Paleontology Hand-out-1_240907_145224.pdf

1 PALEONTOLOGY 2 Paleontology is a rich field, imbued with a long and interesting past and an even more intriguing and hopeful 3 future. Many people think paleontology is the study of fossils. In fact, paleontology is much more. 4 Palaeo: A prefix co...

1 PALEONTOLOGY 2 Paleontology is a rich field, imbued with a long and interesting past and an even more intriguing and hopeful 3 future. Many people think paleontology is the study of fossils. In fact, paleontology is much more. 4 Palaeo: A prefix common in geological terminology, meaning ‘ancient, of past times’, and sometimes 5 suggesting an early primitive nature 6 Ontology : is the philosophical study of the nature of being, existence or reality as such, as well as the basic 7 categories of being and their relations. ontology deals with questions concerning what entities exist or can 8 be said to exist, and how such entities can be grouped, related within a hierarchy, and subdivided according 9 to similarities and differences. 10 Paleontology is the science which deals with studying life of past geologic ages (fossils). It is the study of 11 ancient animal life and how it developed. It is divided into two subdisciplines, invertebrate paleontology and 12 vertebrate paleontology. Paleontologists use two lines of evidence to learn about ancient animals. One is to 13 examine animals that live today, and the other is to study fossils. The study of modern animals includes 14 looking at the earliest stages of development and the way growth occurs (embryology), and comparing 15 different organisms to see how they are related evolutionarily (cladistics). The fossils that paleontologists 16 study may be the actual remains of the organisms, or simply traces the animals have left (tracks or burrows 17 left in fine sediments). Paleontology lies at the boundary of the life sciences and the earth sciences. It is thus 18 useful for dating sediments, reconstructing ancient environments, and testing models of plate tectonics, as 19 well as understanding how modern animals are related to one another. 20 An invertebrate is essentially a multicellular animal that lacks a spinal column encased in vertebrae and a 21 distinct skull. 22 History of paleontology 23 Although paleontology became established around 1800, earlier thinkers had noticed aspects of the fossil 24 record. The ancient Greek philosopher Xenophanes (570–480 BC) concluded from fossil sea shells that 25 some areas of land were once under water. During the Middle Ages the Persian naturalist Ibn Sina, known 26 as Avicenna in Europe, discussed fossils and proposed a theory of petrifying fluids on which Albert of 27 Saxony elaborated in the 14th century. The Chinese naturalist Shen Kuo (1031–1095) proposed a theory of 28 climate change based on the presence of petrified bamboo in regions that in his time were too dry for 29 bamboo. 30 In early modern Europe, the systematic study of fossils emerged as an integral part of the changes in natural 31 philosophy that occurred during the Age of Reason. In the Italian Renaissance, Leonardo Da Vinci made 32 various significant contributions to the field as well designed numerous fossils. At the end of the 18th century 33 Georges Cuvier's work established comparative anatomy as a scientific discipline and, by proving that some 34 fossil animals resembled no living ones, demonstrated that animals could become extinct, leading to the 35 emergence of paleontology. The expanding knowledge of the fossil record also played an increasing role in 36 the development of geology, particularly stratigraphy. 37 The first half of the 19th century saw geological and paleontological activity become increasingly well 38 organised with the growth of geologic societies and museums and an increasing number of professional 39 geologists and fossil specialists. Interest increased for reasons that were not purely scientific, as geology and 40 paleontology helped industrialists to find and exploit natural resources such as coal. 41 This contributed to a rapid increase in knowledge about the history of life on Earth and to progress in the 42 definition of the geologic time scale, largely based on fossil evidence. In 1822 Henri Marie Ducrotay de 43 Blanville, editor of Journal de Physique, coined the word "palaeontology" to refer to the study of ancient 44 living organisms through fossils. As knowledge of life's history continued to improve, it became increasingly 45 obvious that there had been some kind of successive order to the development of life. This encouraged early 46 evolutionary theories on the transmutation of species. After Charles Darwin published Origin of Species in 1 47 1859, much of the focus of paleontology shifted to understanding evolutionary paths, including human 48 evolution, and evolutionary theory. 49 50 Haikouichthys, from about 518million years ago in China, may be the earliest known fish. 51 52 The last half of the 19th century saw a tremendous expansion in paleontological activity, especially in North 53 America. The trend continued in the 20th century with additional regions of the Earth being opened to 54 systematic fossil collection. Fossils found in China near the end of the 20th century have been particularly 55 important as they have provided new information about the earliest evolution of animals, early fish, dinosaurs 56 and the evolution of birds. The last few decades of the 20th century saw a renewed interest in mass extinctions 57 and their role in the evolution of life on Earth. There was also a renewed interest in the Cambrian explosion 58 that apparently saw the development of the body plans of most animal phyla. The discovery of fossils of the 59 Ediacaran biota and developments in paleobiology extended knowledge about the history of life back far 60 before the Cambrian. 61 Increasing awareness of Gregor Mendel's pioneering work in genetics led first to the development of population 62 genetics and then in the mid-20th century to the modern evolutionary synthesis, which explains evolution as the 63 outcome of events such as mutations and horizontal gene transfer, which provide genetic variation, with genetic drift 64 and natural selection driving changes in this variation over time. Within the next few years the role and operation of 65 DNA in genetic inheritance were discovered, leading to what is now known as the "Central Dogma" of molecular 66 biology. In the 1960s molecular phylogenetics, the investigation of evolutionary "family trees" by techniques derived 67 from biochemistry, began to make an impact, particularly when it was proposed that the human lineage had diverged 68 from apes much more recently than was generally thought at the time. Although this early study compared proteins 69 from apes and humans, most molecular phylogenetics research is now based on comparisons of RNA and DNA. 70 Paleontology is traditionally divided into various sub disciplines: 71 Micropaleontology: Study of generally microscopic fossils, regardless of the group to which they belong. 72 Paleobotany: Study of fossil plants; traditionally includes the study of fossil algae and fungi in addition to 73 land plants. 74 Palynology: Study of pollen and spores, both living and fossil, produced by land plants and protists. 75 Invertebrate Paleontology: is the study of fossil animals that lack notochords (non-vertebrates). This 76 includes large, diverse taxonomic groups such as mollusks (e.g., bivalves and gastropods), brachiopods 77 (e.g., lamp shells), corals, arthropods (e.g., crabs, shrimps, and barnacles), echinoderms (e.g., sand dollars, 78 sea urchins, and sea stars), sponges, annelids (worms), foraminifera (single-celled protists), and bryozoans 79 (moss animals). These are all animals that, throughout most of Earth’s geological history, lived in a 80 multitude of habitats including marine, freshwater, and terrestrial. 81 Vertebrate Paleontology: Study of vertebrate fossils, from primitive fishes to mammals. 82 Human Paleontology (Paleoanthropology): The study of prehistoric human and proto-human fossils. 83 Taphonomy: Study of the processes of decay, preservation, and the formation of fossils in general. 84 Ichnology: Study of fossil tracks, trails, and footprints. 85 Paleoecology: Study of the ecology and climate of the past, as revealed both by fossils and by other 86 methods. 87 In short, paleontology is the study of what fossils tell us about the ecologies of the past, about evolution, and 88 about our place, as humans, in the world. Paleontology incorporates knowledge from biology, geology, 89 ecology, anthropology, archaeology, and even computer science to understand the processes that have led to 90 the origination and eventual destruction of the different types of organisms since life arose. 2  91 Paleontology is one of the historical sciences, along with archaeology, geology, astronomy, cosmology, 92 philology and history itself. This means that it aims to describe phenomena of the past and reconstruct their 93 causes. Hence it has three main elements: description of the phenomena; developing a general theory about 94 the causes of various types of change; and applying those theories to specific facts. 95 When trying to explain past phenomena, paleontologists and other historical scientists often construct a set 96 of hypotheses about the causes and then look for a smoking gun, a piece of evidence that indicates that one 97 hypothesis is a better explanation than others. Sometimes the smoking gun is discovered by a fortunate 98 accident during other research. For example, the discovery by Luis Alvarez and Walter Alvarez of an iridium- 99 rich layer at the Cretaceous–Tertiary boundary made asteroid impact and volcanism the most favored 100 explanations for the Cretaceous–Paleogene extinction event. 101 The other main type of science is experimental science, which is often said to work by conducting 102 experiments to disprove hypotheses about the workings and causes of natural phenomena – note that this 103 approach cannot confirm a hypothesis is correct, since some later experiment may disprove it. However, 104 when confronted with totally unexpected phenomena, such as the first evidence for invisible radiation, 105 experimental scientists often use the same approach as historical scientists: construct a set of hypotheses 106 about the causes and then look for a "smoking gun". 107 Paleontology lies on the boundary between biology and geology since paleontology focuses on the record of 108 past life but its main source of evidence is fossils, which are found in rocks. For historical reasons 109 paleontology is part of the geology departments of many universities, because in the 19th century and early 110 20th century geology departments found paleontological evidence important for estimating the ages of rocks 111 while biology departments showed little interest.Paleontology also has some overlap with archaeology, 112 which primarily works with objects made by humans and with human remains, while paleontologists are 113 interested in the characteristics and evolution of humans as organisms. When dealing with evidence about 114 humans, archaeologists and paleontologists may work together – for example paleontologists might identify 115 animal or plant fossils around an archaeological site, to discover what the people who lived there ate; or they 116 might analyze the climate at the time when the site was inhabited by humans. 117 In addition paleontology often uses techniques derived from other sciences, including biology, osteology, 118 ecology, chemistry, physics and mathematics. For example, geochemical signatures from rocks may help to 119 discover when life first arose on Earth, and analyses of carbon-isotope ratios may help to identify climate 120 changes and even to explain major transitions such as the Permian–Triassic extinction event. A relatively 121 recent discipline, molecular phylo-genetics, often helps by using comparisons of different modern organisms' 122 DNA and RNA to re-construct evolutionary "family trees"; it has also been used to estimate the dates of 123 important evolutionary developments, although this approach is controversial because of doubts about the 124 reliability of the "molecular clock". Techniques developed in engineering have been used to analyze how 125 ancient organisms might have worked, for example how fast Tyrannosaurus could move and how powerful 126 its bite was. It is relatively commonplace to study fossils using X-ray micro-tomography. A combination of 127 paleontology, biology, and archaeology, paleo-neurology is the study of endocranial casts (or endocasts) of 128 species related to humans to learn about the evolution of human brains. 129 Paleontology even contributes to astrobiology, the investigation of possible life on other planets, by 130 developing models of how life may have arisen and by providing techniques for detecting evidence of life. 131 Subdivisions 132 As knowledge has increased, paleontology has developed specialized subdivisions. Vertebrate paleontology 133 concentrates on fossils of vertebrates, from the earliest fish to the immediate ancestors of modern mammals. 134 Invertebrate paleontology deals with fossils of invertebrates such as molluscs, arthropods, annelid worms 135 and echinoderms. Paleobotany focuses on the study of fossil plants, but traditionally includes the study of 136 fossil algae and fungi. Palynology, the study of pollen and spores produced by land plants and protists, 3 137 straddles the border between paleontology and botany, as it deals with both living and fossil organisms. 138 Micropaleontology deals with all microscopic fossil organisms, regardless of the group to which they belong. 139 Instead of focusing on individual organisms, paleoecology examines the interactions between different 140 organisms, such as their places in food chains, and the two-way interaction between organisms and their 141 environment. One example is the development of oxygenic photosynthesis by bacteria, which hugely 142 increased the productivity and diversity of ecosystems. This also caused the oxygenation of the atmosphere. 143 Together, these were a prerequisite for the evolution of the most complex eukaryotic cells, from which all 144 multicellular organisms are built. 145 Paleoclimatology, although sometimes treated as part of paleoecology, focuses more on the history of Earth's 146 climate and the mechanisms that have changed it which have sometimes included evolutionary 147 developments, for example the rapid expansion of land plants in the Devonian period removed more carbon 148 dioxide from the atmosphere, reducing the greenhouse effect and thus helping to cause an ice age in the 149 Carboniferous period. Biostratigraphy, the use of fossils to work out the chronological order in which rocks 150 were formed, is useful to both paleontologists and geologists. Biogeography studies the spatial distribution 151 of organisms, and is also linked to geology, which explains how Earth's geography has changed over time. 152 Sources of evidence 153 Fossils: are remains or traces of organisms (animals and plants), which inhabited the globe since the 154 beginning of life. 155 Kinds of fossils: 156 Real fossils: are the remnant of an extinct plant or animal. 157  Range fossils: are those having long range and so can’t be used as time indicators. 158  Index (guide) fossils: are fossils which are characterized by wide geographic distribution, short 159 range, should be common, readily preserved, easily recognizable, spread rapidly and they should 160 have evolved rapidly so that individual species existed during only a short interval of time. They are 161 useful in designating the age of strata. 162 Derived (drifted) fossil: are fossils that are washed out from the original beds and re-deposited in younger 163 strata. 164 Example: Cretaceous and Eocene fossils deposited in the Miocene basins of the Gulf of Suez. 165 Trace fossils. Unlike body fossils, where a portion of the actual organism or its skeleton is preserved, trace fossils 166 are the remains of an organism's activity or behavior. Examples include tracks, trails, burrows, and borings. 167 Pseudo-fossils: are those covered by sediments in recent times and make the impression only of being 168 fossils. 169 Body fossils 170 Fossils of organisms' bodies are usually the most informative type of evidence. The most common types are 171 wood, bones, and shells. Fossilization is a rare event, and most fossils are destroyed by erosion or 172 metamorphism before they can be observed. Hence the fossil record is very incomplete, increasingly so 173 further back in time. Despite this, it is often adequate to illustrate the broader patterns of life's history. There 4 174 are also biases in the fossil record: different environments are more favorable to the preservation of different 175 types of organism or parts of organisms. Further, only the parts of organisms that were already mineralized 176 are usually preserved, such as the shells of molluscs. Since most animal species are soft-bodied, they decay 177 before they can become fossilized. As a result, although there are 30-plus phyla of living animals, two-thirds 178 have never been found as fossils. 179 Occasionally, unusual environments may preserve soft tissues. These lagerstätten allow paleontologists to 180 examine the internal anatomy of animals that in other sediments are represented only by shells, spines, claws, 181 etc. – if they are preserved at all. However, even lagerstätten present an incomplete picture of life at the time. 182 The majority of organisms living at the time are probably not represented because lagerstätten are restricted 183 to a narrow range of environments, e.g. where soft-bodied organisms can be preserved very quickly by events 184 such as mudslides; and the exceptional events that cause quick burial make it difficult to study the normal 185 environments of the animals. The sparseness of the fossil record means that organisms are expected to exist 186 long before and after they are found in the fossil record – this is known as the Signor-Lipps effect. 187 Trace fossils 188 Trace fossils consist mainly of tracks and burrows, but also include coprolites (fossil feces) and marks left 189 by feeding. Trace fossils are particularly significant because they represent a data source that is not limited 190 to animals with easily fossilized hard parts, and they reflect organisms' behaviours. Also many traces date 191 from significantly earlier than the body fossils of animals that are thought to have been capable of making 192 them. Whilst exact assignment of trace fossils to their makers is generally impossible, traces may for example 193 provide the earliest physical evidence of the appearance of moderately complex animals (comparable to 194 earthworms). 195 Geochemical observations 196 Geochemical observations may help to deduce the global level of biological activity, or the affinity of certain 197 fossils. For example, geochemical features of rocks may reveal when life first arose on Earth, and may 198 provide evidence of the presence of eukaryotic cells, the type from which all multicellular organisms are 199 built. Analyses of carbon-isotope ratios may help to explain major transitions such as the Permian–Triassic 200 extinction event. 201 Nature of fossil record: 202 All fossils should occur in sedimentary rocks being abundant in limestone and limy shale but rare in sandstone. Fossils 203 never occur in igneous rocks except when volcanic ash falls or nearly cooled lava have overcome plants and animals. 204 In metamorphic rocks they are also absent except when these rocks were originally fossiliferous and subjected to very 205 low grades of metamorphism. In Nature fossils are found scattered in the rocks, in some cases they are accumulated in 206 layers or patches. Those accumulated in layers or beds are called Biostroms whereas those accumulated without any 207 distinctive layering are called Bioherms. Fossils are one of the most important sources of information about the Earth's 208 past. They can tell us the age of the rocks in which they are found, what the environment was like when the fossilised 209 organisms were alive, and even how the organisms functioned. They can also tell us about Earth movements, such as 210 mountain building, about the former positions of continents (ancient geography), and about the evolution of life on 211 Earth. Some of these uses for fossils are of economic importance, assisting in the search for oil and minerals. 212 Fossils as age indicators 213 Fossils are the most important means of dating sedimentary rock sequences. However, they do not provide an absolute 214 age measured in years, but rather a relative age expressed in terms of the relative geological time scale. The use of 5 215 fossils in this way relies on the fact that individual species evolved into others through time, so that if the time range 216 of a species is known in one particular region, the occurrence of the same species in another region indicates that the 217 rocks there are of the same age. This process of establishing the equivalence in age of two rock sequences in different 218 areas is called correlation. 219 Not all fossils are of equal value in dating rocks; the most useful are called index or zone fossils. Ideally, index fossils 220 should be common, readily preserved and easily recognisable. They should have spread rapidly and widely, and for 221 accuracy of dating, they should have evolved rapidly so that individual species existed during only a short interval of 222 time. Very few index fossils meet all of these criteria. Amongst the most important index fossils are graptolites, 223 ammonites, foraminifera, pollen, conodonts and trilobites. 224 One of the most important groups of index fossils in the Palaeozoic rocks of Victoria is the graptolites. These were 225 extinct marine animals that formed twig-like colonies composed of one or more branches. The colonies were originally 226 three dimensional but usually became completely flattened during fossilisation, though they are still easily 227 recognisable. Some graptolite colonies may have been attached to the sea floor, but most floated freely in the sea. They 228 are of most use in dating rocks ranging in age from Early Ordovician to Early Devonian. 229 The Ordovician rocks of central and eastern Victoria have one of the richest and most diverse graptolite faunas in the 230 world. They have been used to subdivide the rock sequences into 30 intervals, and to correlate these intervals accurately 231 with other sequences in New Zealand, Asia, Europe and North America. 232 233 Fossils as environmental indicators 234 Because fossils are the remains of once living organisms that were adapted to their environments, they can provide 235 valuable information about what past environments were like. We can predict the environmental requirements of 236 organisms in the past from those of closely related organisms living in the present day. Such predictions will be most 237 reliable in the case of younger rocks which contain fossils having representatives alive today. As we go further back 238 in geological time, the predictions become less reliable because we encounter fossils of extinct groups about whose 239 environmental requirements nothing is directly known. 240 The environmental information obtained from fossils may be as simple as whether the rocks in which they occur were 241 deposited in the sea, in a brackish water estuary, in fresh water, or on the land. For example, rocks containing fossils 242 of corals, brachiopods, cephalopods or echinoderms must have been deposited in the sea because living representatives 243 of those groups are found only in the sea today; and fossils of land-dwelling animals such as kangaroos indicate 244 deposition on land or in an adjacent body of fresh water. 245 Fossils of reef-building corals indicate that the rocks in which they occur were deposited in warm, shallow seas 246 because, at the present day, reef-forming corals are found in tropical seas and only at depths of less that 200 m where 247 sunlight can penetrate the water to reach the photosynthesising algae within their cells. 248 The Koonwarra fossil bed of South Gippsland provides a good example of the use of fossils in reconstructing an ancient 249 environment. This fossil bed contains fossilised fish, plants, insects, crustaceans, spiders, bird feathers and a horseshoe 250 crab. There are also bryozoans and a mussel. These fossils tell us that the deposit was formed in the shallow part of a 251 large freshwater lake because the insects include mayflies that are similar to forms living today in cool mountain 252 streams and lakes in Tasmania. The lake may have been frozen in winter because the mass occurrence of fish fossils 253 show no signs of rotting. This conclusion is supported to some extent by the occurrence of a beetle that is similar to a 254 modern species found only in alpine areas. The occurrence of fleas in the fossil fauna suggests that mammals may have 255 been present on the adjacent land, and the occurrence of feathers shows that birds were also present. The small size of 6 256 the fish suggests that they were juveniles or small adults, which inhabit shallow areas in modern bodies of fresh water. 257 The insects are well preserved, even those that were not aquatic, suggesting that they were not transported great 258 distances after death, so that the fossil deposits must have been formed close to the edge of the body of water. 259 260 Fossils as indicators of Earth movements 261 The occurrence of fossils at a particular locality may provide evidence that there has been some movement of the 262 Earth's crust since the fossils were deposited. The movement may have been only slight uplift of the land, as indicated, 263 for example, by the occurrence of fossils of marine shells in cliffs around Port Phillip Bay. Alternatively, the uplift 264 may have been on a much larger scale, as indicated by the occurrence of marine fossils far from present-day oceans 265 and even in the middle of continents, or on high mountains, such as the Himalayas or the European Alps. Movement 266 of the Earth's crust along faults or fractures may be indicated, even if the fracture itself is not evident, by the occurrence 267 of fossils of very different ages at adjacent localities. For example, the Whitelaw Fault on the eastern outskirts of 268 Bendigo is not marked by any obvious landform, but its presence is indicated by the occurrence of graptolites of Middle 269 Ordovician age (about 470 million years old) on one side of the fault, in close proximity to Early Ordovician graptolites 270 (about 493 million years old) on the other side. 271 Fossils as indicators of ancient geography 272 As long ago as the middle of the eighteenth century, it became apparent to some palaeontologists that there were 273 sometimes striking similarities in the assemblages of fossils found in rocks of the same age in widely separated 274 continents. The similarities could not be satisfactorily explained by the migration of organisms across vast expanses 275 of ocean, because the fossils belonged to forms that lived only in shallow marine environments, in fresh water, or even 276 on dry land. A few scientists suggested that these similarities were due to the fact that the continents were once joined 277 together and later split apart, but this suggestion was rejected by most geologists because at that time there was no 278 known mechanism by which the continents could move. The favoured explanation then was that organisms had 279 migrated across 'land bridges', which had connected the continents in ancient times but which had later subsided to 280 form part of the present-day ocean floor. We now know that this could not have occurred, because the Earth's crust on 281 the floor of the oceans differs in composition from that of the continents. With the development of the theory of plate 282 tectonics in the 1960s, leading to the widespread acceptance of continental drift, the similarities in the fossil faunas in 283 different continents could be readily explained by the drifting apart of land masses that formerly lay together. 284 One example of the fossil evidence that the continents were connected in the past is the distribution of the ancient seed- 285 fern Glossopteris and related plants. The fossils of these plants are associated with coal deposits of Permian age in 286 India, Australia, South Africa, South America and Antarctica. The rock sequences in which these coal deposits occur 287 are remarkably similar on all of these continents. The distribution of these plants cannot be explained by wind dispersal 288 of their seeds, as these are too large to have been carried across the ocean. A further line of evidence is the distribution 289 of the reptile Mesosaurus, which is found in Brazil and South Africa at or near the Carboniferous-Permian boundary. 290 Mesosaurus lived in fresh or perhaps brackish water habitats, so it is difficult to imagine that it could have found its 291 way across an ocean as broad as the present day Atlantic. 292 Fossils as evidence for the evolution of life 293 Fossils are the main sources of information on the evolution of life on Earth. Without the information they provide, we 294 would have no knowledge of extinct organisms such as trilobites and dinosaurs, and our knowledge of the history of 295 the development and evolutionary relationships of the modern flora and fauna could be derived only from the living 296 organisms themselves. We would also have no direct knowledge of the timing of critical biological events, such as the 7 297 origin of life, the development of shells or skeletons, the colonisation of the land, the appearance of mammals and 298 flowering plants, the development of flight, and major episodes of extinction. 299 The role that fossils have played in deciphering relationships among organisms can be demonstrated by the evolution 300 of horses, the family of mammals with probably the best fossil record. The development of the modern horse from its 301 oldest known ancestors can be traced via a number of morphological changes, including body size, shape of teeth, and 302 the structure of the feet. These morphological changes reflect changes in habitat and feeding, from browsing on soft 303 leaves in forests to grazing on hard grasses on open plains. The oldest known horse, Hyracotherium, lived during the 304 early Eocene (about 50 million years ago). It was a dog-sized creature with short-crowned teeth, and with four toes on 305 the front feet and three toes on the back feet, each toe having a small hoof. In descendants of Hyracotherium, there was 306 a progressive increase in body size, to the size of the modern horse. The teeth developed long crowns with complex 307 enamel ridges for grinding hard grasses, and the number of toes was progressively reduced to one on both front and 308 hind feet. 309 Estimating the dates of organisms 310 Paleontology seeks to map out how living things have changed through time. A substantial hurdle to this aim is the 311 difficulty of working out how old fossils are. Beds that preserve fossils typically lack the radioactive elements needed 312 for radiometric-dating. This technique is our only means of giving rocks greater than about 50 million years old an 313 absolute age, and can be accurate to within 0.5% or better. Although radiometric dating requires very careful laboratory 314 work, its basic principle is simple: the rates at which various radioactive elements decay are known, and so the ratio 315 of the radioactive element to the element into which it decays shows how long ago the radioactive element was 316 incorporated into the rock. Radioactive elements are common only in rocks with a volcanic origin, and so the only 317 fossil-bearing rocks that can be dated radiometrically are a few volcanic ash layers. 318 Consequently, paleontologists must usually rely on stratigraphy to date fossils. Stratigraphy is the science of 319 deciphering the "layer-cake" that is the sedimentary record, and has been compared to a jigsaw puzzle. Rocks normally 320 form relatively horizontal layers, with each layer younger than the one underneath it. If a fossil is found between two 321 layers whose ages are known, the fossil's age must lie between the two known ages. Because rock sequences are not 322 continuous, but may be broken up by faults or periods of erosion, it is very difficult to match up rock beds that are not 323 directly next to one another. However, fossils of species that survived for a relatively short time can be used to link up 324 isolated rocks: this technique is called biostratigraphy. For instance, the conodont Eoplacognathus pseudoplanus has 325 a short range in the Middle Ordovician period. If rocks of unknown age are found to have traces of E. pseudoplanus, 326 they must have a mid-Ordovician age. Such index fossils must be distinctive, be globally distributed and have a short 327 time range to be useful. However, misleading results are produced if the index fossils turn out to have longer fossil 328 ranges than first thought. Stratigraphy and biostratigraphy can in general provide only relative dating (A was before 329 B), which is often sufficient for studying evolution. However, this is difficult for some time periods, because of the 330 problems involved in matching up rocks of the same age across different continents. 331 Family-tree relationships may also help to narrow down the date when lineages first appeared. For instance, if fossils 332 of B or C date to X million years ago and the calculated "family tree" says A was an ancestor of B and C, then A must 333 have evolved more than X million years ago. 334 It is also possible to estimate how long ago two living clades diverged – i.e. approximately how long ago their last 335 common ancestor must have lived – by assuming that DNA mutations accumulate at a constant rate. These "molecular 336 clocks", however, are fallible, and provide only a very approximate timing: for example, they are not sufficiently 337 precise and reliable for estimating when the groups that feature in the Cambrian explosion first evolved, and estimates 338 produced by different techniques may vary by a factor of two. 8 339 Overview of the history of life 340 The evolutionary history of life stretches back to over 3,000million years ago, possibly as far as 3,800million 341 years ago. Earth formed about 4,570million years ago and, after a collision that formed the Moon about 40 342 million years later, may have cooled quickly enough to have oceans and an atmosphere about 4,440million 343 years ago. However, there is evidence on the Moon of a Late Heavy Bombardment from 344 4,000 to 3,800million years ago. If, as seems likely, such a bombardment struck Earth at the same time, the 345 first atmosphere and oceans may have been stripped away. The oldest clear evidence of life on Earth dates 346 to 3,000million years ago, although there have been reports, often disputed, of fossilbacteria from 347 3,400million years ago and of geochemical evidence for the presence of life 3,800million years ago. Some 348 scientists have proposed that life on Earth was "seeded" from elsewhere, but most research concentrates on 349 various explanations of how life could have arisen independently on Earth. 350 351 For about 2,000 million years microbial mats, multi-layered colonies of different types of bacteria, were the 352 dominant life on Earth. The evolution of oxygenic photosynthesis enabled them to play the major role in the 353 oxygenation of the atmosphere from about 2,400million years ago. This change in the atmosphere increased 354 their effectiveness as nurseries of evolution. While eukaryotes, cells with complex internal structures, may 355 have been present earlier, their evolution speeded up when they acquired the ability to transform oxygen 356 from a poison to a powerful source of energy in their metabolism. This innovation may have come from 357 primitive eukaryotes capturing oxygen-powered bacteria as endosymbionts and transforming them into 358 organelles called mitochondria. The earliest evidence of complex eukaryotes with organelles such as 359 mitochondria, dates from 1,850million years ago. 360 Multicellular life is composed only of eukaryotic cells, and the earliest evidence for it is the Francevillian 361 Group Fossils from 2,100million years ago, although specialisation of cells for different functions first 362 appears between 1,430million years ago (a possible fungus) and 1,200million years ago (a probable red alga). 363 Sexual reproduction may be a prerequisite for specialisation of cells, as an asexual multicellular organism 364 might be at risk of being taken over by rogue cells that retain the ability to reproduce. 365 366 367 368 This wrinkled "elephant skin" texture is a trace fossil of a non-stromatolitemicrobial mat. The image 369 shows the location, in the Burgsvik beds of Sweden, where the texture was first identified as evidence of a 370 microbial mat. 371 9 372 373 Opabinia made the largest single contribution to modern interest in the Cambrian explosion. 374 375 The earliest known animals are cnidarians from about 580million years ago, but these are so modern-looking 376 that the earliest animals must have appeared before then. Early fossils of animals are rare because they did 377 not develop mineralised hard parts that fossilize easily until about 548million years ago. The earliest modern- 378 looking bilaterian animals appear in the Early Cambrian, along with several "weird wonders" that bear little 379 obvious resemblance to any modern animals. There is a long-running debate about whether this Cambrian 380 explosion was truly a very rapid period of evolutionary experimentation; alternative views are that modern- 381 looking animals began evolving earlier but fossils of their precursors have not yet been found, or that the 382 "weird wonders" are evolutionary "aunts" and "cousins" of modern groups. Vertebrates remained an obscure 383 group until the first fish with jaws appeared in the Late Ordovician. 384 The spread of life from water to land required organisms to solve several problems, including protection 385 against drying out and supporting themselves against gravity. The earliest evidence of land plants and land 386 invertebrates date back to about 476million years ago and 490million years ago respectively. The lineage 387 that produced land vertebrates evolved later but very rapidly between 370million years ago and 360million 388 years ago; recent discoveries have overturned earlier ideas about the history and driving forces behind their 389 evolution. Land plants were so successful that they caused an ecological crisis in the Late Devonian, until 390 the evolution and spread of fungi that could digest dead wood. 391 392 At about 13 centimetres (5.1 in) the Early Cretaceous Yanoconodon was longer than the average 393 mammal of the time. 394 10 395 396 Birds are the last surviving dinosaurs. 397 During the Permian period synapsids, including the ancestors of mammals, may have dominated land 398 environments, but the Permian–Triassic extinction event251million years ago came very close to wiping out 399 complex life. The extinctions were apparently fairly sudden, at least among vertebrates. During the slow 400 recovery from this catastrophe a previously obscure group, archosaurs, became the most abundant and 401 diverse terrestrial vertebrates. One archosaur group, the dinosaurs, were the dominant land vertebrates for 402 the rest of the Mesozoic, and birds evolved from one group of dinosaurs. During this time mammals' 403 ancestors survived only as small, mainly nocturnal insectivores, but this apparent set-back may have 404 accelerated the development of mammalian traits such as endothermy and hair. After the Cretaceous– 405 Paleogene extinction event65million years ago killed off the non-avian dinosaurs – birds are the only 406 surviving dinosaurs – mammals increased rapidly in size and diversity, and some took to the air and the sea. 407 408 A modern social insect collects pollen from a modern flowering plant. 409 410 Fossil evidence indicates that flowering plants appeared and rapidly diversified in the Early Cretaceous, 411 between 130million years ago and 90million years ago. Their rapid rise to dominance of terrestrial 412 ecosystems is thought to have been propelled by coevolution with pollinating insects. Social insects appeared 413 around the same time and, although they account for only small parts of the insect "family tree", now form 414 over 50% of the total mass of all insects. 415 Humans evolved from a lineage of upright-walking apes whose earliest fossils date from over 6million years 416 ago. Although early members of this lineage had chimp-sized brains, about 25% as big as modern humans', 417 there are signs of a steady increase in brain size after about 3million years ago. There is a long-running debate 418 about whether modern humans are descendants of a single small population in Africa, which then migrated 419 all over the world less than 200,000 years ago and replaced previous hominine species, or arose worldwide 420 at the same time as a result of interbreeding. 421 422 Mass extinction 423 Life on earth has suffered occasional mass extinctions at least since 542million years ago. Although they 424 are disasters at the time, mass extinctions have sometimes accelerated the evolution of life on earth. When 11 425 dominance of particular ecological niches passes from one group of organisms to another, it is rarely 426 because the new dominant group is "superior" to the old and usually because an extinction event eliminates 427 the old dominant group and makes way for the new one. 428 The fossil record appears to show that the rate of extinction is slowing down, with both the gaps between 429 mass extinctions becoming longer and the average and background rates of extinction decreasing. 430 However, it is not certain whether the actual rate of extinction has altered, since both of these observations 431 could be explained in several ways: 432  The oceans may have become more hospitable to life over the last 500 million years and less 433 vulnerable to mass extinctions: dissolved oxygen became more widespread and penetrated to 434 greater depths; the development of life on land reduced the run-off of nutrients and hence the risk of 435 eutrophication and anoxic events; marine ecosystems became more diversified so that food chains 436 were less likely to be disrupted. 437  Reasonably complete fossils are very rare, most extinct organisms are represented only by partial 438 fossils, and complete fossils are rarest in the oldest rocks. So paleontologists have mistakenly 439 assigned parts of the same organism to different genera, which were often defined solely to 440 accommodate these finds – the story of Anomalocaris is an example of this. The risk of this mistake 441 is higher for older fossils because these are often unlike parts of any living organism. Many 442 "superfluous" genera are represented by fragments that are not found again, and these "superfluous" 443 genera appear to become extinct very quickly. 444 445 Fossilization and Preservation 446 Introduction 447 Ordinarily, only the hard parts of organisms are preserved (for example, only the shells of invertebrates, and only the 448 bones and teeth of vertebrates). In most instances we must make inferences about fossil organisms using only these 449 hard parts. Despite this challenge, we must try to understand the soft-part anatomy of fossil organisms so that we can 450 better appreciate them as organisms that were once alive, that consumed food, breathed oxygen, interacted with their 451 physical and biological environments, etc. Taphonomy is the science that studies the information that is lost between 452 the death of an individual and its final discovery. 453 A fossil is any evidence of a once-living organism. This includes body fossils, casts, molds and traces fossils. This 454 evidence of previous living organisms can then be used to study changes in life forms through time. This includes 455 their evolution, ecology, functional morphology, growth and form, as well as their geographic distribution. Fossils 456 provide us with our best link to the history of life. 457 How do we get fossils? 458 One of the keys to preservation is resistance. Either the conditions are mild enough (calm water, little oxygen) not to 459 destroy much of the organism, or those parts that do get preserved are the most resistant to chemical and physical 460 damage. Good examples of this are the shells of clams and the teeth of mammals. Both of these examples 461 demonstrate that there is a preservational bias for hard parts compared to soft parts. 462 The nature of preservation is dependent upon the interaction of several factors. The composition of the organism and 463 its structure play vital roles in how the body will react to the physical and chemical activities that normally break 464 down or damage dead organisms. Intimately related to this is the sedimentary environment in which the organism 12 465 lived. It will determine the type and intensity of the physical and chemical processes. These all contribute to the post- 466 depositional changes (such as replacement, recrystallization, carbonization, the formation of casts, etc.) that take 467 place during fossilization. And finally, numerical abundance will affect the nature of preservation by increasing or 468 decreasing the chances of something being preserved, simply because of the sheer numbers or lack of certain 469 organisms. 470 The bias of hard parts over soft parts can provide considerable problems for paleontologists. Often, as is the case 471 with most molluscs for example, much of the diagnostic information is in the soft part morphology, making it 472 difficult to say certain specific things about organisms whose only record is in the hard parts. It is then necessary to 473 draw upon recent analogues and extrapolate that information back to the fossil record. This can be dangerous if the 474 past was not entirely like the present in environmental or ecological conditions. We call this the "pull of the Recent 475 analogue" and it can be a serious problem if not recognized at the outset. 476 Conditions of preservation 477 1-possession of hard skeleton: In order to be preserved as fossil, the organism must have a hard skeleton. 478 The soft parts decay after death and only the hard parts are preserved 479 2-Rapid burial: After death, the organism should be directly covered with sediments to prevent its 480 destruction by waves or winds. On land, rapid burial is not common and hence land organisms have little 481 chance of preservation than marine organisms. 482 Types of fossils preservation 483 After death, the organisms are preserved in different forms as follows: 484 I. Unaltered remains: the hard skeleton of the organism or its soft part or both remains unchanged. 485 Soft part (organic compounds): 486 1- Mammoth: in the Pleistocene glaciers of Siberia. 487 2-Insect in Amber: the insects are preserved in the resin (Amber) such as those found in the Oligocene 488 deposits of Baltic province. 489 Hard skeleton (inorganic compounds): 490 This is characteristic for Cenozoic shells which underwent little or no alteration of the original mineral 491 substance. 492 II. Unaltered preservation: implies the preservation of the original composition such as aragonite, calcite, chitin, 493 cellulose, and calcium phosphate. 494 Recrystallization: means that the less stable hard part mineralogies are transformed, through void time, by 495 temperature and pressure to more stable minerals. This is usually a destructive process, where much of the fine 496 morphological detail (e.g. ribs on a clam shell) is lost. The most common form of recrystallization in the invertebrate 497 record is the change from aragonite and/or Mg calcite to the more stable calcite form of CaCO3. 498 Replacement:In contrast to recrystallization, which is a rearrangement of the crystal lattice in which the chemical 499 composition remains the same, replacement is an atom for atom substitution of a mineral's components with the 500 elements composing the replacing mineral. Thus, pyritization, phosphatization, silicification and dolomitization are 13 501 all good examples of the replacement process. One should also note that contrary to recrystallization, replacement is 502 usually NOT destructive; that is, you can see many of the original morphological details. 503 Permineralization: is yet another mode of preservation, where pore-space is in-filled by percolating fluids. The pore- 504 space is usually the xylem and phloem (transport tissues) of woody tissue. Another name for this process is 505 petrification. 506 Carbonization: is often indicated by the shiny black texture of what appears to be an impression of an organism, 507 often a plant leaf or crushed arthropod. This process is due to distillation. An organic film is formed as water is 508 driven off. You can recognize carbonization easily by the shiny black or dark brown color. 509 The next three modes (impression, cast and internal mold) are often confused, but they are distinct both in pattern 510 and process. Impressions or external molds are nothing more than what is produced when something is pressed into 511 soft sediment and that "impression" remains. You can recognize external molds because they show only external 512 detail, and they are negative in relief. 513 A cast on the other hand, is the sediment infilling of an external mold. It will also show only external features, but 514 will be positive in relief, not negative like an external mold. Lastly, internal molds form when sediment in-fills a 515 shell or skeleton, hardens, and the shell is worn away. What is left is molds showing internal features and will most 516 likely have a positive relief. 517 Evidence of the activity: here we don’t have anything of the body fossil itself but only traces of its movement. This 518 branch of paleontology is called Ichnology, which deals with traces of organisms 519 Skeletal mineralogies 520 Before determining how a particular fossil has been preserved, it is important to know the organism's 521 original skeletal mineralogy and the mineralogy present in the fossil. This, for example, enables you to 522 distinguish between recrystallization and replacement. The following display is designed to familiarize you 523 with different types of mineralogies commonly found in fossils. 524  1. Aragonite. Aragonite (CaCO3) is a form of calcium carbonate that is fairly unstable and commonly 525 dissolves away. Skeletons made originally of aragonite are commonly recrystallized to calcite and preserved 526 as molds. Aragonite is easy to recognize. It is usually (not always!) milky white and has no luster. 527  2. Calcite. Calcite (CaCO3) is the more common form of calcium carbonate. It is more stable than aragonite 528 and therefore does not dissolve as readily. Calcite usually has a grayish color and a slight vitreous (or glassy) 529 luster when found as a skeletal mineral. It can be found as an original skeletal material, or as a 530 recrystallization product. 531  3. Silica. Silica (SiO2) is easy to distinguish from the carbonate minerals since it will not react with acid. 532 Skeletons composed of this mineral will commonly have a brown, earthy color, with or without a vitreous 533 luster, and can have a granular texture. Silica is rarely found as an original material and most commonly 534 occurs as a replacement product. 535  4. Pyrite. Pyrite (FeS2) or "fools' gold" is a golden colored mineral with a metallic luster and is therefore 536 identified easily. It always appears as a replacement product. 537 Modes of preservation 538  1. Unaltered hardparts. Occasionally an organism's skeleton is preserved intact without any chemical 539 alteration of the original mineralogy. This mode of preservation becomes increasingly rarer for fossils of 540 older ages. Which skeletal mineralogies would you expect to have better chances of being preserved in this 541 way? 542  2. Mold. Often, a fossil has been encased in sediment and that sediment becomes cemented, and the original 543 skeletal material dissolves away completely leaving a void. The dissolution leaves behind an impression (or 544 mold) of the skeletal remains. 14 545  3. Carbonization. Carbonization occurs when all organic volatiles are distilled away because of the effects of 546 heat and/or pressure, leaving a carbon film remnant of the organism. This usually occurs with organisms rich 547 in carbon that possess thin or no skeletal material. Coal is an example of carbonized plant debris. 548  4. Permineralization. Skeletal material can be quite porous. If the pores are filled in by foreign minerals that 549 precipitate out of solution, the fossil is said to be permineralized. Petrified wood is an example of wood that 550 has been permineralized by silica. 551  5. Replacement. This occurs when skeletal material is replaced, molecule by molecule, by some new alien 552 material. This process occurs gradually over a long period of time as the original mineralogy dissolves away 553 and a new mineral precipitates in its place. Examples include: 554 o (1) silicification - where calcium carbonate is replaced by silica, and 555 o (2) pyritization - where pyrite replaces calcium carbonate. 556  6. Recrystallization. This mode of preservation is probably the most difficult to understand and recognize. 557 Recrystallization occurs when the original mineral crystals are altered in size and/or geometry over time, yet 558 the chemical composition remains unchanged. An example is aragonite recrystallizing to calcite. 559 Taphonomy 560 561 Introduction 562 Taphonomy is the study of postmortem processes on once-living organisms. In addition to determining the type and 563 intensity of the processes and their role in preservation, taphonomy is a way to detect bias in the fossil record. For 564 example, in a hypothetical fossil assemblage of shells certain questions can be asked, such as: 565  1. Is the assemblage a true representation of the original assemblage? 566  2. Was any material lost during the fossilization process? 567  3. Did the material condense? 568  4. How much time is represented in the rocks? 569  5. How can we tell what has happened to the shells from the time of the death of the organisms to their burial 570 and ultimate fossilization? 571  6. What can these preservational features tell us about the depositional environment? 572 These are just some of the questions one could begin to ask about any assemblage of fossilized material. Indeed, it is 573 documented that the relative abundance of species in a fossil assemblage may not be an accurate reflection of the 574 relative abundances in the original assemblage of living populations. 15 575 Taphonomic processes 576 There are three major categories of taphonomic processes of alteration and destruction: physical processes, chemical 577 processes and biological processes. Physical processes involve the mechanical breakdown of organic material via 578 water and/or wind action (storms are an excellent example of a physical process). Chemical processes include any 579 alteration of a material's mineralogy, as well as any leaching of material by the surrounding water or air. Finally, 580 biological processes, such as sponge or algal borings, can help to alter and eventually destroy potential fossil 581 material. 582 Fossil concentrations 583 Fossils can be concentrated in two major ways, first by physical processes mentioned above, such as storms and 584 currents, or winnowing and deflation. Fossils are also concentrated by aggradation, which is a biological process in 585 that it is the piling up of LIVE individuals, such as those found in oyster beds or coral reefs. 586 Konservat-Lagerstatten 587 This term was coined by German paleontologists. It means simply an exceptional preservation in the fossil record. 588 Konservat-Lagerstattenrepresent a preservational endmember in the spectrum of fossilization. Not only are most of the 589 hard and soft parts preserved, the assemblages in these types of deposits are probably the closest approximation to the 590 abundance and diversity of the original assemblage. For Konservat-Lagerstatten to form, all taphonomic processes 591 must be minimized. That is, physical, chemical and biological destruction must be kept to a minimum. 592 Some of the world's most famous fossil deposits happen to be Konservat-Lagerstatten. Faunas such as the Mazon Creek 593 (Illinois), Solnhofen Limestone (Germany), La Brea (tar pits in Southern California; "La Brea tar pits" is a redundant 594 name), insects and others in amber (the Baltic states, Dominican Republic), and Burgess Shale (Canada) are all good 595 examples. The Burgess Shale is located in the Canadian Rockies (British Columbia). The shales and its fossils are dark 596 black in color, suggesting anaerobic conditions (no oxygen) and the fine-grained nature of the sediment indicates quiet 597 water deposition, because there is no disturbance from wave action or burrowing organisms in the sediment. The 598 Solnhofen is also very fine grained. The complete skeletons (e.g. Archaeopteryx) preserved in the limestone indicate 599 very quiet waters too. 600 Although these deposits give us some of our most spectacular fossil deposits, they are important for many other reasons. 601 First of all, they represent a "snapshot" in time, because of probable rapid burial. Secondly, they provide previously 602 unknown anatomical details that can be important from a systematic (evolutionary) point of view. They also can 603 provide an additional test for environmental and diagenetic boundary conditions. And finally, the excellent time 604 resolution may allow true biotic diversity for an assemblage to be observed. This may be the closest that paleoecologists 605 can come to the conditions of modern ecology. 606 Taphonomic grades 607 Some sedimentary environments are better than others when it comes to preserving fossils. The high energy conditions 608 of a river channel or beach may grind and abrade bones or shells so that they are unidentifiable after only a short period 609 of time. The quiet waters of swamps and lagoons, on the other hand, may permit the preservation of the delicate features 610 of many hard parts. 611 Taphonomic grades: It's easy to pick out some shells that show well preserved surface sculpture and growth lines; 612 other shells have had these surface features almost completely worn away; others are in an intermediate condition. 613  Good (1): Well preserved shells that show growth lines and ribs in their original or near-original condition. 614  Fair (2): Shells in which the surface features are somewhat worn, but the growth lines and ribs are still seen. 615  Poor (3): Shells in which most of the surface features are worn away and much of the shell's external surface 616 is smooth and polished. 16 617 Trace Fossils 618 Introduction 619 Trace fossils or ichnofossils represent the effects of organismal activity upon or in the substrate. Tracks, trails and the 620 like are the most commonly encountered traces. A distinction can therefore be made between body fossils, which are 621 actual remains of organisms, and trace fossils that represent an indication of an organism's behavioral activity. 622 Trace fossils, though not preserving the body or necessarily the morphology of the original organism, do have certain 623 advantages over body fossils. In general: 624  Trace fossils are often preserved in environments that are hostile to the preservation of body fossils (e.g. 625 shallow, high energy environments, shallow marine sandstones, and deep marine shales). 626  Trace fossils are generally not affected by diagenesis, and may actually be enhanced (visually) by diagenetic 627 processes. 628  Trace fossils are not transported, and are thus good indicators of the original sedimentary environment. 629 (Exceptions: Feeding damage on body fossils (like damage on bones, coprolites and leaves) are also subject to the 630 same taphonomic processes that affect the body fossil. Sometimes these traces actually facilitate the degredation of 631 the body fossil.) 632 Trace fossils may be preserved in a number of reliefs. They may be preserved in actual 3-dimensional relief, within 633 sediment or sometimes the traces become filled in by a more resistant mineral and are subsequently eroded out of the 634 surrounding sediment in full relief. More often, there is partial preservation caused by the movement of the 635 tracemaker in and out of the depositional interface. These semireliefs may occur on the upper surface of a bed 636 637 Types of trace fossils 638 The major categories of trace fossil ethological classes are described below. If examples are available, you should go 639 through this section while examining the relevant specimens. Also, refer to Table 3.1 and Figure 3.2 for interactive 640 summaries of these categories. 641  Repichnia. These are crawling or walking traces; this group includes any trace that was made during 642 locomotion. Included in this category are examples of amphibian, reptilian and mammalian footprints. 643 Cruziana is an example of a crawling trace made by a trilobite; note the scratch marks made by the trilobite 644 appendages. 645  Fodichnia. These are feeding structures, usually infaunal burrows made by deposit feeders that 646 systematically mine the sediment for food. A typical feature found associated with these types of traces is 647 called spreiten (be sure that you understand how this feature is formed, and its significance). Other fodichnial 648 traces consist of complex burrows that form branching networks where the organism has tunneled 649 systematically below the substrate (e.g. Chondrites). 650  Domichnia. Domichnial traces are burrows used principally for dwelling as opposed to feeding. These types 651 of burrows may be oriented vertically (e.g. Skolithos), or are commonly U-shaped. 652  Cubichnia. This group of behavioral traces includes resting or nestling traces; places where organisms 653 rested temporarily on the substrate. Examples are common impressions left by sea stars and trilobites 654 (Rusophycus). 655  Pascichnia. These types of traces are made by grazing herbivores, usually at the sediment/water interface. 656 Nereites is a systematic sinuous trail made by deep water gastropods. 657  Hard substrate traces. All the above categories are associated with soft substrates. Traces can also be made 658 on hard substrates (i.e. on shells or lithified surfaces) and are usually called borings. Examples are the small 659 boring left behind by clionid sponges boring clam shells, the large holes left by endolithic bivalves, and the 17 660 predatory drill holes of gastropods and octopods. Unfortunately, hard substrate traces are not generally 661 included in the above ethological classification scheme. 662  Stromatolites. Included in this section are the stromatolites. Make sure that you understand how 663 stromatolites are formed, that you can recognize the different morphologies (SS, SH, LH, L), and that you 664 understand their environmental significance. 665 666 Table l. Ethological classification of trace fossils. (Adapted from Frey, 1978) Categories of Definition Characteristic morphology Ichnofossils Troughlike relief, recording to some extent the that temporarily settle onto, or dig lateroventral Resting traces morphology of the animal; on into, the substrate Shallow depressions made by animals (Cubichnia) surface; emphasis structures isolated, ideally, but may intergrade.reclusion with crawling traces or escape structures Linear or sinuous overall structures, some traces horizontal structures made by organisms traveling Crawling Trackways, surficial trails, and shallow from one place to another; branched; footprints or traces borrows, emphasis on locomotion, continuous grooves, commonly annulated; (Repichnia) complete form may be preserved or may appear as cleavage reliefs. Unbranched, nonoverlapping, curved to tightly Grooves, pits and furrows, many of them coiled patterns or delicately constructed spreiten Grazing traces discontinuous, made by mobile deposit dominate; patterns reflect maximum utilization of (Pascichnia) feeders at or near the substrate surface; surficial feeding area; behavior analogous to "strip emphasis on feeding mining" complete form may be preserved. Single, branched or unbranched, cylindrical to Temporary burrows constructed by deposit structures sinuous shafts or U-shaped burrows, or feeders; the structures may also provide complex, parallel to concentric burrow repetitions Feeding traces shelter for the organisms- emphasis on (spreiten structures); walls not commonly lined, (Fodichnia) feeding, behavior analogous to "underground unless by mucus; oriented at various angles with mining" respect to bedding; complete form may be preserved. Simple, bifurcated, or U-shaped structures Burrows or dwelling tubes providing more or Dwelling structures perpendicular or inclined at various less permanent domiciles, mostly for traces angles to bedding, or branching burrow systems hemisessile suspension feeders, or in some (Domichnia) having vertical and horizontal components; walls cases, carnivores; emphasis on habitation typically lined; complete form may be preserved Lebensspuren of various kinds modified or Vertically repetitive resting traces; biogenic made anew by animals in direct response to structures laminae either in echelon or as nested substrate degradation or aggradation; Escape traces funnels or chevrons; U-inUspreiten burrows; and emphasis on readjustment, animals upward or (Fugichnia) other structures reflecting displacement of or downward with respect to the original equilibrium between relative substrate position and substrate surface; complete form may be the configuration of contained traces preserved, especially in aggraded substrates 667 18