17 Palaeontology

Rajeev Patnaik

Palaeontology

 

  1. Learning outcomes
  2. Introduction
  3. Techniques used in Palaeontology
  4. Applications of Palaeontology
    1. Evolution
    2. Biostratigraphy
    3. Palaeoclimate
    4. Palaeoecology
    5. Palaeobiogeography
    6. Palaegeography
  5. Summary

 

  1. Learning outcomes:

 

After going through the module, one should be able to get a basic idea what fossils are and how they can be used for understanding past evolution, environment and biodiversity by applying state of the art techniques.

 

  1. Introduction:

 

In order to appreciate the extent of present biodiversity and to visualise its future evolution, we need to have a deep understanding of the past. And palaeontology is one such science that provides us a window into the past life, going back to almost 4 billion years. It is basically the study of prehistoric life by examining fossils. Fossils include both body parts and traces of ancient animals and plants. A body fossil preserves the actual organic and inorganic remain of an organism. For example a bird limb bone, a leaf of a tree or an insect embedded inside an amber come under body fossils (Figure 1).

 

Usually, hard parts such as bones, teeth and shells that are more durable get preserved as body fossils. But, sometimes soft tissues as in the case of deeply frozen Siberian Mammoth, mummified (air dried) animals and plants are also fossilised. A trace fossil on the other hand provides an indirect evidence of the existence of an ancient organism. For instance, tracks made by early humans, burrows of ancient earthworms or fossilised dung of dinosaurs can be classed under trace fossils (Figure 2).

The negative impression of an ancient organism is termed as its mould and when this mould gets filled up by another material forming a replica of the original organism or a part of it, then it is called as its cast (Figure 3). A fossil is termed as petrified if the pore spaces and the original tissue are  replaced  by  foreign  minerals.  However,  the  original  structure  of  the  organism  remains preserved. For example, a petrified tree, millions of years old, would still contain the annual growth rings. The process where the volatiles are lost and the fossil turns into carbon is called as carbonisation.

 

The branches of palaeontology that deal with animals with and without back-bone are called vertebrate and invertebrate palaeontology, respectively. The study of fossil plants is called palaeobotany. Studies of microscopic fossils are dealt by micropalaeontology. Molecular palaeontology involves extraction and analyses of DNA, proteins, carbohydrates, lipids and their diagenetic products from early humans, animals and plants. Palaeontological methods include fieldwork, excavations, laboratory investigations and interpretations. Fossils have several uses, from exciting the minds of young ones such as dinosaur exhibitions in museums, assisting in oil exploration, understanding climate change to recreating extinct species.

 

  1. Techniques used in Palaeontology

 

The study of fossils involves methods developed in other sciences and technologies. Using biology, maths, physics, chemistry and engineering, to satellite-based remote sensing and computer modelling, a palaeontologist searches for important clues to interpret the natural history of past life. Synchrotron based micro-CT scanning can now reveal fossil of a juvenile animal preserved inside a fossilised egg without damaging the specimen. Another exciting field is molecular palaeontology that uses polymerase chain reaction (PCR) to extract ancient DNA molecules from extinct animals including early humans and the potential of recreating them in near future. More importantly, palaeontology requires the understanding of very basic techniques and can be mastered by young students. Fossils attract the imagination of young minds and by getting involved in the discovery, preparation, preservation, and analysis of these, students can understand the process of science and even contribute new knowledge to this fascinating field.

Fossils are first discovered in the field, where natural erosion or excavations exposes fossil-bearing rocks. Fragile fossils require extreme care and often are glued with the matrix to avoid damage. Large bones/skeletons are excavated using Plaster Of Paris jackets. A field palaeontologist is required to take photographs by placing a scale close to the fossil before removing it. Collected fossils are catalogued in the field diary and information about the precise geographic location using a GPS is noted. The stratigraphic context of the fossil yielding layer is also noted down. For microfossils bulk samples are collected from specified layers. These field procedures are crucial for all types of fossil collections.

 

In the laboratory fossils are cleaned and restored if damaged. If fragmented or disarticulated, they are rejoined and reconstructed as close as possible to their original state. Microfossils are recovered from sediments by soaking them in water or other solvents and screened through sieves. The specimens are then compared with their closest extinct and extant relatives for their taxonomic identification. Fossils are then numbered and catalogued in a book or a computer. Replicas are made in order to minimise the use of the original and for carrying out comparisons with specimens housed elsewhere. Photographs and micro-CT scanned images are also taken for publication and sharing the results. Few of the very important and well preserved ones are exhibited in Natural History Museums and the rest are archived in stores. Specimens are archived in well protected cabinets/boxes for safe handling and preservation for future students and scientists.

 

  1. Applications of Palaeontology

 

The general perception about fossils is that these are the dead remains of long extinct organisms, and only have historical value. But the subject of palaeontology is very much relevant to the modern and future world. We get to know how, when and why the early organisms developed. How land masses have moved across the globe and how climate has changed over the years influencing the past organisms. Conversely, how past organisms have changed the physical features of this world. Fossils tell us about extinctions, evolutionary changes, and changes in the biodiversity over the past several million years. For example, the effects of climate change such as global warming on the past biodiversity can provide clues to probable changes in future ecosystems and changes that will affect human land-use. Similarly, one can visualize how alteration in human land-use and exponential population growth might result in the similar effects we observe in the fossil record due to physical environmental change. This has a far reaching consequence as urbanization and loss of habitats are going to radically change the course of life on this planet. Therefore the subject of palaeontology can provide a deep understanding of the historical record of the climate and ecological changes that has happened at various time scales.

 

The study of palaeontology has got several applications. For instance, fossils can be of economic, as well as academic value. Since petroleum and coal are basically the product of ancient organisms, the  study of fossils is imperative for oil and coal exploration and exploitation. To begin with, fossils provide an idea about the age of the petroleum source rock. Secondly, they also provide vital information about the temperature and pressure conditions crucial for the formation of coal and petroleum. Besides microfossils are of immense use in logging of wells during oil exploration.

 

Natural history museums all over the world have generated a lot of revenue and promoted the tourism industry, by displaying rare fossils. A better understanding of the ancient organisms, their habitat and extinctions has facilitated making of commercially block buster movies such as Jurassic Park and Ice Age.

 

4.1 Evolution

 

Fossils add another very important dimension of time, to the understanding of the organic evolution. Life has not evolved in a linear fashion from simple unicellular organisms to complex multicellular ones. In fact, fossils teach us that there were several experiments at several stages and life as such has gone through several trial-errors and bottle necks. Almost 99 % of all the species that existed in the past are now extinct. Whether life came from space or originated on the earth, is not known for sure. But early life appears fairly quickly within 500 million years of the formation of earth around 4.6 billion years ago, suggesting that the universe already had life before and earth only provided a conducive environment for it to make a foothold.

 

 

Earliest fossils were those of unicellular prokaryote (without a nucleus) bacteria. Eukaryotes (having a nucleus) came much later. The spread of photosynthetic cyanobacteria in the form of stromatolites in the ocean, since 3.5 billion years ago changed everything. This created oxygen rich ocean and atmosphere for future organisms to emerge and diversify. Early multicellular organisms were soft-bodied and jelly fish-like appearing around 700 Ma ago. It is only around the beginning of Cambrian time (~550-540 Ma) that invertebrates acquired shells. Soon thereafter, earliest vertebrates having a notochord (a flexible cartilaginous structure), in the form of jawless fishes appear ~ 525 million years ago (Figure 4).

 

Figure 4. Early jawless fish Hakouichthys from China.

 

Gradually one group of fishes gave rise to early amphibious tetrapods (Figure 5) ~ 375 million years ago, that would later go on to invade the land. A strong muscular system to hold the neck and ribcage bones against gravity facilitated movement on the land. A suitable ecology to support these early land vertebrates was already in place on land that was occupied by early land plants (seed ferns) and insects. The development of amniotic eggs (eggs laid on land having a cover and a sac to aid the development of embryo) some 330 Ma ago further allowed the land vertebrates to adapt to terrestrial conditions.

A major extinction towards the end of Permian (252 Ma) wiped out most of the land vertebrates and other fauna and flora. From the Permian remnant synapsids ( reptiles bearing a skull with one hole behind the orbit) went on to give rise to mammals, whereas, diapsids ( reptiles bearing a skull with two holes behind the orbit) gave rise to dinosaurs and crocodiles. Dinosaurs and seed-producing gymnosperms dominate the landscape in the Jurassic and Cretaceous time (200-65

 

Ma). A little later ~ 130 Ma ago angiosperms, the flowering plants arose. Mammals during this period were diminutive and dormant. Early birds developed from theropod (carnivorous) dinosaurs and some of them had teeth and long tail (Figure 6).

 

Another major extinction the K-T event (65 Ma) almost eradicated 50% all the animals, including all the dinosaurs. Mammals start occupying the niches left by the dinosaurs and dominate the land. Between 65-60 Ma the earliest rodents (gnawing mammals), primates (our ancestors) and creodonts (carnivores) originate. Flightless birds diversify. By 55 Ma modern birds (parrots, woodpeckers, songbirds) diversify, early elephants, rabbits, odd and even toed mammals including the earliest whales appear. Fossils from India, Pakistan and Egypt have revealed that the early whales were terrestrial mammals with four limbs. In fact rudimentary limb bones are still present inside the whale flippers (Figure 7).

 

 

 

Fossils of early humans and apes have revolutionised our understanding about our own evolution. We often are worried about the present day climate change. However, fossil finds show that in fact we have evolved from quadrupedal, low brain sized primates to bipedal, large brained hominins, to meet the demands of changing climate and habitat over the past 20 million years. Our small arboreal ancestors lived in tropical rain forests during the Eocene times (~50 Ma) that received rain throughout the year. Such conditions prevailed till the Oligocene (~25 Ma) but gradually climate became cooler and drier as continents moved northwards and atmospheric carbon dioxide kept on reducing. Combined with these conditions, an intensification of monsoon led to seasonality in rainfall that reduced the forest cover further and facilitated the spread of grassland in the Late Miocene (10-7 Ma) both in Asia and Africa. African ape-like primates had to start adapting to grassland conditions and one such lineage gave rise to the first bipedal hominid. Bipedalism (walking on two legs) allowed the hands to be free for other works. Further deterioration of climate and ecological conditions in East Africa most likely led to the birth of our own genus Homo, that began using stone tools. Harsher conditions thereafter forced us to diversify our diet. Living under highly fluctuating and unpredictable climatic conditions, led to several innovations including use of stone tools, better communications for making hunting strategies followed by cultural evolution most probably led to the increase in the brain size manifold (Figure 8).

 

4.2 Biostratigraphy

 

Fossils provide relative ages to the rock they occur in. Fossils that were short lived and had a wider geographical distribution are best for such assignments. These are also called as index fossils. The occurrence of different fossils together also helps providing relative ages to sedimentary rocks. The level of evolution and relative abundance of fossils are other factors that help assign an age to a particular stratum. Similar fossil content helps correlating rocks of different regions, sub-continents and even continents. Once fossil zones are tied to geochronologically dated sections, they can be used  for precise dating of similar fossil bearing non-dated sections. This branch is called biochronology. Free floating foraminifers in marine and land mammals in terrestrial deposits are best for biochronolgy.

 

 

4.3 Palaeoclimate

 

Past organisms sensitive to climate changes can give us useful information about the past climatic conditions. They are kind of proxies for past climatic events. For example if one were to find fossils of crocodile in 1 million year old rock occurring in Antarctica today, it would indicate Antarctica had a tropical climate 1 Million years ago. Oceanic micro-organisms such as foraminifers flourish during monsoons. Therefore their abundance in the past is a good indicator of monsoon intensification. Leaf shape is another good indicator of climate. Needle shaped leaves such as those of pines reflect cold conditions whereas broad leaves of Sal suggest sub-tropical warm and humid conditions. Tree rings are also good indicators of past climate. A wide tree ring would suggest a wet season whereas a narrow one indicates a dry season. Fossils can tell us about past climate at various time scales. Climate led evolutionary changes happen at millions and hundreds of thousand year scales. Trends in general abundance or changes in coiling patterns in foraminifers can resolve past climate up to thousands of years.

 

 

Stable oxygen isotopes preserved in shells, bones and teeth of fossils are very good for reconstructing past temperatures and rainfall. In water, the lighter variety of oxygen (16O) occurs in high percentage compared to the heavier one (18O). A rise in temperature leads to evaporation of the water molecules having lighter isotope (H216O). Rainfall will bring back any heavier isotope left in the clouds. Therefore, the ratio of these isotopes (18O/16O) in water is controlled by temperature and rainfall. At higher temperatures water contains more 18O compared to 16O.Therefore, 18O/16O values of shells (CaCO3) or bones/teeth (CaPO4) of fossils will reflect the ambient temperature and rainfall at the ancient time of their formation. Close sampling of growth lines of fossil shells, corals, tree rings and teeth can provide very high resolution climate proxy data of ancient times (Figure 9). PO4 bonds are considered stronger compared to CO3 bonds and therefore bones and teeth are less susceptible to diagenetic alterations.

 

4.4 Palaeoecology

 

A better understanding of relationship among present day co-existing animals and plants and their environment can help us reconstructing past ecological conditions. The study includes the reconstruction of past community structures, life history strategies, death and burial. Overall such data added with physical parameters help reconstructing palaeoenvironmental conditions. Past ecological conditions can be built on the principal of actualism. That is by assuming that the extinct species must have lived under similar ecological conditions as their closest extant relatives do. Other procedures involve the analysis of functional morphology. For example longer legs are meant for running and if we find several fossil mammal species with long legs at one particular time, it would indicate presence of an open habitat.

 

Carbon isotopes are good indicators of past vegetation types. Photosynthetic pathways of trees and shrubs (C3 plants) differ significantly from those of warm season grasses (C4 plants). Delta 13C values that depends on the ratio of 13C/12C, found in C3 plants range between -24 to -33 ‰, and that in C4 plants ranges between -16 to -10 ‰. These values get further enriched by 14 ‰ in shells or teeth of animals due to their metabolism. Therefore, diet of ancient animals can be reconstructed based on their stable carbon contents (Figure 10). This in turn provides information about the habitat of these organisms.

Abrasive food leave scratches and pits on tooth surface. Grassy diet leaves more scratches, whereas a diet dominated by fruits and nuts leave a high percentage of pits (Figure 11). Studies on microwear on fossil teeth helps reconstructing their diet.

4.5 Palaeobiogeography

 

Geographic and temporal distribution of fossils provides information about the centre of their origins and dispersal patterns. Elephants arrived in India from Africa only after Africa got attached to Eurasia some 20 Ma ago. Likewise, horses and camels arrive in India from America when a connection between Asia and America developed through the Bering Strait. We humans have also come from Africa as per the fossil and genetic record.

 

4.6 Palaeogeography

 

Fossils also inform where oceans and continents were in ancient times. Distribution of similar land animals and plants in different continents such as South America, Antarctica, Africa, India and Australia 200 Ma ago indicates presence of one landmass the Gondwanaland (Figure 12).

 

  1. Summary

 

Palaeontology is a fascinating branch of science that provides a window into the deep time, showing glimpses of early life. Fossils teach us about evolution, extinctions, migrations, climate changes, movement of continents and rise and fall in the sea levels. State of the art technology is now being applied to the study of fossils. In near future palaeontology is going to be at the forefront of research in astrobiology.

 

References