paradigms based on different mechanical principles to provide the same function can be tested. ... sculpture on erosional scour around the exposed shell regions (Stanley. 1977, 1981, Bottjer ... Raup (1971) proposed to add chance (in a ...
ACTA UNIVERSITATIS UPSALIENSIS Abstracts of Uppsala Dissertations from the Faculty of Science 680
ASPECTS OF THE FUNCTIONAL MORPHOLOGY OF FOSSIL AND LIVING INVERTEBRATES (BIVALVES AND DECAPODS)
by
ENRICO SAVAZZI
UPPSALA 1983
2
Aspects of functional morphology
Enrico Savazzi
3
is to individuate first one or more functions required by the organism, and subsequently look for the corresponding morphological adaptations. INTROD UCTION
In this context, a character is defined as functional if it can be shown to increase the adaptiveness of the organism as a whole.
Since
This publication summarizes the papers in functional morphology
an absolute statement of non-functionality of a morphological character
previously published by the author (see below), and provides an
is not falsifiable (it cannot be excluded that a function will
overview of the general themes and research guidelines followed.
eventually be found; Reif 1975, 1982), the definition of functionality
Unpublished data relevant to the discussion and further research
is often restricted to characters bearing a direct relationship with
possibilities connected with the subject are briefly covered. The
the 1ife habits and eco1ogi ea 1 niche of the organism.
present paper has been produced in fulfillment of the requirements for
colour pattern of some burrowing bivalves has been defined as non
doctoral dissertations at Uppsala University, Sweden.
functional (Seilacher 1972), since it has no conceivable effect in
The studies presented here deal with two distinct basic themes in functiona1 JIDrphology.
Thus, the
The first of these concerns burrowing
sculptures; i. e., external sculptural patterns that aid hard-shelled
enhancing the fitness of the organism to the infaunal life habit. In this restricted sense, a statement of non-functionality does not imply any connotation of neutralist evolution (see dicussion in Reif Contrary to the opinion of Gould & Lewontin (1979), I do not
invertebrates in burrowing through soft substrates, and which are
1982).
compared among different taxonomic groups.
accept the idea that the formulation and testing of functional
The phylogenetic,
developmental and constructional constraints of each group impose
hypotheses presupposes the
different limitations to the optimization of burrowing sculptures
organism.
(Savazzi 198lb, 1982a, 1982b, Savazzi, Jefferies & Signor
1 982).
a priori
assumption of adaptiveness of the
Rather, adaptiveness itself is testable as part of the
working hypothesis ( Reif 1932, and below).
However, the indirect
The second theme deals with the adaptive strategies of bivalves,
inference of function from morphology in fossils does presuppose the
originally adapted to life on or within solid substrates, and which
acceptance of a selectionist point of view, and is not compatible with
evolved secondarily into soft-bottom dwellers (Savazzi 198la, 1982c,
strongly neutralist theories (Lewontin 1978, Reif 1982).
1982d and unpublished manuscript; Chinzei, Savazzi & Seilacher 1982).
In living organisms, a functional hypothesis may often be directly
These papers show also that only a limited number of conceptual tools
testable by observing the result of experimental modifications of
is available for the study of functional morphology in macro
selected morphological characters and/or environmental conditions.
invertebrates.
When modification of the experimental conditions is not feasible (typically, when conditions cannot be altered singly), indirect informations can still be collected by monitoring the organism's activity by unobtrusive techniques.
HETHODS IN F UNCTIONAL MORPHOLOGY
The typical case is repr·esented by
complex behavioural changes induced by experimental �Iterations of JIDrphological characters.
When studying the relationships between morphological characters of an organism and its immediately surrounding ecosystem, a working
For examples of these two methods, see
Savazzi 1982a and 1982b, respectively). The methods of functional analysis in fossils must obviously be less
hypothesis is usually formulated about the adaptive value of an
direct, but the same basic principles can be used.
observed morphology.
indirect methods developed by palaeobiologists can often be profitably
The functional hypothesis can subsequently be
tested in a variety of ways (see below).
A less immediate but equally
valid approach (C. R. C. Paul (Liverpool), verbal communication 1980)
applied to living organisms as well.
In fact, the
These methods can be summarized
as follows (a somewhat different scheme was proposed by Reif 1982) .
4
5
Aspects of functionaZ morphoZogy
Enriao Savazzi
may prevent the evolution of the optimal morphology. a) Comparison of fossil organisms with Recent counterparts possessing assumedly analogous or homologous characters that are susceptible to direct testing of functional hypotheses.
Trade-offs may
occur also in favour of non-morphological characters (e. g., fertility, growth rate) which may be difficult or impossible to assess in fossils. It has been suggested that constraints may be taken into account
b) Comparison of morphological characters with one or more alternative mechanical analogues (paradigms) designed to provide the hypothesized function with maximum efficiency (Rudwick 1964) . c) Experiments with actual fossil specimens or physical models to test the properties of morphological characters.
when selecting the paradigm (Cowen 1979).
However, phylogenetic and
constructional constraints are often inferred from the general lack of certain features in a taxonomic group (cf. Signor 1982a). to a circular reasoning (circuZue vitiosus).
d) Informations on the life habits, ontogeny and ecological
Therefore,
allowing for these constraints while selecting the paradigm may lead Signor (1982a) rejects
the paradigm method for this reason, and because a negative outcome of
relationships derived from the occurrence and preservation of fossil
the comparison of the organism with the paradigm does not necessarily
organisms, traces of predation and parasitism, composition of the
falsify a general hypothesis of functionality.
thanatocoenosis and sedimentological evidence inferred from the
adopt constructional morphology (see following section) in place of
enclosing rock.
the paradigm method.
While these methods may be applied to single characters, the
He further proposes to
I do not agree with Signor that constructional
morphology can be regarded as a substitute to the paradigm method
conclusions should take into account the organism as a whole (cf.
(or to any of the other methods here discussed ) .
Savazzi l982a) .
falsification of the functional hypothesis is not the necessary
The comparison of fossil organisms with Recent
Furthermore,
counterparts can only be as good as the extent of our knowledge of the
outcome of a negative result in any of the approaches listed above, so
degree of similarity and the understanding of organs and soft parts
that there appears to be no flawless substitute for the paradigm
normally not preserved in fossils.
method.
The validity of the actualistic or
uniformitarian principle has to be assumed.
Obviously, the method is
Therefore, I prefer to retain it, allowing only for constraints
dictated by concurrent functions and coadaptive characters, which can
least useful with organisms lacking taxonomically related Recent
be incorporated in the functional hypothesis.
representatives.
compliance with the paradigm, while not excluding a general hypothesis
The paradigm method (b) , as presented by Carter
(1967) consists of four distinct steps (see also Paul 1975) : 1) A functional hypothesis is made,
2) a theoretical model (paradigm)
In this way, a lack o f
of fu�ctionality, does, indeed, exclude that the hypothesized function is carried out in the way envisioned by the paradigm. Constructional
is designed to provide the hypothesized function with maximum
and phylogenetic constraints can be considered later on, to explain
efficiency (constraints due to the materials available are allowed for
why
at this stage) ,
3) the actual organism is compared with the paradigm,
the actual morphology does not comply with the paradigm.
While the choice of the appropriate paradigm presupposes a fairly
4) the functional hypothesis is accepted or rejected according to the
good understanding of the laws or principles involved, the use of
goodness of fit of the actual morphology to the paradigm. Alternative
fossil specimens or physical models in actual experiments (method c)
paradigms based on different mechanical principles to provide the same
is suitable when the engineering prohlem of designing a paradigm is
function can be tested.
too complex, or the principles involved are little understood.
While a good fit between the paradigm and the real organism is a strong suggestion in favour of the functional hypothesis, a bad fit is not necessarily an indication of non-functionality.
A particular
Since
the inferential process is e;sentially the opposite to that of the paradigm method, I prefer to regard these two approaches as separate. Weak points of this latter approach are that it can be applied with
morphology need not necessarily be optimized at an engineering scale in
confidence only to skeletal parts, and that only passive properties
order to provide the function at the level required by the organism.
are accurately simulated.
r�oreover, trade-offs between different or contrasting functional needs
Ideal fields for the application of this
6
Enrico Savazzi
Aspects of functionaL TN)rphotogy
method are problems in fluidodynamics, such as the effect of shell sculpture on erosional scour around the exposed shell regions (Stanley 1977, 1981, Bottjer & Carter 1980) or passive re-orientation of organisms (Fisher 1977, Savazzi l982c) .
This method can be also
successfully used to study active properties of morphological traits, such as the effect of shell sculpture on the burrowing process in bivalves (Stanley 1975, 1977) .
In this case, the resulting
oversimplification in reconstructing and simulating the behaviour of the organism may cause the results to be not directly comparable with biological data. Evidence derived from.traces of predation, parasitism or association with other organisms and from taphonomical (Efremov 1940) aspects (method d) can be usefully compared with data from Recent organisms. This method is most useful for assessing the life habits of fossils in relation to the immediately surrounding habitat.
For instance,
information on the burrowing depth and shell orientation of infaunal bivalves can be obtained from the presence of sessile epibionts on the
habits and ecosystem.
7
As examples of this method, see Savazzi (1981a,
1981b, 1982c, 1982d, and manuscript).
Raup (1971) proposed to add
chance (in a neutralist sense) and ecophenotypic factors as separate aspects.
The inherent difficulty of including random-walk evolution
or other neutralist concepts is that their existence cannot be directly derived from the observed morphology (except perhaps through statistical treatment of large data sets).
Most of the further aspects
proposed by Hickman (1980) can be grouped under the general heading of constructional and developmental mechanisms.
In the present and
associated papers, the original definition of constructional morphology is adhered to. I do not agree with Signor (1982a) in regarding constructional morphology as an alternative to other methods in functional morphology. Given its original definition, constructional morphology is rather a broader attempt to explain morphology, employing functional morphology as one of its basic tools.
exposed shell regions and from the pattern of repaired damage to the shell margins (Savazzi l98la, 1982b, l982c, and manuscript) .
Care
must be taken to identify all post-mortem and preservational alterations. It is evident from the above discussion that the different approaches to functional morphology are not mutually exclusive, and that they should rather be used in conjunction with each other.
The reasoning
involved is not circular, since new data are collected along the way by testing and refining the functional hypothesis.
BURROWING SCULPTURES Organisms with mineralized skeletal parts, possessing a high fossilization potential, are favourite subjects for studies in functional morphology.
In particular, the exoskeleton of invertebrates,
being in direct contact or close proximity with the environment, is likely to possess morphological characters functionally related with the life habits of the organism.
This is especially true of burrowing
or boring organisms, since the forces necessary to penetrate the medium are high enough to require special adaptations. CONSTRUCTIONAL t-10RPHOLOGY
In the present context,
the term burrowing is used restrictively to indicate movement through loose sediments of variable grain-size and cohesiveness, but in which
As defined by Seilacher (1970), constructional morphology is an attempt to explain the morphology of an organism as the interaction of three
the sediment particles are never cemented to each other.
The term
boring is reserved for locomotion through a solid substrate, whose
factors ("aspects") : the phylogenetic heritage of the organism, its
shear force may easily approach the mechanical strength of the boring
constructional and developmental mechanisms (including the morphogenetic
parts.
programmes, the properties of the materials involved and ecophenotypic
place for the growth of the organism.
characters) , and its functional morphology in relation to the life
burrowing and boring is to a certain extent artificial, it complies
Actually, in most cases, the boring activity simply provides a Although the distinction between
8
Aspeets of funetional morphology
En:rico Savazzi
well with the observed variety of locomotory patterns involved. The types of bUrrowing processes were summarized by Seilacher (l982a), mostly according to the nature of the appendages involved. processes can be divided into two basic types:
These
eontinuous, in
which the
the opposite direction.
9
The higher friction in the opposite direction
would be functional in bracing the shell against the substrate while the
foot
probes forward.
The hypothesis was further supported by
observations on living bivalves showing that the orientation of the
·organism proceeds through the substrate at a steady rate (e. g.,
terrace-lines was in agreement with the burrowing direction.
burrowing echinoids; see Ghiold 1 979, 1982, and Ghiold
defining the paradigm for burrowing sculptures, Seilacher (1973) used
1 982), and
intermittent,
Seilacher
&
in which one part of the body acts as an
anchor against backslippage, while the actively burrowing part probes forward through the sediment.
The role of the two parts is
observations on a wide range of burrowing invertebrates. requirements of
this
The
paradigm were summarized by Savazzi (1981b, 1982a,
1982b) and Savazzi, Jefferies and Signor (1982).
subsequently exchanged in the next phase of the burrowing process.
In
The compliance of
the actual organisms with the paradigm was so good, that "burrowing
Together with other auxiliary movements, the two complementary phases
sculptures" came to be regarded almost as a synonym of "terrace
constitute a burrowing sequence (Trueman
sculptures" (evidence for this statement can be found in Seilacher
Savazzi 1 982a).
&
Ansell lg69; cf. also
The anterior and posterior parts of the organism
1972, 1973, 1976, Schmalfuss 1976b, 1978a).
Terrace patterns with
active in the burrowing process were originally called retraction and
different functions have been described (Schmalfuss 1978a), and a
penetration anchor, respectively (Trueman
variety of functions have been hypothesized for other types of
&
Ansell ' 1 969).
Occasionally,
these two parts are located side by side (e. g., the dorsal and ventral
sculptures (e. g., Schmalfuss 1975, 1977, 1978b), but evidence fot·
surfaces of the Recent reptile OphisaUI'US; Frey 1982).
burrowing sculptures totally different from terraces came only recently
All the organisms considered in the present connection employ (or are supposed to) an intermittent burrowing process.
The two burrowing
anchors can be either soft (foot of bivalves, whole body of polychaetes) or rigid (mollusc shell, crustacean cuticle).
Basically, progression
(Stanley 1981, Savazzi 1982a).
Reasons for this bias may be found in
the fact that comparisons with a range of actual organisms was used in the early phase of choosing the characters of the paradigm (which should rather be derived from theoretical principles).
A tendency to
through the substrate is achieved by alternately shifting the maximum
treat burrowing sculptures independently of the context of the whole
friction against the substrate between the two anchors.
organism may also be responsible.
While a soft
organ can be alternately inflated and contracted (e. g., the bivalve foot), rigid parts may have a fixed volume (the valves of the bivalve shell can be adducted together.to reduce the cross-sectional area, but in gastropods, the shell is a single unit).
Passive morphological
In decapod crustaceans, sculptural features interpreted as burrowing sculptures range from asymmetrical tubercles (Savazzi 1982b) to fully formed terrace lines (Seilacher 1961, 1973, 1976, Schmalfuss 1978a, Savazzi 1981b, 1982b, Savazzi, Jefferies & Signor 1982; see Schmalfuss
features may aid in burrowing, provided that they exert a consistently
1975, 1981 on other crustaceans).
different friction in opposite directions.
sculptural patterns differing from terraces have only recently received
Based on these requirements,
a paradigm for burrowing sculptures can be designed.
Stanley (1969)
observed that several Recent bivalve species possess growth-unconformable
attention.
For the reasons expressed above,
Future research may show that they occm· far more
frequently than well-developed terraces (Savazzi, unpublished data).
ridges (i. e., neither commarginal (="concentric") nor radial), which
It is easy to imagine an evolutionary process leading from isolated
are oriented perpendicularly to the burrowing direction and are
asymmetrical tubercles to terrace-shaped ridges formed by the fusion
terrace- or sawtooth-shaped in cross-section. of these ridges faces away from the
b urrowing
Since the steeper side direction, he suggested
of transversely aligned tubercles (Savazzi 198lb, 1982b).
In most
cases, this is further suggested by the facts that the edge of the
that these ridges also possess a lower grade of friction when moved in
terraces is crenulated (Savazzi l98lb: Fig. 2; 1982b: Figs. 4, 8,
the burrowing direction (thus not hindering forward movement) than in
and that the terraces in the anomuran
Emerita
11),
increase ontogenetical ly
10
Aspects of functiona� morpho�ogy
Enrico Savazzi
11
in length by the addition of new crenulations at the sides (Seilacher
Kauffman 1969; retardation of scour around the exposed or shallowly
1973, 1976).
buried posterior shell margins: Stanley 1977, 1981, Bottjer & Carter
The terraces of the brachyuran
Ranina (Lophoranina)
are
identical, in cross-section and general appear·ance, to the randomly
1980; camouflage and preventing or favouring the attachment of epibionts
scattered isolated tubercles of
to exposed shell regions: Stanley 1970, Bottjer & Carter 1980;
Ranina (s.
evolved (Savazzi 198lb and unpublished). Corystes
s.) ,
from which they likely
Also, in the brachyuran
deterring predators: Carter 1967; facilitating the respiratory activity
the well-formed terraces in the posterior region of the
by increasing the length of the commissure or by sustaining induced
carapace (where they are most needed for burrowing) grade into shorter
flow: Stanley 1970, Paul 1975), but experiments with living bivalves
terraces and isolated tubercles in the anterior region (Savazzi 1982b).
(Stanley 1981, Savazzi 1982) show that also sculptures not optimally
The smoothly edged terraces present in part of the grapsid crabs may
designed as burrowing ribs can indeed be functional in burrowing.
have evolved through a different evolutionary pathway (Savazzi, unpublished).
The reason why sculptural traits that are extremely coarse (with
These terraces are not burrowing sculptures, but are
respect to the grain-size of the sediment) and lacking any visible
functional in increasing friction against the substrate when the crabs
reason for exerting a different frictional force in opposite directions
wedge themselves in rock crevices.
should be more efficient than a smooth surface (as shown by the actual
The increased friction prevents them
from being extracted by predators (Schmalfuss 1978a).
experiments) lies in other coadaptive characters of the burrowing
Since the physical size of the burrowing sculptures is related to the grain-size of the surrounding sediment, the physical size of
the
process.
The hydraulic pumping action that complements the mechanical
rocking of the shell during the burrowing process (see Stanley 1975,
burrowing ribs (in particular, their relief and spacing) should remain
1977, Savazzi 1982a) periodically changes the physical properties of
constant during growth ("allometric densing" of Seilacher 1973).
the surrounding sediment.
requires an allometric growth process. aspect of the paradigm in the brachyuran
This
Failure to comply with this may
Ranina (Lophoranina)
Therefore, the mechanical asymmetry of the
sculpture is substituted by a temporary asymmetry in the properties of Lhe sediment, which change in different moments of the burrowing
indicate the existence of constructional or phylogenetic constraints
sequence.
(Savazzi 198lb, 1982b).
(Signor 1980, l982b), trilobites (Schmalfuss 1981; see Miller 1975 for
Experiments with artificial models of terrace
Burrowing sculptures have also been described in gastropods
patterns (Savazzi 198lb) show that, although apparently lacking the
a totally different interpretation) and calcichordates (Savazzi,
capability of introducing new terraces among preexisting ones during
Jeffet·ies & Signor 1982).
growth,
brachiopods appear to satisfy all the requirements of the paradigm
Lophoranina
nonetheless underwent ontogenetic adjustments to
partially maintain the efficiency of the burrowing sculptures. randomly distributed tubercles in
Ranina (s.
s. ) ,
The
unlike the
Terrace patterns in obolid inarticulate
for burrowing sculptures (Seilacher 1973).
However, the steep faces
of the terraces are directed away from the peduncule, suggesting that
asymmetrical tubercles in other crabs (cf. Savazzi l982b), are
they burrowed with the distal commissure posteriormost.
approximately five times longer (in the burrowing direction) than wide.
their closest Recent counterparts, the Lingulidae, are known to burrow
Placing these tubercles side by side to form terraces running from one
in the opposite direction (Thayer & Steele-Petrovic 1975j. This may
side of the carapace to the other, as seen in
Lophoranina
(including
juveniles) would leave no space for further introduction of sculptural elements in the way observed in
Emerita
(Seilacher 1973) and
Corystes
(Savazzi 1982b) (Savazzi, unpublished).
suggest either a totally different burrowing mechanism for the Obolidae, or a function of the terraces other than burrow·i ng. According to the interpretation offered by Schmalfuss (1981), the terraces on the ventral side of trilobites are used to prevent sediment
Many burrowing bivalves show a variety of sculptural traits that do not comply with the paradigm for burrowing sculptures.
Actually,
Other functions
of bivalve sculptures are well documented (mechanical strengthening:
particles from sliding in the cavity excavated by the appendages under the body to allow filter feeding, and accordingly are directed outwards along the entire perimeter of the ventral surface.
Since these
12
Enrico Savazzi
Aspects of functional morphology
13
terraces do not have to move in opposite directions with respect to the sediment, as true burrowing sculptures do, their requirements should be different.
SECONDARY SOFT-BOTTOM BIVALVES
In fact, the ventral terraces in trilobites seem
to be consistently more projecting than the terraces on the dorsal
Bivalves are believed to have originally evolved from rostroconch
side (which are true burrowing sculptures facing the posterior
ancestors.
direction) .
secondarily adapted to life on, or within, solid substrates (Stanley
It is interesting to note that, in bivalves that burrow
Since the Palaeozoic, a variety of forms have become
with the commissure plane horizontal (e. g., Tellinidae) , the terraces
1968, 1972, Pojeta & Palmer 1976) .
on the lowermost lying valve are actually less conspicuous than on the
advantages of being mechanically stable and of providing concealment
The hard bottoms offer the principal
dorsa1 one, probably in order to compensate for the difference in 1 oad
and mechanical protection against predators and natural agents (at
exerted by the sediment (Seilacher 1972, 1973, Savazzi 198la) .
least in nestling and boring forms) .
In the Recent crabs legs and, in
Gecarcinus
Ge car>cinus,
and
Ocypode,
terraces occur on the
along the sides of the carapace.
Both these
genera excavate burrows in subaerial environments. The terraces in Gecarcinus
were said by Schmalfuss (1978a) to be functional in
relevance in certain environments.
Better aereation may be of
Most hard-bottom bivalves have
entirely lost their locomotory abilities, at least in the adults, and are directly cemented or byssally attached to the substrate. Mechanically and/or chemically boring forms have evolved in several
preventing sediment being carried out of the burrow from slipping
lineages (cf. Purchon 1954, 1955, Yonge 1955, Pojeta & Palmer 1976,
between the first leg and the terraced ventro-lateral region of the
Roder 1977, Carter 1978, Kleemann 1980, Savazzi 1980, 1982c, 1982d,
carapace.
Observations on both genera in their natural environment in
1982e) .
A number of other taxa have adapted as nestlers in empty
Bermuda and on live specimens in the laboratory (Savazzi, unpublished)
boreholes.
show that the sediment being carried out from the burrow is kept
from the adaptations to the hard bottoms prevented the return to a
Usually, the extent of the morphological changes resulting
pressed between the first pair of legs and the mouthparts, and is
free-burrowing or endobyssate habit comparable with that of their
never in contact with the terraced regions.
soft-bottom ancestors.
It was possible to show
that the terraces in these crabs become functional in wedging the organism against the wall of the burrow when threatened by a predator or an intruder.
The wedging behaviour in adult
Ocypode
is usually
abandoned in favour of running or fighting, and the terraces are accordingly degenerated with respect to juveniles of the same species
Therefore, the secondary return of these
organisms to the soft bottoms is particularly interesting, since it can be expected that the different adaptive strategies would channel evnl ut.i on into
a
variety of areas unexploited by primary soft-bottom
dwellers. The soft-bottoms environments are mostly characterized by their
(in particular, they tend to loose the asymmetrical edge and to become
intrinsic instability as compared with solid substrates.
rounded in cross-section) .
soft-bottom dweller incapable of active movements should therefore
Further studies may show other differences
from true burrowing sculptures. Gecai'cinus
It is interesting to note that in
the terraces in a restricted area of the pterigastomi a·l
exploit one or more passive strategies in order to maintain or re-establish a suitable life position (cf. Savazzi 1982c, Seilacher
region are secondarily modified into a stridulatory apparatus (Klaassen
1982a, Chinzei, Savazzi & Seilacher 1982) .
1973) .
are summarized as follows:
Terrace patterns in other groups (e. g. , eurypterids) still
await detailed investigation.
A
These adaptive strategies
1) attachment to solid objects (shells, wood, seaweeds) ; 2) building a structure so heavy or deeply anchored that it is unlikely to be disturbed in the first instance; 3) adopting a morphology whose geometrical properties facilitate the re-establishment of the life position by gravity and/or water
14
15
Aspects of functionaL morphoLogy
Enriao Savazzi
secondary forms (e. g., the Anadarinae).
currents;
4) association with an actively moving organism which assures a life
A peculiar adaptive shell
morphology featuring a commissure plane twisted into a three-dimensional s tructure has repeatedly evolved in the pteriomorphiid families Arcidae
position suitable for the host. S olutions 1) and 4) are often accompanied by a reduction in adult size.
(McGhee 1978, Teves z & Carter 1979, Savazzi 198la), Bakevelliidae
The first solution requires few morphological changes with respect to
(McGhee 1978) and r�ytilidae (Savazzi, manus cript).
the hard-bottom ancestors, and appears to have often been used as a
of this s hell morphology and the underlying morphogenetic mechanisms
stepping evolutionary mechanism into the new ecological zone.
are discuss ed in the papers cited above.
Representatives of all major groups of boring bivalves are known to facultatively bore into shells or other objects lying on the soft bottoms.
Although this represents a straightforward evolutionary step
The adaptive v alue
It is interesting to note how the method of constructional morphology allows an explanation of a certain given morphology accounting for the possibility of parallel evolution and the characteristics of the {the set of basic characters shared by a whole taxon).
Since
towards the permanent adaptation to the soft bottoms, the overall
BaupLan
stability of the organism remains rather low.
the conclusions reached for one organism or one group can be checked
In true soft-bottom
dwellers, boring into small objects is most often seen as being no
against other organisms, they are experimentally testable in a full
more than a juvenile adaptation to increase the overall size and
scientific s ense.
therefore lessening the risk of accidental disturbances of the life position.
Later in ontogeny, this is complemented or substituted by
more efficient strategies (Savazzi 1982, Chinzei, Savazzi & Seilacher 1982).
The calcareous secretion that originarily enabled a few
ACKNOWLEOGEI-1ENTS
families of boring bivalves to repair the walls of accidentally damaged boreholes (Carter 1978; cf. also Massari & Savazzi 1980,
I thank Richard Reynent for reviewing the present paper and the
Savazzi 1980) has evolved into calcareous envelopes
manuscript on s hell torsion in the �1ytilidae, and Adolf Seilacher for
(crypts;
Savazzi
1982c, 1982e) lying freely within soft sediments (tube-dwelling bivalves; Carter 1978).
The coadaptations required by the new life
habit are discussed by S avazzi (1982c; cf. also Carter 1978).
The
following and supporting my scientific work in functional morphology since its very beginning.
The research work was financially supported
by the Deuts cher Akademischer Austauschdienst, the Sonderfors chungs
different adaptations to the tube-dwelling habit are best explained as
bereich 53 "Paliikologie" of TUbingen (West Germany), the Exxon
the result of convergent evolution.
Corporation through the Bermuda Biological Station, and the Swedish
Commensalism with actively moving
Institutions that loaned or made available material and
soft-bottom corals evolved convergently at least three times in the
Institute.
r4ytilidae (Savazzi 1982d).
facilities are acknowledged in the s eparate papers.
The evolutionary pathway that lead to
commensalis m in other bivalves (e. g.,
JousseaumieLLa)
is still
unclear. The adaptive strategies adopted by other sessile bivalves secondarily adapted to the soft bottoms are discussed by Chinzei 1982, Chinzei, Savazzi & Seilacher 1982 and Seilacher l982b. soft-bottom bivalves,
Among the secondary
representatives of the Pteriomorphia may be
regarded as the least specialized.
Several lineages within the Arcidae
never completely lost the capability for active locomotion (Stanley 1970, Thomas 1976, 1978), and often evolved into truly burrowing
16
Enrico Savazzi
Aspects of functional rnoPphology
pusiZlus REFERENCES
(Mollusca).
Functional and phylogenetic
structures.
Journal of Paleontology 54, 200-216.
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