AMER. ZOOL., 28:775-793 (1988)
Analysis of Form and Function in Fossils1
CAROLE S. HICKMAN
Department of Paleontology, University of California,
Berkeley, California 94720
SYNOPSIS. Paleontology and the fossil record contribute an historical perspective on the
form-function relationship that is essential to an understanding of the particular range of
biological forms and functions that exist in the living world. The record contains rich
evidence of forms and functions that do not exist in the modern world and provides a
context for exploring arenas of theoretical possibility and impossibility for organisms.
Although the basic data of the fossil record are static forms and patterns in the rocks,
paleontologists have developed methods of inferring function. Analogy is the most important source of hypotheses for the function of extinct organisms and enigmatic structures.
Paleontological analogy frequently extends beyond biological form and structure to engineering solutions in familiar simple machines and a variety of other human artifacts.
Three tools have proved especially useful in the analysis and interpretation of form in
fossils. The paradigm method is a useful procedure for rigorous evaluation of alternative
functional possibilities for enigmatic structures in a predominantly adaptive context. Constructional morphology reaches beyond the adaptive context to provide a conceptual framework for understanding the full range of factors that contribute to organic form. Theoretical
morphology provides the basis for examining the range of forms and functions that have
actually evolved against possible morphological and functional space. This essay is structured to provide applications of these paleontological tools and to encourage incorporation
of paleontological data and perspective into instruction in introductory biology courses.
raises an important question about how we
know what we know.
Modern empirical science has emphasized knowing by carefully designed experimentation and measurement. Because the
relationship between form and function in
living organisms is subject to direct investigation, should it not be simple to understand how living organisms work? And,
conversely, should it not be impossible to
assess the performance of a fossilized
organism or structure that has been reduced to a static and imperfectly preserved
pattern in the rocks?
In fact, much of what we think we know
about the function of living organisms is
derived from the data of routine systematics and static images of form. Imaging
processes, like the fossilization process, filter the information that we retrieve about
form. And some of what we think we know
about the function of fossil organisms is
derived from simple experiments. The
problem of knowing a living organism and
the problem of knowing a fossil organism
are not really so different in the sense that
both must procede from some level of
1
From the Symposium on Science as a Way of Know- understanding of form itself.
ing—Form and Function presented at the Annual
There are four misconceptions about
Meeting of the American Society of Zoologists, 27—
knowing organisms that are common to
30 December 1987, at New Orleans, Louisiana.
INTRODUCTION
Life on earth has not always had the same
shape. Living organisms represent a small
range of design and function relative to
what is theoretically possible and relative
to what has been realized over some three
billion years of evolutionary experimentation. Full appreciation of form-function
relationships in living organisms requires
placing them in their historical perspective.
Historical perspective on form is relatively easy. The fossil record is a record of
form and pattern. There is a wealth of evidence that ancient organisms experimented with unique designs, patterns of
assembly, and even materials. The functional significance of these ancient experiments is less obvious. Paleontologists have
had to devise their own methods of analyzing and interpreting form and inferring
function. But how strong is paleontological
inference of function? This question is of
some interest in the context of the Science
as a Way of Knowing project because it
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CAROLE S. HICKMAN
both neontology and paleontology. The information loss must be taken into account
first is that we know the organism when we in any analysis of fossil organisms, the
have named it and put it in a drawer with incompleteness of the fossil record does
similar organisms. The second is that we not detract from the wealth of exquisitely
know the organism when we have traced preserved material available for analysis
its genealogy and legitimized it in terms of and interpretation. Taphonomy, the study
its ancestry. The third is that we know the of the controls on preservation and the
organism when we have developed a tech- information content of the fossil record,
nical descriptive vocabulary and can name has taken an increasingly positive view of
and catalog its parts. The fourth is that we the amount and quality of available data
know the organism when we have recorded (Behrensmeyer and Kidwell, 1985). It is
true that soft parts are seldom preserved,
stories about how it makes a living.
The purpose of this essay is to explore that three-dimensional forms frequently
the contributions of paleontology and the are reduced to two-dimensional films, and
fossil record to our understanding of the that many kinds of organisms and habitats
form-function relationship and to provide are almost never represented. On the other
suggestions as to how a paleontological hand, there are common forms of preserperspective can enrich both instruction in vation that record details of cellular strucbiology and instruction in the way science ture and numerous situations in which
exceptional preservation has frozen entire
operates.
There are several major themes in the biotas of unusual organisms. The classic
essay. One is the importance of analogy as Lagerstatten, such as the Cambrian Bura source of insight into the functional gess Shale and Pennsylvanian Mazon Creek
potential of extinct organisms and enig- biotas, provide unique opportunities to
matic structures. Another is the utility of examine bizarre extinct organisms.
certain tools or formalities that paleontoloOne of the most important conclusions
gists have developed to explore form and that paleontology has to offer is that ancient
its relationship to function. The first tool organisms were no less elegantly designed
is the paradigm method, which provides a than modern organisms. The record does
recipe for arriving at the most likely func- not support the notion that organismal
tion of an enigmatic structure. The second design and function have undergone
is the framework of constructional morphol- increasing perfection. Some ancient strucogy, which is not so much an analytic tool tures appear to have solved complicated
as it is a conceptual framework for thinking functional problems that are unsolved at
about form outside of a strictly functional this moment in the living world; and the
context, in terms of the factors that inter- history of life is recorded as a complicated
act to produce form and pattern. The third pattern of evolutionary expansions and
is theoretical morphology, which defines contractions of life through structural and
boundaries between theoretically possible functional design space.
and impossible form and provides a basis
for examining the range of forms and func- ENIGMATIC ORGANISMS AND STRUCTURES
tions that have actually evolved against theThe fossil record contains many bizarre
oretically possible morphological and func- and enigmatic organisms. These are
tional space.
organisms that have puzzled paleontologists not because they are exotic or unfaHow TO APPROACH THE
miliar but because they confound homoFOSSIL RECORD
logical comparison. They are organisms
Discussions of the nature of the fossil that are constructed on unique plans or
record, modes of preservation, and pres- which have peculiar structures in peculiar
ervational biases are readily available in positions or configurations.
standard paleontology and historical geolSome enigmatic organisms have been difogy textbooks (see also Jablonski et al., ficult to reconstruct with respect to even
1986). Although alteration of form and the most basic properties of life orientation
FORM AND FUNCTION IN FOSSILS
777
B
FIG. 1. Enigmatic Early Paleozoic fossil animals having no clear homologies with other organisms, living or
extinct. A. Ainiktozoon loganense, from the Silurian of Scotland (after Ritchie, 1985 and Sourfield, 1937). B.
Hallucigenia sparsa, from the Cambrian Burgess Shale (after Whittington, 1985).
and polarity. Hallucigenia (Fig. IB), from
the Middle Cambrian Burgess Shale, is an
example of a bilaterally symmetrical
organism with a plausible dorsal and ventral polarity, but it has structures that make
it more difficult to determine which is anterior and which is posterior. Hallucigenia is
sufficiently peculiar in form that Briggs and
Conway Morris (1986) reserve the possibility that individual fossil specimens could
have radiated from the central region of a
larger composite organism.
Many classes of enigmatic mineralized
objects in the early fossil record may rep-
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CAROLE S. HICKMAN
FIG. 2. Two early experiments in the arrangement of calcareous plates. A. A Cambrian helicoplacoid echinoderm. B. A Cambrian wiwaxiid.
resent skeletal elements rather than entire
skeletons. Wiwaxia (Fig. 2B), from the Burgess Shale fauna, is an example of a bilaterally symmetrical animal covered with
transverse rows and longitudinal columns
of calcareous sclerites. The organization of
Wiwaxia is preserved intact in this exceptional Cambrian fauna, but there are a
number of classes of isolated calcareous
objects in the Paleozoic fossil record (e.g.,
tommotiids, sachitids, and other groups
whose names will be largely unfamiliar to
the biologist) that may occur in large numbers and represent similar organisms (see
Dzik, 1986).
Helicoplacus (Fig. 2A) is a radially symmetrical animal with a peculiar spiral
arrangement of numerous calcareous elements, representing a short-lived Early
Cambrian experiment in echinoderm evolution. The peculiar arrangement of plates
cannot be reconstructed from arrays of disarticulated plates, and knowledge of the
helicoplacoid anatomical plan, essential to
any reconstruction of helicoplacoid life orientation and function, is dependent upon
specimens preserved intact.
The traditional "problem" of enigmatic
and bizarre fossils has been viewed as one
of classification rather than one of functional analysis. The so-called problematica
are studied with highest priority on resolution of taxonomic placement. Patterson
(1977) has expressed an urgency to classify
problematic fossils in still existing taxa with
the threat that any fossil existing outside
the biologist's hierarchy "will yield virtually no useful information." This is a
shortsighted view of "useful information,"
but it prevails in the typical detailed study
of enigmatic fossil material. The first step
is one of reconstruction and "naming the
parts" and delineating characters: bringing the organism into the framework of
the comparative method so that homologies can be considered systematically.
However, some fossils are so bizarre that
use of the comparative method leads to
frustrating dilemmas.
The peculiar Silurian animal Ainiktozoon
loganense (Fig. 1 A) is an example of a bizarre
fossil in which cataloging the parts and
comparison with extant taxa has been
something of an exercise in futility. Ritch-
FORM AND FUNCTION IN FOSSILS
ie's (1985) study of Ainiktozoon is an exemplar of careful use of the comparative
method, pushing the tool to its limits in the
attempt to find homology and resolve the
affinities of this unlikely object. The animal
is now known from several dozen specimens preserving sufficient structural detail
to generate a catalog of unique terms: an
ovoid capsule, palisade, shagreen patch,
articulate organ, oblique bands, primary
and secondary segmented rods with lobes
and connecting pieces, the hook, the tube,
and the vague area. The search for homology can give us a beast of arthropod affinities if we accept the shagreen patch as a
compound eye (a medial cyclopean eye at
that!), or it can give us some form of vertebrate if we interpret the segmented rods
as having a common origin with the vertebral column.
Animals like Ainiktozoon often provoke
more amusement than scientific curiosity.
Briggs and Conway Morris (1986, p. 174)
note that the morphology of the Burgess
Shale animal Opabinia (Fig. 3D) provoked
"loud laughter from a Palaeontological
Association audience in Oxford in 1972."
In addition to its peculiar trunk-like proboscis, Opabinia has five pairs of eyes, and
in spite of early suggestions of crustacean
affinities (Walcott, 1912; Stormer, 1944) it
remains an animal in search of a phylum.
Forcing homology can be used to settle
matters of classification or to avoid the proliferation of new higher taxa. But if this
solution mistakes analogy for homology, it
can lead to a serious underestimation of
taxonomic diversity. Although there are at
least 20 animals in the Burgess Shale fauna
that cannot be assigned comfortably to any
living phylum (see Briggs and Conway
Morris, 1986), new phyla have not been
proposed. The focus of debate over each
of the Burgess Shale animals continues to
be the question of taxonomic affinities, and
this theme prevails in the alphabetical bestiary-style review of Briggs and Conway
Morris. Was Amiskwia (Fig. 3E) a chaetognath or a pelagic nemertean? Was Wiwaxia
(Fig. 2B) a polychaete, a mollusk, or a turbellarian derivative? Odontogriphus (Fig. 3B)
has been classified as a lophophorate (Conway Morris, 1976) on the basis of a loop-
779
shaped organ on the ventral surface surrounding the mouth. This is an unusual
position for a lophophore, and the basic
form of Odontogriphus, a dorso-ventrally
flattened animal that is annulate and possibly segmented, is very different from that
of familiar living lophophorates (phoronids, brachiopods, and bryozoans). At a gross
structural and functional level it makes
more sense to compare Odontogriphus with
other dorso-ventrally flattened vermiform
enigmata.
BASIC PATTERNS IN THE FORM OF
ENIGMATIC ANIMALS
There is no reason why functional interpretation of enigmatic animals cannot precede their classification and phylogenetic
interpretation. The basic biology of an
organism can be studied without putting a
name on it. The only prerequisite to functional inference is a careful study and accurate understanding of the form itself.
One approach to the form of whole
organisms is to back off from peculiarities
of structure and to search for recurring
patterns in such fundamental attributes as
size, shape, symmetry, and ratios of surface
area to volume.
A survey of basic body plans and organization in bizarre and enigmatic living and
fossil invertebrates has led me to recognize
some recurring patterns that represent a
very basic way of knowing the organism
(Hickman, 1981 and unpublished manuscript; Lipps and Hickman, 1982). An example is the flat, vermiform body plan that
recurs in a number of enigmatic fossils,
sometimes at relatively large body size.
Odontogriphus and Amiskwia, discussed
above, are representatives of this plan that
occur together in one phase of the Burgess
Shale.
The most extraordinary example of this
basic form is in the Late Precambrian worm
Dickinsonia (Fig. 3 A), one of the dominant
animals in the Ediacaran fauna and now
known from several hundred specimens
from at least three continents. This circular to elliptical worm reached lengths of
nearly 1 m while remaining less than 3 mm
in thickness (Runnegar, 1982). It pushes
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CAROLE S. HICKMAN
FORM AND FUNCTION IN FOSSILS
781
the limits of two-dimensionality at large two-dimensional experiment in arthropod
size. There are a number of ways to look evolution (Fig. 3F). Cambrian trilobites and
at this animal. Wade (1972) was impressed the initial trilobite radiation are especially
with the similarity in form between Dick- flat. The Cambrian Period has been called
insonia and the living aberrant polychaete "The Age of Trilobites." From a construcSpinther. It was originally interpreted as a tional point of view it could be called "The
jellyfish (Sprigg, 1947). Its similarities to Age of Flattness." In terms of volume, twofree-living flatworms have been debated dimensional experiments are relatively
(see Runnegar, 1982), and Seilacher (un- empty designs. The organisms we have
published manuscript) has moved in the been considering are experiments in mindirection of treating all of the sheet-like imal construction relative to their linear
Ediacaran animals as an experiment at the dimensions.
level of an extinct kingdom. It is the physThe recognition of patterns in basic coniological approach of Runnegar (1982) that struction of ancient organisms may lead to
has contributed the most to our knowledge functional hypotheses; but detailed analyof Dickinsonia in terms of basic biology and sis requires study of form and structure at
functional potential.
a more refined level. Respiration by difIt is not unreasonable, in the context of fusion is a common correlate of flat body
body plan, to compare Dickinsonia with form, but the interesting functional conanother enigmatic dorso-ventrally flat- clusion that Dickinsonia could have respired
tened animal, Tullimonstrum gregarium (Fig. in sea water containing one tenth the present amount of oxygen (Runnegar, 1982),
3C).
Tullimonstrum is a Pennsylvanian animal is based upon detailed study of the geomfrom the Mazon Creek fauna (Richardson, etry of the animal and calculations based
1966; Johnson and Richardson, 1969), on measurements of oxygen consumption
another of the classic Lager statten. It is per unit of body weight across a range of
one of the dominant animals in the fauna living aquatic worms. Likewise, the inferand is now known from hundreds of spec- ence that several families of Cambrian triimens. Its most striking anatomical pecu- lobites developed auxiliary diffusion sysliarities are the "bar" and paired "bar tems, represented morphologically by
organs" and the long flexible proboscis with structures generally referred to as "caecal
its distal "claw" and impressions of minute networks" (Jell, 1978) is strengthened by
conical "stylets." Attempts to force homol- the observation of analogous diffusion sysogy with living animals do not help us tems in living anthropods.
understand Tullimonstrum as an organism.
Analogy is a powerful tool in the analysis
Although Tullimonstrum and Dickinsonia of the form-function relationship in fossils
have nothing in common in structural and deserves its own chapter in this essay.
detail, they are both large dorso-ventrally
flattened "worms" that dominated the
ancient marine environments in which they
ANALOGY AS A SOURCE OF
lived.
FUNCTIONAL HYPOTHESES
The dorso-ventrally flattened body plan
Analogy is invariably the first approach
also occurs in a number of early evolution- to functional interpretation of an enigary experiments in skeletonized metazoan matic structure. If similar structures do not
groups. Trilobites represent a relatively exist in living organisms, analogy can be
Fie. 3. Diagrammatic representations of Paleozoic fossil invertebrates, mostly Problematica, with two-dimensional body plans. Reconstructions are from specimens or photographs unless otherwise noted. A. Dickinsonia,
a Precambrian worm from the Ediacaran fauna. B. Odontogriphus, from the Cambrian Burgess Shale (after
Conway Morris, 1976). C. Tullimonstrum, from the Pennsylvanian Mazon Creek Fauna. D. Opabinia, from the
Burgess Shale (after Whittington, 1977). E. Amiskwia, from the Burgess Shale. F. A generalized flat Cambrian
trilobite.
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CAROLE S. HICKMAN
explored with simple machines, architecture, industrial design, communication systems, transportation systems, and other
man-made systems designed for efficient
and cost-effective function.
There is no recipe for using analogy,
although it is a major ingredient in the
paradigm method that I describe in a later
section. It is used most effectively on structures that bear an obvious resemblance to
a simple machine, class of engineering
solutions, or body of engineering theory.
Analogy can be used to explore the range
of form of living as well as fossil organisms
and structures. For example, beam theory
provides a way of looking functionally or
mechanically at patterns of distribution of
materials in teeth in the gastropod radula
(Hickman, 1980), and filtration theory
provides a basis for understanding both the
set of problems encountered in separating
particles from fluids and the range of solutions that have evolved in living and fossil
organisms (Hickman, 1984).
Three examples of the use of analogy
demonstrate both the detail with which
fossil functional potential can be explored
and the evolutionary insight that can
emerge from functional analysis.
Optic theory and lens construction
Optic theory and analogy with lenses has
contributed to a body of knowledge about
the potential for image formation and visual
acuity of trilobite eyes. The eyes of many
Paleozoic trilobites differed in structural
detail from the eyes of all living arthropods
(Clarkson, 1966a, b, 1967; Clarkson and
Levi Setti, 1975; Stockton and Cowen,
1976). An especially intriguing system is
represented by the schizochroal eyes of trilobites in the family Phacopidae. The schizochroal eye differs in form from the more
typical compound trilobite eye in consisting of a few relatively large lenses that are
separated from one another by cuticle
instead of being closely packed and covered by a single corneal membrane. Reconstruction of the imaging process led Stockton and Cowen (1976) to the conclusion
that the overlapping images formed by the
individual lenses permitted stereoscopic
vision in one eye.
Detailed examination of the structure
and composition of the individual lenses in
the phacopid eye and comparison of functional design of lenses led Clarkson and
Levi Setti (1975) to another striking conclusion. Each individual lens had developed a peculiar doublet structure with an
upper unit of pure calcite and a lower unit
of calcite with an organic admixture. The
design of the upper unit, in terms of shape
and orientation of the major optic axes
(calcite is a birefringent mineral—i.e., light
travels at different speeds along the major
optic axes) would have been ideal for use
in air. The lower lens would have compensated in the imaging process for the higher
refractive index of seawater. Furthermore,
the upper lens has a biconvex design that
closely approximates a lens design published in the 17th century by Descartes and
Huygens (Clarkson and Levi Setti, 1975).
The design is one that produces an "aplanatic" lens, one that eliminates spherical
abberation.
Harvesting analogy
Cowen (1981) used a harvesting analogy
to analyze an unusual branching pattern in
the arms of a Devonian crinoid. He reasoned that the crinoid filtration net is made
up of primary, secondary, and tertiary
branches that intercept and collect food
particles and transport them to the mouth,
which is located in the center of the net.
As a problem in constructional design, the
filtration net involves a material investment in filling space with transport routes
of differing dimensions and branching
angles (analogous to roads of differing
width and length, surfacing, and branching pattern) leading to a central mouth
(analogous to a central terminal or processing plant).
Cowen found his analog in a road plan
created for harvesting bananas and transporting them with optimal efficiency to the
central processing plant in a British colonial banana plantation. The major evolutionary insight from the analogy derives
from a second analogy in constructional
economics: the hypothetical road system
was never actually constructed. It may have
been an optimally efficient design, but it
FORM AND FUNCTION IN FOSSILS
was apparently too expensive to build. The
plantation could "get by" with a less than
optimal road system and still make a go of
harvesting bananas. It should not be surprising, therefore, that the fossil record
contains examples of elegant solutions during times when animals have been permitted the luxury of pursuing costly constructional edges without impairing fitness.
Pumps and fluid dynamics
783
and out of respiratory and feeding chambers. The problem can be defined as one
of separating inflow from outflow, which
can be done either spatially (e.g., with separate inhalant and exhalant regions and
structures) or temporally (e.g., through volume changes in the chamber). This provides a framework in which to consider
movement of water through the mantle
cavity of a mollusk or air in and out of a
vertebrate lung.
Analogies with pumps are an obvious
possibility for analyzing the function of When analogy fails
The language that descriptive morpholstructures that might have been involved
in the movement of fluids in connection ogy uses to label the parts of living and
with respiration and feeding. In the case fossil organisms is frequently based on very
of analysis of the peculiar, flat, cap-shaped superficial similarity of appearance, with
dorsal calcium carbonate shell valve, fitted no intended implication of either function
within the larger conical ventral valve of or ancestry. Jellyfish have bells that don't
the Permian brachiopod Prorichthofenia, the ring, and sea urchins have lanterns that
pump analogy proved a poor one. How- don't light. Snails and sharks and leaf marever, the process of comparison was gins have "teeth." And some enigmatic
instructive.
Cambrian shells had "snorkels."
Cambrian "snorkel animals" (Fig. 4)
Grant (1972) argued that the dorsal valve
was not involved in circulating water, based provide an example of a situation in which
on the non-existence of a pump designed functional inference does not readily folto meet the needs of circulating water in low from an obvious analogy. The function
and out of the chamber housing the respi- of the tubes that project for varying disratory and feeding apparatus. A pump tances and in varying orientations is diffitransports a fluid by raising and lowering cult to reconstruct in the absence of any
fluid pressures, and Grant could find no knowledge of the animals that inhabited
analog of the single-bladed oscillating pump these relatively simple conical shells. They
that would be required to move fluid in have, however, been classified as a family
of mollusks in the Monoplacophora (Pojeta
and out of a single chamber.
Cowen (1975) subsequently argued that and Runnegar, 1976; Runnegar and Jell,
the Chinese had indeed invented such a 1976).
The term "snorkel" is used primarily in
pump, and he reproduced an illustration
of a metallurgical blast furnace from the a descriptive sense, and there has been no
Nung Su, a 14th century agricultural trea- suggestion that it enabled an animal in
tise. Grant (1975) countered Cowen's water to intake or exhaust air. Debate about
argument with an alternative account of the function of the "snorkel" has centered
the manner in which the Chinese furnace on whether water flowed in or out of it and
whether it was anterior or posterior. By
would have functioned.
Although this exercise could be used to analogy with extant mollusks, holes, slits,
argue the dangers or futility of using emarginations and tubes are standardly
mechanical analogies in functional analy- involved in channeling the flow of water
sis, it is instructive in its insight into the in or out of the respiratory chamber of the
way science progresses. The discarded mantle cavity. The analogy is hazardous
analogies are falsified working hypotheses. for three reasons: (1) the form and orienCowen's (1975) pursuit of the pump anal- tation of the snorkel differs from that of
ogy in fact led him to look beyond pumps the inhalant and exhalant structures in the
to other fluid flow systems, in particular shells of other mollusks, (2) the snorkel ani"tidal flow systems" for moving fluids in mal was minute, with a shell only a few
784
CAROLE S. HICKMAN
FIG. 4. Cambrian snorkel animals, drawn from photographs in Runnegar and Jell (1976). Note that the
drawings range from 30-40 x natural size.
millimeters high, posing a scaling problem
in functional comparison with larger mollusks, and (3) the snorkel animal may not
have had a discrete mantle "cavity" or discrete respiratory structures.
Snorkel animals are ideal candidates for
mechanical analysis using physical models,
and I have predicted that such an approach
will eliminate one or more of the postulated functions of the structure if proper
attention is paid to scaling (Hickman, 1983).
It is likely that the diameter of the snorkel
of this already minute shell cannot function effectively in circulating water, particularly in the inhalant direction. The appropriate experiments (Lindberg and Denny,
in progress) cannot confirm the function
of the snorkel, but they can set definite
limits on its functional potential or demonstrate the effects of the snorkel on water
movement at different velocities.
PALEONTOLOGICAL TOOLS FOR
MORPHOLOGICAL ANALYSIS
The paradigm method
Rudwick (1960, 1961, 1964a, b) proposed a formal analytic procedure for evaluating the function of fossil structures that
has been hailed as a major breakthrough
in evolutionary paleontology (Gould, 1970).
The procedure, usually referred to as "the
paradigm method" relies heavily upon the
use of analogy with machines in formulating hypotheses and is attractive primarily
because it invites experimentation and
quantification in judging the performance
of enigmatic structures in alternative functional settings (see Cowen, 1979).
The paradigm, as defined by Rudwick
(19646) is the form of a structure that will
perform a given function with optimal efficiency, given the limitations of the biological materials involved. Ideally, the method
involves testing alternative functional
hypotheses. After the actual form of the
structure has been described carefully and
accurately, the range of possible functions
is identified. For each of the possible functions, an ideal form (paradigmatic form) is
specified. Finally, the actual structure is
compared in turn with each paradigmatic
structure. Hypotheses that do not have a
close fit can be eliminated, and the hypothesis with the best fit is favored as the best
approximation of a functional explanation.
The closer the fit between the actual and
paradigmatic structure, the stronger the
inference of function.
Most applications of the method have
involved testing a single functional hypothesis involving a single paradigmatic structure. For example, Rudwick (1964a) interpreted the crenulate commissures of fossil
brachiopods as a close fit to the ideal solution to maximizing the flow of water over
the food-collecting organ or lophophore
while simultaneously eliminating large particles that would clog the mechanism. In
his study of the Permian brachiopod Prorichthofenia, which was discussed above,
Rudwick (1961) constructed a working
FORM AND FUNCTION IN FOSSILS
model that he tested in a tank of water with
suspended oil droplets. The efficiency with
which water moved in and out of the cavity
in the model during "flapping" of the dorsal valve was used as strong inference of
the actual function of the valve.
Although Rudwick's (1961) classic "flapping valve hypothesis" was subsequently
questioned and debated (Grant, 1972;
Cowen, 1975) and falsified (Grant, 1975),
falsification of functional hypotheses is in
the aim and spirit of the method and does
not constitute a valid argument against its
use.
Paul (1968) compared the pore structure
and canal structure in the calcareous walls
of extinct Paleozoic cystoids with several
kinds of exchange systems. He found that
the cystoid configuration closely approximated the configuration of an ideal (paradigmatic) counter-current exchange system—one in which 100% of the exchange
substance (in this case oxygen) is transferred from donor to acceptor current.
The most careful attempt to clarify and
follow the steps in the method, including
the testing of multiple functional hypotheses and their paradigms, is Carter's (1967)
analysis of the peculiar long spines on the
bivalve Hysterioconcha. Four alternative
functions were proposed: (1) stabilization
of the shell in a shifting substrate, (2)
hydrodynamic influence on water entering
and exiting the inhalant and exhalant
siphons, (3) support for sensory tissue that
served as an "early warning system," and
(4) structural defense against predatory
attacks on soft tissue. Carter eliminated the
first three hypothetical functions because
their paradigms did not closely approximate the actual structure of the spines. He
concluded that a protective function is most
likely because it meets the following criteria: (1) they should be in a position that
would guard the most exposed and vulnerable tissue (the siphons), (2) they should
be as long as possible and still have strength,
(3) they should taper to sharp points, and
(4) they should be oriented in the direction
of approach of the predator.
The major criticism of the paradigm
method has been its emphasis on adaptation and the optimality of form. It has
785
become popular to label the functional
approach as "adaptationist" or "hyperselectionist" {e.g., Gould and Lewontin,
1979), and a few critics have gone so far
as to argue that the paradigm method is
invalid and should be used only as a "last
resort" (Signor, 1982). Detailed discussion
of the strengths and weaknesses of the paradigm method are beyond the scope of this
essay, but see Fisher (1985) for a careful
justification of the logic of the method in
terms of Bayes' Theorem, relating the posterior probability of a hypothesis to its prior
probability.
The paradigm method can be used in
the broader context of constructional morphology, discussed below. I have argued
(Hickman, 1980) that the functional
approach is excellent for assessing aspects
of form that are predominantly and
obviously mechanical, but dangerous if it
leads to neglect or lack of recognition of
the influences on form that have little to
do with immediate function.
Some structures and some aspects of form
and pattern in organisms are poor candidates for functional analysis and use of the
paradigm method. They may be rich
sources of information about phylogenetic
history, development, or ecological plasticity and phenotypic response. Effective
use of the paradigm method follows primarily from recognizing structures that are
good candidates for functional analysis.
The paradigm method has great strength
as a teaching tool. I have found that students enjoy original problems in inferring
the function of enigmatic or bizarre structures. The best students can be challenged
to design, build, and test their own models.
The method (1) provides experience in formulating multiple working hypotheses, (2)
provides experience in simple experimental design and hypothesis testing, (3) stimulates curiosity and creativity and often
generates novel ideas, and (4) fosters practical understanding of the strengths and
limitations of functional analysis and its
underlying assumptions. Students without
biological background may be quick to
question the assumption that organisms are
optimally designed or to recognize that
structures may have compromised designs
786
CAROLE S. HICKMAN
to look at form first of all for clues as to
the kinds of information that prevail. Some
aspects of form contain a predominance of
information about mechanical function,
while other aspects may be rich in information about growth and assembly or about
an individual's history.
There is no logically rigorous or formal
method for evaluating the relative contributions of different factors to form.
Attempting to use it as an analytic tool is
Constructional morphology
an invitation to circular reasoning. The
Constructional morphology (Konstruk- contributions of constructional morpholtionsmorphologie) (Seilacher, 1970; Thomas, ogy as an attitude have come from studies
1979) is one of paleontology's major con- of particular groups of organisms or structributions to the analysis of form. It extends tures. Some of these contributions have
the study of form from a limited functional taken the form of new principles and conperspective, which restricts explanations to cepts.
performance pressure in an evolutionary
The concept of the "constructional articontext, to its broader biological context. fact" is one of the major results of studies
Form is influenced by function, but it results of skeletal fabrication in echinoderms,
from the interaction of functional necessi- mollusks, and arthropods (Seilacher, 1973).
ties (adaptiver Aspekt) with traditional fac- A constructional artifact may be highly
tors (historischer Aspekt, also referred to as ordered, such as the pattern of alternating
phylogenetic, historical, or bauplan fac- grooves and ridges in the shell of a clam
tors) and fabricational factors (bautechni- that result from environmentally induced
scher Aspekt, also referred to as construc- growth rhythms in the deposition of caltional or architectural factors, which cium carbonate. The ribs have a fundainclude building materials and rules for mentally developmental rather than funcconstruction or assembly).
tional explanation. They are artifacts of
Constructional morphology is not an construction. This does not, however, mean
analytic method. It is a framework in which that growth ribs have no immediate functo consider the influences on form. There tion for a clam. They can become secondare a number of ways to classify the factors arily functional.
that influence form and pattern, and the
The principle that evolution will operate
triangular model that was originally pro- on fabricational features is illustrated in
posed (Seilacher, 1970) is no more "cor- the frictional asymmetry that has been
rect" or useful than the expanded models superimposed on the growth ribs of some
of Raup (1972) and Hickman (1980).
burrowing clams. These evolutionarily
It has become popular to refer to the modified ribs are rounded in the direction
influences on form as "constraints." Inso- of substrate penetration and terraced in
far as living and fossil organisms have failed the opposite direction in a way that resists
to exhaust the possibilities of form and pat- dislodging of the shell from the sediment.
tern that are theoretically open to them, it
If the embellishment of fabricational
is equally appropriate to view the interac- components of form increases fitness, then
tion of function, phylogenetic heritage, and embellishment will occur. It does not always
fabrication as "opportunities" to extend occur. For example, the evolutionary
the known limits of diversity (Hickman, refinement of the simplest pigment depounpublished manuscript).
sition patterns in mollusk shells that can be
Constructional morphology can be char- generated by purely fabricational models
acterized as an attitude with which to (e.g., Meinhardt, 1984; Ermentrout et al,
approach the study of form and pattern. 1986) occurs in some species and not in
Its main value is in alerting the observer others. It is most pronounced in the elabfor the performance of multiple functions.
Students all have an every-day familiarity
with a wide range of simple mechanical
devices that can be used in exploring
organismal function by analogy. Browsing
through catalogs or the shelves of the local
hardware store can be a source of analogs
of structures illustrated in taxonomic
monographs or observed directly in living
and fossil organisms.
FORM AND FUNCTION IN FOSSILS
787
FIG. 5. Comparison of constructional aspects of form in the tests of unicellular organisms using different
building materials and different modes of growth: A benthic foraminifer (A, C, E) and radiolarians (B, D, F).
A. Bar = 200 pm. B. Bar = 40 (im. C. Bar = 100 ^m. D. Bar = 50 urn. E. Bar = 10 /im. F. Bar = 40 ion.
tia
CAROLE S. HICKMAN
FIG. 6. Constructional aspects of form on a microscopic scale. A. Silicon dioxide lattice of an Eocene glass
sponge (bar = 400 ^m). B. Enrolled radular tooth of a predatory cone shell (bar = 200 ion). C. Segment of
FORM AND FUNCTION IN FOSSILS
789
oration of pigment patterns that approach are drawn in the same manner as from fosparadigms for protective functions against sils.
visual predators and least evident in shells
The radula is a complex mechanical
of animals that spend their lives in the sed- apparatus that offers diverse information
iment or in the dark.
in the sometimes thousands of minute chiUnicellular organisms that secrete sim- tinous teeth that prepare and gather food
ple unitary skeletons of calcium carbonate as the organ is protracted and retracted.
or silicon dioxide provide an opportunity A variety of radular groundplans have
to compare several aspects of form that are evolved in different major groups (as
related to materials and fabrication (Fig. defined on other anatomical characters and
5). The spiral form of the benthic fora- shell characters) such that the traditional
minifer (Fig. 5A, C, E) and the successive (bauplan) aspects of form can be identified.
rows of pores in the skeleton are a product As a mechanical apparatus that undergoes
of an accretionary mode of fabrication complex movements during protraction
along a growing edge, while the spherical and retraction, there are aspects of tooth
or subspherical skeletons of radiolarians form that meet the paradigms for struc(Fig. 5B, D, F) are the product of a single tures from the viewpoint of beam theory
phase of deposition. In addition to the two (Hickman, 1980). There are separate
different fabricational modes, reflected in aspects of tooth form that relate less to the
the overall form, there are differences in movement of the apparatus than to the spethe kinds of skeletal substructure con- cific function of dealing with food.
structed with calcium carbonate and silicon
The highly modified single teeth of
dioxide. The delicate networks and crisp predatory snails of the genus Conus (Fig.
edges that commonly are produced in sil- 6B) provide an example of evolutionary
icon dioxide differ from the more solid departure and fine tuning of a highly spestructure and rounded edges characteristic cialized tooth that is loosely analogous to
of most forms of calcium carbonate.
some combination of a hypodermic needle
Multicellular organisms that secrete sil- and a harpoon. The reverse barbs at the
icon dioxide can create massive but light- "business end" of the tooth approximate
weight skeletal structures that suggest a the paradigm for a structure that will penclose relationship between form and func- etrate the flesh of the prey readily (species
tion. The grid of vertical and horizontal of Conus specialize on worms, fish, or other
elements in the skeleton of the Eocene glass mollusks), but they resist removal. The
sponge illustrated in Figure 6 A is of a form introduction of a potent neurotoxin is
that invites comparison with grid systems facilitated by the enrollment of the chitin
common in the skeletal design of buildings into a tubular structure. Although the
and constructional paradigms for the enrolled chitin represents a major departure from other radular groundplans,
transmission of forces.
developmental
and traditional factors preI have used the gastropod feeding organ
clude
the
formation
of a leak-proof cylinor radula to illustrate how the constructional morphological approach contributes der and ideal hypodermic system that would
to a multifactorial understanding of form optimally pressurize the introduction of the
(Hickman, 1980). Although this represents neurotoxin. It is, nevertheless, a highly
a departure from the subject of interpret- effective predatory weapon.
ing form in fossils, I have used the radula
The radular developmental program is
as a static structure from which inferences fundamentally a code for producing a sin-
the radula of a littorine snail, showing repetition of the basic row or constructional unit along the length of
the ribbon (bar =100 Mm). D. An abnormal fusion of the central and left lateral teeth through a programming
error in radular construction (bar = 20 Mm). E, F. Comparison of a fully formed radular tooth (E) from a
limpet radula with a worn tooth (F) from the same radula. Note the appearance of a distinct angulation in
the worn tooth that is not present in the pristine tooth (bars = 40 Mm).
790
CAROLE S. HICKMAN
gle transverse row of teeth of different
types. The row is repeated many times (Fig.
6C). This does not mean, however, that
form is constant in each basic unit throughout the length of the radula. Differences
in form reflect different stages of formation in a structure that is continually secreting new rows at one end and sloughing off
old and worn teeth at the other end. Reading anteriorly from the posterior end of
the radula, each row is at a slightly different stage of formation. This developing
portion of the radula is rich in information
about the fabricational process. Fully
formed teeth in the mid-region of the radula provide information about what the
developmental plan produces for function.
It is at this region that the systematist should
look for comparative data. And it is at the
anterior end, where teeth are worn from
interaction with the substrate, that a great
deal of functional information is contained. Comparison of a fully-formed tooth
from a limpet radula (Fig. 6E) and a worn
tooth from the anterior end of the same
radula (Fig. 6F) suggest the following principle: tools are not necessarily produced in
the ideal form for use (Hickman, 1980).
The shape of the tooth is modified by its
contact with the substrate into the appropriate functional form. The tooth is like a
new unsharpened pencil; and the form that
comes from the "factory" is not the form
that tells us the most about how it is used.
Pristine form is not necessarily the best
indicator of functional potential.
The radula illustrated in Figure 6D is
abnormal and unlike radulae extracted
from other animals in the same population.
The central tooth is normally bilaterally
symmetrical and flanked by pairs of lateral
teeth that are mirror images of one
another. In this radula, the central tooth
and inner lateral tooth consistently have
been secreted as a single, fused, asymmetric unit. The adult animal was alive and
apparently healthy when collected. We do
not know whether the deviation was fixed
genetically or whether it was somatic, perhaps a result of damage to the generative
epithelium. Nor do we know for sure how
such animals normally fare in terms of fitness; but changes in functional potential,
recognized through changes in form, are
raw materials of evolution.
Theoretical morphology
Theoretical morphology explores what
is possible. It is based on the idea that complex form and pattern can be generated by
a few simple rules or instructions. Although
theoretical morphology does not play a
direct role in the inference of function in
fossils, it can be used to generate and
organize functional hypotheses and to
compare the range of designs that have
evolved with what is theoretically possible.
Most importantly, it provides a means of
examining temporal distributions in morphology as patterns of occupation of various types of design space.
Theoretical morphologists have dealt
primarily with computer simulation of form
and pattern in fossils, including branching
patterns in plants (Niklas, 1982) and bryozoans (Cheatham et al., 1980, 1981;
McKinney and Raup, 1982); the geometry
of spirally coiled shells (Raup and Michelson, 1965; Raup, 1966, 1967); shell form
in brachiopods (McGhee, 1980); plate patterns in echinoids (Raup, 1968); patterns
of multiple radiating spiral holes in extinct
receptaculitids (Gould and Katz, 1975); and
patterns in fossil feeding tracks.
Outside of paleontology, the primary
emphasis in theoretical approaches to morphology has been developmental and aimed
at understanding morphogenesis (Cohen,
1967; Jacobson, 1980). Paleontologists
have also treated simulations as growth
models (e.g., Raup, 1968; Gardiner and
Taylor, 1980; McKinney and Raup, 1982).
Although seldom stated, there is an
assumption that the growth models are
close approximations of the actual genetic
instructions. Although simulation modeling may produce forms that closely resemble living or fossil forms, there may be
alternative pathways to the same result.
This is illustrated in two different models
for generating pigment patterns in mollusk
shells (Meinhardt, 1984; Ermentrout et al.,
1986). Meinhardt's model, based on diffusion and reaction of chemical morphogens produces simulations that reproduce
actual shell patterns as well as the neural
791
FORM AND FUNCTION IN FOSSILS
activity model of Ermentrout and his colleagues.
Paleontology's main contribution to theoretical morphology is the concept of morphospace, in the sense that it was developed
by Raup (1966). Morphospace is a set of
all possible results from a given set of
instructions. Morphospace can have any
number of dimensions, although it is most
easily explored and depicted two- or threedimensionally. The definition of a morphospace or design space may be an end
in itself to show what is theoretically possible. However, morphospace becomes
more interesting when it is used to explore
the distribution of living and fossil organisms. For example, McKinney and Raup
(1982) explored several three-dimensional
design spaces based on variations in parameters that produce arrays of spiraled uniformly-spaced branches and produced
forms that are similar to fossil and living
spiraled bryozoan colonies as well as forms
that are not known to have existed. Ward
(1980) generated a design space for coiled
cephalopods and examined the patterns of
changing occupation of the space by
ammonites and nautiloids during the
Mesozoic and Cenozoic. And McGhee
(1980) generated a design space for brachiopods and examined the regions occupied by 324 genera of fossil articulate brachiopods.
There is much room for additional
research in theoretical morphology. In
particular, there is a need to examine alternative design spaces, based on different
parameters, for the same basic forms and
patterns. There is also a major opportunity
to begin to explore the development of
corresponding functional spaces (Hickman, 1985) based on the properties (see
Wainwright, 1988) of different designs and
materials.
CONCLUDING REMARKS
Paleontology and the fossil record are
frequently omitted from introductory biology courses. Separate units dealing with
geologic time and the fossil record only
tend to set paleontology artificially apart
from the rest of biology. Most of the renaissance of incorporation of paleontological
data into the mainstream of modern biology is in the realm of evolutionary theory
(Maynard Smith, 1984) and beyond the
level of introductory courses of instruction. If paleontological data are to be
incorporated into a unified science of biology that is properly set in its historical context, it is in the area of form and the interpretation of the form-function relationship
that this process can and should be brought
immediately into the classroom.
ACKNOWLEDGMENTS
I am especially grateful to the following
people at the Australian Museum, Sydney,
and the University of Western Australia,
Perth, who have provided extraordinary
support and assistance with the final production, under nearly impossible conditions, of this manuscript: M. Colman, P.
Colman, S. Hong, and D. Walker. I thank
M. Taylor for preparing the line drawings.
The scanning electron micrographs are
selected from a set of several thousand negatives produced under support from past
NSF grants DEB 77-14519 and DEB 8020992. I also thank S. Walker for invaluable assistance from Berkeley in meeting
deadlines.
I thank Professors W. V. Mayer and J.
A. Moore for the opportunity to participate in the Symposium on Science as a Way
of Knowing—Form and Function, and for
all that the project represents in making
the results of scientific inquiry more accessible to our students.
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