and other corrosives which through dissolution, structurally weaken the hard parts, rendering them more prone to fragmentation and abrasion.
In some cases, the orientations of fossil remains can indicate aspects of the environment in which they were deposited. Current-aligned shells of the nautiloid Orthoceras (indicates unidirectional current) Convex-up bivalve shells with bidirectional orientation of umbos (bidirectional currents related to wave action)
Factors That Favour Fossilization Special conditions are therefore required to preserve dead tissues (most organisms have < 5 % chance of leaving any trace of their existence in the fossil record). Two of the most important factors that promote the preservation of remains are: 1.Rapid burial/entombment – This isolates remains from the work of scavengers and long-term physical disturbance. 2.Low oxygen – This also allows remains to be isolated from scavengers (most of which need oxygen to survive) and slows down bacterial decay. The precipitation of certain minerals that enhance preservation also tends to occur in oxygen-free environments. In many cases, rapid burial and low-oxygen conditions go hand-in-hand.
Hard Parts: Common mineral components Calcium carbonate CaCO 3 Principal mineral components of most seashells. Two common calcareous minerals in seashells: Aragonite – unstable over geological time. Aragonitic shells commonly are dissolved (preserved as moulds) or transform to calcite (poor preservation of primary textures). Calcite – stable over geological time. Consequently calcitic (e.g. brachiopod) shells tends to be well-preserved. Silica SiO 2 – Usually amorphous hydrated silica (Opal A) which commonly transforms to quartz and other silica minerals following death. Opal may be well preserved in pelagic sediments, if deeply buried. This is the principal “mineral” component of the skeletons of some sponges, and micro-organisms such as diatoms and radiolarians. Skeletons may be lost or degraded through opal dissolution and obliteration of the original amorphous structure through quartz crystallization. Calcium phosphate (apatite) Ca 3 (F.Cl.OH)(PO 4 ) 3 –stable over geological time (tends to be well- preserved). Principal mineral component of bones, teeth and some shells.
Post-Burial Processes: Modes of Preservation
Unaltered/Actual Remains Skeletal remains that are composed of stable minerals (e.g. calcite, calcium phosphate) can be preserved without significant change in chemical makeup or internal structure. Under rare circumstances, soft tissues can also be preserved without significant alteration.
These brachiopod shells are made of calcite. This is the same calcite that was present in the shells when the animals died. Even though these shells are about 375 million years old, they have undergone no significant change in their chemical/mineralogical and physical character. Unaltered/ Actual Remains Hardpart Preservation
Unaltered/ Actual Remains Hardpart Preservation Although less stable than calcite, aragonite is occasionally preserved (at least over shorter periods of geological time). This ammonite (the shell of a squid-like mollusc) preserves its original nacreous layer (mother of pearl), composed of sheets of aragonite crystals. These fossils are only of Late Cretaceous age (~70 million years) and therefore may still be within the window of common aragonite preservation, though other ammonites of this age show incipient, or more advanced stages of alteration to calcite. This suggests that these well-preserved aragonitic fossils represent depositional environments conducive to long-term aragonite stability. The oldest known aragonite in the rock record is of Late Palaeozoic age (~350 Ma).
Soft Tissue Preservation: Refrigeration 20,000 year old Siberian Wooly Mammoth preserving the original hair and flesh. The flesh is so well preserved that it is still edible, and has apparently been served to various people on several occasions in the last century. This is certainly a highly exceptional case of unaltered preservation! Unaltered/Actual Remains
Soft Tissue Preservation: Amber Entombment ant scorpion Unaltered/Actual Remains
Soft Tissue Preservation: Tar Immersion/Impregnation Beetle encased in solidified tar Unaltered/Actual Remains
Altered Remains More often than not, fossil remains are physically and/or chemically altered in some way. Four main types of alteration processes are 1.Recrystallization 2.Petrification/Permineralization 3.Replacement 4.Carbonization
Increased size of crystals Due to the coarser overall texture of the skeleton, finer textural details (e.g. ribs in this case) will be lost or obscured. With burial, and over time, the very fine crystals of the original skeleton tend to increase in size, and/or change to more stable crystal morphologies. Recrystallization Although some hard parts can be preserved with minimal change, most experience at least some degree of recrystallization after burial (crystals tend to increase in size due to the higher temperatures encountered below Earth’s surface).
Petrification/Permineralization Petrified wood Petrified dinosaur bone This occurs when mineral matter (carried in solution) in percolating ground waters precipitates in voids and pores in the remains of an organism. This does not necessarily involve replacement of the original skeletal material which may remain entombed in the new mineral matrix. Common petrifying minerals: silica (quartz, chalcedony), calcite.
In some cases, organic matter or minerals of an organism can be replaced by different mineral substances. This replacement occurs at a microscopic (molecule for molecule) level. Depending on the chemistry of pore waters within sediment, a number of minerals can replace the original material. These transformations may occur at earlier (before or during lithification), or later (after lithification) stages of fossilization. Calcareous (calcitic and aragonitic) shells are commonly replaced by silica minerals (silicon dioxide), pyrite (iron sulphide), or calcium phosphate minerals. Replacement
Replacement: Silicification (replacement by silica) Fossil calcareous sponge (originally calcitic, now siliceous) Fossil brachiopod (calcite replaced by microcrystalline quartz).
Replacement: Pyritization (replacement by pyrite) Original calcitic brachiopodPyritized brachiopod
Replacement: Phosphatization (replacement by calcium phosphate minerals) shark vertebrae (originally cartilage (organic), now phosphatic)
Replacement: Phosphatization (replacement by apatite) Phosphatized embryo of an arthropod (crustacean?) in an egg capsule over 500 million years old. As suggested by the previous example, replacement is not restricted to mineralized hard parts. In rare instances, relatively durable organic tissues can be replaced by minerals, thereby enhancing their preservation potential.
Carbonization Carbonized fern leaves and coal Carbonization refers to the process of carbon enrichment of organic-rich remains through their burial and heating. Organic remains, when buried to relatively shallow (few kms) depths, are lightly heated due to higher temperatures closer to the centre of the Earth. During this low-grade cooking, the volatile elements in the original organic molecules such as oxygen, hydrogen, and nitrogen are released as gasses, while carbon (non-volatile) is left behind. As a result, the remains are increasingly enriched in carbon. Coal is the typical product of this process. The characteristics (variety) of coal varies based upon the amount of cooking and degassing that has occurred.
Moulds and Casts
Moulds When remains are buried, they are surrounded with sediment. The impression that the buried object made in the surrounding sediment is called an external mould. (impression of shell exterior surface) If the buried object is hollow, it can also be infilled with sediment. The impression of the interior of the buried object is called an internal mould (also known as a “steinkern”). (impression of shell interior surface) In many cases, the actual buried object (in this case a shell) decays or is dissolved, leaving only internal and external moulds. External mould Internal mould Note: the original shell of this ammonite has completely dissolved away, but its former presence is indicated by external and internal moulds.
Moulds External mould Internal mould External mould Internal mould Clam Fossil clam Fossil snail Snail Fossil snail 2.Shell is dissolved 1. Sediment surrounding the shell and filling the shell cavity hardens 2.Shell is dissolved 1. Sediment surrounding the shell and filling the shell cavity hardens
Casts Cast of Tree Trunk Casts are formed when the void within an exterior mould is filled in by siliciclastic sediment or minerals precipitated from ground water. The product of this infilling (a cast) will consequently take the form of the original remains. In this case, the sediment surrounding a tree trunk hardened sufficiently to hold its shape after the tree trunk completely decayed. Sediment was later washed into the resulting hole and hardened, producing a natural cast. In this case, the cast is made entirely of sandstone (none of the original tree tissue has been preserved). Casts, primarily replicate only the shape of the original remains, though textural impressions of the exterior of the remains may preserved.
Effects of tectonic stress: The shape of fossils can become distorted due to differential pressure associated with tectonic activity. Such changes are normally observed in low grade regional metamorphic rocks (e.g. slates and low grade marbles). Post-burial processes
Biological Classification of Fossils As fossils clearly represent the the remains of ancient biological organisms, it only makes sense that they should also be classified in the same manner as living organisms. The fundamental unit of biological classification is the species. Members of a species are able to interbreed and give rise to fertile offspring. Palaeontologists, lacking evidence of reproductive isolation of ancient “species”, focus on morphological definitions of species. Above the species level are increasingly more inclusive groups which are defined by certain characteristics possessed by all their members. These various groupings are as follows: Kingdom: (e.g. Animalia) Phylum: (e.g. Chordata) Class: (e.g. Mammalia) Order: (e.g. Primates) Family: (e.g. Hominidae) Genus (e.g. Homo) Species: (e.g. Homo sapiens) This classification heirarchy applies mainly to body fossils. As you will see, trace fossils classification is more limited in this respect.
Thus far, we have concentrated on the physical remains of organisms. As these fossils represent remains of the actual body tissues of ancient organisms, we call them body fossils. As we already know, body fossils aren’t the only types of fossils that we have to work with. Palaeontologist also commonly concerned themselves with trace fossils. Trace fossils record the activities of ancient organisms. Whereas body fossils tell us things about the anatomy of ancient organisms, trace fossils provide evidence of their behaviour. Body Fossils vs. Trace Fossils
Identification of Trace Fossils Note: a single organism may produce multiple types of traces (e.g. trilobites produce resting, ploughing (feeding), and walking traces, all of which look very different). Similarly, different types of organisms may produce very similar traces which reflect similar activities. Consequently, identification of the trace maker is not easily established and is not the primary goal of trace fossil nomenclature and classification. resting ploughingwalking
Naming Trace fossils Trace fossils are named and classified under a different set of rules from those governing body fossils, though the hierarchy of classification still holds to the generic level and the species have binomial names consisting of the generic name, followed by the species name. Using the example of Homo sapiens (clearly not a trace fossil species!), Homo is the generic name (tells us the genus) and sapiens is the specific name (tells us the species of that particular genus). Trace fossils are also known as “Ichnofossils”; consequently, the prefix “ichno” is used in reference to the two categories of classificaton, i.e ichnogenus and ichnospecies. The names of trace fossils reflect the morphology or other key characteristics of the impressions themselves, and have no necessary relation to the organism(s) believed to have produced them.
Behaviours leading to the formation of different ichnogenera (as produced by the same organism)
Trace fossils that don’t fit the mould (no pun intended). Fossils which qualify as trace fossils based on the definition of the term, but which don’t correspond to the normal notion of trace fossils, and are not named or classified in the same manner, include the following.
Ancient stromatolitesModern stromatolites Trace fossils formed by daily sediment accretion and cementation on successive layers of bacterial mats. Some bacterial may be preserved within the layers. (composite body-trace fossils?) Stromatolites
Dinosaur coprolite Coprolites: Fossil excrement Fish coprolites Coprolites represent the solid end-products of digestion of food by ancient organisms. Contrary to the normal procedure for trace fossils, they are classified primarily on the basis of the organism interpreted to have produced them. These may be very revealing of the diet and food preferences of the producing organisms.