Boggs S. Principles of Sedimentology and Stratigraphy
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Coe, A. L. (ed.), 2003, The sedimentary record of sea-level change:
Cambridge t:niversity Press, Cambridge, 287 p.
Emery, 0., and K J. Meyers (eds.), 1996, Sequence stratigraphy:
Blackwell Science Ltd., Oxford, 297 p.
Haq, B. t:. (ed.), 1995, Sequence stratigraphy and depositional re
sponse to eustatic, tectonic and climatic forcing: Kluwer Aca�
Jemie Publishers, Dordrecht, 381 p.
Miall, A. 0., 1997, The geology of stratigraphic sequences:
Springer-Verlag, Berlin, 433 p.
Pamentier, H. W., and G. P. Allen, 1999, Siliciclastic sequence
stratigraphy-Concepts and Applications: SEPM Concepts in
Sedimentology and Paleontology, Tulsa, Okla., 210 p.
Posamentier, H. W., C. Summerhayes, B. C. Haq, and G. P.
Allen (eds.), 1993, Sequence stratigraphy and facies associa
tions: Inteational Association Sedimentologists Spec. Pub.
18, Blackwe Scientific Publications, Oxford, 4 p.
Van Wagoner, J. C., R. M. Mitchum, K. M. Campion, and D.
Rahmanian, 1990, Siliciclastic sequence stratigraphy in well
logs, cores, and outcrops: Concepts for high-resolution corre
lation of time and facies: AAPG Methods in Exploration Series
No. 7, Am. Assoc. Petroleum Geologists, Tu lsa, Okla., 55 p.
Weimer, P., and H. Posamener (eds.), 1993, Siliciclastic sequence
stratigraphy: Recent developments and applications: Am.
Assoc. Petroleum Geologists Memoir 58, Tu lsa, Okla., 492 p.
Wilgus, C. K., B. S. Hastings, C. G. St. C. Kendall, H. W. Posa
mentier, C. A. Ross, and J. C. Van Wagoner (eds.), 1988, Sea·
level changes: integrated approach: Society of Economic
Paleontologists and Mineralogists Sc. Pub. 42, Tulsa, Okla.,
407 p.
Further Reading 477
Williams, G. 0., and A Oobb (eds.), 1993, Te ctonics and sequence
stratigraphy: e Geological Society, London, 226 p.
Magnetostratigraphy
Aissaoui, D. M., D. McNeill, and N. Hurley (eds.), 1993, Ap
plications of paleomagnetism to sedimentary geology: SEPM
Spec. PubL 49, Tulsa, Okla., 216 p.
Berggren, W. A, D. V. Kent, M- Aubry, and J. Hardenbol, 1995,
Geochronology, time scales and global stratigraphic correla
tion: SEPM Spec. Pub. 54, Tulsa, Okla., 386 p.
Butler, R , 1992, Paleomagnetism: Magnetic domains to geolog
ic terranes: Blackwell, Boston, 319 p.
Hail wood, E. A., 1989, Magnetostratigraphy: GeoL Soc. Spec. Re
port 19, Blackwell, Oxford, 84 p.
McElhinny, M. , and P. L McFa dden, 2000, Paleomagnetism:
Continents and oceans: Academic Press, San Diego, 386 p.
Opdyke, N. D., and J. E. Channe, 1996, Magnetic stratigra
phy:
Academic Press, San Diego, 346 p.
Salvador, A. (ed.), 1994, Inteational stratigraphic guide: A
guide to stratigraphic classification, terminology, and proce
dure: Inteational Union of Geological Sciences and Geolog
ical Society of America, Inc., 'Irondheim, Norway, 214 p.
Ta rling, D. H., and P. Tuer (s.), 1999, Paleomaetism and di
agenesis in sediments: Geological Society Special Publication
No. 151, London, 214 p.
Tauxe, L., 2002, Paleomagnetic principles and practice: Kluw
Academic Publishers, Dordrecht, 299 p.
Van der Voo, R., 1993, Paleomagnetism of the Atlantic, Tethys, und
Iapetus Oceans: Cambridge University Press, New Yo rk, 411 p.
Biostratigraphy
478
14.1 INTRODUCTION
T
he preceding two chapters focus on stratigraphic relations in sedimentary
successions that exist owing to the physical characteristic of sedimentary
rocks, either lithology or physical properties that can be remotely sensed by
seismic or magnetic instrumentation. We tum now in the present chapter to exam
ine the exceedingly important role that fossil organisms play in stratigraphy. First,
fossils provide an additional and highly useful method for subdividing sedimen
tary rocks into identifiable stragraphic (biostratigraphic) units. In addition, they
make possible the ordering and relative-age dating of strata and their correlation
on both a continental and (in some cases) a global scale. The characterization and
correlation of rock units on the basis of their fossil content is called biostratigra
phy. Stratigraphy based on the paleontologic characteristics of sedimentary rocks
is also referred to as stratigraphic paleontology, the study of fossils and their dis
tributions in various geologic formations.
Separation of rock units on the basis of fossil content may or may not yield
stratigraphic units whose boundaries coincide with the boundaries of liic strati
graphic units. In fact, lithostratigraphic units such as formations commonly can be
subdivided by distinctive fossil assemblages into several smaer biostratigraphic
units. Indeed, one of the primary objectives of biostratigraphy is to make possible
differentiation of strata into small-scale subunits or zones that.can be dated and
correlated throughout wide geographic areas, allowing interpretation of Earth
history within a precise framework of geologic time. On the other hand, it is qui
common for some biologically defined stratigraphic units to span the boundaries
of formally defined lithostratigraphic units. Some biostratigraphic units may thus
include parts of two members or formations or even encompass two or more en
tire members or formations. Note in Figure 14.1, for example, that the Naheola
Formation contains two biostratigraphic units, the Pr. pusil/a pusilla I.Z. and M. an
gulata I.Z. planktonic foraminiferal zones, whereas the Pr. pseudomenardii R.Z.
foraminiferal zone spans the Nanafalia Formation and part of the Tuscahoma
Formation and includes several dierent rock types (sandstone, shale, marl).
The concept of biostratigraphy is based on the principle that organisms have
undergone successive changes throughout geologic time. Thus, any unit of strata
can be dated and characterized by its fossil content. That is, on the basis of its con
tained fossils, any stratigraphic unit can be differentiated from stratigraphically
Lithostratigraphic Unit
Age
Lith- Form-
Member/
ology ation informal unit
.
�
c
z
"buhrstone"
Meridian
Sand
"upper
'
"Bashi sand"
"upper"
Bells Landing
Marl
"Boar Creek
marlu
Grampian
Hills
"Osta thirsae
beds"
Gravel Creek
Sand
11
Upper
ll
Coal Bluff
Marl
"Coal Bluff
sand
Oak Hill
Biostratigraphic
Unit
(Planktonic Foram-
1
Figure 14.1
14.1 Introduction
479
Illustration of the relationship between biostratigraphic units
and lithostratigraphic units, Te rtiary sedimentary rocks of the
eastern Gulf
Coastal Plain, U.S.A. [Based on Mancini, E. A., and
Tew, B. H., 1995, Geochronology, biostratigraphy and
sequence stratigraphy of a marginal marine shelf stratigraphic
succession: Upper Paleocene and lower Eocene, Wilcox Group,
eastern Gulf Coastal Plain, U.S.A., in Berggren, W. A., D. V.
Kent, M-P. Aubry, and J. Hardenbol (eds.), Geochronology and
global stratigraphic correlation, Fig. 1, p. 282, SEPM Spec.
Publ. 54, Tulsa, Okla.]
younger and older units. Biostratigraphy is obviously closely allied to paleontol
ogy, and a skilled biostratigrapher must also be a well-trained paleontologist.
fact, the application of biostratigraphy is for speciasts who have timate knowl
edge of large groups of organisms and their temporal and spatial disibution. Be
use satigraphic paleontology is such a complex field, comprehensive treatment
of is subject is beyond the scope of is book. The aim of this chapter is to intro
duce some very basic concepts and principles of biostratigraphy. Readers who
wish more in-depth treatment of biostratigraphy should consult standard refer
ence works on paleontology as well as more specialized, biostratigraphically ori
ented monographs such as those listed under "Further Reading" at e end of this
chapter.
480
Chapter 14 I Biostratigraphy
We begin discussion of biostratigraphy by examining the concept that fossils
constitute a valid basis for stratigraphic subdivision. As part of this examinaon,
the ogin and development of methods for biostratigraphic zonation are traced,
and the stragraphic procedures currently in use for classifying, naming, and de
scribing biostratigraphic units are discussed. Organic evolution and the distribu
tion of organisms in both time and space are explored next. We conclude the
chapter with a discussion of the extremely important role that biostratigraphy
plays in correlation of stratigraphic units. The use of fossils for calibrating the ge
ologic time scale, called biochronology, is discussed in Chapter 15.
14.2 FOSSILS A BASIS FOR STRATIGRAPHIC
SUBDIVISION
inciple of Faunal Succession
An English surveyor and civil engineer named Wilam Smith, who worked
England and Wa les the late 1700s, is credited with discovering the fundamental
principle of biostratigraphy. Previous workers had recognized that fossils are e
remains of once-living organisms, and some workers had even suggested the pos
sibility that certain species of marine-shelled organisms had become extinct.
Smith was evidently the first to utilize fossils as a practical tool for characterizing,
subdividing, and correlating strata from one area to another. In his work as a sur
veyor and canal builder, he had discovered by about 1796 that the strata in and
around Bath in Somerset and for some distance outward were always found in e
same order of supeosition-the order in which rocks are placed above one an
other. Furthermo, he noted that each layer in the stratigraphic succession was
characterized by the same distinctive fossil assemblage wherever it was found
throughout the region. Soon, Smith was able to assign any fossil-bearing rock to
its proper superpositional interval by comparing its fossils with others whose
stragraphic posion he knew from previous study. He thus discovered that fsil
bearing sata occur in a definite and determinable order. the basis of Smi's
discovery; we now know that rocks formed dung any particular interval of geo
logic time can be cognized and distinguished by their fossil content from rocks
formed during other time intervals. This concept has consequently become
known as the pnciple (law) of faunal succession. Even without assigning
names to fossils, Smith successfully used them to establish a stratigraphic succes
sion and to subdivide the rocks into mappable its by a combination of lithologic
characteristics and fossil assemblages.
It is important to stress that Smi did not subdivide rock successions on the
basis of fossils alone. His strata were first delineated and named according to their
lithology. en, their characterisc fossils were collected and studied. The use of
foss
ils alone to subdivide thick, essentially lithologically homogeneous forma
tions did not come about for another fteen yea. The French scientist Geoe
Cuvier, a contemporary of Smith's, recognized the desirability of using fossils
subdivide rocks but did not attempt this process himself. Subdivision of rock suc
csions on the basis of fossils was first carried out on sediments of Te rtiary age in
the early 1830s. Deshayes in France (1830), Bronn in Germany (1831), and Lyell in
England (1833) all proposed subdivisions of Te rtiary strata based on fossils (re
ported in Hancock, 1977). Lyell's subdivisions are storically noteworthy. He
split the Tertiary strata into four units on the basis of the proportions of living to
extinct species in the rocks (Table 14.1). Thus, we see here for apparently the first
time the use of fossils as an essential part of the definition of units of geologic me
and the possibility of biostratigraphy freed from lithologic controL
Name of subdivision
Pliocene (more recent)
Newer Pliocene
Older Pliocene
Miocene (less recent)
Eocene (dawn of t)
Concept of Stage
14.2 Fossils as a Basis for Stratigraphic Subdivision
481
Extant species in the rocks (%)
90
33-50
18
3.5
Although Smith's principle of faunal succession was to be the coerstone for all
subsequent
biostratigraphy, his own work on biostratigraphic successions led to
only vaguely defined time units. Lyell's subdivisions were similarly vague and,
in any case, were confined to the Tertiary. A closer look at fossil successions was
needed to refine their use in dating and correlation. This important step came
with introduction of the concept of stage, credited to the French paleontologist
Alcide d'Orbigny. About 1842, d'Orbigny came up with the idea of erecting
major subdivisions of strata, each systematically following the other and each
bearing a unique assemblage of fossils. Like Smith, d'Orbigny recoized that
similarity of fossil assemblages was the key to correlating rock units, but he
went a step further to propose that strata characterized by distinctive and
unique fossil assemblages might include many formations (lithostratigraphic
units) in one place or only a single formation or part of a formation in another
place. He defined as stages groups of strata containing the same major fossil as
semblages. He named these stages aer geographic localities with particularly
good sections of rock that bear the characteristic fossils on which the stages are
based. Using the stage concept, he was able to divide the rocks of the Jurassic
system into ten stages and the Cretaceous rocks into seven stages, each charac
terized strictly by its fossil fauna.
The boundaries of d'Orbigny's stages were defined at intervals marked by
the last appearance, or disappearance, of distinctive assemblages of life forms
and their replacement in the rock record by other assemblages. He conceived
these stages as having worldwide extent and to be the result of repeated cata
strophic destruction of life on Earth followed by new creations. His ideas on
catastrophic destruction and special new creation, like those of Cuvier who pre
ceded him, failed to gain lasting acceptance among geologists, and subsequent
study has shown that his stages and their characteristic faunas are local rather
than worldwide. Nonetheless, d'Orbigny's concept of stages as major bodies of
strata characterized by large assemblages of fossils unique to that part of the total
stratigraphic column was a significant and lasting contribution to the growing
discipline of biostratigraphy. Somewhat different interpretations of the meaning
of stage have been used by subsequent workers, but d'Orbigny's basic concept is
still valid.
Concept of Zone
The stage concept of d'Orbigny permitted subdivision of strata into major succes
sions on the basis of fossils. What they did not provide was a method by which
fo ssiliferous strata could be divided into small-magnitude, clearly delimited units.
Friedrich Quenstedt in Germany was particularly critical of d'Orbigny's stages
because, according to him, d'Orbigny's method "centered around the acceptance,
482
Chapter 14 I Biostratigraphy
Fi 14.2
as the diagnostic faunal aggregate, of species of many strata many localities,
lumped together without enough regard for their precise stratigraphic ranges''
(Berry, 1987, p. 125). Quenstedt maintained that only by extremely detailed study
of strata on essentially a centimeter-by-centimeter basis could full understanding
of the succession of faunas be developed. Quenstedt's own work did not bring
this notion of detailed biostratigraphic subdivision into full fruition. It remained
for his student, Albert Oppel, to expand, synthesize, and meld Quenstedt's ideas
into the concept of the zone.
Oppel inoduced the concept of zone in 1856 and thereby altered for all time
the practice of biostratigraphy. Working with Jurassic rocks in various parts of
Germany, he conceived the idea of small-scale units defined by the stratigraphic
ranges of fossil species irrespective of lithology of the fossil-bearing beds. Oppel
noted that the vertical ranges of some species were very short; that , the species
existed for only a very short time geologically. Others were quite long, but most
were of some intermediate length. Oppel noted also that the assemblages of fossils
that characterized the strata were made up of overlapping ranges of fossils. He
defined his zones by exploring the vertical range of each separate species. Each
zone was characterized by the joint occurrence of species not found together
above or below this zone. Thus, the range of some species began at the base of a
zone (the first appearance of a species), others ended at the top of a zone (the last
appearance of a species), whereas still others ranged throughout the zone or even
extended beyond it. Using species ranges, Oppel discovered that he could delin
eate the boundaries between small-scale rock units and distinguish a succession of
unique fossil assemblages. Each of these assemblages was bounded at its base by
the appearance of distinctive new species and at its top, that is, the base of the suc
ceeding section, by the appearance of other new species. It is, however, the over
lapping stratigraphic ranges of the species that make up the fossil assemblage
that typifies a zone (Fig. 14.2). Because a zone represents the time between the
appearance of species chosen as the base of the zone and the appearance of other
species chosen as the base of the next succeeding zone, recoition of zones thus
permits delineation of clear-cut, small-scale time units. Each of Oppel's zones
was named after a particular disnctive fossil species, called an index fossil, or
index species, which is but one fossil species in the assemblage of species that
characterize the zone.
The concept of zone thus allowed subdivision of stages into two or more
smaller, distinctive biostratigraphic units that could be recognized and correlated
Last
appearance
0
0
Oppel Zone
Diagrammatic illustration of an Oppel zone
defined by overlapping ranges of two or more
taxa.
First
appearance
14.3 Blostratlgraphlc Units
483
over long distances. Oppel was able, for example, to subdivide the Jurassic rocks of
weste Europe into 33 zones. It should be noted that Oppel did not start with
d'Orbiy's stages and subdivide them into zones. Instead, he delineated zones
the basis of fossil ranges and then combined the zones into stages, all of which did
not necessarily fit into one of d'Orbiy's stages as then dened (Hancock, 1977).
Zones were slow to be adopted into stratigraphic practice, especially in the
United States, but with minor modifications of Oppel's method they have now be
come the common denominator of biostratigraphic study. Zones have been ex
tended to all parts of the fossil record, not just the Jurassic, and to all areas of the
world. It is now recognized, however, that ere are definite geographic limits be
yond which most zones cannot be traced. e area within which a zone can be rec
oized is a biogeographic province. Because zones constitute the basic it of
biostratigraphic classication, considerable work has gone into efforts to stan
dardize their usage. e current usage of zone and the different kinds of zones
now recognized are described in the following section.
14.3 BIOSTRATIGRAPHIC UNITS
As the preceding discussion makes clear, a biostratigraphic unit is a body of rock
strata characterized by its fossil content that distinguishes and differentiates it
from adjacent strata. Furthermore, the zone, or biozone, is the fundamental bios
tratigraphic unit. Biozones do not have any prescribed thickness or geographic ex
tent. They may range in thickness from thin beds a few meters thick to units
thousands of meters thick and in geographic extent from local units to those with
nearly worldwide distribution. Recent attempts to standardize nomenclature and
usage of biozones have been made by the International Subcommission on Strati
graphic Classification (Hedberg, 1976; Salvador, 1994) in the International Strati
graphic Guide and by the North American Commission on Stratigraphic
Nomenclature in the 1983 North American Stragraphic Code and pendg revi
sions. The usage in this book follows that recommended in the Nor Amecan
Stratigraphic Code, but reference is made to usage in the International Strati
graphic Guide where appropriate.
Principal Categories of Zones
Revisions to the 1983 North Amecan Stratigraphic Code (Appendix C) contained
in North American Stratigraphic Commission Note 64 (Lenz et al., 2001; see also
http://www.agiweb.org/nacsn/) subdivides biostratigraphic units into five
principal
kinds of biozones: range biozones (two kinds), interval biozones (two
more kinds), lineage biozones, assemblage zones, and abundance biozones
(Fig. 14.3). Each of ese zones is distinguished by different criteria, as explained
below.
A taxon-range biozone (Fig. 14.3A) is a body of rock presenting the known
stratigraphic and geographic range of occurrence of a single taxon. A concurrent
range biozone (Fig. 14.3B) is a body of rock that includes the concurrent, coinci
dent, or overlapping part of the ranges of two specified taxa.
interval biozone, or subzone, is the body of strata between two specific,
biostratigraphic surfaces (biohorizons of the Inteational Stratigraphic Guide,
p. 56). e features on which biohorizons are commonly based include lowest oc
currences (Fig. 14.3C) and highest occurrences (Fig. 14.3D); however, they may also
include disncve occurrences and changes in the character of individual taxa, such
as changes in the direction of coiling in foraminifers or in number of septa in corals.
A lineage biozone (Fig. 14.3E) is a body of rock containing species repre
senting a specific segment of an evolutionary lineage.
484 Chapter 14 I Biostratigraphy
Figure 14.3
A - Ta xon-range Biozone (based
on the range of a taxon)
C - Inteal biozone (based
on lowest occurrences of taxa)
t
Jz
s
y
r
X
B -Concurrent-range Biozone
(based on range of concurrent
occurrence of two taxa)
0 - Inteal Biozone (based on
highest occurrences of taxa)
Diagram illustrating the principal kinds
of biozones as defined in North Ameri
can Stratigraphic Commission Note 64,
a revision to the 1983 North American
Stratigraphic Code. [After Lenz et al.,
2001, Note 64-Application for revision
of articles 48-54, biostratigraphic units,
of the North American Stratigraphic
Code: PG Bull., v. 85, Figures 4 and
5,
p. 373.] (see also
E - Lineage Biozone (based
on successive species in a
segment of an evolutiona
lineage)
G - Abundance Biozone (thickened
line denotes range of increased
abundance of the taxon).
F - Assemblage Biozone (based on
overlapping ranges of an assemblage
co-occurring taxa)
EXPNATION
-
Lower and upper range of taxon
D
r, s, t
X
, y, Z
Vertical range of taxon
Biozone boundaries:
lower, upper, lateral
Ta xa
http:/ /www.agiweb.org/nacsn/).
An assemblage biozone (Fig 14.3F) is a body of rock characterized by a
unique association of three or more taxa, the association of which distinguishes it
in biogeographic character from adjacent strata. An assemblage biozone may be
based on a single taxonomic group, for example, trilobites, or on more than one
group, such as acritarchs and chitinozoans.
An abundance biozone (Fig. 14.3G) is a body of rock in which the abun
dance of a particular taxon or specified group of taxa is significantly greater than
in adjacent parts of the section. Figure 14.4 further illustrates subdivision of strata
on the basis of abundance zones. Note that gaps may exist between zones defined
on the basis of taxon abundance.
Rank of Biostratigraphic Units
The biozone is the fundamental unit of biostratigraphic classification. Other bio
stratigraphic units are formed by either grouping or subdividing biozones. The
ternational Stratigraphic Guide (Salvador, 1994) suggests that some kinds of
biozones may be subdivided into subbiozones (subzones) and/or grouped into
superbiozones (superzones). The North American Stratigraphic Code provides
that a biozone may be completely or partly divided into subbiozones.
y
14.4 The Basis for Biostratigraphic Zonation: Changes in Organisms Through Time
485
ndance Zone
�
-
--
Relat1ve abunnce
Figure 14.4
Schematic illustration or the abundance zones of three hy
pothetical fossil species. Note that each species reaches
pea k abundance (total number of individuals) at a particu
lar time and then declines in abundance. ges of the stra
ta increase downward, and relative abundance increases
to the right.
Namin
g
Biostrati
g
raphic Units
The n,1me of .1 biozone consists of the n,imc oi one or more distinctive tax found in
the biozone, followed by the word Biozone (e.g., Tu rborulalia ccrrozaulensis Biozont').
e name of the species whose lowest occurrence defines the b,1se of the zone i� the
most common choice for the biozone name.
14.4 THE BASIS FOR BIOSTRATIGRAPHIC ZONATION:
CHANGES IN ORGANISMS THROUGH TIME
Evolution
1e practicality of biosh·a ti
g
r,1phy as a tool for characterizing and COITe lating strata
had been clerly established by Smith, d'Orbigny, ,m d Oppel by the middle of tl1e
19th cenh1ry. Prob,1bly, none of these workers fully understood why fossil ssem
blages cl1ngcd frorn one stratigraphic layer to another, although d'Orbigny cl p
pMently gave considerable thought to the origin of his stilge boundaries. Charles
Darwin provided the cl nSWer to this puzzle a few years after Oppel conceived the
zone concept. In his monumental work on the origin of species published in 1859,
D
arwin denwnstriltcd tht' existence of orgc1nic evolution and thereby greiltly
changed subsequent geologicl nd philosophical thought, although his ideas
were by no mens accepted by all of his contemporaries (nor arc they ccepted by
some people outside the field of science today).
Darwin WilS not the first to conceive the general idea of evolution, but previ
ous workers held marsh,, lied little supporting evidence for their ide,1s. By contr<1St,
Darwin drew together data on the fossil records of extinct organisms, the n�sults
486 Chapter 14 I Biostratigraphy
of selective breeding of domestic animals, observations on ecological adaptations
and variations among living organisms, and details on comparative anatomy. He
then wove these data into a powerful argument for organic evolution. Darwin
pointed out that all organisms have high reproductive rates, yet populations of
these organisms remain essentially constant in e long run. He explained this ob
servation by suggesting at not all orgasms of the same kind (species) are equally
well equipped to survive, and therefore many individuals die before reproducing.
Each individual of a species differs from other individuals as a result of variations
that arise within organism entirely by chance. Some of these chance variations
may be an advantage to the organism in coping with its environment in its strug
gle for existence. Others may be a disadvantage. Successful variations help organ
isms survive and extend their environment and
Unsuccessful variations
result in extinction.
Darwin termed this process of weeding out the unfit and survival of the
fittest natural selection. Furthermore, he proposed that these favorable varia
tions are inheritable and can be transmitted from one generation to the next.
Darwin's fundamental contribution to understanding evolution was thus recog
nizing that natural selection was the process by which new species arise. New
species appear because the composition of populations changes with time be
cause those individuals that undergo favorable adaptations will stand a better
chance of surviving and reproducing. He likely did not understand how varia
tions arose or how these traits were passed on from one generation of organisms
to the next. The concept of spontaneous changes in genes that we now call
mutations was not well known at the time Darwin published The Origin of
Species, although the basic laws of genetics had been set forth in a paper by the
Austrian monk Gregor Mendel in 1865.
xonomic Classification and Importance of Species
Organisms can be classified in a variety of ways, including habitat (planktonic,
nektonic, benthonic) and environmental distribution (littoral, neritic, bathyal,
etc.); however, taxonomic classification based on morphological and develop
mental similarities and presumed genetic relationships is most pertinent to recog
nizing evolution and biostratigraphic zonation. The basic system of taxonomic
classification now in use was introduced in 1735 by the Swedish naturalist Lin
naeus, who grouped organisms into a hierarchy of different categories based on
the number of distinctive characteristics shared in common. Organisms in the
lowest, or least inclusive, category have the greatest number of common charac
terists; those in the next highest category have fewer common characteristics;
and so on, until the highest, or most inclusive, category is reached. In the last cat
egory
, organisms share only a very few common characteristics or traits. Lin
naeus's system of classification, as modified by some later additions, is illustrated
schematically in Figure 14.5.
The Linnaean system of taxonomic classification brought to light the fact
that degrees of similarities among organisms differ at different levels of classifica
tion. Differences among groups of organisms are greatest at the kingdom level
and least at the species level. Species have thus become the fundamental entity of
biostragraphy. Biologists define species as a breeding community that preserves
its genetic identity by its ability to exchange genes with other breeding communi
ties. In other words, all members of a given species have the ability to interbreed,
but they do not normally breed with members of a different
Thus, a
species constitutes a group of interbreeding organisms that are reproductively
isolated from other such groups. The criteria for identifying a biologic species are
difficult to apply to fossil organisms. Therefore, fossil species are commonly