Microfossils E-Book

Howard Armstrong

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 586

Veröffentlichungsjahr: 2013

Beschreibung

This is a new and completely rewritten edition of the well-known text Microfossils (first published in 1980) covering all the major microfossil groups, with information on taxonomy, phylogeny, ecology and palaeoecology.

• particular attention is given to the uses of microfossils in environmental reconstruction and biostratigraphy
• numerous line and half-tone illustrations
• emphasis on practical applications of micropalaeontology
• only student-friendly micropaleontology text available

Leseprobe

Contents

Preface

Part 1 Applied micropalaeontology

CHAPTER 1 Introduction

Microfossils – what are they?

Why study microfossils?

The cell

Nutrition

Reproduction

The empires of life

CHAPTER 2 Micropalaeontology, evolution and biodiversity

Mechanisms of cladogenesis

Biodiversity in the marine plankton

Reconstructing phylogeny

CHAPTER 3 Microfossils in stratigraphy

The stratigraphical column

Microfossils and biostratigraphy

Quantitative biostratigraphy

Microfossils in sequence stratigraphy

CHAPTER 4 Microfossils, stable isotopes and ocean-atmosphere history

Introduction

Methodology

Oxygen isotopes

Palaeosalinity

Palaeotemperature

Carbon isotopes

CHAPTER 5 Microfossils as thermal metamorphic indicators

Part 2 The rise of the biosphere

CHAPTER 6 The origin of life and the early biosphere

Origins of life

Geochemical proxies for early life

Archean fossils

CHAPTER 7 Emergence of eukaryotes to the Cambrian explosion

Emergence of eukaryotes

The Cambrian explosion

CHAPTER 8 Bacterial ecosystems and microbial sediments

Bacterial habitats

The living bacterium

Bacterial taxonomy

Cyanobacteria

Anaerobic bacteria

Some geologically significant bacteria

Hints for collection and study

Part 3 Organic-walled microfossils

CHAPTER 9 Acritarchs and prasinophytes

The vesicle

Classification

Acritarch affinities and biology

Acritarch ecology

General history of acritarchs

Applications of acritarchs

Phylum Prasinophyta

Further reading

Hints for collection and study

CHAPTER 10 Dinoflagellates and ebridians

The living dinoflagellate

Cyst surface features

Dinoflagellate life history

Dinoflagellate ecology

Classification

General history of dinoflagellates

Applications of dinoflagellate cysts

Further reading

Hints for collection and study

Ebridians

CHAPTER 11 Chitinozoa

Morphology

Distribution and ecology of chitinozoans

Classification

Affinities of chitinozoans

General history of chitinozoans

Applications

Further reading

Hints for collection and study

CHAPTER 12 Scolecodonts

Morphology and classification

Geological history and applications

CHAPTER 13 Spores and pollen

Life cycles of ‘lower’ land plants

Life cycle of the ‘higher plants’

Spore and pollen taxonomy

Distribution and ecology

Geological history

Applications of fossil spores and pollen

Further reading

Hints for collection and study

Part 4 Inorganic-walled microfossils

CHAPTER 14 Calcareous nannoplankton: coccolithophores and discoasters

The living coccolithophore

Coccoliths

Ecology of coccolithophores

Coccoliths and sedimentology

Classification

General history of coccolithophores

Applications of coccoliths

Further reading

Hints for collection and study

CHAPTER 15 Foraminifera

Living foraminifera

Life cycle

The test

Foraminiferal ecology

Planktonic foram ecology

Classification

Molecular phylogeny of Foraminifera

Geological history of foraminifera

Applications of foraminifera

Further reading

Hints for collection and study

CHAPTER 16 Radiozoa (Acantharia, Phaeodaria and Radiolaria) and Heliozoa

Phylum Radiozoa

Classification of radiolarians

General history of radiolarians

Applications of radiolarians

Phylum Heliozoa

Further reading

Hints for collection and study

CHAPTER 17 Diatoms

The living diatom

The frustule

Diatom distribution and ecology

Diatomaceous sediments

Classification

Evolutionary history

Applications of diatoms

Further reading

Hints for collection and study

CHAPTER 18 Silicoflagellates and chrysophytes

The living silicoflagellate

The silicoflagellate skeleton

Classification

Geological history of silicoflagellates

Applications of silicoflagellates

Chrysophyte cysts

Further reading

Hints for collection and study

CHAPTER 19 Ciliophora: tintinnids and calpionellids

The living tintinnid

The lorica

Distribution and ecology of tintinnids

Classification

Geological history of tintinnids

Applications

Further reading

Hints for collection and study

CHAPTER 20 Ostracods

Soft body structure

The ostracod carapace

Ostracod reproduction and ontogeny

Ostracod distribution and ecology

Classification

General history of ostracods

Applications of ostracods

Further reading

Hints for collection and study

CHAPTER 21 Conodonts

Soft anatomy

Conodont elements

Classification

Conodont affinities

Mode of life palaeoecology and palaeobiogeography

Evolutionary history

Applications

Further reading

Appendix – Extraction methods

Sample collection

Sample preparation

Sorting and concentration

Separation by heavy liquids

Systematic Index

General Index

Wonder is the first of all passions

René Descartes, 1645

© 2005 Howard A. Armstrong and Martin D. Brasier

BLACKWELL PUBLISHING

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The right of Howard A. Armstrong and Martin D. Brasier to be identified as the Authors of this Work has been asserted in accordance with the UK Copyright, Designs, and Patents Act 1988.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs, and Patents Act 1988, without the prior permission of the publisher.

First edition published 1980 by George Allen & Unwin, © M.D. Brasier 1980

Second edition published 2005 by Blackwell Publishing Ltd

Library of Congress Cataloging-in-Publication Data

Armstrong, Howard, 1957–

Microfossils. – 2nd ed./Howard A. Armstrong and Martin D. Brasier.

p. cm.

Rev. ed. of: Microfossils / M.D. Brasier. 1980.

Includes bibliographical references and index.

ISBN 0-632-05279-1 (pbk. : alk. paper)

1. Micropaleontology. I. Brasier, M.D. Microfossils. II. Title.

QE719.A76 2004

560–dc22    2004003936

A catalogue record for this title is available from the British Library.

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Preface

In the 25 years since the first, highly successful, edition of Microfossils was published there have been significant advances in all the areas of understanding of microscopic life and their fossil counterparts. Our new knowledge has led to major changes in the classification, applications and in some cases the biological affinities, of the major groups covered in this book. Greater understanding of species concepts, stratigraphical ranges and the completeness of the microfossil record means all of the Phanerozoic and parts of the Proterozoic can now be dated using microfossils. The high fidelity of the microfossil record provides the best test bed for numerous evolutionary studies. Microfossils remain an indispensable part of any sedimentary basin study, providing the biostratigraphical and palaeoecological framework and, increasingly, a measure of maturity of hydrocarbonprone rocks. The rise of palaeoclimatology has given micropalaeontology a new impetus too, with calcareous-walled groups providing stable isotope and geochemical proxies for oceanographic, palaeoenvironmental and palaeo-climatic change. Indeed it is now widely accepted that some microscopic groups are responsible for maintaining the Earth as a habitable planet and have been doing so since the early Proterozoic and perhaps before. Micropalaeontology therefore now occupies a central role in the modern Earth and environmental sciences and increasingly a much wider group of Earth scientists are likely to come across the work of micropalaeontologists. We hope this second edition provides an inexpensive introductory textbook that will be of use to students, teachers and non-specialists alike.

We have not changed the main motivation of this book, which is to provide a manual for somebody with little micropalaeontological background working at the microscope. Morphology and classification lie at the core of the book, supported by more derivative information on geological history, palaeoecology and applications, with supporting references. An addition to this book are selected photomicrographs, which are not intended to give a comprehensive coverage of the taxa discussed but to supplement the line drawings.

Conscious of the adage that for every expert there is a different classification we have favoured the use of those schemes published in the Fossil Record II (Renton, M. (ed.), 1993, Chapman & Hall, London), a volume compiled by experts in the respective groups and a statement of the familial level classification at the time of publication. Students will therefore have access to a much more detailed treatment of family level stratigraphical ranges than can be provided by this text. Mindful also of the value of collecting and processing microfossil material, the section on preparatory methods has been retained. This focuses on techniques that are simple, safe and possible with a minimum of sophisticated equipment.

In order to compile this book we have relied on the work and advice freely given by our many colleagues past and present. We are particularly indebted to those who have commented on the various parts of the manuscript: Professor R.J. Aldridge, Professor D.J. Batten, Dr D.J. Horne, Professor A.R. Lord, Dr G. Miller, Dr S.J. Molyneau, Dr H.E. Presig, Dr J.B. Riding and Dr J. Remane. Mrs K.L. Atkinson prepared the diagrams and new line drawings. In addition, a special thankyou is offered to all these authors and publishers who have kindly allowed the use of their illustrations and photomicrographs; formal acknowledgement is provided throughout the text. Without all these people this project would never have been completed and we are most grateful for their help.

Blackwell Publishing and the Natural History Museum London are the publishers of PaleoBase: Microfossils, a powerful illustrated database of microfossils designed for student use. Please see www.paleobase.com for ordering details, oremail [email protected]

PART 1

Applied micropalaeontology

CHAPTER 1

Introduction

Microfossils – what are they?

A thin blanket of soft white to buff-coloured ooze covers one-sixth of the Earth’s surface. Seen under the microscope this sediment can be a truly impressive sight. It contains countless numbers of tiny shells variously resembling miniature flügelhorns, shuttlecocks, water wheels, hip flasks, footballs, garden sieves, space ships and chinese lanterns. Some of these gleam with a hard glassy lustre, others are sugary white or strawberry coloured. This aesthetically pleasing world of microscopic fossils or microfossils is a very ancient one and, at the biological level, a very important one.

Any dead organism that is vulnerable to the natural processes of sedimentation and erosion may be called a fossil, irrespective of the way it is preserved or of how recently it died. It is common to divide this fossil world into larger macrofossils and smaller microfossils, each kind with its own methods of collection, preparation and study. This distinction is, in practice, rather arbitrary and we shall largely confine the term ‘microfossil’ to those discrete remains whose study requires the use of a microscope throughout. Hence bivalve shells or dinosaur bones seen down a microscope do not constitute microfossils. The study of microfossils usually requires bulk collecting and processing to concentrate remains prior to study.

The study of microfossils is properly called micropalaeontology. There has, however, been a tendency to restrict this term to studies of mineral-walled microfossils (such as foraminifera and ostracods), as distinct from palynology the study of organic-walled microfossils (such as pollen grains, dinoflagellates and acritarchs). This division, which arises largely from differences in bulk processing techniques, is again rather arbitrary. It must be emphasized that macropalaeontology, micropalaeontology and palynology share identical aims: to unravel the history of life and the external surface of the planet. These are achieved more speedily and with greater reward when they proceed together.

Why study microfossils?

Most sediments contain microfossils, the kind depending largely on the original age, environment of deposition and burial history of the sediment. At their most abundant, as for example in back-reef sands, 10 cm3 of sediment can yield over 10,000 individual specimens and over 300 species. By implication, the number of ecological niches and biological generations represented can extend into the hundreds and the sample may represent thousands if not hundreds of thousands of years of accumulation of specimens. By contrast, macrofossils from such a small sample are unlikely to exceed a few tens of specimens or generations. Because microfossils are so small and abundant (mostly less then 1 mm) they can be recovered from small samples. Hence when a geologist wishes to know the age of a rock or the salinity and depth of water under which it was laid down, it is to microfossils that they will turn for a quick and reliable answer. Geological surveys, deep sea drilling programmes, oil and mining companies working with the small samples available from borehole cores and drill cuttings have all therefore employed micropalaeontologists to learn more about the rocks they are handling. This commercial side to micropalaeontology has undoubtedly been a major stimulus to its growth. There are some philosophical and sociological sides to the subject, however. Our understanding of the development and stability of the present global ecosystem has much to learn from the microfossil record, especially since many microfossil groups have occupied a place at or near to the base of the food web. Studies into the nature of evolution cannot afford to overlook the microfossil record either, for it contains a wealth of examples. The importance of understanding microfossils is further augmented by discoveries in Precambrian rocks; microfossils now provide the main evidence for organic evolution through more than three-quarters of the history of life on Earth. It is also to microfossils that science will turn in the search for life on other planets such as Mars.

The cell

A great many microfossils are the product of single-celled (unicellular) organisms. A little knowledge of these cells can therefore help us to understand their way of life and, from this, their potential value to Earth scientists. Unicells are usually provided with a relatively elastic outer cell membrane (Fig. 1.1) that binds and protects the softer cell material within, called the cytoplasm (or protoplasm). Small ‘bubbles’ within the cytoplasm, called vacuoles, are filled with food, excretory products or water and serve to nourish the cell or to regulate the salt and water balance. A darker, membrane-bound body, termed the nucleus, helps to control both vegetative and sexual division of the cell and the manufacture of proteins. Other small bodies concerned with vital functions within the cell are known as organelles. The whip-like thread that protrudes from some cells, called a flagellum, is a locomotory organelle. Some unicells bear many short flagella, collectively called cilia, whilst others get about by means of foot-like extensions of the cell wall and cytoplasm, known as pseudopodia. Other organelles that can occur in abundance are the chromoplasts (or chloroplasts). These small structures contain chlorophyll or similar pigments for the process of photosynthesis.

Fig. 1.1 The living cell. (a) Eukaryotic cell structure showing organelles. (b) Cross-section through a flagellum showing paired 9+2 structure of the microfibrils. (Reproduced with permission from Clarkson 2000.)

Nutrition

There are two basic ways by which an organism can build up its body: by heterotrophy or by autotrophy. In heterotrophy, the creature captures and consumes living or dead organic matter, as we do ourselves. In autotrophy, the organism synthesizes organic matter from inorganic CO2, for example, by utilizing the effect of sunlight in the presence of chlorophylllike pigments, a process known as photosynthesis. Quite a number of microfossil groups employ these two strategies together and are therefore known as mixotrophic.

Reproduction

Asexual (or vegetative) and sexual reproduction are the two basic modes of cellular increase. The simple division of the cell found in asexual reproduction results in the production of two or more daughter cells with nuclear contents similar in proportion to those of the parent. In sexual reproduction, the aim is to halve these normal nuclear proportions so that sexual fusion with another ‘halved’ cell can eventually take place. Information contained in each cell can then be passed around to the advantage of the species. This halving process is achieved by a fourfold division of the cell, called meiosis, which results in four daughter cells rather than two.

The empires of life

Living individuals all belong to naturally isolated units called species. Ideally, these species are freely interbreeding populations that share a common ecological niche. Even those lowly organisms that disdain sexual reproduction (such as the silicoflagellates) or do not have the organization for it (such as the cyanobacteria), occur in discrete morphological and ecological species. Obviously it is impossible to prove that a population of microfossils was freely interbreeding but, if specimens are sufficiently plentiful, it is possible to recognize both morphological and ecological discontinuities. These can serve as the basis for distinguishing one fossil species from another.

Whereas the species is a functioning unit, the higher taxonomic categories in the hierarchical system of classification are mere abstractions, implying varying degrees of shared ancestry. All species are placed within a genus that contains one or more closely related species. These will differ from other species in neighbouring genera by a distinct morphological, ecological or biochemical gap. Genera (plural of genus) tend to be more widely distributed in time and space than do species, so they are not greatly valued for stratigraphical correlation. They are, however, of considerable value in palaeoecological and palaeogeographical studies. The successively higher categories of family, order and class (often with intervening sub- or super-categories) should each contain clusters of taxa with similar grades of body organization and a common ancestor. They are of relatively little value in biostratigraphy and palaeoenvironmental studies. In ‘animals’ the phylum taxon is defined on the basis of major structural differences, whereas in ‘plants’ the corresponding division has been defined largely on structure, life history and photosynthetic pigments.

An even higher category is the kingdom. In the nineteenth century it was usual to recognize only the two kingdoms: Plantae and Animalia. Plants were considered to be mainly non-motile, feeding by photosynthesis. Animals were considered to be motile, feeding by ingestion of pre-formed organic matter. Although these distinctions are evident amongst macroscopic organisms living on land, the largely aqueous world of microscopic life abounds with organisms that appear to straddle the plant–animal boundary. The classification shown in Box 1.1 overcomes these anomalies by recognizing seven kingdoms: the Eubacteria, Archaebacteria, Protozoa, Plantae, Animalia, Fungi and Chromista.

The highest category is the empire. The classification of the empire Bacteria will be considered further in Chapter 8. The Bacteria are single celled but they lack a nucleus, cell vacuoles and organelles. This primitive prokaryotic condition, in which proper sexual reproduction is unknown, is characteristic of such forms as cyanobacteria. The empire is currently divided into two kingdoms, the Archaebacteria and the Eubacteria. The other five kingdoms are eukaryotic. That is their cells have a nucleus, vacuoles and other organelles and are capable of properly coordinated cell division and sexual reproduction. Attempts to divide unicellular eukaryotic organisms, often called protists, into plants or animals based on feeding style were abandoned when it was recognized that dinoflagellates, euglenoids and heterokonts have members that are both photosynthetic and heterotrophic, feeding by engulfing. Since the 1970s both ultrastructural analysis under the scanning electron microscope and molecular sequences have been used to elucidate protistan phylogenies and develop a largescale classification. The new classification of Cavalier-Smith (1981, 1987a, 1987b, 2002) has put forward two new categories: the predominantly photosynthetic kingdom Chromista (brown algae, diatoms and their various relatives) and the primitive superkingdom Archezoa (which lack mitochondria (amitochondrial)). He has also proposed an ultrastructurally based redefinition of the kingdom Plantae which requires the exclusion of many aerobic protists that feed by ingestion (phagotropy). The kingdom Protozoa is now considered to contain as many as 18 phlya (Cavalier-Smith 1993, 2002) and their classification and phylogenetic relationships, which is in a state of flux, is largely based upon cell ultrastructure and increasingly sophisticated analyses of new molecular sequences. The kingdom

Protozoa includes two subkingdoms, the Gymnomyxa and Corticata. Members of the Gymnomyxa have a ‘soft’ cell wall often with pseudopodia or axopodia (e.g. foraminifera). The Corticata are ancestorally biciliate (e.g. dinoflagellates).

Fig. 1.2 The empires of life. (Modified from Cavalier-Smith 1993.)

Members of the superkingdom Archezoa differ from most Protozoa in having ribosomes, the RNA-protein structures on which messenger RNA is ‘read’ during protein synthesis, found in all other eukaryotes, and they also lack certain other organelles (e.g. mitochondria, Golgi bodies). The Archezoa comprise three phyla: the Archamoebae, Metamonada and Microsporidia. There is reasonable rDNA phylogenetic evidence to suggest that the latter two represent surviving relics of a very early stage in eukaryote evolution. The evolution of the eukayotes can thus be divided into two major phases. The origin of the eukaryote cell (the first archezoan) is marked by the appearance of the membrane-bounded organelles, cytoskeleton, a three-dimensional network of fibrous proteins that give order and structure in the cytoplasm, nucleus and cilia with a 9+2 structure (Fig. 1.1). This was apparently followed by the symbiotic origin of mitochondria and peroxisomes (Margulis 1981; Cavalier-Smith 1987c) to produce the first aerobically respiring protozoan. The change in their ribosomes may have occurred somewhat later in their evolution.

The kingdom Chromista is a predominantly photosynthetic category in which the chromoplasts are located in the endoplasmic reticulum but separated by a unique smooth membrane, thought to be a relic of the cell membrane of the photosynthetic eukaryotic symbiont that was ‘engulfed’ by the protozoan host, leading to the emergence of the Chromista (Cavalier-Smith 1981, 1987c). The Chromista contains a number of important microfossil groups such as the silicoflagellates, diatoms and calcareous nannoplankton.

The kingdon Plantae is taken to comprise two subkingdoms. The subkingdom Viriplantae includes the green plants including the green algae (Chlorophyta), the Charophyta and the ‘land plants’ or the Embryophyta. The subkingdom Biliphyta includes the red algae (Rhodophyta) and the Glaucophyta. It is not yet clear whether these two subkingdoms are correctly placed together in a single kingdom or should be separate kingdoms. The Viriplantae have starch-containing chloroplasts and contain chlorophylls a and b. The Biliphyta have similar chloroplasts but there is a total absence of phagotrophy in this group.

The kingdom Fungi comprises heterotrophic eukaryotes that feed by the adsorption of pre-formed organic matter. They are rarely preserved in the fossil record and have received little study as fossils and are not considered further in this book.

The kingdom Animalia comprises multicellular invertebrate and vertebrate animals that feed by the ingestion of pre-formed organic matter, either alive or dead. Invertebrates that are microscopic when fully grown, for example the ostracods, are considered as microfossils, but we are obliged to leave aside the microscopic remains of larger animals (such as sponge spicules, echinoderm ossicles and juvenile individuals). For more information on the macro-invertebrate fossil record the reader is referred to our companion volume written by Clarkson (2000).

Microfossils that cannot easily be placed within the existing hierarchical classification, for example acritarchs, chitinozoa and scolecodonts, are accorded the informal and temporary status of a group in this book.

REFERENCES

Cavalier-Smith, T. 1981. Eukaryote kingdoms: seven or nine? Biosystems14, 461–481.

Cavalier-Smith, T. 1987a. Eukaryotes without mitochondria. Nature (London) 326, 332–333.

Cavalier-Smith, T. 1987b. Glaucophyeae and the origin of plants. Evolutionary Trends in Plants2, 75–78.

Cavalier-Smith, T. 1987c. The simultaneous symbiotic origin of mitochondria, chloroplasts and microbodies. Annals of the New York Academy of Sciences503, 55–71.

Cavalier-Smith, T. 1993. Kingdom Protozoa and its 18 phyla. Microbiological Reviews57, 953–994.

Cavalier-Smith, T. 2002. The phagotrophic origin of eukaryotes and phylogenetic classification of protozoa. International Journal of Systematic and Evolutionary Microbiology52, 297–354.

Clarkson, E.N.K. 2000. Invertebrate Palaeontology and Evolution, 4th edition. Blackwell, Oxford.

Margulis, L. 1981. Symbiosis in cell evolution. Life and its Environment on the Earth. Freeman, San Francisco.

CHAPTER 2

Micropalaeontology, evolution and biodiversity

Micropalaeontology brings three unique perspectives to the study of evolution: the dimension of time, abundance of specimens (allowing statistical analysis of trends) and long complete fossil records, particularly in marine groups. Despite these features giving special insights into the nature of evolutionary processes, micropalaeontologists have until recently concentrated mainly on documenting the ascent of evolutionary lineages, such are described in the separate chapters in this book.

Micro- and macroevolution are the two main modes of evolution. Microevolution describes smallscale changes within species, particularly the origin of new species. Speciation occurs as the result of anagenesis (gradual shifts in morphology through time) or cladogenesis, rapid splitting of a pre-existing lineage. Which of these is the dominant mode has remained one of the most controversial questions in palaeobiology in the last 30 years.

Some of the best recorded examples of anagenesis have been documented in planktonic foraminifera (Malmgren & Kennett 1981; Lohmann & Malmgren 1983; Malmgren et al. 1983; Hunter et al. 1987; Malmgren & Berggren 1987; Norris et al. 1996; Kucera & Malmgren 1998), whilst examples of cladogenesis (e.g. Wei & Kennett 1988; Lazarus et al. 1995; Malmgren et al. 1996) are less widely cited. Similar studies have been conducted on Radiolaria (Lazarus 1983, 1986) and diatoms (Sorhannus 1990a, 1990b). Where morphological change has been mapped onto an ecological gradient (such as temperature/depth gradients measured by oxygen isotope analysis) it appears that gradual morphological trends do not strictly reflect the rate of speciation or its mode. For example, Kucera & Malmgren (1998) showed that gradual change in the Cretaceous planktonic foraminifera Contusotruncana fornicata probably resulted in a shift in the relative proportion of high conical to low conical forms through time. High conical forms evolved rapidly and gradually replaced the low conical morphs, though at any one time the abundances of different morphs were normally distributed. Similarly, Norris et al. (1996) documented a gradual shift in the average morphology of Fohsella fohshi over ∼400 kyr, suggesting only one taxon was present at any given time (Fig. 2.1), yet isotopic data indicated a rapid separation of the population, into surface- and thermoclinedwelling populations and reproductive isolation midway through the anagenetic trend. During the same interval keeled individuals gradually replaced unkeeled forms, a clear example of both anagenesis and cladogenesis occurring in the same population. Another ‘classic’ example of anagenetic change, that of Globorotalia plesiotumida and the descendant G. tumida (Malmgren et al. 1983, 1984), has been challenged by Norris (2000). G. plesiotumida ranges well into the range of G. tumida (e.g. Chaisson & Leckie 1993; Chaisson & Pearson 1997) and therefore cannot have given rise to G. tumida by the complete replacement of the ancestor population. An alternative explanation to this and probably all examples of anagenetic trends is that cladogenesis is quickly followed by a rapid change in the relative proportions of the ancestor and descendant populations. Apparently gradual changes in ‘mean form’ may be caused by natural selection operating on a continuous range of variation in populations living in environments lacking barriers to gene flow.

Macroevolution is concerned with evolution above the species level, the origins and extinctions of major groups and adaptive radiations. Microevolution and macroevolution processes are decoupled (Stanley 1979). This is because the individual is the basic unit of selection in microevolution whilst selection between species may occur at higher levels, although the notion of competition and natural selection occurring between higher taxonomic categories is not unanimously accepted (see Kemp 1999). New structures, body plans and biochemical systems, and the characters of high taxonomic categories, appear suddenly in the fossil record, for example the appearance of calcification in the calcareous nannoplankton in the Early Mesozoic. The evolutionary mechanisms behind these changes are the least well understood of evolutionary phenomena. Explanations invoke mutation in regulatory genes, which encode for hormones and other rateeffecting proteins and wholesale changes in chromosomal structure.

Fig. 2.1 Changes in morphology and habitat during the evolution of the planktonic foraminifera Fohsella from the mid-Miocene. On the left, frequency histograms show the gradual (anagenetic) change in the morphology of the shell outline. On the right, stable oxygen isotope data from the same specimens show an abrupt appearance of a new thermocline-reproducing species (cladogenesis). The ancestor became extinct ∼70 kyr after the appearance of the descendant species. Morphological data suggest that no more than one population was present at any one time. (Redrawn from Norris et al. 1996 with permission.)

Mass extinctions are probably the most widely studied of the macroevolutionary patterns. These differ from ‘background’ extinction events in their speed (commonly <5 Myr) and intensity (where 20–50% of marine biodiversity may disappear in a single event). The Cretaceous–Tertiary boundary mass extinction provides the best-studied example of a mass extinction event. This been documented globally and has been attributed to a variety of terrestrial (including climate change) and extraterrestrial (meteorite impact) causes (see Hallam & Wignall 1997 for a review). A comprehensive review of the biological effects of the K-T mass extinction event is provided by MacLeod & Keller (1996). Patterns of extinction in individual groups add little to the debate on the cause of the K-T mass extinction. For example, extinctions in planktonic foraminifera extend over an interval of 30 cm (<100,000 years) that spans the boundary and exhibit a preferential extinction of large ornate forms. Benthic foraminifera declined in diversity but were much less affected. Coccolithophorids were once thought to become almost extinct at this boundary, however Cretaceous species found in the lower Tertiary are now considered to have survived the event (Perch-Nielsen et al. 1982). Dinoflagellates were evidently less affected by events at the boundary. Dinoflagellate cysts are extremely abundant in the boundary clay, indicating that environmental conditions were ideal for stimulating dinoflagellate blooms. Diversity and species turnover rates are also high across the boundary. Plants on the other hand show major changes, Wolfe & Upchurch (1986) noted the decline in pollen and a sharp peak in fern spores, suggesting the influence of wildfires, though increasing humidity could also have caused an increase in fern abundance.

Mechanisms of cladogenesis

Models of cladogenesis rely upon the genetic isolation of a population. Random mutations in these small populations (peripheral isolates) are then quickly spread and eventually lead to the development of a new species, a process known as allopatric speciation (Fig. 2.2). In the marine realm genetic isolation would at first sight seem less probable. However a number of ecological barriers are present in the oceans. For example, ocean frontal systems, such as the Tasman Front, a boundary between tropical and subtropical water masses, have been proposed as effective barriers to dispersal and may have been important in promoting allopatric speciation in globoconelid planktonic foraminifera during the Pliocene (Wei & Kennett 1988). Vicariant models of speciation similarly subdivide an original population into smaller units through the development of physical barriers such as land barriers, sea-level fall and the strengthening of water mass boundaries. Knowlton & Weight (1998) have documented many examples of vicariant speciation in the marine realm following the separation of the Atlantic and the Pacific oceans through circulation changes during the Pleistocene. Low sea levels during the Pleistocene have also been implicated in the speciation of copepods on either side of the Indonesian Seaway (Fleminger 1986). However, many planktonic foraminifera species have the ability to cross such major barriers; Pullenatina obliquiloculata and other related species repeatedly reinvaded the tropical Atlantic from the Indo-Pacific during Pleistocene glacial cycles. Neither equatorial upwelling in the Atlantic nor the Isthmus of Panama were sufficient barriers to dispersal.

Many microfossil groups are planktonic and have high population sizes and high dispersal potential. These features would seem contrary to the conditions required for allopatric speciation. Species models that allow restricted genetic exchange may therefore be better explanations of speciation in these types of organisms.

Variation in morphology along geographical gradients (clines) can result in limited interaction between the ends of the cline and effective genetic isolation (‘isolation-by-distance’ or parapatric speciation). Clinal trends have been described in a wide range of marine planktonic organisms (van Soest 1975; Lohmann & Malmgren 1983; Lohmann 1992), though some believe these may represent geographical successions of distinct species (see below). Even the classical latitudinal morphological cline of Globorotalia truncatulinoides, originally described as continuous (Lohmann & Malmgren 1983) may contain distinct species (Healy-Williams et al. 1985; de Vargas et al. 2001).

Similarly ‘isolation-by-ecology’ appears common, and is particularly well documented for depth in foraminifera. Many forams reproduce by sinking (Norris et al. 1996), during which they cross the large number of physical and chemical barriers in the ocean. It seems plausible that speciation could occur by changes in the depth of reproduction, though confirmatory evidence is still rather sparse. Norris et al. (1993, 1996) used stable oxygen isotopes to show that the evolution of Fohsella fohsi in the mid-Miocene involved a rapid shift in reproductive depth habitat (Fig. 2.1). Using similar methodology Pearson et al. (1997) calculated 1–2°C differences in the temperature at which calcification occurred in closely related species, relating this to differences in either season or depth of growth. As the seasonal range in temperature of surface waters in the tropics and subtropics can be greater than this it is reasonable that divergence in these species could have occurred as the result of a shift in timing of reproduction and growth (‘seasonal sympatry’).

Fig. 2.2 Speciation models. (a) Allopatry, created by divergence on either side of a hydrographic boundary. (b,d) Parapatry in which species diverge along a gradual hydrographic gradient, for example a gradually changing thermocline depth (b) or depth (d). (c) Vicariance, occurs where a physical boundary creates isolation and the formation of a new species. (e) ‘Seasonal sympatry’ in which isolation is caused by a change in the timing of reproduction. In marine planktonic species complete genetic isolation as indicated in (a) and (c) is unlikely. (Redrawn after Norris 2000 with permission.)

Theoretical and empirical studies (e.g. Howard & Berlocher 1998) have also indicated sympatric speciation may be more common in the marine realm than has been hitherto considered. Sympatry may have resulted from individuals evolving different strategies to avoid strong competition for a single food source (Dieckmann & Doebell 1999), or from disruptive selection which favours individuals with extreme characters, for example large and small predators at the behest of medium sized individuals (e.g. Kondrashov & Kondrashov 1999; Tregenza & Butlin 1999).

Biodiversity in the marine plankton

Briggs (1994) calculated there are approximately 12 million terrestrial multicellular species (approximately 10 million of which are insects!) but only 200,000 marine taxa. These are surprising numbers when models of ecosystem size, energy flow and environmental stability predict substantially higher numbers of marine to terrestrial taxa (Briggs 1994). Are the models or numbers incorrect?

Results of molecular phylogenetic analyses indicate there is a high cryptic biodiversity in the oceans. Numerous sibling species can be diagnosed using molecular sequence data but show few if any morphological differences (e.g. Bucklin 1986; Bucklin et al. 1996; Bucklin & Wiebe 1998), a feature that probably extends into the cyanobacteria (Moore et al. 1998) and bacterio-plankton (De Long et al. 1994). Cryptic speciation and high genetic diversity has also been documented for planktonic foraminifera (Huber et al. 1997; de Vargas & Pawlowski 1998; Darling et al. 1999; de Vargas et al. 1999) and, surprisingly, many morphologically similar taxa have ancient divergences. Distinguishing sibling species in the fossil record is extremely difficult and many previously defined ecological variants (ecophenotypes) may be distinct species; if this is the case then planktonic biodiversity has been grossly underestimated.

Reconstructing phylogeny

The higher classification (above species level) of a group of organisms should reflect their evolution. The taxonomic hierarchy is expressed as an upwardly inclusive nested heirarchy, similar species are grouped into genera, similar genera into families, families into orders, orders into classes and classes into phyla and where necessary subdivisions of these major categories, for example subfamily and superfamily, are also used. Higher taxonomic categories are distinguished by their suffix (i.e. -ae, -a, etc.) and many examples are included in subsequent chapters.

Defining higher taxonomic groupings is a largely subjective exercise. Until the 1970s classical taxonomists used a combination of morphological (or phenetic) similarity and phylogenetic (evolutionary) resemblance, based on ill-defined notions of ancestor–descendant relationships. Stratigraphical succession of species and their geographical distribution played an important role in establishing phylogenetic relationships. Since the 1970s an attempt has been made to reduce the subjectivity inherent in the classical method and two philosophical approaches have been followed. Phenetics (or numerical taxonomy) relies on scoring of characters. Cluster analysis and distance statistics can then be used on the resulting character matrix to quantify the similarities between taxa and groupings into higher taxonomic categories. Cladistics (or phylogenetic systematics), founded by W. Hennig (1966) has been much more widely applied to palaeontology though less so in micropalaeontology. The reader is referred to Smith (1994) for a comprehensive explanation of the methodology. At the heart of cladistics is the concept that organisms contain a combination of ‘primitive’ (symplesiomorphic) and evolutionary novelties (synapomorphic) or ‘derived characters’. Closely related groups will share derived characters and these will distinguish them from other groups. For example, humans have a backbone, a primitive character of all vertebrates, and an opposable thumb, a derived character shared with our nearest relatives the great apes. A primitive character for all vertebrates, the backbone, is of course a derived character as compared to invertebrates. Synapomorphy and symplesiomorphy are therefore relative conditions of particular characters with reference to a particular phylogenetic reconstruction.

The results of a phylogenetic analysis are expressed in a cladogram, in which branching points are arranged in nested hierarchies. In the example in Fig. 2.3 C and D share a unique common ancestor, they are sister groups and share a synapomorphy not possessed by B. Thus B is the sister group of the combined grouping C + D and A is the sister group of B + C + D. If a large number of characters and taxa are being analysed the character matrix is routinely manipulated by computer programs such as PAUP (Phylogenetic Analysis Using Parsimony). This is a technique that makes the fewest assumptions (parsimony) to rank the set of observations and produce the cladogram. A cladogram is not an evolutionary tree but a hypothesis of relationships. Stratigraphical succession is explicitly ignored in the analysis. Once the cladogram has been produced stratigraphical succession can be used in the analysis of the cladogram (see Smith 1994) and to constrain the splitting of lineages in time. At this point the cladogram becomes a phylogenetic tree.

Fig. 2.3 A cladogram showing the phylogenetic relationship between A and D (see text for explanation).

Distinguishing shared primitive (sympleisiomorphic) and shared derived (synapomorphic) characters is achieved by outgroup analysis. Here the ingroup, the group being studied, is compared to a closely related outgroup. In Fig. 2.3 B + C + D could be the ingroup and A the outgroup. Any character present in a variable state in the ingroup and found in the outgroup must be plesiomorphic (primitive). Apomorphic characters are those only found in the ingroup.

Three types of cladistic groups have been defined: monophyletic groups contain the common ancestor and all of its subsequent descendants; paraphyletic groups are descended from a common ancestor (usually extinct) but do not include all the descendants; polyphyletic groups are the result of convergent evolution. In the latter, their representatives are descended from different ancestors and though looking superficially similar, there is no close phylogenetic relationship.

Subjectivity cannot be entirely removed from phylogenetic reconstruction and higher taxonomic categories. In cladistics equally parsimonious cladograms can result from the analysis and choosing between these may become subjective. In numerical taxonomy the methods of measurement and the relative weighting given to characters are also subjective decisions. The possibility of morphological convergence during evolution is a problem for all taxonomic methods and ultimately molecular sequence data may be required to distinguish between polyphyletic and sibling species. Unfortunately such data are not available in extinct groups.

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CHAPTER 3

Microfossils in stratigraphy

The stratigraphical column

The succession of rocks exposed at the surface of the Earth can be arranged into a stratigraphical column, with the oldest rocks at the base and the youngest ones at the top (Fig. 3.1). Although the absolute ages have been determined from studies of radioactive isotopes, it is customary to use the names of stratigraphical units, mostly distinguished on the basis of differences in their included fossils. These units are arranged into a number of hierarchies relating to rock-based stratigraphy (lithostratigraphy), fossil-based stratigraphy (biostratigraphy) and time-based stratigraphy (chronostratigraphy).

Lithostratigraphical units, such as beds, members and formations, are widely used in geological mapping but will not concern us further here. The biozone is the fundamental biostratigraphical unit and comprises those rocks that are characterized by the occurrence of one or more specified kinds of fossil known as zone fossils.

Formal chronostratigraphical time units are also important and include, in ascending order of importance, the age, epoch, period and era. For example we may cite the Messinian Age, of the Miocene Epoch, of the Neogene Period, of the Cenozoic Era. Rock units laid down during these times are properly referred to as stages, series, systems and erathems (i.e. the Messinian Stage, of the Miocene Series, etc.). Less formal divisions are also widely used so that we may talk of the lower Neogene rocks laid down during Early Neogene times. In the following text, these informal subdivisions are abbreviated as follows: lower (L.), middle (M.) and upper (U.) and their equivalents for chronostratigraphy early (E.), middle and late.

Microfossils and biostratigraphy

Biostratigraphy is the grouping of strata into units based on their fossil content with the aim of zonation and correlation. As such biostratigraphy is concerned primarily with the identification of taxa, tracing their lateral and vertical extent and dividing the geological column into units defined on their fossil content.

Microfossils are among the best fossils for biostratigraphical analysis because they can be extremely abundant in rocks (a particular consideration when dealing with drill cuttings) and they can be extracted by relatively simple bulk processing methods. Many groups are geographically widespread and relatively free from facies control (e.g. plankton, airborne spores and pollen). Many of the groups evolved rapidly, allowing a high level of subdivision of the rock record and a high level of stratigraphical resolution. It should also be emphasized that spores, pollen, diatoms and ostracods are indispensable for the biostratigraphy of terrestrial and lacustrine successions, where macrofossils can be scarce.

Detailed biostratigraphical zonations, using the groups mentioned in this book, have been developed for the entire Phanerozoic. Some areas of the column are better subdivided than others, for example the Cretaceous to Recent can be subdivided into approximately 70 biozones, based on calcareous nannoplankton and planktonic foraminifers, with an average duration of 2 million years per biozone. In comparison the Lower Palaeozoic has only been divided into 39 conodont biozones at an average duration of 3 million years. Detailed biostratigraphical zonations for the Mesozoic and Cenozoic are to be found in the two volumes of Plankton Stratigraphy (Bolli et al. 1985). The biostratigraphy of selected microfossil groups can be found in the ‘Stratigraphic Index’ series published by The Micropalaeontological Society and a host of specialist papers in scientific journals. The additional reading lists in this book provide an entry into this literature.

Fig. 3.1 The stratigraphical column (modified from the IUGS correlation chart). British stage/age names have been retained for the Ordovician and Cambrian systems/periods as these have to be internationally agreed. Whittaker et al. (1991) gives further information on stratigraphical terminology.

Fig. 3.2 Categories of biozones. (After Bassett in Briggs & Crowther 1987 with permission.)

The basic unit of biostratigraphy is the biozone and fossils that characterize and give their names to a particular biozone are called zone or index fossils, for example the Orbulina universa Biozone of the Miocene. There are three basic types of biozone: the assemblage, abundance and interval biozones (Fig. 3.2). An assemblage biozone is based on the association between three or more species (though this concept is often more loosely applied) with little regard to the stratigraphical range of each. As species associations are strongly dependent upon local ecology, this type of biozone is most suitable for local or intra-basinal applications. The majority of defined biozones are interval biozones based upon the first appearance datum (FAD) and last appearance datum (LAD) of the named species. There are five types of interval biozone (Fig. 3.2), the most commonly used being the local range zone and the concurrent range zone. The latter comprises that interval which lies above the FAD of one species and below the LAD of a second species. The interval between two successive LADs is called a successive last appearance zone and is the most commonly used zone in commercial biostratigraphy where most of the samples are from borehole cores or cuttings and the FAD of a species cannot always be determined due to down-hole contamination (‘caving’).

Quantitative biostratigraphy

Because microfossils can occur in large numbers they are ideal for use in quantitative methods of biostratigraphy. Over the past 20 years a large number of techniques have become available for measuring biostratigraphical utility, defining and testing the error on a biozone and developing and testing correlations (Armstrong 1999). Typically quantitative methods are best applied to planktonic groups from continuous sections where FADs and LADs can be accurately determined. The most commonly used methods are semi-quantitative methods such as the graphical correlation technique developed by Shaw (1964). Details of this technique can be found in Armstrong (1999).

Graphical correlation uses a two-axis graph to compare the FADs and LADs of taxa found in common between two sections (Fig. 3.3). The heights of the first and last appearances of species are plotted as coordinates in the field of the graph. A line of correlation (LOC) is then drawn through the scatter of points either by hand or using a variety of statistical techniques (e.g. least squares, linear regression or principal components analysis). The LOC is then used to transfer species range data from one section to the other. The latter becomes the composite standard reference section (CSRS). Additional sections are similarly correlated with the CSRS and included range data is also transferred to the composite, so that species ranges are progressively extended with the addition of new sections. When all the data from all available sections have been added, further rounds of correlation are undertaken to refine and stabilize the position of the LOCs. If only a small number of sections are to be correlated then the graphical correlation can be carried out by hand; computer packages are available for correlating large numbers of sections.

Fig. 3.3 Example of a graphical correlation. Shows the correlation of a new section with the composite standard reference section (CSRS). Sections have been correlated using the 25 and 30 standard time unit (stu) datum lines via a line of correlation (LOC) which exhibits changes in sedimentation rate and an unconformity plateau. The changing slope of the LOC Curve shows an increased rate of deposits above the unconformity, relative to the CSRS. Once the correlation has been made, other data, for example radiometric dates (85 Ma, 120 Ma) or isotope excursions, can be transferred into the CSRS via the LOC. Open circles, base of range; crosses, top of range.

Species ranges within the CSRS should span the maximum within the included sections. Where sections are included that cover a wide range of geographical and palaeoecological settings, then these ranges should approach the full temporal span of that species. Lithological, geochemical and palaeomagnetic data can also be included in the CSRS and help strengthen the correlations.

The CSRS can be divided into units of equal length (standard time units-stu). The resultant chronometric timescale can then be transferred into the original sections using the LOCs. Standard time unit datum planes can be matched to provide a high resolution correlation of all the sections. This method of correlation is particularly useful for illustrating diachronous lithostratigraphical events: those that appear to be the same but occur at different times in different localities, between sections (e.g. progradation of sedimentary strata or facies or the diachronous nature of an unconformity).

The high resolution available using graphical correlation (limited only by the accuracy in placing the LOC) provides the only means by which the predictions of sequence stratigraphical correlation models can be independently tested (see below). The slope and geometry of the LOC is taken to reflect the relative rates of sedimentation between the two sections being compared. Strata that are missing, owing to faulting or a hiatus, or a highly condensed sequence, will appear as a plateau in the LOC (Fig. 3.3).

Fig. 3.4 Palaeoenvironmental distribution of some of the main microfossil groups through time. These are placed in a sequence stratigraphical framework. Insert shows the principle sequence statigraphical terms related to rising and falling sea level. HST, highstand systems tract; LST, lowstand systems tract; mfs, maximum flooding surface; sl, sea level; ts, transgressive surface; TST, transgressive systems tract; SB, sequence boundary. (After Hogg in Emery & Myers 1996 with permission.)

Microfossils in sequence stratigraphy

Sequence stratigraphy represents a powerful method for analysing familiar stratigraphical concepts such as transgression, regression and eustatic cycles and microfossils have a key role to play in sequence interpretation. The methods were largely developed as an extension of seismic stratigraphy and the need for correlation in the subsurface, but are equally applicable to outcrop geology where they have proved invaluable in understanding the influence of climate change on sedimentary successions. The reader is directed to Emery & Myers (1998) for a more detailed review of the principles of sequence stratigraphy. The basic philosophies of sequence stratigraphy are, firstly, that sediment accumulation occurs in discrete sequences, which are relatively conformable successions bounded by unconformities (or the correlative conformities in deep water). A sequence is considered to represent all the sediments deposited in an interval of time (0.5–5 Ma) and the sequence boundaries (intervals of no or very slow deposition) are considered effectively synchronous over large areas and can be used for matching sections. Secondly, the interaction of the rates of relative sea-level changes (eustasy), basin subsidence and sediment supply lead to variations in accommodation space, which is the space potentially available for sediment accumulation. The fundamental building blocks of sequences are parasequences, which generally represent shallowing or coarsening upwards cycles of short duration (10–100 kyr).

Every sequence comprises three systems tracts and potentially has a distinctive assemblage of microfossils (Fig. 3.4): a lower one representing periods of rapid but decelerating sea-level fall (LST, lowstand systems tract); a middle one relating to increasing acceleration in sea-level rise (TST, transgressive systems tract); and an upper one relating to a decreasing rate of sea-level rise and initial sea-level fall (HST, highstand systems tract). The base of each systems tract is defined as the sequence boundary, transgressive surface and maximum flooding surface respectively.

The interplay of environmental conditions, biological evolution, preservation potential of the microfossil group and cyclic changes in depositional style control the microfossil content of different sedimentary sequences. In a sequence stratigraphical analysis, it is the primary role of the micropalaeontologist to document changes in biofacies, and hence palaeoenvironment, and to provide a high-resolution biostratigraphical framework.

In the oil industry benthic foraminifera are commonly used to define marine benthic palaeoenvironments, although conodonts, ostracods and benthonic algae have also been used. Palynofacies analysis is most useful in defining fluvio-deltaic subenvironments (e.g. Brent Field, North Sea, Parry et al. 1981; see also Tyson 1995 for a review of palynofacies in sequence stratigraphy). Terrestrial microfossil assemblages can also provide a detailed record of climate changes around the margins of the sedimentary basin. With increasing knowledge of the ecological controls on microfossil groups, the relative abundances of different marine groups can be used to elucidate the changing palaeooceanography.

The transport or reworking of species into the marine environment by wind (e.g. bisaccate pollen) or rivers (e.g. miospores, charophytes, ostracods and woody material) or tides (e.g. foraminifera, dinoflagellates) can be problematic in biostratigraphy and palaeoenvironmental analysis. However the abundance gradients and size range of these derived fossils can be used to indicate the proximity of the source, location of palaeo-shorelines and exposure and uplift histories of the hinterland.

Few published studies have integrated the biostratigraphy, biofacies analysis and sequence stratigraphy. Exceptions include Armentrout (1987), Loutit et al. (1988), McNeil et al. (1990), Allen et al. (1991), Armentrout & Clement (1991), Armentrout et al. (1991), Jones et al. (1993) and Partington et al. (1993).

Sequence boundaries (SB) and correlative conformities

A sequence boundary is produced by a fall in relative sea level and may be associated with considerable erosion of the underlying sequence. It can be recognized by discrepancies in age and palaeoenvironment across the SB. The scale of these differences reflects the magnitude of the sea-level fall and location within the basin (McNeil et al. 1990). For example a SB can be characterized by a marked hiatus in nearshore sections or by subtle changes in biofacies across the correlative conformity within deep basinal settings. Our ability to resolve sequence boundaries biostratigraphically is limited by the biozonal resolution of the index fossils, commonly ~1 Ma or less if graphical correlation is used. Absence of preserved microfauna may mark the period of maximum regression. Reworking of specimens associated with erosion is commonplace above sequence boundaries.

Lowstand systems tract

This comprises two components, the lowstand wedge and fan. Both are produced by gravity sliding as sediment provided by rivers bypasses the shelf and upper slope through incised valleys and canyons which cut the continental shelf. Consequently both wedge and fan deposits will contain reworked terrestrially derived material and older, often polycyclic, marine microfossil assemblages when compared with adjacent shales with indigenous microfossil assemblages. Lowstand fan deposits in the Palaeogene of the North Sea, for example, only contained an impoverished microfauna comprising long-ranging agglutinated foraminifera.

The lowstand wedge is initiated as sea level begins to rise and can be progradational (sediment supply is greater than the rate of relative sea-level rise; facies belts migrate basinwards) or aggradational