ENVIRONMENTAL STABILITY AND MORPHOGENETIC
RELAXATION IN BRYOZOAN COLONIES FROM THE EDEN
SHALE (ORDOVICIAN, OHIO VALLEY):

A DEVELOPMENTAL EXPLANATION OF STABILITY-
DIVERSITY - VARIATION HYPOTHESES

Dissertation for the Degree of Ph. D.
MICHIGAN STATE UNIVERSITY
JOSEPH F. PACHUT. JR
1977

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thesis entitled

ENVIRONMENTAL STABILITY AND MORPHOGENETIC RELAXATION
IN BRYOZOAN COLONIES FROM THE EDEN SHALE
(ORDOVICIAN, OHIO VALLEY): A DEVELOPMENTAL
EXPLANATION OF STABILITY—DIVERSITY-VARIATION HYPOTHESES

presented by

Joseph F. Pachut, Jr.

 

has been accepted towards fulfillment
of the requirements for

FA . D. degree in%

 

MW

Major professor J

 

Date.%déig 271‘ /]Z7

0-7639

 

ABSTRACT
ENVIRONIVENTAL STABEITY AND IVDRPHOGENETIC RELAXATICN
IN BRYOZOAN CDLONIE‘S FFDM 'IHE BEEN SHALE
(OHIDVICIAN, OHIO VAIIEY): A EEVELOPMENTAL
EWLANATION OF STABIIITY-DIVERSITY—VARIATION HYPOTHESES
By

Joseph F. Pachut, Jr.

Growth features that are morphogenetically regulated provide a
developmental insight into the gerietic-mr'phologic paradox of stability-
diversity-variation hypotheses. Morphogenetic systems within Ordovician
bryozoan colonies are recog'lized through morphologic gradients that
allometrically changed in "field potential" through astogeny. 'Ihis
developmental regulaticn is additionally documented by means of newly
recognized monarchic zooids at field centers, a test of position effect
on zooidal differentiation, and a test of field induction and suppression
as a distance effect. For each species, least squares regression analy-
sis permits the measurement of the rates of field growth, and the residual
variance from regression measures morphogenetic relaxation, a faculta—
tive response to environmental differences. In four stratigraphically
pervasive species in the Eden Shale the level of mrphogenetic relaxation
is homogeneous within species, but varies significantly across taxa.

The two species with the highest residual variability fit the concept
of r—selected opportunistic species and are most abundant in cormrunities
of lowest diversity. The other two species have much lower residual

variance, fit the concept of K-selected equilibrium species, and are

Joseph F. Pachut, Jr.

most abundant in the communities of highest diversity. Within—colony
variability is higher in the opportunistic rather than in the equilibrium
species, indicating that the higher morphologic variability observed in
unstable environments is non-genetic in nature and not the result of
higher genetic polymorphism. Equilibrium species in stable environments
have lower levels of morphologic deregulation whose relationship to
genetic diversity could not be conclusively tested with the available
data. However, several genetic mechanisms, such as maintenance of
developmental homeostasis through heterozygosity, are available to explain
the apparently negative correlation of genetic and morphologic variabil-
ity.

ENVIRONMENTAL STABILITY AND IVDRPHOCENETIC IEIAXATION
IN BRYOZOAN COIDNIES FIDM 'IHE EIEN SHALE
(ORIDVICIAN, OHIO VALLEY): A EVELOPIVENTAL
EXPLANATION OF STABILITY-DIVERSITY—VARIATION HYPOTHESES
By

Joseph F. Pachut, Jr.

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

mCTOR OF PHILOSOPHY

Department of Geology

(I; I).

11

Th Betty and
my Parents

iii

Acmowrsmmrs

I would like to express my appreciation and thanks to
Dr. Robert L. Anstey for guiding this research project, and to
Dr. Dmcan F. Sibley, Dr. Thomas A. Vogel, Dr. John T. Wilband,
Dr. Chilton E. Prouty, and Dr. J. Alan Holman for their constructive

suggestions regarding study methods and manuscript improvements.

iv

TABLE OF CONTENTS

LIST OF TABLES . . . ..... . . . . . . . . . . . . . . . .' . . . V
LIST OF FIGURES . . . . ....... . vi
INTIDDUCTION . . . . . . . ..... . . . . . . . ..... l
DDRPHOCENEI‘IC SYSIEIVB IN ORIDVICIAN BRYOZOAN COLDNIES 3
Introduction . . ..... . . . ................ 3
Monarchiccontrol........ ....... 5
Position effect within a field ..... . 11
Field growth . . . . ...... . 16
Field initiation and suppression . 18
Regeneration . 20
NEASUREMENT OF DEVELOPMENTAL RELAXATION ...... . 23
Genetic basis . . . ......... . 23
Measurements . ...... . ..... . . . . ......... 2“
Results and inferences . ............. . 25
COMPARISON OF EVEIDPMENTAL RELAXATION WITH DIVEFEITY . 27
Sampling and diversity indices . 27
Species dominance and opportunism . . . . . . ..... . 33
Taxonomic diversity . . . . . 37
Developmental relaxation and homeostasis ..... . ’40
Developmental relaxation and diversity . “I
CONCLUSIONS ....... . 5A
BIBLIOGRAPHY , ,,,,,, , 56

Table

LIST OF TABLES

Species growth rates, developmental relaxation, and
intraspecific character variability ...........

Relative species' biomass measured in cm3 . . .....

Diversity indices and weighted deve10pmental relaxation
for Eden Shale sampling intervals .......... .

Genetic variability in living benthic marine invertebrates

tested at 15 or more genetic loci .......

Aspects of the major stability-diversity-variation models . .

.26
.30

.3LI

.143
.47

LIST OF FIGURES

Figure

10.

ll.

Zooidal arrangement in the ancestrular and monticular

regions of PraSOpora conoidea . . . ..............

 

Monticular structure in tangential sectims in eigit

genera of Ordovician trepostome bryozoans ..........

Monticular structure in lmgitudinal sections of six

genera of Paleozoic trepostome bryozoans . . . . . . . . . .

Monticular structure in surficial views of Ordovician

and Recent colonies .....................

Changes in zooecial outline of five zooecia from a
monticule of Applexopora filiasa, NBUlOOl.II-1001.9,
through six serial acetate peels cut a 0.5 mm intervals .

 

Lateral displacement of a monticular zooid in
Amplexopora filiasa, FBU1001.5-1001.9, througi five

 

serial acetate peels (A—E) cut at 0.5 mm intervals .....

Correlation of the thickness of lamellar growth zones
in Armlempora filiasa, MSUlOOl, with the mean radius

 

of subcolonies at the top of each zone ..........

Induction and suppression of mcnticular fields through
five serial acetate peels cut at 0.5 mm intervals in

@lexopora filiasa ........... . .........

 

Developmental regressions and variability in the four

pervasive Eden trepostome species . .............

levels of species dominance across 6 sections of the

Eden Shale .........................

Levels of species Opportunism (A) and taxonomic diversity

(B) across 6 sections of the Eden Shale ...........

Correlation of developmental relaxation within species
and the percentage of total Eden biomass ...... .

u

7

13

.15

17

.19

21

28

36

39

All

13.

114.

15.

l6.

1?.

vii
LIST OF FIGURES (con't)

Correlation of the average percent heterozygous and
average diversity of inhabited associations for extant
benthic marine invertebrates tested at 15 or more
genetic loci ..................

Correlations of the percentages of opportunistic
and equilibrium species with taxonomic diversity .....

Correlations of the percentages of opportunistic
and equilibrium species with species evenness . y ......

Correlation of weighted developmental relaxation and

taxonomic diversity in 17 of the 21 Eden sampling intervals .

Correlation of weighted developmental relaxation and
species evenness in 17 of the 21 Eden sampling intervals

“5

A9

.50

.52

.53

IN‘TFDIIJCTION

Hypotheses concerning the relationship between environmental sta-
bility, morphologic variability and genetic polymorphism have consider-
able potential in providing more ecologically based explanations for
many episodes in the history of life, as well as simplifying a great
number of other paleontological problems. Stability-diversity-variation
models (Ashton and Rowell, 1975) have predicted that communities in un-
stable environments have low levels of taxonomic diversity and high mor-
phological variation within species. Opinions have differed, however,
as to the relationship of genetic variability within species to morpho-
logic variability across stability gradients. Ievins (1968), in his
"niche-variation" model, predicted on theoretical grounds that a popu-
lation's genetic variation should be positively correlated with the
degree of fluctuation of environmental parameters, so that species in-
habiting unstable environments would be characterized by high levels of
both genetic and morphologic variation, while low levels of both would be
expected in stable habitats. Several other authors have supported this
hypothesis, based upon the variability of morphologic features or protein
polymorphism (Van Valen, 1965; Bretslqy and Lorenz, 1970; Powell, 1971;
Burns and Johnson, 1971; Grassle, 1972; Grassle and Sanders, 1973;
Levinton, 1973; Iorenz, 1973; ROthstein, 1973; Soulé et al., 1973;
Johnson, 1973). The only previous assessment of stability-diversity-
variation models in a paleontological context (Ashton and Rowell, 1975)

failed to reveal any relationship, based upon the comparison of'
l

2

coefficients of variability of eight cranidial features of seventeen Late
Cambrian trilobite species across a diversity gradient. The authors
viewed their failure to find any differences in variability as the result
of a dearth of attributes capable of illuminating potential differences.

The work of Ayala, valentine et al. (1975a, b, 0), based upon pro-
tein polymorphism in extant benthic marine invertebrates, indicates a
possible paradox; genetic variability within species was Observed to be
positively correlated with taxonomic diversity and, presumably environ?
mental stability, while the opposite trend.was Observed for’morphologic
variability (Ebyle, 1971, 1972; SchOpf' and Gooch, 1971, 1972; Gooch and
Schopf, 1973; Somero and Soulé, 197A; Valentine and Ayala, 197A). High
genetic variability, therefore, appears to be characteristic of species
exhibiting low levels of morphologic variation.

Developmental deregulation possibly provides an explanation of this
conflict: through greater complexity in regulatory rather than structural
genes, species in unstable environments could facultatively deregulate
development in all individuals , thereby conferring high morphologic
variation within species in which the choice of developmental pathway
would be ecophenotypically rather than genetically determined. Conversely,
species in stable environments could have more canalized development and
therefbre exhibit more constant withinrspecies morphology. Higher gene-
tic polymorphism could be maintained within the latter species, either
because the value of deregulated development in stable environments would
be adaptively neutral, or because constant morphology is better achieved
through the "greater ability of the heterozygote to stay within the norms
of canalized development" (Ierner, 195A; Eldredge and Gould, 1972).

The morphogenetic fields of Ordovician trepostome bryozoan colonies

provide a test of this hypothesis in the fossil record. Their "ontogeny"

3
is not simply growth-related, but is developmentally regulated and has

components of variation both within and among species. Very similar
developmental fields or gradients in living organisms are known to be
regulated by specific growth hormones. It is possible that these bryo-
zoan fields give access to an important regulatory sector of the genome.
Changes in "field potential" resulted from both colony growth and en—
virmmental conditions. Separaticn of these two sources of variability
and a comparison of the latter component with taxonomic diversity pro-
vides a developmental insight into the nature of the genetic-morphologic
paradox. It is the purpose of this paper to document the existence and
characteristics of morphogenetic regulation in fossil bryozoan colonies
from the Eden Shale (Ordovician, Ohio Valley), and to carpare levels of
developmental deregulaticn with the taxonomic diversity, and by inference
with the environmental stability, of the communities in which they lived.

FDRPHOCENETIC SYS‘IEB IN ORIIJVICIAN BRYOZOAN CDLDNIES

Introduction. - Gradients of substances are responsible for much

 

cellular differentiation beginning during embryogenesis and continuing
until maturity. Differentiation along a gradient can be a process of
selective genetic expressim in response to changing concentratim levels
of substances diffused away from the point of origin. The establish-
ment of the existence of this type of regulation requires that polar
points (field centers) and potential fields (zones of influence) be
recognizable and testable. These criteria, and others as well, have been
recogiized in the morphogenesis of both living and fossil cheilostare
bryozoans (Bronstein, 1939; Dzik, 1975), for fossil trepostome bryo-
zoans (Anstey and Pachut, 1976; Anstey et al. , 1976), hydroids (Burnett,
1966; Bravermen and Schrandt, 1966), graptolites (Urbanek, 1973

 

Figure];

0.5 mm

Zooidal arrangement in the ancestrular and monticular regions
of Prasopora conoidea. A. Transverse section through the
ancestrula (A), fit generation of zooids (1-3) and second
generation (a-e). B. Transverse secticn through the same
area as in A, but at a sligitly hiQ1er level. C. Trans-
verse sectim througm the same area as in A and B, but at a
level 1.5 mm higher in the zooarium; the structure of the
primary zooids at this level is indistinguishable from a
monticule. All figures redrawn from Cumings (1912): A,

from Plate 19, Figure 5; from Plate 19, Figure 6; C, from Plate
21, Figure 31.

5
inter alia), fossil crinoid stems (Seilacher et a1. , 1968), mammalian

 

molars (Van Valen, 1962; reviewed in Gould and Garwood, 1969), for the
spacing of punctae in fossil brachiopods (Cowen, 1966), and for the
morphogenesis of a variety of living animals and plants (reviewed by
Bonner, 19714).

rIhe recognition of morphogenetic fields in Paleozoic stenolaemate
bryozoans has been centered around the structure and spatial arrangement
of structures called monticules. These structures are surficially
expressed as regularly arranged prominences of polymorphic zooecial
chambers. Specialists have generally recogiized the polymorphic character
of the zooids in these small clusters (Boardman and Cheetham, 1973,
p. 156; Utgaard, 1973, p. 324; Astrova, 1973, P. 1I; Banta et al., 19714).
However, utilizing detailed computer mapping around several monticules

from a colony of Agplexgmra filiasa (D'Orbigiy), Anstey et a1. (1976)

 

demonstrated, at least in the species studied, more or less radial gra-
dients in zooecial morphology centered on the monticules. Computer
mapping also showed that field boundaries could be obj ectively determined
by the change in slope of the morphologic gradient going from one mon-
ticule to another, and that a large number of brown bodies (preserved
polypide degeneration remnants) were located along the field boundaries .
They also showed, within four other species, that the fields gradually
enlarged themselves with colony growth, and that the polar zooidal clus-
ters, or monticules, grew larger as the polar points became more widely
separated. In this paper the most critical evidence for polar regulation
from previous work is restated, and new information is presented as well.

Monarchic control. - The recognition of a Special monarchic zooid

 

or pseudoancestrula within each monticule was caSLally made by Cummings

Figure 2:

Monticular structure in tangential sections in eight genera
of Ordovician trepostome bryozoans. A. Hallopora nodulosa,
IU8975.LI102II. B. Mglexogora septosa, IUF976. 17005. C.
Heterotrypa ulrichi, 97 .25007. D. Peronopora vera,

IU 897A.1001. E. Generalized zooid pattern, using the same
symbols as Figure 1. F. Balticoporella whitfieldi,
IU8975.38001. G. Batostoma Jamesi, IU897TI.100A. H. Stig—
matella clavis, IU8972.3008. I. Eridotrypa mutabilis,
IU8971I.100117 Monarchic zooids marked by stars. All thin
sections are in the paleontological collections of Indiana
University.

 

 

 

 

 

 

8
(1912, p. 366) and by no one else since; he observed the same arrange-
ment of zooid types in both the zone of earliest colony development from
the founding zooid or ancestrula, and within a monticule of the Ordovi-

cian trepostome Prasopora conoidea (Figure l) . The structure within each

 

monticule repeats, with some modification, the structure of the zone of
early development , including a special zoom within the monticule that
"mimics" or replicates the ancestrula. In several species examined for
this study, including one of Prasomra and eight other genera from the
Eden Shale, one large zooid within each monticule has been observed

to be in the same spatial arrangement with respect to the other monticu-
lar zooids as the ancestrula of the entire colony is positioned with
respect to the first nine or so zooids that were budded from it

(Figure 2). In very small disklike colonies (50-100 zooids) examined
for this study, the ancestrula and its surromding ring of nine or so
polymorphic zooids form, in essence, the first monticule of the colony.
The zooids outside of the initial monticule display radial morphologic

gradients, identical to those mapped in Amplexopora filiasa. Thus, the

 

morphologic gradients in the zone of early astogeny are polarized on the
ancestrula, Just as the gradients in the zcnes of later development
(astogenetic repetition) are polarized on the monarchic zooid (pseudo-
ancestrula) in each monticule (Anstey and Pachut, 1977). As zooidal
arrangement in trepostome monticules may vary, the monarchic zooid can-
not always be easily identified. Several recognition criteria, however,
have been found to be useful in combination if not alone: 1) the
monarchic zooid is usually larger in diameter than any other monticular
zooid, 2) small zooids in the monticular center (frequently termed

mesopores) are commonly smaller near the monarchic zooid and become

Figure 3:

Monticular structure in longitudinal sections of six genera
of Paleozoic trepostome bryozoans. a, b. Amplexopora filiasa,
IVBU1001. c. Peronopora vera, IU8976.19007. d. Heterot a
urlichi, IU8976.21021I. e. HallOpora nodulosa, IU3975.15009.
f. Generalized zooidal pattern, using same symbols as in
Figure l. g. Atactoggrella variant, IU897LI.2001I. h. ___Ta_b_-
ulepora penerudis, 0 .111b. Specimen illustrated in a
and b is in the paleontological collection of Michigan State
University; all others are in the paleontological collection
of Indiana University. Horizontal bar in each illustration
is 0.5 mm.

 

10

a
.

“in ..

.wWWw

‘4

a...

_ we... 1:...
v “3y“
3.

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11

larger away from it; the opposite size gradient has also been observed
as well as a radial structure resembling the spokes of a wheel, 3) the
monarchic zooid is usually offcenter in the monticule, althougn its
position varies taxonomically, and A) monticules are generally bilat—
erally symmetrical and fan-shaped, with the monarchic zooid at the fan
apex; the very large zooids (termed by some authors megazooecia) around
the margin of the central cluster are regularly arranged in symmetric
pairs, one member on either side of the monticular axis, duplicating the
pattern of early astogeny (Figure 3). rThese patterns can be illustrated
in at least six families of Paleozoic bryozoans and possibly in a Recent
heteroporid (Figures l-lI) . There is, therefore, a strong analogy between
the function of the monarchic zooid of a trepostore monticule and that

of the siculozooid of the graptolite morphoregulatory systems demonstra-
ted so carefully by Urbanek (1973, summarizing a series of earlier
papers) through "natwr'al experiments" provided by damaged and regenerated

colonies.

 

Position effect within a field. — Inferences have been made concern—
ing the reproductive, feeding, and sanitation functions of the poly-

morphic zooids within monticules (Banta et a1. , 197A, inter alia). 'llne

 

control of reproductive differentiation by a morphogenetic field has
been illustrated in the colonial hydroid Podocoryne (Braverman, 1963)

in which the reproductive zooids form only in the center of the colony,
and asexual feeding zooids occupy the periphery. As the colony grows
marginally, the sexual center expands by the progressive differentiation
of the asexual zooids caused by an increase in field strength (potential).
Urbanek (1973) has postulated reproductive differentiation of graptolite

zooids as a position effect along a morphogenetic gradient arising from

Figure II:

12

Monticular structure in surficial views of Ordovician and
Recent colonies. A. Peronopora vera, IU8976.30001.

B. Agplexopora septosa, IU8976.30002. C. Heteropora sp.,
BMVHlBB9,l.l.G (Port Elizabeth). A and B are in the
collection of Indiana University; 0 is from the British
Museum (Natural History), photograph courtes of Paul D.
Taylor, University of Dnrham. Above each photograph is a
slightly enlarged diagrammatic drawing of the monticular
pattern, using the same symbols as in Figure 1. Third
generation zooids marked by an x. Small zooids in monticular
center are shaded. Vertical bar in each photographic illus-
tration is 1.0 mm.

 

 

 

114
the siculozooid. The position effect on polymorphic differentiation of
zooids in a morphogenetic field can be tested only in bryozoans having
monticular budding. In such colonies, the monticule functions as a
meristem, and as new buds arise at the growth pole, older zooids are
displaced outward in the field. The position effect on zooids can be
tested directly by tracing the actual history of specific zooids through

serial thin sections or peels. In the trepostome Amplex0pora filiasa,

 

the vertical development of five zooids was traced through nine serial
tangential peels cut at 0.5 mm intervals (Figure 5). All of these were
monticular polymorphs at the lowest level (9) ; three (2, 3, 5) were the
very small variety of polymorphs found in the monticular center, and

two (1, A) were the very large polymorph characteristic of the monti-
cular border. All three of the very small ones greatly increased in size
through only one level , and all eventually became standard nonmonticular
autozooids (above level 6).

In the trepostomes, zooid migation takes place within morphogenetic
fields as a consequence of zooidal budding within growth centers (monti-
cules or branch tips). New zooids displace older ones outward in the
field, producing a form of morphogenetic movement induced by growth at
the polar point . To document the lateral movement away from a monticule
during zooidal ontogeny, the spatial position of the five zooids of
figure 5 were mapped througn the same nine levels; all originate in a
monticule and are gradually displaced away from it (Figure 6), inducing
the morphologic differences shown above. rIhese growth displacements are
likewise observable in longitudinal section (Figure 3). Polymorphism may
be induced by the displacements of zooids within a field, but also by

displacement of the fields themselves, discussed below. This test

Figure 5:

15

 

4 O Q O O O

35 0 O O O 0
if O O 0 <3 0
o o O o o

8 O O D <3 O
9Q o O D o

1 2 3 4 5

 

 

 

Changes in zooecial outline of five zooecia from a monticule
of Amplexopora filiasa, IVBU1001.lI-1001.9, through six serial
acetate peels cut at 0.5 mm intervals. Zooeciun 1, level A,
is 0.20 mm in maximum diameter.

16
demonstrates that polymorphic differentiation is a function of position
in a morphogenetic field. The newly budded individuals are initially
very small, undergo a dramatic size increase, and then decrease in size
thereafter as they migrate from the monticular pole toward the field's
periphery.

Preliminary observations indicate that the monarchic zooid is, at
least in several species, the only zooid that is not displaced out of
its original position, remaining in the same general location, even
through successive lamellar growth zones of accretionary colonies (Anstey
and Pachut, 1977) . This unique quality partially indicates its dominance
over the morphogenetic field, and if the fields are maintained by the
release and diffusion of a growth hormone (as, for example, in plant
phyllotaxis) , then the monarchic zooid is the most likely source of that
substance. In addition, it is the locus of zooidal budding within
monticules in certain taxa (Figure 3), implying that asexual reproduction
within a colony is also a function of this polymorph.

Field gowth. - Morphogenetic field size (potential) in these colo—

 

nies is not static; it changes allometrically during colony growth

(astogeny). In a massive lamellar colony of Amplexopora filiasa

 

(illustrated by Delmet and Anstey, 197A, Figure l), the thickness of each
lamellar growth zone is very significantly correlated with the mean
radius of the morphogenetic fields at the top of each zone (Figure 7).
This colony, about 8 cm thick, experienced 17 cycles (probably seasonal)
of growth, each of which added another layer of skeletal material to the
massive zoarium. This correlation indicates that, after an exponential
increase in field size within each growth zone, the fields deteriorated.

Because the field strengths achieved at the end of each growth cycle are

Figure 6:

17

C
. 5...... '0

0...:
’ O
A." .45

 

 

 

 

 

 

 

 

 

I
a ‘4‘;

 

 

 

'A-Is

Lateral displacement of a monticular zooid in W
filiasa, NBUlOOl.5-1001.9, througn five serial acetate peels
I“) cut at 0.5 mm intervals. Generalized displacement
pattern illustrated at lower right; monticules marked as M.
Shaded regions in A-E are monticules; displaced zooid centered
within circle. Each illustration represents an area 3.7 mm
by 5.0 mm.

18
completely non-monotonic through the zoarium, the fields must have dis-
appeared at the end of each cycle, and their buildup begun _de [1933
at the onset of the next. This type of field growth suggests the
diffusion of a morphoregulatory substance rather than other types of
field maintenance.

Monticules are absent in the lower part of each growth zone, and
generally appear about 2 mm above the base of each zone. In the lower
part of each layer (the immature zone or endozone) , the differentiating
morphogen, by extrapolation from figure 7, is inferred to be present in
concentrations below a critical level; differentiation of zooids into
monticular polymorphs is concomitant with the appearance of the exozone
(nature zme) and presumably reproductive maturity in the upper part of
each layer.

Field initiation and suppression. - One of the most conventional

 

observations regarding monticules is that of their generally hexagonal
spacing over a colony surface, an effective space-filling strategy with
the morphogenetic implication of a distance effect. Cowen (1966) in-
ferred regulation in the insertion of hexagonally spaced brachiOpod
punctae, but could not test it because the fields remained static after
insertion. Because the monticular fields grow within the colony, and the
colony itself may not be able to expand uniformly in all directions, the
fields locally interfere with one another, resulting in the suppression
of some fields and the actual disappearance of a monticule. Local irreg-
ularities in field arrangements develop, and new monticules actually
appear in relatively empty areas (Figure 8). This suggests that thres-
hold levels are required to induce new fields in interareas that have

become too large, and likewise for field suppression in areas that have

Figure 7:

19

 

 

 

.-

7J
0
O
U
z
I
2
3
.-
u 5"
u
y
<
d

5:

6 *lo
I I r I
1.5 1.6 1.7 1.3

FlllD RADIUS

Correlation of the thickness of lamellar growth zones in
Amplexgora filiasa, NSUlOOl, with the mean radius of subcolo-
nies at the top of each zone. Means computed from N8 radii
measured from the calculated center of gravity of each
subcolony; layer means based on fran 2-21 subcolonies per

thin section, and 2-14 thin sections per layer. Stars in-
dicate growth zones within the zooarium, numbered in
sequential order.

20
become too crowded. The histories of “1 specific fields were traced
through five serial acetate peels in g, filiasa (Figure 8). The closest
observed spacing between field centers in this interval was 1.2H~mmn
and the largest 14.21% mm (mean, 2.27 mm). The close spacing of 1.21% mm
was measured between fields 11 and 14 (Figure 8) at the 140 mmldepth,
above which, between the 1.0 and 0.5 levels, field 11 was suppressed.
The large spacing of n.2u.mm1was measured at the 0.5 level, between
field 6 and an unnumbered field, above which a.new field was induced in
the interarea which appeared at the 0.0 level. The cross-sections in
figure 8 illustrate the induction of new fields (2, 10, 3, H, 8, 9) in
empty interareas, and the suppression of an old field (11) in a crowded
region. The budding of new zooids in the field centers of this colony
caused lateral displacement of the monticules and their fields, so that
some of the fields illustrated (5, 7) are displaced into and out of’the
cross-secticnal planes. Field suppression and.induction imply that the
zooids in the areas affected must be repolarized toward the new field
arrangement. These patterns illustrate the inhibitory and polarizing
effects of the field agent. It can be concluded that, in the absence
of the inhibitor, any autozooid could become a monarchic zooid and in-
stitute a field of’its own. Conversely, in the presence of too much
inhibitor a monarchic zooid could produce a normal polypide and become
a.normal autozooid.polarized.towards the nearest monticule. Therefbre,
the morphogenetic agent could be considered either an.inhibitor with
respect to monarchic zooids, or an inducer with respect to autozooids.

Regeneration. - Anstey et al. (1976, Figures 8. 9) and Anstey and

 

Pachut (1976) have shown that brown bodies, the residues of polypide

degeneration, occurred.in.Amplexopora filiasa.mostly in zooecia near the

 

Figure 8:

21

 

 

 

 

 

Induction and suppression of monticular fields through five
serial acetate peels cut at 0.5 mm intervals in Alplexgpora
filiasa. Center, plan view of 13 monticular fields at upper-
most level. Cross sections along transect A—A' and B-B'
shown above and below plan view, respectively. Monticular
fields represented by stippled areas, the diameter estimated
by the mean intermonticular spacing in all five peels. Areas
of no data are vertically lined. Stars, loci of field
induction; crosses, loci of field suppression; arrows, loci
of field displacement. Cross sections are drawn without
vertical exaggeration.

22
boundaries of the monticular fields. A few anomalous zones were observed
wherein all the zooecia contained brown bodies except those in the man-
ticules. Such zones provide further support for the fOrmer existence of
these morphogenetic systems. They occur at random locaticns within
the colony's repetitive growth layers and are thus unrelated to seasonal
growth. They most likely resulted from external environmental pertur-
bations (e.g. starvation) with the monticular zooids remaining unaffected,
possibly because some kind of physiologic gradient ran parallel with the
morphogenetic one, concentrating the colony's resources at the polar
points. Therefore, the source of the morphogenetic substances would
be unaffected, and.the gradient could be maintained. The Observed
regeneration of the extramonticular zooids supports this contention: the
size and shape of an individual skeletal chamber did not change after
regeneration, and must have been dependent upon its position in the
field polarized on the monticule (Anstey et al., 1976). The regenerated
zooids adopted the form.(and.inferred fUnctions) of their defbnct pre-
decessors in the same zooecium above several layers of’mass degeneration
in A, filiasa, indicating that the gradients were stable enough.to be
maintained, even though most of the colony's autozooids had experienced
degeneration.

In summary, evidence for the existence of a regulatory system in
Ordovician bryozoan colonies includes: 1) radial morphologic gradients
in zooecial morphology polarized on each monticule, 2) the presence
of a monarchic zooid or pseudoancestrula.within each monticule that
provides a specific point of origin fbr each morphogenetic field, 3) a
position effect fOr polymorphic differentiation.with respect to the

monarchic zooid, U) nonlinear (allometric) growth of fields, suggesting

23

diffusion as a likely cause of field.mamntenance, 5) a distance effect
for the induction of new fields and suppression of old ones, illustrating
the polarizing and inhibitory effects of the field agent, and 6) the
maintenance of morphogenetic fields following an episode of mass degenera-

tion not at the end of a normal growth cycle.
MEASUREMENT OF DEVELOPMENTAL RELAXATION

Genetic basis. - In.many organisms, gradient or field characteris-

 

tics, including morphogen concentration, diffusion rate, threshold levels
and so on, appear to be involved.in the localization of differentiated
cells or body parts, and pattern formation in general. Therefore, envi—
ronmentally or genetically induced changes in these characteristics

could produce different phenotypes. Genetic alterations of the basic
gradient or field pattern have the capacity to cause changes in the rates
and amounts of growth, in the localization, degree of expression, or even
the presence or absence of particular cellular (or intracolonial)
differentiations (Child, l9fl1). Such quantitative alterations of basic
gradient patterns may explain growth and form.differences in related
species, its recognition thus assuming importance in phyletic reconstruc-
tion (as, for example, in heterochrony). Underlying genetic changes

need not be complex or extensive, however. It has been suggested
(DeBeer, 1958, ch. 3; Gould, 1968, p. 92; Wilson, 1976) that general
developmental rates may be regulated by simple genetic mechanisms, minor
alterations potentially having substantial phenotypic effects. Valentine
and Campbell (1975) have applied the concept of gene regulation to
explain the major’advances in animal evolution, viewing evolutionary

changes as expansions or repatternings of the regulatory portions of the

214
genome rather than changes in structural genes. It is plausible that

similar controls may represent a measurable aspect of the regulatory
phenotype on which natural selection has acted, a conclusion which
Urbanek (1973, p. 1498) reached regarding graptolite phyletics. The
morphogenetic field dynamics of bryozoans are capable of producing
sigrlficant differences in colorw form, both within and among species.
This study seeks to determine whether or not relaxation of these develop-
mental systems is in any way correlated with environmental stability (as
measured by taxonomic diversity). To test this hypothesis, the develop-
mental variation of the bryozoan assemblages of the Eden Shale (Ordo-
vician, Ohio Valley) will be compared with taxonomic diversity.

Measurements. - A preliminary investigatim (Anstey et al. , 1976)

 

illustrated significant correlations among field size, colony size, and
monticular diameter in four species of Eden trepostorres. These relation-
ships indicated that field dynamics were related to overall colony

form and monticule size. Therefore different growth rates of the fields
could produce colonies having different stature, erectness, and poly-
morphic ratios. In each of the four stratigraphically pervasive Eden
species (Heterotrypa ulrichi, Hallopora nodulosa, Amplexopora septosa,

and Peronopora vera) twenty measures of monticular diameter and inter-

 

monticular distance (i.e. field diameter) were obtained from each colony,
allowing the calculation of colony means. These means were subsequently
converted to natural logarithms to remove non-linear effects, as allo-
metric field growth had been previously established. Colony size (branch
diameter or frond thickness) measures were not utilized because of low
reliability caused by differences in colony growth forms and multiple

growth increments (as in Amplexopora) , thickness variations along the

 

25
length of branching colonies' axes, and deformation resulting from

sediment compaction. Additionally, the species Peronopora vera,

 

which has a very thin endozone, would possess diameter measures varying
totally with the realized stage of astogeny. Monticule and subcolony
(field) sizes, on the other hand, are measurable irrespective of growth
form and circumvent such problems.

The developmental pattern of each species and the degree of deviation
from a strictly canalized development (develOpmental relaxation) may be
measured through least-squares regression analysis: the calculated
regression line itself estimates the "stan " developmental pathway,
and the residual variance from regression measures the degree of relaxa-
tion from normal development. Additionally, the slope (allometric
exponent) estimates the relative rates of growth, and the value of the
Y-intercept provides the size of the monticules at a field diameter of
1 mm (the log of 1 mm is zero). Fiom the slope an indication of poly-
morphic ratios and general colony form may be obtained. Samples from
each of the four species listed above were subjected to regression
analysis, yielding within—species statistics (Table 1). Developmental
relaxation values (residual variances) were converted into standard
deviations for comparative purposes.

Results and inferences. - For each species (Table l) a significant

 

correlation between monticule and subcolony size was obtained, confirm-
ing the developmental character of field growth. Both the relative growth
rate (allometric exponent or slope) and monticule size, at a field dia—
meter of 1 mm (Y-intercept), differ among species so that different
species have different overall growth forms. The scatter of points

within a species represents the deviations from normal development

26

Table 1. Species growth rates, developmental relaxation, and intra-
specific character variability.

 

4§pecies N R P AE YI DR IV
Peronopora 38 .5990 .01 .7860 .6339 .11718 “.65
vera
Amplexopora 55 . 571m . 01 . 7212 . 7052 . 121211 8 . 20
septosa
Hallopora H3 .5782 .01 .H663 .8790 .152H1 5.78
nodulosa
Heterotrypa 66 . 3963 . 01 . 6521 . 7836 . 21823 6 . 72
ulrichi

 

N: number of specimens. R: correlation coefficient. P: significance
level of correlation. AE: allometric exponent (log mm/ log mm) ; pro-
portional to colony growth rate. YI: y—intercept ; proportional to
monticular size at a field diameter of 1 mm. DR: developmental
relaxation; square root of residual variance from.regression. IV: mean
percent intraspecific variability of the 80 two-state characters used
by Anstey and Perry (1973, Tables 2—5. pp. 17—20).

27
(Figure 9); those lying along the regression line are representative of
the "normal" developmental pathway (astogeny) While deviations from this
trend are inferred to reflect ecophenotypic deregulatim. Although these
species overlap one another in the specific aspects of development
being measured, each species is sufficiently distinct in terms of slope,
intercept, and residual variance, implying that each displays a certain
degree of developmental homostasis.

The strength of the correlations suggests that these systems were
functionally important; across the four species, a higrly significant
correlation is obtained between monticule and field size (r = .70,

p = .01). As these species represent four different families, such a
developmental economy must have a big: degree of generality in des-

cribing trepostome morphogenesis, and has implications in the areas of
functional morphology, autecology, and phyletics that are undeveloped

in this study .

(DMPARISON OF EVEIDPMENTAL RELAXATION WITH DIVERSITY

Samplirg and diversity indices. - Taxonomic diversity has been

 

considered to be one of the more important aspects of community organi-
zation and structure. Indices are reasonably sensitive to such ecologic
parameters as the number of ways and how evenly the environment is
partitioned among species and, therefore, how effectively it is being
utilized by the community (Lorenz, 1973). Nargalef (1968) has further
sted the implications of diversity measures as being an expression
of the possibilities of constructing feedback mechanisms. Based upon
these ideas, diversity should be proportional to food chain length, the

degree of symbiosis, and the possibility of negative feedback control,

28

 

 

 

 

2.0K. . .
o
d} o
z_m..
2.11140
1.1.1
:1 ..
(O
5% 1.0324»
_1
D
8
g...
2
O
1‘2 1.405»
1.
1.174%
c
monol- ’
4.
0
11.819 : + + J. : + + 4 : : J. t : :
1.3411 1.610 1.035 2.318 2.113 3.3211 3.915 4.15!
HCNTICULRR SPRCING
Figure 9: Developmental regressim and variability in' the four pervasive

Eden trepostme species. The length of each regression line
equals the range of values for the species. Circles, HU,
Heterotrypa ulrichi; solid squares, HN, Hallopora nodulosa;
asterisks, AS, Amplexopora septosa; open squares, PV, E911”
onopora vera; all values are in mm. Values of the slope ,
intercept, and developmental variability are listed in Table
l‘.

 

 

 

 

 

29
thus causing a reduction in oscillations and increasing stability when
diversity is high (Odum, 1971).

In order to assess diversity, the collections of Anstey and Perry
(1973), utilized above, and additional unsectioned material in the
paleontological collections of Michigan State University were censused.
Locality data is listed and figured in Anstey and Perry (1973, Appendix
1, p. 77; Figure 1, p. 8); all measured sections included recognizable
formational boundaries to exclude any "floating" sections. Samples were
obtained from strictly in—place materials throughout the lateral extent
of each collecting interval. The separation of specimens from matrix
was accomplished through kerosene immersion and washing through nested
sieves; materials that passed through the 2 mm sieve were discarded.
External debris was removed ultrasonically and as the taxonomy of these
materials had previously been worked out on a numerical basis by Anstey
and Perry (1973), external identification was possible. The species
involved have several externally observable diagnostic features. All
materials (over 6000 colony f‘ragrents) were so identified using a dissect-
ing microscope.

The high degree of fragrantation of the resulting material would
have led to systematic overestimation of diversity if simple numbers
of colony fragments had been used. An additional size bias would also

enter: of the dominant species present, Heterotrypa ulrichi and Hallopora

 

nodulosa were considerably smaller in estimated living colony size than

Mplexopora septosa and Peronopora vera. Tb circumvent these potential

 

 

biases, species' skeletal volumes from each of 21 sampling intervals
were determined by water displacement (Table 2) . This yielded a measure
of relative skeletal biomass, presumably proportional to resource uptake

in filterefeeding organisms and more closely approximating the actual

3%)

 

 

 

 

 

 

a.nam «dd «.4» can 5.05 so“. “am mad as am sad o.oa n.~au ~.a: mam mm ~.Hea “.mn so: sad inn .1..."
1 nm on :m n n a N .n m 1 1 2. 0a. 1 :H w. n “A m ad «3205530
1 1 1 1 1 1 1 1 1 1 A 1 1 1 1 1 1 1 1 1 1 £3.75.“ 3:53.300
1 1 1 1 1 1 1 1 1 1 a 1 1 1 1 1 1 1 1 1 1 maummmmmmu.mmmmmmumw
1 - - - - - - - . - a - - - - - - - . - . a a
1 1 1 1 1 1 1 1 1 1 m 1 n m 1 1 1 1 1 1 1 mmwummumqummnmmmmmmun
1 1 1 1 1 1 1 1 1 1 A m 1 1 m 1 1 1 1 1 1 nu>Qfivmmmmmmmflmm
saw 1 1 1 1 1 1 1 1 1 1 1 a «a 1 m 1 1 1 1 1 «mmummmmmm.ummmmmmmmmmmmm
1 1 1 m a m a a 1 1 1 1 1 1 1 1 1 1 1 a o mumamwmflfimwm
1 3 a 1 1 a. 1 1 1 1 1 1 1 1 1 3 1 n. n 1 1 «a a
ha a N «N a on so mm m w o« m. m o 1 4H m a HH mm «w «an» ummmmmmmwm
mo n - me e. do“ «a mm ma m Ha 1 ma a mod ca on a m 1 ma mummmuw mmmmmmmmumq
m. a m. a o «a m a: a: as mm «a a m. on ma :4« mm mm mm a: mummmmmm_ummm1mmmm
« on «u use «m an: .;m on ca a 1 4 an n mm mm m e mam awa oaa mmmmmmmuumnmmmmummm
om1o m01w ad-» na1w «H10 ”~1w mo1n 4o1n mo1m HH1m no-3 “a-“ ~H1n H1a m~1n mnrn11,as1w, :41“ nH1~ no-” ward .oaovmmx

”abacus“ and“ I.»

m

. 50 pH omnzmmms.mmm50fio .mofiooom m>HpmHom .m magma

31

community relationships (Lloyd and Ghelardi, 19611). Macrofossils other
than bryozoans were very minor in abundance in the intervals sampled
except for the brachiopod Onniella which was as common as any of the
trepostome species, and in some intervals, fragrented crinoid columnals.

In this study, Brillouin's equation, based upon information theory,
is utilized as a measure of taxonomic diversity rather than the more
widely used Shannon index. The reasons for this are: l) Brillouin
values are determined, not estimated, with a standard error of zero;
Shannon values are an approximation of Brillouin's, 2) Kaesler and
Brondos (1975) have found that Shannon's index is a biased estimator of
diversity, always resulting in overestimation, and suggested the use of
Brillouin's equation for paleontological purposes, and 3) the Brillouin
equation is ideally suited for studies such as the present one wherein
collections may be treated as discrete entities and not as subsets of
larger pOpulations (Pielou, 19711).

The Brillouin equation measures the information content per symbol
of a message composed of N symbols of 3 different kinds, of which NJ are

of the 3th kind as follows (Pielou, 1969, p. 232):

 

H l lpgN

=11 (N12N22...............NS!)

The base of the logarithm employed in this study is 2, H being expressed
as bits per individual.

This index, like Shannon's, confounds simple species diversity and
species evenness, a measure of how equally (numerically) species are
represented in a community (Pielou, 19711). A finite collection has
maximum evenness and diversity when individuals are distributed among

the species as evenly as possible. A separate measure of evenness, also

32
termed "dominance cmcentration" (Whittaker, 19614) and "equitability"

(Lloyd and Ghelardi, 19611), is therefore desirable as communities of
similar diversity may exhibit different patterns of species distribution.
The ratio of observed diversity to the maxirmmn possible diversity for

the same number of species was suggested by Pielou (1966) for this

 

purpose:
J = “max where,
l N!
H = —- log
“33‘ N {EN/31!} S r{(D\1/s}+ our
and,
N = total of importance values (e.g. number, biomass,
productivity)
s = number of species
[N/s] = integer portion of N/s

N-s [N/SJ
A related measure, inversely proporticnal to diversity, is eco-

1"

logical dominance. Its importance as a separate index resides in the
fact that not all of the organisms present in a community exert the same
influence over the nature and fimction of the entire community (Odum,
1971). In general, a few species or groups of species disproportionately
affect energy flow owing to their numbers, size, productivity, and other
factors. Relative importance additionally tends to cross taxonomic lines,
involving organisms belonging to widely differing groups. An index of
dominance based upon relative resource utilizatim measures (i.e. biomass)
may closely approximate the degree to which the energy flow of a commu-
nity is controlled by one or several species. Such an index is defined
by Odum (1971, p. 11111) as:

33
c = (mi/N)2

where, ni = importance value for each species (e.g. numbers, biomass,
productivity), and

N

sum of importance values.
Therefore, c is proportional to the sum of each species' importance
to the community as a whole.

Species dominance and ggportunism. - Examining these indices
(Table 3) in more detail reveals several previously unobserved aspects
of the Eden Shale communities. Species dominance measures indicated
that domination was consistently attributable to three species: 1m-

trypa ulrichi, Hallcpora nodulosa, and in a few instances to Balticom-

 

rella whitfieldi (Pachut and Anstey, 1977). Intervals of lowered domi-

 

nance (less than 50%) exhibited more equal abundances of up to ten addi-
tional species (Table 2). ' The latter range of dominance values was
subdivided into three statistically differing levels whose correlations
across sections are shown in figure 10. The three "dominating" taxa
exhibit several of the relative abtndances and distributional character-
istics listed by Levinton (1970, p. 76) as useful in recognizing oppor-
tunistic (r—selected) species: 1) random orientation and lack of size
sorting in individual beds, but a tendency toward size-group agregations
of the dominant species (see Waage, 1968, p. 162), 2) limited areal
distribution, beyond which the horizon is unfossiliferous (Waage, 1968),
3) individual species aggregated into clusters (especially if sessile),
11) presence of species in thin but widespread isochronous horizons,
indicative of a brief invasion, 5) species found abundantly in several
otherwise distinct famal assemblages (eurytopic), 6) species appearing

in great abundance in a facies with which it is not usually associated,

39

 

Table 3. Diversity indices and weighted developmental relaxaticn for
Eden Shale sampling intervals.

Uhit SP PO DIV EVN DOM WDR

1—01 27.85 75.6 1.57 0.71 0.38 0.15737
1—03 31.35 82.9 1.09 0.69 0.52 0.17591
2-13 28.20 90.6 0.71 0.39 0.75 0.18612
5-99 32.75 78.0 0.86 0.88 0.99 0.12028
5—91 35.80 85.3 0.67 0.99 0.69 0.19815
5-38 90.15 93.0 1.85 0.97 0.18 0.07376
5—23 55.05 29.3 0.92 0.66 0.58 0.13391
5-18 69.65 38.9 1.92 1.00 0.22 0.03019
5—17 65.90 19.2 1.00 0.65 0.52 0.01578
5-15 67.90 95.0 0.53 1.00 0.56 0.15731
9-01 85.95 29.8 1.29 0.70 0.91 0.02208
3—11 0.65 87.9 0.62 0.50 0.69 0.19887
3—09 5.80 75.9 0.92 0.81 0.97 0.19627
3-09 13.10 56.6 1.53 0.96 0.28 0.19261
3—03 19.30 97.9 1.97 0.79 0.35 0.15551
6-21 29.50 76.5 1.20 0.97 0.55 0.18593
6-19 31.20 82.0 0.95 0.65 0.52 0.16075
6-15 35.75 93.8 1.60 0.88 0.32 0.09389
6-ll 38.25 39.9 0.73 0.95 0.53 0.03010
6-09 90.25 27.7 1.57 0.86 0.28 0.02579
6—30 99.60 75.1 0.89 0.61 0.60 0.00802

 

Uhit: codes used by Anstey and Perry (1973). SP: stratigraphic position
of sampling intervals in meters below Eden-Dillsboro contact. PO:
percentage of opportunistic species. DIV; Brillouin Diversity index.
EVN: species evenness. DOM: species dominance. WDR: weighted develop-
mental relaxatim of species occurring in sampling interval.

35

7) a species numerically dominates an assemblage by 85-1001. Specific-
ally, characteristics 1, 2, 3, and 7 apply to the species of HalloEra,
Heterotrypa and Balticoporella in the Eden Shale.

 

 

MacArthur (1960) distinguished between opportunistic (eurytopic)
and equilibrium (stenotopic) species . Opportmists are generally small,
mspecialized, r—selected species capable of rapid population expansion
as a result of elevated birth rates, enhanced larval survival, short
generation length and generality with respect to resource utilization.
They are common in unpredictable environments and areas with unoccupied
or newly opened niches, and are usually not constrained by density-
dependent factors such as oxygen level, food supply or living space.
Equilibrium species, or the other hand, are larger, more stable, K-
selected taxa, inhabit more uniform or predictable enviromrents and
maintain their numbers near the carrying capacity of the environment
by producing the minimum number of offspring to ensure survival. Hallam
(1972, Figures 9-8, p. 79) graphically illustrated the patterns of
variation in the abundances of both opportmistic and equilibrium species:
equilibrium species maintain small or moderate populaticn sizes while
undergoing minor oscillations in abundance througr time ; conversely,
opportunists are characterized by abrupt oscillations resulting either in
very low or extremely high numbers of individuals in response to ecolo-
gical opportunities hindering or favoring proliferation. Therefore, cm-
sistent extreme dominaticn of comunities by the same taxa suggests that
such species were opportunistic.

This premise was tested by calculating the percentage of the total
biorass of each sampling interval accounted for by the three possibly

opportunistic species (Tables 2 and 3). Two statistically different

36

.m 0.33. CH 696.: mHmPHmufi
mafiaosmm Hm oop.mo mosam> connedsoo ooxeop no coamfi>fioosm Hooapmfiawpm on» so oomph who mHm>oq

.oonmomefi who mcofipmamnnoo .mamew doom one no mooapaom m mmopoo possessoo moaooom mo mam>oa "oa.msswam

 

 

 

 

’oJ

 

1. 0.0-00.350.6-

 

 

 

 

 

OJ...”

 

 

 

 

 

 

 

 

37
(t = 10.25, p = .01) subsets of sampling intervals resulted: a group

whose total biorasses were at least 75% made up of the three opportunists;
and a subset wherein less than 57% of the biomass was attributable to the
opportunists and which contained more equal abmdances of up to 8 addi-
tional taxa (Table 2). This dichotomy bears out the presence of oppor-
tunistic species in the Eden Shale. The subgroup of sampling intervals
characterized by equilibrium species was further subdivided to yield a
total of three levels of opportunism whose patterns across sectims are
illustrated in figure 11A. It is proposed that the opportunists' ad-
vantages could have been related to a greater physiologic tolerance of
physical changes in the environment which greatly enhanced larval survival.
Simply elevating birth rates would have had little direct influence on
population bursts as average fecundity (hmdreds or thousands of repro—
ductive individuals per colony) could easily have accounted for contin—
ually large populatims if larval mortality had been sligrt (Levinton,
1970) .

Taxonomic diversity. - In contrast to dominance and opportunism,
Brillouin diversity shows a higmly mosaic pattern in the Eden Shale
(Figure 11B) . Rollins and Donahue (1975) summarized community charac-
teristics and recogrized three successional levels; opportunistic
(immature), mature (equilibrium), and relict-mature communities. Through
time, maturity and stability tend to increase (Valentire, 1969;

Levinton, 1970). Sharp changes in stress conditions, however, would have
the effect of producing a quantum change in stability, stenotopic taxa
being rapidly winnowed, and thus reducing the probability of their being
preserved. The general absence of monotonic gradients of increasing

community stability (diversity) in the Eden (Figure 11B) suggests that

Figure 11:

38

Levels of species opportmism (A) and taxonomic diversity
(B) across 6 sections of the Eden Shale. Correlations are
inferred. Levels in each instance are based upon the
statistical subdivision of ranked values of opporttnism and
diversity listed in Table 3.

 

 

 

 

 

 

 

 

 

an Lana-o I".

 

 

 

 

 

90
fluctuaticns in limiting factors may have been quite sudden.

Therefore, monotonic trends in both species dominance and the
percentage of opportunists are observable within the Eden, and the latter
trend is especially distinct. Althougi Brillouin diversity is signifi-
cantly (negatively) correlated with both quantities, its levels show a
pronounced mosaic pattern probably owing to abrupt changes in stress
conditions. In contrast, the monotonic trends in dominance and opportun-
ism most likely persist because those measures are less sensitive to
low level environmental fluctuations .

Developmental relaxation and homeostasis. - If such a dichotomy
in their adaptive strategies existed, differences might be predicted in
the developmental strategies of the opportunists and equilibrists.
Opportunists, adapted to Instable environments, should display less
canalized development than the equilibrists. To ascertain if statis-
tically significant differences existed among the four pervasive species,
a homogeneity of variance test was performed. Sigificant inhomogeneity
(Fmax = 3.97, p = .05) was present, indicative of differing degrees of
developmental relaxation in these taxa.

Examining each species' value of developmental relaxation (Table 1),

one finds that Peronopora vera and Amplexgmra septosa have the lower

 

 

values, whereas Hallopora nodulosa and Heterotrypa ulrichi have higher

 

 

values. This accords well with qualitative observations of the morphology
of Peronopora, which is the least variable in overall form (bilaminate,
frondescent) .

These values are independent (r = .13, p = .93) of the average intra-
specific variability of the 80 two-state characters (Table l) examined by

Anstey and Perry (1973, Tables 2-5, pp. 17-20). Relaxation of

91

 

HU

35.800

1
q
,u
18

PERCENTRGE 0F TOTRL BIOMRSS

 

 

 

'7 .40 4e 1 i 1
111700 114000 116300 118600 120900

SPECIES DEVELOPMENTRL RELRXRTION

 

Figure 12: Correlation of developmental relaxation within species and
the percentage of total Eden biorass. HN, Hallopgra
nodulosa; HU, Heterotrypa ulrichi; AS, A_mp1exopora
septosa; PV, Peronopora vera.

92

morphoregulatory restraints, therefore, does not appear to be genetically
linked to these phenetic traits, although these traits were successfully
used by Anstey and Perry in the taxonomic discrimination of a diverse
group of trepostomes.

Developmental homeostasis is, therefore, present to varying degrees
in these taxa. It apparently ftnctioned by canalizing development,
buffering the bryozoans against deviations in astogeny which could have
eventually resulted in reduced overall fitness. It is by definition,
inversely proportional to the degee of developmental relaxation, being

higest in Peronopora vera and lowest in Heterotrypa ulrichi.

 

 

Both Heterotrypa ulrichi and Hallopora nodulosa have been considered

 

 

to be Opportunistic, while Agplexopora septosa and Peronopora vera have

 

 

the attributes of equilibrium taxa. Levels of developmental relaxation

support these assertions (Table 1): Heterotrypa and Hallopora possess

 

the mguest levels with correspondingly lower homeostasis. In particular,
Heterotrypa ulrichi, which has the highest range of variability and the

 

lowest level of homeostasis of any of the four, must have been capable

of withstanding extremes in environmental conditions through substantial
developmental deregulatim. Further evidence for opportunism is obtained
through an evaluation of allometric exponents (or slopes), and Y-inter-
cepts (Table 1). The opportunists are characterized by a lower field
growth rate and have proportionately larger monticules (and more of them)
than the equilibrium species . If hypotheses concerning the presence of
reproductive individuals in the monticules are valid, then opportunistic
colonies would have had a higher ratio of reproducers to feeders. Equil-
ibrists, on the other hand, would have a much lower ratio of reproducers

to feeders, in accord with their being attuned to effective resource

93

Table 9. Genetic variability in living benthic marine invertebrates
tested at 15 or more genetic loci *.

 

NUmber Average

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Species Percent of Loci Source
Heterozygpsity_
1. Asterias vulgaris 26 1.1 Schopf & Murphy,
verrill 1973
2. Cancer magister 29 1.9 Hedgecock & Nelson,
Dana unpublished
3. Asterias forbesi 27 2.1 Schopf & Murphy,
(Desor) 1973
9. Liothyrella notorcadensis 39 3.9 Ayala et al., 1975
Jackson
5. Homarus americanus 37 3.9 Hedgecock & Nelson,
NilneeEdwards unpublished
6. Crangpn negricata 30 9.9 Ibid.
(Stimpson)
7. meulus polyphemus 25 5.7 Selander et. al.,
(Linne) 1970
8. Upggebia pugettensis 39 6.5 Hedgecock & Nelson,
Dana unpublished
9. Callianassa califbrniensis 38 8.2 Ibid.
Dana
10. Phoronopsis viridis 39 9.9 Ayala et. al., 1979
Hilton
11. Crassostrea virginica 32 12.0 W. W. Anderson,
(Gmelin) unpublished
12. Asteroidea, four deep-sea 29 16.9 Ayala et. al., 1975
species
13. Frieleia halli 18 16.9 valentine & Ayala,
Dell 1979
19. Qphiomusium.lymani 15 17.0 Ayala & valentine,
Thompson 1979
15. Tridacna.maxnma 37 21.6 Ayala et. al., 1973;
Roding Campbell et. al., 1975
* After valentine (1976, p. 86)

99

utilizaticn rather than reproduction. If such inferences are correct,
the estimated productivity of the Opportunists should be higher than that
of the equilibrists. Figure 12 indicates that this appears to be so;
the opportunists account for a proportionately higher percentage of the
Eden's skeletal biomass in spite of being the two smallest taxa in terms
of individual colony size, an additional attribute of opportunistic
species (Rollins and Donahue, 1975; Schoener, 1969).

Developmental relaxation and diversity. - Ayala et al., (1975a)

noted that the recent brachiOpod Liothyrella notorcadensis is rather

 

variable morphologically while exhibiting low genetic polymorphism. In

contrast, the deep-sea brachiopod Frieleia halli is quite variable

 

genetically but possesses little morphologic diversity. A paradox thus
becomes apparent; phenotypic and genotypic variability may be inversely
related. In living populations, genetic variability may be directly
assessed througl an analysis of enzyme polymorphism utilizing the tech-
nique of starch-gel electrophoresis (see Ayala et a1. , 1972 for methodol-
ogy). Table 9 summarizes the available data in genetic variation in
extant benthic marine invertebrates, expressed as the percentage of loci
at which an average individual is heterozygous where 15 or more loci
were examined. As correlative data on resource stability is as yet un-
available, diversity measures have been employed as first-order approxi-
mations (Valentine, 1971, 1972, 1976). A highly siglificant positive
correlation (Figure 13) between average heterozygosity and the average
diversity of associations in which the species ranges today results (see
Valentine, 1976, pp. 89-90) , indicatirg a trend of increasing genetic
variability with increasing stability. Such a relationship (Table 5)

is contrary to the expectations of the Bretsky-Iorenz model, but

RVERRGE PERCENT HETEROZYGOUS

Fugue 13:

18.700

14.300

9.900

95

 

    

T

T

 

 

 

.5001
10 i!— 4, ; :7
2.000 2.800 3.600 4.400 51200
RVERRGE DIVERSITY
Correlation of the average percent heterozygous and average

diversity of inhabited associations fer extant benthic marine
invertebrates tested at 15 or more genetic loci. Numbers
refer to the species listed in Table 5.

96
consistent with the predictions of Ayala et al. (1975a, b, c) and
Valentine and Ayala (1979) .

Additional supportive data is available from pelagic organisms;
Sorero and Soulé (1979) found diversity and heterozygosity to be posi—
tively associated in 13 species of marine teleosts, while Ayala et al.
(19750) and Valentine and Ayala (in press) noted that, in three species
of krill (Egphausia) , average heterozygosity was negatively correlated
with trophic resource seasonality. Doyle (1971, 1972) found considerable
variability in three species of ophiuroids from the deep-sea considering
only 9 to 6 loci, and Schopf and Gooch (1971, 1972) and Gooch and SchOpf
(1973), also examining deep-sea populations, found genetic polymorphism
to be significantly higl at 9 to 15 loci in small populatiols of eight
species.

As colonial organisms, bryozoans offer the possibility of partitioning
their morphologic variability into withinecolony (ecologic, polymorphic,
or developmental) and between-colony (genetic) components of variance
(Schopf 1976). Schopf and Dutton (1976), for the recent cheilostome
SchiZOporella errata, and Farmer and Rowell (1973) , for the Paleozoic

crystoporate Fistulipora decora, found within-colony variance to be

 

greater than between-colony variance. Schopf (1976) attributed this to
gradients in environmental stability; forms in variable environments
exhibited variation assignable to ecophenotypic rather than genetic
causes, while the reverse appeared to be true for stable habitats.

For the bryozoans of the Eden Shale , the relationship between
developmental relaxation and taxonoric diversity (stability) may be
assessed in two ways: 1) through an examination of opportunistic and

equilibrium species abundances, and 2) through a weighted average value

117

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98
of developmental relaxatiol for each sampling interval. It was shown

above that the opportunistic species (Heterotrypa ulrichi and Hallogra

 

nodulosa) exhibited higler levels of develOpmental relaxation than the

equilibrium species (Amplex0pora septosa and Peronopora vera) . There-

 

 

fore, high percentages of opportunists should represent environments
favoring greater developmental relaxation (variability), whereas a
preponderance of equilibrium species would be indicative of developmental
restraint (canalization).

A weighted average of developmental relaxation, based upon relative
biomass contributios of the above four taxa follows similar logic.
However, inasmuch as the biomass of an interval may have substantial
contributios from up to nine other taxa having the characteristics of
equilibrium species, but whose levels of developmental relaxation were
not measurable due to small sample sizes, corrections were made to count—
eract a consistent overestimation of the overall developmental relaxation.
To accomplish this, the developmental relaxatiol value of each of the
four measurable taxa was multiplied by their biomasses, summed, and
divided by the total biomass of the sampling interval (Table 3) . This
has the effect of lowering the weighted value of intervals containing
substantial numbers of equilibrists while leaving the values for highly
dominated intervals unchanged. Four intervals were deleted: 5-18,

6-30, and 5-17, 6-11, dotinated by Balticqiorella and undifferentiated

 

ceramoporoids respectively, which appear to have been opportunists but

whose developmental relaxation values have not been calculated.
Figures 19 and 15 illustrate siglificant correlations between the

percentages of opportunistic and equilibrium species in each sampling

interval and species diversity and evenness of distribution, respectively,

PERCENTRGE 0F BIOHRSS

Figure 19:

99

 

 

 

 

 

x
11 11
a» 3" m
an: a:
{ll * El X
a
60+ 9
1::
*1; 1111
44"- “ m
x
:11
2‘1’ an. m 7 m
m m m
m a:
m
.500 .600 1.100 1.100 1.700

SPECIES DIVERSITY

Correlatios of the percentages of opportunistic and equili-
brium species with taxonoric diversity. Asterisks denote
opportunists (r = -50, p = .01); squares, equilibrists

(r = .98, p = .01).

 

PERCENTRGE 0F BIOHRSS

Figure 15:

50

 

 

 

 

 

:x
:x
a *5 ‘3
fi
*1 a: 2x
111 as a)"
6+~ “1
a] III
E)
11 4
41- 3* gym
1!
241- m [D X U E! 1!]
11.1 M
E] I!
ll
1 t. : 1 : 1“
.340 .490 .840 .790 .940

SPECIES EVENNESS

Correlations of the percentages of opportmistic and equili-
brium species with species evenness . Asterisks , denote
wmmmmm(r=-1W1P=.09;mmmme®flmm$s

(r = 37. p = .015)

51
while figures 16 and 17 indicate similar relationships between taxonomic
diversity and evenness, and weiglted developmental relaxation throughout
the Eden Shale. Through their wider ranges of developmental responses,
opportunists characterize the lower diversity (more unstable) habitats
where they dominate biomass. Conversely, higcer diversity (more stable)
regimes favor a more even distribution of taxa and greater niche subdi-
vision by equilibrium species.

Developmental relaxatiol is, therefore, negatively associated with
diversity and, presumably, stability. The genetic-morphologic paradox
remains to be explained. For the two extant species so corpared to date
(the brachiopods Liothyrella notorcadensis and Frieleia halli) , genotypic
and phenotypic variability were inversely related. In the Eden Shale
bryozoans, the values of developmental relaxatio: measure departures
between-colonies from "normal" development. The variability 113112.
colonies provides a test of the genetics of deregulation. The intra-
colonial variability is not of a genetic nature because each colony
is a clone, and every zooid is genetically identical to all the others;
it must be attributable to developmental deregulation at the subcolony
level.

Within—coloy variability was estimated for each species by calcu-
lating an average coefficient of variability based on 10 to 15 colonies,
and 20 measures of monticular spacing from each zoarium. Measured
colonies were the same as those utilized for the calculatim of between-
colony developmental relaxation; the within-colony variances employed
are normalized for astogeny because all fields on each colony were

measured at the same astogenetic stage.

52

 

.16200

RELRXRTION

.12700

.09200

W

 

T

.05700

 

11220 1 ‘ # 1 * ‘

.500 .800 1.100 1.400 1.700

 

 

WEIGHTED DEVELOPMENTRL

SPECIES DIVERSITY

Figure 16: Correlation of weighted developmental relaxation and taxonomic
diversity in 17 of the 21 Eden sampling intervals. Intervals
5-17, 5-18, 6-11, 6-30, possibly dominated by additional
opportunists, ommited as their developmental relaxation
values could not be calculated.

53

 

.162001

.12700'

 

 

 

WEIGHTED UEVELOPHENTHL RELRXRTION

 

.09200- rz—Js "‘
a1:
p=.03
.06700»
.0220 1 4. 111 1 " J.
.340 .490 .640 .790 .940

SPECIES EVENNESS

Figure 17: Correlation of weighted developmental relaxation and species
evenness in 17 of the 21 Eden sampling intervals. Omitted
intervals are the same as those in Figure 16.

59

The opportunistic species, Heterotrypa ulrichi (CV = 19.25) and

 

Ha110pora nodulosa (CV - 19.99), have higner within-colony variability
than the equilibrium species, Amplexopora septosa (CV - 16.61) and
Peronopora vera (CV - 16.01) . This result indicates that developmental
deregulation is correlated with the non-genetic variability within
colonies; therefore, the higher morphological variability observed in
unstable environments is concluded to be non-genetic in nature and not
the result of higher genetic polymorphism. Conversely, stable habitats
favored the development of communities of equilibrium species having
lower levels of morphologic deregulation. Whether species in stable
environments have higher levels of genetic polymorphism cannot be con-
clusively tested with the available data. However, several genetic
mechanisms , such as the maintenance of developmental homeostasis through
heterozygosity, are available to explain the apparently negative corre-
lation of genetic and morphologic variability.

CONCLUSICNS

1) Evidence for the existence of developmental regulation in Ordovician
bryozoan coloies includes: a) radial morphologic gradients in
zooecial morpholog polarized on each monticule, b) the presence
of a monarchic zooid or pseudoancestrula within each monticule
that provides a specific point origin of each morphogenetic field,
c) a position effect for polymorphic differentiation with respect to
the monarchic zooid, d) nonlinear (alloretric) growth fields,
suggesting diffusion as the likely cause of field maintenance,

e) a distance effect for the induction of new fields and suppressim
of old ones, illustrating the polarizing and inhibitory effects of

2)

3)

9)

5)

55
the field agent, and f) the maintenance of morphogenetic fields

following an episode of mass degereration not at the end of a
normal growth cycle.

For each of the four stratigraphically pervasive Eden species,
colony growth rates and the proportion of monticular to extra-
monticular zooids differs, resulting in different overall growth
forms. Additionally, the degree of developmental relaxation is
homogeneous within species but varies significantly across taxa.
The two species with the highest levels of developmental relaxat ion

(Heterotrypa ulrichi and Hallopora nocmlosa) fit the concept of

 

r-selected, opportunistic species, and are most abmdant in the
communities of lower diversity and evenness. The other two species
(Amplexopora septosa and Peronopora vera) have much lower levels of
developmental relaxation, fit the concept of K—selected equilibrium
species, and are most abundant in the cormnities of higner diversity
and species evenness.

A major cause of the negtive correlatim between morphologic
variability and environmental stability is apparently related to

the selection of species having different levels of morphological
regulation in different environments .

Within-colony variability is higher in the opportunistic rather than
in the equilibrium species , indicating that the higner morphologic
variability observed in unstable environments is non-genetic in
nature and not the result of higner genetic polymorphism. Species
in stable environments (equilibrium species) have lower levels of
morphologic deregulation whose relationship to genetic diversity could

not be conclusively tested with the abailable data.

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56

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