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from the 2.5 Ga Ghaap Plateau Dolomite in South

Africa and the 2.1–1.8 Ga Gunflint Formation in

western Ontario, Canada. These microfossils display

coccoid (roughly spherical), septate filamentous, un-

branched tubular, and budding bacteria-like structures

(Figure 3).

Biomarkers

Biomarkers are biologically formed (biosynthesized)

organic molecules that can be attributed to a specific

biological origin. They possess molecular structures

and isotopic characteristics that enable them to be

distinguished from abiogenic organic compounds that

Figure 12 Scanning electron microphotographs of microfossil-like structures that were produced abiologically in the laboratory.

These ‘biomorphs’ display many complex characteristics that mimic structures of biological origin. Photographs courtesy of Stephen

Hyde, Australian National University.

Figure 13 Putative microfossil from the Apex Chert, Pilbara region, Western Australia. Such structures have been interpreted as the

fossilized remains of bacteria that flourished around 3.5 billion years ago. Photograph courtesy of J. W. Schopf.

292 BIOSEDIMENTS AND BIOFILMS

are distributed throughout the cosmos. In other words,

they are a kind of molecular fossil that can: (1) provide

evidence of life; and (2) provide information about

the nature of those life forms, even in the absence of

recognizable fossils. Figure 14 describes the formation

of biomarker molecules as organisms degrade during

burial and heating.

The oldest known biomarkers are from organic ma-

terial in the 2.7 Ga Fortescue Group of the Pilbara

region, Western Australia. The molecules were identi-

fied as originating from the group of organisms known

as eukaryotes (see Precambrian: Eukaryote Fossils)

(Figure 1).

Chemical Fossils

Microbial activity can be traced, in some instances,

by the chemical signatures left behind in rocks. How-

ever, at present, our ability to detect these signatures

may exceed our ability to interpret them. We know

that microbes act as agents of dispersion, concentra-

tion, or fractionation of different chemical compon-

ents and, in doing so, may impart specific chemical

signatures upon their environment. However, we also

know that similar patterns may be produced by abio-

logical mechanisms. In the following paragraphs, we

investigate some of the chemical signatures that can

be used to trace biological activity, and some of the

proposed problems with their interpretation.

Biological isotopic fractionation signatures form

when organisms discriminate between light and heavy

isotopes of elements, such as carbon, hydrogen, nitro-

gen, and sulphur. Both the light and heavy isotopes

are involved in abiotic reactions, whereas organisms

that utilize carbon compounds in their metabolic re-

actions almost always prefer the lighter

C isotope.

The carbonaceous remains of the organisms are en-

riched in

C, whereas the source material from

which they derived their carbon is enriched in

Carbon-bearing minerals (e.g., calcium carbonate

or limestone) that precipitate from the residual

C-enriched material will acquire the heavy carbon

signature. Other known biological fractionation pro-

cesses involve the preferential assimilation of

N over

S over

S, and hydrogen over deuterium.

Thus, isotopic analyses of organic matter and their

host mineral deposits can yield evidence of bio-

logical activity through fractionated C, N, S, and H

signatures.

Carbon isotope fractionation patterns have been

found in Early Archaean carbon-bearing rocks, such

as the Strelley Pool Chert and Dresser Formation

(Pilbara, Western Australia) and the Buck Reef Chert

(Barberton, South Africa). Carbonaceous material

(biogenic material?) with low values for

from these formations may have derived from the

biological fractionation of carbon isotopes. However,

Fischer–Tropsch synthesis has been proposed as an

alternative mechanism for carbon isotope fraction-

ation. Fischer–Tropsch synthesis is a chemical reac-

tion process involving carbon that is thought to occur

in the mantle in the presence of certain catalytic com-

pounds, such as Fe and Mn. Because Fischer–Tropsch

synthesis is a high temperature process, and requires

the presence of certain compounds, the geological

setting of a fractionated C isotope signature may pro-

vide clues to differentiate Fischer–Tropsch-type occur-

rences from biological occurrences. However, the

distinction is not always easily made and controversy

persists with regard to the biological interpretation

of light carbon signatures in Early Archaean rocks.

Beyond the morphological fossil record, which

extends back to around 3.5 Ga, evidence for life

may only be found via chemical fossils in ca. 3.8 Ga

rocks from Greenland. These rocks are too meta-

morphosed to contain any morphological remains.

However, carbon within the rocks is enriched in the

light isotope,

C. This enrichment may have resulted

from the biological fractionation of carbon, and has

Figure 14 Schematic diagram of biomarker formation from the degradation of biological organic material. As the organic material

is heated over millions of years, functional groups are lost from the molecular structure. In some cases, the remaining structure may

still provide clues to the organism from which the organic material was derived. Adapted from original unpublished illustration by

J. Brocks, with permission.

BIOSEDIMENTS AND BIOFILMS 293

thus been interpreted as the oldest evidence for life on

Earth. However, in the absence of original sediment-

ary features and context, the interpretation of these

signatures is contentious.

Clearly, the fossil that sets the irrefutable starting

point for life on Earth has not been found. However, if

the interpreted complexity of microbial communities

seen in the fossil record at around 3.5 Ga is anything

to go by, it seems likely that life on Earth began well

before then. The remains of the very first microbes, or

the rocks that they were part of, have been destroyed

or metamorphosed beyond recognition.

Glossary

biogenic Of biological origin.

Ga Billion years before present/billion years of age.

lacustrine Associated with lake environments.

metamorphism Pressure- and temperature-related

alteration of rocks during burial (involving pro-

cesses that result in new textural and mineralogical

characteristics).

Further Reading

Banfield JF and Nealson KH (1997) Geomicrobiology:

Interactions Between Microbes and Minerals. Washing-

ton: Mineralogical Society of America.

Brocks JJ and Summons RE (2003) Sedimentary hydro-

carbons, biomarkers for early life. In: Holland HD and

Turekian K (eds.) Treatise in Geochemistry, pp. 63–115.

Amsterdam: Elsevier.

Buick R (1990) Microfossil recognition in Archaean rocks:

an appraisal of spheroids and filaments from a 3500 M.Y.

old chert–barite unit at North Pole, Western Australia

Palaios 5: 441–459.

Ehrlich HL (2000) Geomicrobiology. New York: Marcel

Dekker.

Grotzinger JP and Knoll AH (1999) Stromatolites in Pre-

cambrian carbonates: evolutionary mileposts or environ-

mental dipsticks? Annual Reviews, Earth and Planetary

Science Letters 27: 313–358.

Hoffman HJ (2000) Archaean stromatolites as microbial

archives. In: Riding R and Awramik S (eds.) Microbial

Sediments, pp. 315–327. Berlin, Heidelberg: Springer-

Verlag.

Lowe DR (1994) Abiological origin of described stromato-

lites older than 3.2 Ga. Geology 22: 387–390.

Schopf JW (1983) Earth’s Earliest Biosphere. Princeton,

NJ: Princeton University Press.

Schopf JW and Packer BM (1987) Early Archaean

(3.3 billion to 3.5 billion year old) microfossils from

Warrawoona Group, Australia. Science 237: 70–73.

Walter MR (1976) Stromatolites. Amsterdam: Elsevier.

BIOZONES

N MacLeod, The Natural History Museum,

London, UK

Introduction

The contemporary concept of biozone is that of a

stratigraphical interval defined on the basis of its biotic

content. This deceptively simple definition serves as

the door to vast realms of complexity, access to which

is important not only because those realms contain

much of the early history of geology, but also because

they encompasses the single most important set of

tools yet devised for reconstructing earth history. The

biozone concept is built on the work of William Smith,

who first demonstrated the importance of fossils for

establishing sequences of geological events. Smith’s

ideas were extended by many nineteenth-century

geologists and were instrumental in creating the

geological time-scale (see Time Scale): one of the

greatest scientific achievements of that century. For

the last 100 years, various biozone types – range bio-

zones of diverse types, assemblage biozones, interval

biozones, acme biozones – have been employed

throughout the Earth sciences where they have proven

their worth in fields as diverse as palaeoceanography,

palaeogeography, palaeoecology, and palaeoclimate

studies; wherever there is a need for relative time cor-

relation. In the twenty-first century, use of biozones

remains at the forefront of stratigraphical analy-

sis, with methodological improvements being made

through use of the concept in quantitative stratigraphy

(see Stratigraphical Principles).

In order to understand the concept of a biozone,

one must understand its development. As noted

above, foundations for an understanding of biozones

were laid by William Smith who, in 1796, realised

that the rock layers cropping out around the southern

English town of Bath always occurred in the same

294 BIOZONES

superposed order and that individual strata contained

the same types of fossils. This insight provided Smith,

who was engaged in surveying the routes of canals,

with a practical tool whereby he could predict what

rock types would be encountered during a canal’scon-

struction. Smith’s observations of fossils were particu-

larly important to his method because strata that

superficially appeared similar (e.g., limestones, sand-

stones, shales), but that contained different sets of

fossils, could be distinguished from one another and

assigned to their correct positions within the overall

sequence (see Sequence Stratigraphy). Using this ap-

proach, Smith was, by 1799, able to reconstruct

the basic sequence of strata from the British Coal Meas-

ures (Carboniferous (see Palaeozoic: Carboniferous))

to the Chalk (Cretaceous (see Mesozoic: Cretaceous)).

Extension of this method to other regions resulted in

Smith producing the first geological map in 1815, the

fossil evidence for which was publishing in his 1817

book Stratigraphical System of Organized Fossils.

Smith’s work contained two further insights that

proved crucial to development of the biozone con-

cept. The first of these was that even a single stratum

could be subdivided on the basis of its fossil content.

This observation established variations in lithological

type and fossil content as independent of one another,

with fossil content often providing the more refined

basis for subdivision. The second was that fossils

from the same lithological types in different parts of

the sequence often resembled one another. This ob-

servation established that there was a relation be-

tween the general types of fossils that occurred in

different depositional environments.

In 1808 Smith showed his collection of fossils ar-

ranged stratigraphically to members of the Geological

Society of London. By the 1820s Smith’s methods had

been accepted by most geologists and extensions had

begun to appear, most notably by the work of

Georges Cuvier and Alexander Brongniart (in 1822)

in establishing the stratigraphical sequence of the

Paris Basin. Later, Gerard Deshayes (in 1830),

Heinrich Georg Bronn (in 1831), and especially

Charles Lyell (in 1833) extended Smith’s concepts

by formulating subdivisions of Tertiary strata based

on the sequence of fossils alone. This represented an

important step toward the conceptualisation of the

biozone in that it demonstrated the independence of

palaeontological and lithological observations. Since

changes in fossil morphologies were arranged in a

recognisable and predictable sequence over both

time and space, careful comparison of fossils with

established sequences allowed stratigraphers to infer

relative time correctly. Sequences of lithological

changes failed to exhibit a similarly predictable

pattern of variation over time.

The term ‘zone’ had been used informally by a

number of geologists in the early and mid-1800s to

denote a vertical interval of strata or assemblage of

co-occurring fossil species. For example, Alcide

d’Orbigny (1842–1851) used the term ‘stage’ and

‘zone’ interchangeably as subdivisions of Jurassic

strata based on their ammonite content (see Mesozoic:

Jurassic). The modern concept of the biostratigraphic

zone, however, can be traced to the writings of Albert

Oppel (e.g., Die Juraforation Englands, Frankreichs

und des su

dwestlichen Deutchlands, 1856–1858) who

used the term to denote ‘the constant and exclusive

occurrence of certain species [that] mark themselves

off from their neighbours as distinct horizons’ (trans-

lated in Arkell, The Jurassic System of Great Britain).

Critical to Oppel’s biozone concept was its abstract

nature. Oppel-type zones existed independently of

variations in the local lithological or palaeontological

succession.

The success of Oppel’s formulation for establishing

long-range correlations within Europe and even be-

tween Europe and North America was beyond ques-

tion by the late nineteenth century. Controversy

remained though, regarding just what Oppel-type

zones represented. In particular, Thomas Henry

Huxley pointed out that, whereas Oppel’s biozones

could be construed to document the identity of, or

homotaxic, arrangement in remote stratigraphical suc-

cessions, the extent to which such zones represented

equivalent intervals of time within such successions

was unclear. In terms of practicality, such distinctions

rarely mattered. Biostratigraphical analysis using

Oppel-type zones had been established as an accurate

method for reconstructing the local, regional, and

(at least in principal) global sequence of strata, all of

which were understood to represent some measure of

geological time. Nevertheless, interest in the relation

between biozones and measures of absolute time

remained unabated, especially insofar as estimates

of absolute time were critical to support for the con-

cept of uniformitarianism that was held to underpin

so much of late nineteenth-century geological theory.

The tools for radiometric – later radioisotopic –

dating (see Analytical Methods: Geochronological

Techniques), this volume) that were developed in the

early 1900s offered an empirical way to resolve this

controversy. Development of these methods was later

augmented with other physio-chemical techniques,

including magnetostratigraphy, chemostratigraphy,

isotopic stratigraphy, and most recently, orbital stra-

tigraphy. It should be noted though, that all of these

latter, supplementary methods are, to a greater or

lesser extent, dependent on accurate biostratigra-

phical analysis before their unique properties can be

exploited with confidence. What has been made

BIOZONES 295

abundantly clear by the various methods of absolute

dating, however, is that the boundaries between most

biozones are often measurably diachronous on re-

gional and global scales. This diachrony is often

modest and, in many cases, can be ignored without

fatally compromising the results of a biostratigraphi-

cal analyses. Nevertheless, biozones (Figure 1) are

now conceptually recognised to have no necessary

chronological implication in the sense that identifica-

tion of the same biozone or biozone boundary cannot

be taken as evidence of contemporaneous sediment

deposition. This, in turn, has led to a proliferation of

methods that can be used to combine various types of

stratigraphical data into synthetic and mutually re-

inforcing comparison systems that can be used to

make accurate geochronological assessments of strati-

graphical successions (see below).

Types of Biozones

A wide variety of biozone types have been used to

subdivide stratigraphical successions. Biozones are

defined by single or combinations of biostratigraphi-

cal ranges for fossil species, genera, families, and so

forth. Naturally, the ability to define and to recognise

biozones is also strictly dependent on the existence of

accurate systematic descriptions of fossil taxa such

that individuals from locations remote from the

primary study area can be quickly, easily, and cor-

rectly identified. A biostratigraphical range is the

body of strata delineated by a fossil taxon’s first

appearance horizon or datum (FAD) and its last ap-

pearance horizon or datum (LAD, Figure 2). This

‘first appearance’ refers to the taxon’s initial, or stra-

tigraphically lowest appearance in a local succession

while its ‘last appearance’ refers to the taxon’s local-

ised extinction at the stratigraphically highest level.

Unfortunately, these first-lowest and last-highest con-

ventions are usually reversed when working with

wells that are being drilled, in which case the LAD

is referred to as the ‘first appearance’ because it will

be encountered before the FAD, and the FAD is re-

ferred to as the ‘last appearance’ because it will be

encountered after the LAD.

Range Zones

A biostratigraphical range zone is a body of strata

delineated by the total range of occurrences of any

selected element(s) of the total fossil assemblage re-

covered from the body of strata (Figure 3). Although

the conventional representation of a range zone is

that of a one-dimensional interval (Figure 3A), a

better mental image of a range zone is that of a

two-dimensional body of rock delineated by the cor-

relation of fossil distributions between stratigra-

phic successions (Figure 3B) or a three-dimensional

Figure 1 Relation between the spatio-temporal concept of a biozone and various local and global events. A biozone is a three-

dimensional concept (two dimensions represented here) that encompasses all strata delimited by the zone’s boundary definitions. All

local identifications of the zone span the interval from the first appearance (equivalent to the appearance or stratigraphically lowest

juxtaposition of defining criteria) to the last appearance (equivalent to disappearance or stratigraphically highest juxtaposition of

defining criteria). The global maxima and minima of these juxtapositions also define a combined chronostratigraphical-geographical

interval within which the zone exists. Note that local first and last appearances of zone-defining criteria are all diachronous. Note also

that, even though the stratigraphic level or chronostratographic time represented in any local succession can, in principle, be

correlated outside the geographic limits of the zone, the zone itself is unrecognizable outside these limits.

296 BIOZONES

conceptualization that takes variation in occurrence

and co-occurrence patterns across two geographical

axes into account. This distinction between the

conventional (one-dimensional) and actual (three-

dimensional) conceptualization of biozone geometries

holds true for all types of biozones. As better tools

become available to reconstruct the three-dimen-

sional geometries of stratigraphical bodies, the need

for accurate three-dimensional concepts of biostrati-

graphical units will increase. There are four basic

types of range zones: taxon range zones, concurrent

range zones, Oppel zones, and lineage zones.

Taxon range zone A taxon range zone (also referred

to as a teilzone, local zone, local-range zone, or

topozone) is a body of strata delineated by the total

occurrence range of any specimens belonging to a

selected taxon (e.g., species, genus, family, Figure 3).

Another way of thinking of this type of zone is as

the region defined by all local appearances and ex-

tinctions of a selected fossil taxon (see Figure 1).

While the geographic scope of such zones is set by

the spatial occurrence pattern of their defining

taxon, their boundaries are inherently diachronous

insofar as the taxon’s speciation and final extinction

events will almost always vary from one locality to

another (see Mesozoic: End Cretaceous Extinctions;

Palaeozoic: End Permian Extinctions). Moreover, the

boundaries of such zones are imprecise because taxon

abundances usually diminish as both local and global

speciation/appearance and extinction events are ap-

proached from within the zone. As a result, estimates

of the local appearance and disappearance horizons

of a taxon can be effected by sampling frequency and

sample size near both ends of a taxon’s stratigraphical

range. Indeed, any abrupt appearance or disappear-

ance of a taxon from a local succession is often taken

as evidence of artificial truncation due to facies shift

or the presence of a depositional hiatus.

Biostratigraphers estimate the boundaries of taxon

range zones by assessing the boundaries of local range

zones and correlating these boundaries between suc-

cessions. By convention the names of taxon range

zones refer to the defining taxon (e.g., Dictyoha acu-

leata Total Range Zone, Parvularugoglobigerina

Total Range Zone).

Concurrent range zone A concurrent range zone

(also referred to as an overlap zone or range-overlap

zone) is a body of strata delineated by the those parts

of the biostratigraphic ranges of two or more taxa

that coincide in space and time (Figure 4). The term

concurrent is a somewhat unfortunate choice for the

name of this range-zone type since the colloquial

definition of ‘concurrent’ is to ‘happen at the same

time’. This would, in principle, allow a concurrent

range zone to be defined on the basis of two taxa that

existed at the same time, but that never shared the

same environment (e.g., a hypothetical Tyranno-

saurus rex–Abathomphalus mayaroensis Concurrent

Range Zone could be constructed as that interval of

time represented by the overlap between these dino-

saurian and planktonic foraminiferal species regard-

less of the fact that these two species have rarely, if

ever, been found in the same stratigraphical succes-

sion). Such a zone would be of limited practical utility

however, and the concept is understood to be re-

stricted to those species whose stratigraphical ranges

overlap in time and that occur together in the same

samples.

Figure 2 The manner in which the limits of a local biozone

are established. Samples are taken up through a stratigraphic

succession (or down through a core) and analyzed for their fo-

ssil content. The stratigraphically lowest sample containing a

fossil species marks that fossil’s local first appearance and

the stratigraphically highest sample containing the species

marks the local last appearance. The interval between these

two datums represents the species’ local biostratigraphic

range. Note that gaps in the appearance pattern can occur within

the species’ biostratigraphical range. These gaps can be the

result of many factors (e.g., low absolute abundance, small

sample size, variable preservation, environmental exclusion,

structural complications). The existence of such gaps means

that the observed end-points of each species’ range represent

minimum approximations of the true endpoints.

BIOZONES 297

Figure 3 One-dimensional (A) and two-dimensional (B) representations of a taxon range biozone. The zone begins at the speciation

event that gave rise to the taxon, encompasses all strata containing fossils assignable to the taxon, and ends at the taxon’s global

extinction horizon. Globally, these speciation and extinction events, in additional to the taxon’s geographic limits of distribution, define

a spatio-temporal region within which the taxon’s inferred Total Stratigraphic Range (vertical) and Total Geographic Range (horizon-

tal) are defined (dashed box). In local sections however, the biozone will be represented by a one-dimensional interval whose

boundaries are included within the taxon’s global biozone. Note the strong diachrony of the biozone boundaries in this example.

Intervals of strata that lie outside the biozones’ geographical boundaries cannot be referred to the biozone

per se, but can be placed

within the taxon’s range biochronozone (¼ time interval represented by the Total Stratigraphic Range).

Figure 4 One-dimensional (A) and two-dimensional (B) representations of a concurrent range biozone. In this example the zone

begins at the speciation event of the hypothetical species Exus betus and encompasses all strata containing fossils assignable to the

taxon range zones of Exus alphus and Exus betus. Globally, the speciation event of Exus betus, the extinction event of Exus alphus, and the

geographic limits of the concurrent distribution of these two species’ range biozones define a spatio-temporal region within which the

taxon’s inferred Total Stratigraphic Range (vertical) and Total Geographic Range (horizontal) are defined (dashed box). In local

sections however, the biozone will be represented by a one-dimensional interval whose boundaries are included within this

concurrent range biozone. Note the strong diachrony of the biozone boundaries. Intervals of strata that lie outside the biozones’

geographical boundaries cannot be referred to the biozone

per se, but can be placed within the taxons’ concurrent range biochrono-

zone (= time interval represented by the Total Concurrent Stratigraphic Range). Other definitional geometries are also possible (e.g.,

two taxa, one of whose biozone is completely enclosed by the stratigraphic and geographic ranges of another) so long as the

concurrent range criterion is respected.

298 BIOZONES

The concurrent range zone concept is distinguished

from the taxon range zone concept by being based on

more than a single taxon, and from the assemblage

zone concept by not including all elements of a natur-

ally occurring assemblage of fossil organisms. Ideal

taxa to define a concurrent range zone are those

whose range overlap represents a unit with advanta-

geous recognizability, similar ecological tolerance,

wide geographic scope, and limited temporal dur-

ation. In order to distinguish the concurrent range

zone from an Oppel zone, the presence of all zone-

defining taxa should be used in recognising the zone,

though this can serve to limit the concurrent range

zone’s scope. In practice, many biostratigraphers

adopt a flexible approach to the use of defining taxa

when recognising concurrent range zones, in which

case the concept intrudes inevitably into the realm of

the Oppel zone.

Properly defined concurrent range zones can have

boundaries that are less diachronous than taxon

range zones and less facies-controlled that many as-

semblage range zones. They also typically delineate

finer stratigraphic and temporal intervals than taxon

range zones. By convention, current range zones are

named for the taxa that define them (e.g., Globiger-

ina selli–Pseudohastigerina barbadoensis Concurrent

Range Zone, Neodenticula koizumii–Neodenticula

kamtschatica Concurrent Range Zone).

Oppel zone An Oppel zone is a body of strata delin-

eated by the ranges and range-limits of a group of

fossil taxa selected in such a way as to minimize zonal

boundary diachrony and maximize the geographic

scope of the interval so defined (Figure 5). This type

of zone was made popular by Albert Oppel in studies

of Jurassic stratigraphy in the mid-1900s and served

as a ‘type’ example of a biozone in the days when

distinctions between biostratigraphy and chrono-

stratigraphy were less clearly drawn. Oppel even

made use of certain lithological beds (e.g., ‘spongy

limestones’) that he felt had chronostratigraphic

significance in defining his zones.

Many stratigraphic reference works still list the

Oppel zone as the ‘preferred’ or ‘most useful’ type of

biozone; particularly those written by biostratigra-

phers specializing in invertebrate macrofossil taxa.

This author’s experience, however, suggests that in

modern, high-resolution stratigraphical applications,

and especially in the context of microfossil zonations

which represent the contemporary stratigraphic

standard for Jurassic through Recent marine sedi-

ments, explicit Oppel zones are rarely established in

Figure 5 One-dimensional (A) and two-dimensional (B) representations of an Oppel range biozone. In this example the zone begins

at the speciation event of the hypothetical species Exus omegus (representing the lowest interval of a concurrent range based on Exus

alphus, Exus betus

, and Exus omegus), and encompasses all strata containing fossils assignable to the upper limit of the Exus alphus-Exus

beta-Exus gamus

concurrent range zone. As is always the case with Oppel-type biozones, these boundary definitions are chosen so as to

enhance the ability of this zone to approximate a biochronozone. Globally, these two concurrent range biozones define a spatio-

temporal region within which the Oppel biozone’s inferred Total Stratigraphic Range (vertical) and Total Geographic Range (horizontal)

are defined (dashed box). In local sections however, the biozone will be represented by a one-dimensional interval whose boundaries

are included within this total Oppel biozone. Note the relatively modest diachrony of the biozone boundaries. Intervals of strata that lie

outside the biozones’ geographical boundaries cannot be referred to the biozone

per se, but can be placed within the corresponding

biochronozone (= time interval represented by the Total Stratigraphic Range). Many other definitional geometries are also possible.

BIOZONES 299

principle. Practice can be different though and, with

the rise of neocatastrophism in the latter decades of

the twentieth century, the common usage of certain

standard microfossil zones has acquired a distinctly

Oppelian character (e.g., the lowermost Paleocene

planktonic foraminiferal Zone P0, also called the

Guembelitria cretacea Zone) (see Tertiary To Present:

Paleocene).

Oppel zones are more like abstract or Gestalt con-

structs than geometric juxtapositions of biostrati-

graphic ranges. They are largely constructed from

sets of coincident range segments among taxa selected

for the expressed purpose – or under the unexpressed

assumption – of representing chronostratigraphic

intervals. Unlike concurrent range zones (sensu

stricto), Oppel zones do not rely on the necessary

presence of all defining taxa to be recognised.

Uniquely, this zonal concept embraces the idea that

the biostratigrapher’s judgement of the chronostrati-

graphical value of a particular taxon’s presence (or

absence) can be used in recognizing the zone. The

rationale underlying this concept is that chronostrati-

graphical correlations are desirable and that, through

long years of patient study, biostratigraphers can

become sufficiently familiar with subtle morpho-

logical signals within their fossil faunas and floras

that a higher level of time-based correlation than

that afforded by any other zone concept can be

achieved. The danger, of course, is that recognition

of such a zone becomes an exercise in aesthetics with

no obvious way to adjudicate judgemental disagree-

ments between equally experienced stratigraphers.

Interestingly, many – if not most – biostratigraphers

approach the identification of fossil species in a simi-

lar manner. Employment of this Gestalt aesthetic in

systematic palaeontology has led to the realisation

that the reproducibility of many species lists is quite

low. One cannot help but suspect that Oppel-zone

recognisability is likely beset by similar problems.

By convention, Oppel zones are named for the

dominant taxon used to recognise the zone’s presence

(e.g., Subcolumbites-Prohungarites Oppel Zone,

Globigerina selli–Pseudohastigerina barbadoensis

Oppel Zone).

Lineage zone A lineage zone (also referred to as a

phylozone, evolutionary zone, lineage zone, morpho-

genic zone, phylogenetic zone, or morphoseries zone)

is defined as a body of strata delineated by the com-

bined biostratigraphic range of a series of ancestor–

descendant species couplets (Figure 6). As such, a

lineage zone is a type of taxon range zone in which

Figure 6 One-dimensional (A) and two-dimensional (B) representations of a lineage range biozone. In this example the zone begins

at the speciation event of the hypothetical species Exus alphus (representing the oldest species of the lineage) and encompasses all

strata containing fossils assignable to the lineage. In this example, the stacked taxon range biozones for a three-species lineage

define the overall lineage biozone. Globally, these taxon range biozones define a spatio-temporal region within which the lineage’s

inferred Total Stratigraphic Range (vertical) and Total Geographic Range (horizontal) are defined (dashed box). In local sections,

however, the biozone will be represented by a one-dimensional interval whose boundaries are included within the range of the global

biozone. Note the strong diachrony of the biozone boundaries. Intervals of strata that lie outside the biozones’ geographical

boundaries cannot be referred to the biozone

per se, but can be placed within the corresponding biochronozone (¼ time interval

represented by the lineage’s Total Stratigraphic Range). Lineage range zones also have the added uncertainty that, since lineages are

inferred—not observed, their composition may change with new evidence and/or analyses.

300 BIOZONES

the ‘taxon’ concept has been replaced by the evolution-

ary concept of a lineage or lineage segment. Given the

controversial nature of attempts to identify ancestors

in the fossil record using modern methods of phylo-

genetic systematics, one suspects that, were these sys-

tematic methods applied to some lineage zone-

defining taxa, the basic rationale for recognizing

many lineage zones might be called into question.

Nevertheless, there are a number of instances in

which morphological transitions between stratigra-

phically successive taxa are so well structured as to

pass even the most stringent morphological tests.

A number of previous writers have commented that

lineage zones should be particularly useful for chron-

ostratigraphical correlations. I do not believe this to

be the case. Lineage zones are prone to all the prob-

lems of taxon range zones (see above), and to the

problem of being uniquely susceptible to the discov-

ery of new fossils, or new characters that, by altering

our understanding of phylogenetic relations between

fossil species, would serve to change the zone’scon-

cept or rationale. A strictly comparative and geomet-

ric approach to the determination of FAD/LAD

sequences in local sections, along with efforts to esti-

mate reliable chronostratigraphic ages for those

events based on global composite standard sections

(see below), are likely to be of more practical use in

achieving a chronostratigraphically-based biostrati-

graphy than efforts to mix phylogenetic inferences

with biostratigraphic observations. Notwithstanding

the issues highlighted above, lineage-based zonations

remain popular, particularly among students of Ter-

tiary planktonic microfossils where the tradition of

qualitative phylogenetic analysis remains strong.

By convention, lineage zones are named for the

lineages on which they are defined (e.g., Globorotalia

foshi Lineage Zone, Allevium praegallowayi–

galloway-superbum Lineage Zone).

Assemblage Zone

An assemblage zone (also referred to as a cenozone,

ecozone, ecological zone, faunizone, biofacies zone,

and association zone) is defined as that body of strata

delineated by a natural assemblage of fossils (Figure 7).

This assemblage can be defined in a variety of ways.

For example, it may represent an ensemble of many

groups (e.g., a coral–algal assemblage zone) or of a

single group (e.g., a mollusc assemblage zone). It

may also take a specific habitat into consideration

(e.g., a planktonic foraminiferal assemblage zone),

or cross habitat boundaries. The distinguishing char-

acter of this zone is that the assemblage be composed

Figure 7 One-dimensional (A) and two-dimensional (B) representations of a assemblage range biozone. In this example the zone

begins at the speciation event of the hypothetical species

Exus alphus (representing the oldest species in an assemblage of six fossil

species usually found together in the same environment) and encompasses all strata containing fossils assignable to this assem-

blage. As is always the case with assemblage range biozones, the boundary definitions are chosen so as to base the zone on a series

of ecologically related taxa. Globally, this assemblage of taxon range biozones defines a spatio-temporal region within which the

assemblage biozone’s inferred Total Stratigraphic Range (vertical) and Total Geographic Range (horizontal) are defined (dashed

box). In local sections, however, the biozone will be represented by a one-dimensional interval whose boundaries are included within

this global biozone. Note the strong diachrony of the biozone boundaries. Intervals of strata that lie outside the biozones’ geographical

boundaries cannot be referred to the biozone

per se, but can be placed within the corresponding biochronozone (¼ time interval

represented by the Total Stratigraphic Range).

BIOZONES 301

Elsevier Encyclopedia of Geology - vol I A-E

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