Preservation of Mollusca in Copano Bay, Texas. The long-term record

Palaeogeography, Palaeoclimatology, Palaeoecology, 95 (1992): 209-228

209

Elsevier Science Publishers B.V., Amsterdam

Preservation of Mollusca in Copano Bay, Texas. The longterm record Eric N. Powell a, R o b e r t J. Stanton Jr. b, Anna Logan b and M. Alison Craig a aDepartment of Oceanography, Texas A&M University, College Station, TX 77843, USA bDepartment of Geology, Texas A&M University, College Station, TX 77843, USA (Received June 4, 1991; revised version accepted April 10, 1992)

ABSTRACT Powell, E. N., Stanton Jr., R. J., Logan, A. and Craig, M. A., 1992. Preservation of Mollusca in Copano Bay, Texas. The longterm record. Palaeogeogr., Palaeoclimatol., Palaeoecol., 95: 209-228. The stratigraphic, taphonomic and biologic records from two cores in Copano Bay, Texas were analyzed to determine (1) whether variations in shell content with depth were caused by variations in carbonate preservation, carbonate production or sedimentation rate and (2) the extent to which characteristics of fossil assemblages, such as species composition, numerical abundance, biomass, and trophic and habitat structure, identified similar or different trends. Below the top few cm of the sedimentary column, variations in carbonate content with depth could be attributed to variations in carbonate production. Most biological attributes varied similarly with depth and, hence, time on both long (>~ 100 yr) and short ( ~ 10 yr) temporal scales. These variations could not be explained by any taphonomic process, sediment reworking and burial, or sedimentation rate. Despite a vigorous taphonomic milieu, to obtain the shell content of Copano Bay sediments requires the preservtion of nearly all carbonate produced. Preservation of a large fraction of the shell carbonate added requires the preservation of most of the relatively large-shelled biota (large species and adults) which retain important evidence of changes in the community's history in this area. The results reemphasize the importance of large individuals and biomass in paleontologic reconstruction and suggest that changes in community productivity, which in paleontologic usage, must be carbonate productivity, are preserved in the fossil record.

Introduction

How fossil assemblages form is an important question in paleontology. The processes of hardpart production, taphonomic loss and time averaging (by transportation and burial) have come under close scrutiny in recent years (Powell et al., 1989b). With few exceptions, studies of modern marine communities and their death assemblages have concentrated on the upper 10 to 20cm of the sedimentary column where most mortality and, accordingly, hardpart addition occurs. The goals of these studies have been to (1) understand how these assemblages came to be and (2) obtain rules to apply to fossil assemblages for paleocommunity Correspondence to: E. N. Powell, Department of Oceanography, Texas A & M University, College Station, TX 77843, USA. 0031-0182/92/$05.00

reconstruction, by comparing the living community to the death assemblage. Community attributes such as diversity, numerical abundance, biomass and trophic structure have been examined and taphonomic analysis and taphofacies descriptions have been used to develop criteria for interpreting the fossil record. Kidwell (1986) defined the depth of final burial as "a sufficient depth ... for a hardpart: (1) to attain a refuge from further small-scale episodes of exhumation and exposure ... and (2) to escape destructive early diagenetic porewater regimes." Berger et al. (1979), Powell et al. (1982) and Denne and Gupta (1989) described similar zonations conceptually and mathematically. For a bay like Copano Bay, Texas, to be discussed later, the depth of final burial is probably about 20 cm for epifauna and somewhat deeper for infauna and

© 1992 - - Elsevier Science Publishers B.V. All rights reserved.

210

the time required for shells to reach this horizon is on the order of 60 to 70 yr. The assemblage below the depth of final burial differs from that higher up in the sedimentary column in two important ways. (1) The assemblage lacks recently added shells many of which will not be indefinitely preserved (Powell et al., 1989b); (2) The processes of time averaging and taphonomy have ceased; hence the assemblage has a more or less permanent disposition and taphonomic signature. Certainly, this assemblage will undergo some long-term diagenetic changes (e.g. Koch and Sohl, 1983), but the rate of change will be very slow in comparison to the rapid changes that occurred during its production; accordingly this assemblage contains a quasi-permanent record of the community comparable to that in the fossil record rather than the constantly changing record present in its earlier formative stages. Most sedimentary records are characterized by vertical variations in carbonate content. Above the depth of final burial, variations in carbonate content originate from (1) the continuing processes of carbonate addition, loss and reorganization by physical reworking or bioturbation and (2) the time-dependency of the formative processes operating in the assemblage; carbonate may be added in pulses, for example, rather than at a constant rate (Powell et al., 1989b). That is, the processes forming the assemblage are not just time-varying but ongoing and involve temporal variations of both short and long term. Below the depth of final burial, save for gradual diagenetic changes, no processes are ongoing. Variations in carbonate content originate from those time-varying processes producing the assemblage, such as cyclic variations in the rate of carbonate production, that were of sufficiently long period and of sufficiently strong signal to be preserved. Sedimentation rate determines the period required for a timevarying process to be preserved and variations in biological production and taphonomic processes are the primary determinants of signal strength. The sedimentary horizons below the depth of final burial provide an important intermediate link between the upper, easily studied sedimentary horizons and the fossil record. Unfortunately, few studies have taken advantage of this intermediary

E . N . POWELL ET AL.

record. The purpose of this research was to examine this link to the fossil record in Copano Bay, Texas, an area where detailed research on the processes forming a death assemblage from the living community provides a wealth of supporting data (Staff et al., 1986; Cummins et al., 1986b). The death assemblage in Copano Bay is parautochthonous, a type of assemblage common in the fossil record. Time averaging has been relatively unimportant (Powell et al., 1989a), despite the importance of local reworking and burial (Cummins et al., 1986a), and taphonomic loss has greatly affected the assemblage's composition (Cummins et al., 1986b,c). Sedimentation rate, about 30 cm 100 yr-1, is reasonably well known (Shepard and Moore, 1960). Long-term changes in community composition have occurred associated with drought cycles (Parker, 1955). Unidirectional shifts due to man, shrimping and the building of causeways, may also have affected the area. It is essential to recognize the potential biases imposed by studying nearsurface assemblages affected by man; these assemblages may not always reflect the processes that were important in forming assemblages in prehistoric times (Powell et al., 1989b). The assemblage at Copano Bay, like most assemblages, is characterized by downcore variations in carbonate content, species composition and numerical abundance. Such variations might originate from and retain evidence of temporal variability in the living community that produced the assemblage. Alternatively, they might originate from taphonomic or sedimentological processes that were time-varying. Dilution by increased sedimentation rate would be an example. Such changes, if their origin was unrecognized, would produce false information about the history of the community. We asked the question: to what extent are downcore changes in community attributes such as numerical abundance indicative of temporal changes in the living community that produced the assemblage. We were particularly interested in determining (1) whether variations in shell content with depth are determined by variations in carbonate preservation, carbonate production or sedimentation rate and (2) the extent to which the community attributes normally measured by paleontologists, viz. species composition, numerical

PRESERVATION OF MOLLUSCA IN COPANOBAY. TEXAS

21 [

abundance, biomass, trophic and habitat structure, identified similar or different depth- (and time-) dependent changes in assemblage composition.

were assigned to the uppermost 1-cm layer which they occupied. Taphonomic analysis followed Davies et al. (1990). To tally numerical abundance, whole shells as defined by Davies et al. (1990) and fragments with intact apexes (for gastropods) or beaks (for bivalves) were tallied. Disarticulated valves and articulated specimens were treated equivalently for all computations. Powell et al. (1986) suggested that both disarticulated valves of juveniles are unlikely to be present, but both valves of adults might be. Hence the analysis probably overestimates adult numbers and biomass. Biomass-at-death was computed from linear measurements as described by Powell and Stanton (1985). Adult individuals were defined as those whole shells exceeding some fraction of the species' maximum size as described by Powell and Stanton (1985) using maximum size data obtained as in Staff et al. (1986). Criteria for dissolution were described by Davies et al. (1990). Only whole shells

Materials and methods

Two 7.6 cm OD vibracores were taken from the upper end of Copano Bay near the mouth of the Aransas River, Texas, at the site described by Staff et al. (1986) (Fig.l). Water depth was about 1 m. One core was 82 cm long; the other 97 cm long. Based on Shepard and Moore's (1960) determination of sedimentation rate, these cores covered the last 300-400 yr of community history. It is relative to this time scale that the term "long-term" is used. Both cores were sectioned in 1-cm intervals for analysis. Faunal analysis followed Staff et al. (1985). Those specimens greater than i cm in size

N Bayslde,.,

N

Sampling baseline ( ill),/,';: [/,J.

Sampling location

Corpus

":i:!

I

Gulf

KM

:

~

,

J

Mexico

!) I

:

Aransas River

of

tqii I

Fig.l. Location map showing the sampling site in Copano Bay.

212

E.N. POWELL ET AL.

were examined. encountered.

Abraded , shells

were

not

abundances as low as in the uppermost section (Fig.3). The stratigraphic distribution of individuals was nonrandom in each case (Milenkovi6's method, P<0.001); like sediment texture, equivalent changes in numerical abundance occurred 10-15 cm higher in core 1. Additionally, the increase in numerical abundance occurred well above (20-30 cm) the depth where mud content increased, and the highes~!abunffances were more or less coincident with the sandiest sedimentary horizons. As might be expected, species richness paralleled numerical abundance in most respects (Fig.4). Downcore variations in species richness were nonrandom in both cores (P<0.001); like sediment texture, equivalent changes in species richness occurred 10-15 cm higher in core 1. The ratio of whole shells to fragments fluctuated widely downcore. Nevertheless, the stratigraphic variations were non-randomly distributed in both cores (P<0.001, core 1; 0.01 < P < 0 . 0 5 , core 2) as were variations in the fraction of the assemblage that was adult and the fraction that showed evidence of dissolution (P<0.001) (Figs.5-7). The number of fragments was generally two to three times the number of whole shells. Highest ratios of whole shells to fragments occurred more commonly in the upper part of the core. Adults generally accounted for about half of the whole shells. Adjacent horizons with few or no adult individuals in the upper part of core 2 and, to a much lesser extent, in the upper part of core 1, had few

Results

General overview

We first determined whether the stratigraphic variations in each of the biologic, sedimentological and taphonomic attributes were randomly distributed within each core or whether some pattern existed in their distribution, certain areas averaging higher than others for example. We used Milenkovi6's' (1989) method. Results of this and other analyses were considered further only if the results were signific~ant in both cores. Downcore changes in sediment texture were nonrandomly distributed in both cores (each core, P<0.001). Mud content decreased from already low values at the surface ( ~ 2 0 % ) to about 10% at a depth of 40-50 cm. Below 50-60 cm, mud content increased dramatically, averaging about 60%, to the depth of core penetration, up to 97 cm, and showed wide variations. The point where the mud content initially increased was about 10 cm higher in core 1 than in core 2 (Fig.2). The number of individuals was generally below 10 per 1 cm interval in the upper 20-25 cm of each core, rose dramatically to generally exceed 30 per 1 cm interval in the central part o f each core, and then decreased gradually, with large fluctuations, over the remainder of the core, but never reached

0 1.0~

:

4 Core

#

20 '

'

40 '

'

60 '

'

80 '

100 I

0.6 ~

0.4

08"

0G. 0.40.2" 0.0 20

Fig.2. The ratio of mud to sand with depth-in-core.

40

Depth

60

80

100

213

PRESERVATION OF M O L L U S C A I N COPANO BAY, T E X A S

0 70

m

I'

40

I

i

I

60

80

100

I

I

I

Core

60-

"O

20 i

5040-

T,

30. 20-

--

100 Core

2

.

.

.

.

.

60

40 Z

30

20 10 0 0

20

40

Depth

60

80

100

60

80

100

I

I

I

Fig.3. The number of individuals with depth-in-core.

i

20

'--

o ~-~

Core

20

40

I

I

20

40

i

1

10

~0

"~ 20

Core 2

e-~

E Z

0

,

0

.

Depth

i

•

J

60

80

60 I

80 I

60

8o

.

100

Fig,4. Species richness with depth-in-core.

4

,

20 1

40

2o

4o

I

i

100 I

Core 1

--

2

0

3

2

0

Fig.5. The ratio of whole shells to fragments with depth-in-core.

Depth

loo

214

E . N . P O W E L L E T AL.

=. 0

20

O3

40 i

60

I

i

100

80

I

I

i

6O

8O

I

¢¢ 0 8 0.6 0,!.

z= O3

0.2 0.0.

Core

2

0,8.

0

0.6" 0.4" 0.2"

A 20

0

40

Depth

10

2

Ii

Fig.6. The fraction of whole shells that were adults, as defined by Powell and Stanton (1985), with depth-in-core. Note from Fig. 5 that some zeros indicate horizons without whole shells rather than the absence of adults.

e-

,

.o

. . . . . . . .

1 o3

20 i . . . . . . . . .

40 i . . . . . . . . .

60 i . . . . . . . . .

80 i . . . . . . . . .

100

";ore 1

0

L 2d

: 40

Depth

60

8o

1o0

Fig.7. The fraction of whole shells showing evidence of dissolution, as defined by Davies et al. (1990), with depth-in-core. Note from Fig. 5 that some zeros indicate horizons without whole shells rather than the absence of dissolved shells.

individuals of any size. Most whole shells showed some evidence of dissolution. Stratigraphic variations in biomass-at-death were nonrandomly distributed in both cores (P<0.001) (Fig.8). Biomass was low in the upper 30-40 cm except for two spikes in core 1 produced by adult stout razor clams, Tagelus plebeius, that died in place in 1981 (Staff et al., 1985). Biomass increased at depths of 30 cm in core 1 and 37 cm in core 2 coincident with the increase in numerical abundance. Throughout the remainder of the cores, biomass displayed large peaks separated by sedimentary horizons which themselves averaged higher than the extremely low values typical of the upper 30 cm. Not all peaks in numerical abundance

produced significant peaks in biomass; some were composed exclusively of small individuals. The peaks in biomass below 30-37 cm, about four in each core, were predominantly produced by layers enriched in adult lucinids, Phacoides pectinatus. As was typical for most biological attributes, the lucinid layers in core 1 were consistently shallower than in core 2. The T. plebeius above were lone specimens; no coincidence between cores was present nor was any expected. Infaunal species comprised stout razor clams (Tagelus plebeius), tellinids of several species, and lucinids. Variations in the number of infauna were nonrandomly distributed downcore in both cores (P< 0.001). In both cores, the fraction of infauna

215

PRESERVATION OF MOLLUSCA IN COPANO BAY, TEXAS 20

40

60

100

80

I

300

j

Core

1'

200-

E

lOO

e-

N ~

o

Core 2

00

¢,~ 200 I O0 m

0

20

0

40

60

100

80

Depth

Fig.8. Biomass-at-death with depth-in-core.

fluctuated widely in the upper 20-30 cm because total abundance was low in this horizon; below this level fluctuations better reflect changes in the structure of the community as preserved (Fig.9). The taxonomic composition of the assemblage was detailed by Staff et al. (1985, 1986). The assemblage comprised oligohaline species like the hydrobiids Littoridina sphinctostoma and Littoridina barretti and the tellinid Macoma mitchelli; euhaline/hypersaline species like the venerid Chione cancellata, the cardiid Laevicardium rnortoni, and Tellina tampaensis; and mesohaline/polyhaline species like Tagelus plebeius, Tellina texana and the cerithiid Diastoma varium. Both the oligohaline and the euhaline/hypersaline individuals were patchily distributed stratigraphically in each core (P<0.001, each group, both cores) (Figs.10 and 11). Not surprisingly, few common species were 0 , 070 0.60 1 m

20 I

I

40 I

randomly distributed within the cores (Figs. 12-18 as examples). Oligohaline and euhaline/hypersaline species tended to occur in a quasi-cyclic pattern in core 1 and euhaline/hypersaline fauna tended to predominate deeper in the core. (By quasi-cyclic, we mean that the downcore trends in an attribute like numerical abundance were characterized by alternating horizons of high and low values which were nonrandomly distributed downcore. However, no sine-wave pattern could be discerned; accordingly cycles in the statistical sense were not observed.) The pattern was similar, but less obvious in core 2. Peaks in abundance of oligohaline and euhaline/hypersaline fauna did not coincide significantly more often than would be expected by chance (binomial test, P<0.05). That is, peaks in abundance of oligohaline and euhaline/hypersa-

i

60 I

,

80 I

100 I

,

Core f

0.50

o oo o1\ _AM 0.50 040 0.30 020 ~

o 1o

.

Z" i

LL

20

40

tn-ep-U

Fig.9. The fraction of individuals that were infaunal with depth-in-core.

6o

80

~oo

,216

E. N. P O W E L L E T A L .

0 25 J

20 i

'

40

~ Oligohaline Fauna

,

I

60 I

80 i

6o

80

100 I

A

So ~° "o

,

5

"o

Euhaline/Hypersaline Fauna

20

R

E

s 20

Depth

40

loo

Fig. 10. The number of individuals of oligohaline and euhaline/hypersaline species with depth-in-core, core 1. 0 15

20 I

,

~

,

40 I

60 I

,

80 I

,

100 i

Oligohaline Fauna

"0

"r,

~ --

] Euhaline/Hypersaline

~

5

Fauna

•

4o

20

0

Depth

so

80

loo

Fig.l 1. The number of individuals of oligohaline and euhaline/hypersaline species with depth-in-core, core 2.

=

25.

~-

20

40

I

I

,

60 I

20

40

Depth

6o

80 ,

100

I

I

so

100

E 20 -~ o_=

~, g ls ¢--.>

Core 2

0

t, 2°

•~._1

5 0

Fig.12. The d i s t r i b u t i o n o f L i t t o r i d i n a

i

s p h i n c t o s t o m a , ~ an oligohaline species, w i t h depth-in-core.

line species were frequently offset on the cm scale, so that the flip-flop in c o m m u n i t y structure between the two extreme salinity faunas was frequently preserved. I n a s m u c h as time averaging should militate against such a trend, the data

suggest that t e m p o r a l resolution frequently occurred on the 1 cm scale, a time period of 3 - 4 yr. In addition, larger-scale changes in the relative a b u n d a n c e o f oligohaline and euhaline/hypersaline species occurred over the entire core. Euhaline/

PRESERVATION

OF

M O L L U S C A

IN

COPANO

BAY,

217

TEXAS

20

40

I

,

453 Core 1'

l

60

80

I

I

i

100 I

i

~

.>_ 0

m

o

......

* . . . . . . . . . . . . . . . . . . . . . . . .

Core 2

4. 3 ~.

~E

~-

A

A/ ....

20

40

Depth

.A ..M A so

80

loo

Fig. 13. The distribution of Phacoides pectinatus, a euhaline/hvpersaline infaunal species, with depth-in-core.

20

co

,,

.

.

40

.

60

.

'

80 "

100 '

'

¢/)

"o

0 C.ore 2 4

0)

E Z

0

. . . . . . .

0

- ....

1

20

.

.

.

AAAMAAA .

.

i

-

•

Depth

40

-

1

-

-

so

-

|--

-

--

8o

i

loo

Fig. 14. The distribution of Tagelus plebeius, a mesohaline/polyhaline infaunal species, with depth-in-core.

Core 1

15

20

40

60

8'0

100

I

I

i

i

I

20

40

60

80

~00

¢1)

~'~10

~'o G)

._

.~--~ t- 0

5

0

Core 2

~10 E

Z 5

0

Depth

Fig. 15. The distribution of Mulinia lateralis, a euryhaline species, with depth-in-core,

218

E.

0

20

40

60

80

100 I

4

Core 1

.~- <,>

E.~ E'0 h.

Core 2

0

4

2

, J Z

I 0

. . . .

i

o

20

40

Depth

AAAA 6o

J

8o

100

Fig.16. The distribution of Laevicardium mortoni, a euhaline/hypersaline species, with depth-in-core.

20

40

60

80

100

x . ~

•

,,.

"~ o

0

.

a.

?ore 2

.

.

.

.

.

.

.

.

.

.

.

.

~Z

0

.

.

.

A.

.

1

.

-

.

20

. 40

f

Depth

.

.

60

.

.

=

/'

•

80

i

100

Fig. 17. The distribution of Brachidontes exustus, a mesohaline/polyhaline species, with depth-in-core.

20

0

25

I=

I

40 =

I

60 =

I

80 =

I

,

,

,

100 I

Core 20-

E"~ 15. P: ,,..

2o

0 Core 2

.t~ ,,.. 2 0

" "~ E-, ~s1 ~,

10

i

0

.

.

.

.

,

20

.- ....

i

40

.

Depth

,

6o

.

8o

Fig. 18. The distribution of Diastoma varium, a mesohaline/polyhaline species, with depth-in-core.

-

i

~oo

N.

POWELL

ET

AL

219

PRESERVATION OF MOLLUSCA IN COPANO BAY. TEXAS

hypersaline species gradually increased in importance downcore. A few relatively rare euhaline species were absent from the upper third of both cores, the infaunal lucinids being an important example. Only in this case did first or last occurrences change species composition downcore. Oligohaline taxa were most abundant in the central region of both cores. Although the two cores were taken within about 30 m of one another and water depth did not differ by more than a few cm, their stratigraphic sequences were offset by about 15 cm. We compared sediment texture in each core to determine how frequently equivalent depths both fell above or below their respective median values for the entire core and then dropped 1 cm from the top of core 2 at a time from the analysis. Best fits between the two cores were obtained when the top 14 cm of core 2 were deleted from the analysis (67 of 82 comparisons both fell above or below their respective medians). Accordingly, apparently about 14 cm of extra sediment has been deposited on top of core 2. Such local variability in the thickness of lithologic units is typical of marginal marine settings. The lower numerical abundances near the top of core 2 probably result from dilution produced by a more rapid sedimentation rate in the recent past at this location.

Temporal coincidence among biological and taphonomic attributes The values for most of the attributes vary on two different depth (and time) scales. (1) Large, comparable trends occurred over the length of both cores. Figure 2, for example, shows that the mud-sand ratio is low in the upper portion of both cores and high in the lower portion. Figures 10 and 11 show that species composition varied from dominantly mesohaline/polyhaline and oligohaline taxa to dominantly mesohaline/polyhaline and euhaline/hypersaline species over the length of the cores. Assuming sedimentation rate was on the order of 30 cm 100 yr- 1, these trends occurred on temporal scales of a hundred years or more; (2) Small scale, but significantly nonrandom, variation occurred as well, on scales of 2-4 cm (7-15 yr as estimated from long-term sedimentation rates

and amino acid analysis; Powell et al., 1989a). These shorter-term effects were examined separately. Even smaller scale changes certainly occur (Staff et al., 1985), but seasonal cycles, even if preserved, could not have been resolved by our sampling (Powell et al., 1986). To investigate the larger of the two temporal scales, we compared, cm by cm, combinations of biological and taphonomic attributes, asking in each case whether any two fell simultaneously above or below their respective medians for the entire core. This analysis tested only whether peaks and valleys co-occurred stratigraphically. We did not attempt to assess the degree to which the absolute values were correlated because so many processes affect the value of an attribute at any stratigraphic level. For some attributes, the size of peaks varied widely. This was particularly true for biomass, but such variations may have originated from the fortuitous collection of large but rare individuals. Consequently, we judged that analyzing peak location was the most conservative approach to identifying fundamental changes in community, taphonomic and environmental processes. We asked two questions. (1) Did peaks and valleys co-occur more frequently than expected by chance, as tested by a binomial test? (2) Were the coincidences randomly distributed in the core as tested by Milenkovi6's (1989) Markov-chain method? For the most part, we considered only those cases where both cores yielded significant results for the same pair of attributes. We used the same approach for small scale changes, but we compared the attribute's value for each cm to a local mean generated as a 5-cm running mean (Waldron, 1987). Five-point running medians, when used in the same analysis, produced identical trends.

Temporal coincidence among biological and taphonomic attributes - - large temporal scales High or low values of species richness, numerical abundance, biomass, the fraction of adult whole shells, fragmentation, the fraction of whole shells showing evidence of dissolution, the fraction infaunal and the abundance of euhaline/hypersaline

220

taxa were associated as frequently with muddy intervals as with sandy intervals (binomial test, P > 0.05). The decline of euhaline/hypersaline taxa was persistent throughout the core. The abundance of low salinity taxa was significantly higher in sandier sediments (P<0.05). Overall then, sediment texture had little effect on biological or taphonomic attributes. The association of highest total numerical abundance with sandiest sediments was due to a disproportionately high number of oligohaline individuals in the central sandy section of the cores. Most of these species were not necessarily found exclusively in sandy substrata, however; the relationship is probably a fortuitous coincidence of environmental and sedimentological processes. As might be expected, peaks in species richness and numerical abundance co-occurred more frequently than expected by chance (P < 0.05), regardless of the species' salinity preferences (P < 0.05). Peaks in biomass and numerical abundance occurring together more frequently than expected by chance (P<0.05) indicates that occasional large shells, the razor clam T. plebeius in the upper part of core 1 for example, did not significantly impact the biomass data on the scale analyzed here. When numerical abundances were divided among the salinity preferences, covariance with biomass was found with both of the extreme salinity faunas as it was with species richness so that, over the entire core, both euhaline/hypersaline and oligohaline taxa contributed to horizons with high total numerical abundance, total biomass, and speciesrich horizons. As significant peaks in biomass were due to euhaline/hypersaline species, the analysis indicates that biomass peaks coincided for both faunas. Coincident peaks were nonrandomly distributed in every case (P < 0.001). Coincident peaks occurred more frequently in the lower two-thirds of both cores. All three taphonomic attributes, the fraction adult, the fraction dissolved, and the ratio of whole to fragmented individuals tended to have coincident peaks in core 2, but no significant trends were observed in core 1. We discounted the suggestion from core 2 that fragments and juveniles were poorly preserved in some horizons because the same relationship could not be demonstrated ~in

Z. N, POWELL ET AL.

core 1. The ratio of whole to fragmented shells averaged higher in horizons with high biomass suggesting that high biomass might be due, in part, to decreased fragmentation rates. No consistent trends between biomass and the fraction adult or fraction dissolved suggests that selectivelyenhanced preservation o f large shells in some intervals probably did not occur. No relationship existed among the number of individuals and the fraction of whole shells showing evidence of dissolution or the ratio of whole shells to fragments. Adults were proportionately more common in intervals of high numerical abundance in core 2, but not in core 1. The same trends were present in the euhaline/hypersaline and oligohaline fractions of the assemblage. Infaunal abundance and species richness were unrelated to any taphonomic attribute. Overall then, the attributes fell into two categories. Most community attributes covaried: peaks co-occurred for biomass, numerical abundance and species richness. Taphonomic attributes, including fragmentation, dissolution and size-frequency [see Cummins et al. (1986c) for arguments favoring this categorization] were rarely consistently related to each other or to any biological attribute. Only peaks in the ratio of whole shells to fragments and biomass co-occurred consistently. Horizons with high biomass (above the median) had a highei proportion of whole shells. Temporal coincidence among biologicaland taphonomic attributes - - small temporal scales As might be expected, local (a 5-cm scale) peaks in species richness and numerical abundance cooccurred more frequently than might be expected by chance (P<0.05). The number of euhaline/ hypersaline individuals were similarly distributed with respect to species richness, probably because they contributed a large fraction of the total species to the assemblage. Oligohaline species were not. Local peaks and particularly local valleys in species richness coincided with equivalent changes in biomass as well (P<0.05). The same pattern existed for numerical abundance and biomass. Coincidence of local valleys occurred more frequently than coincidence of local peaks, so that the lowest

PRESERVATION OF MOLLUSCA IN COPANO BAY, TEXAS

values obtained by the attributes, on a 5 cm scale, frequently fell in the same cm interval whereas the highest values obtained were often offset by a c m or two. Valleys again tended to be separated by 1-3 cm. However, the abundance of oligohaline or euhaline/hypersaline fauna alone did not coincide with biomass in either core (P>0.05); hence the frequent offset between peaks in oligohaline and euhaline/hypersaline abundances compromised any coincidence with biomass. At this scale, both contributed significantly to biomass fluctuations in the core. None of the taphonomic attributes covaried with one another consistently (P>0.05). The ratio of whole shells to fragments covaried with biomass, but the fraction of whole shells dissolved or adult did not. No trend between biomass and the fraction of whole shells of adult size suggests that many adult shells were small and many large shells were not adults. Taphonomic attributes bore no relationship with local changes in grain size, the fraction infaunal, species richness or numerical abundance of all the fauna or the oligohaline or euhaline/hypersaline components. Sediment texture also bore no relationship with any biological attribute. So, at both temporal scales, most biological attributes covaried among themselves; most taphonomic attributes did not nor did they covary with biological attributes; the single exception being biomass and the ratio of whole shells to fragments. Sediment grain size bore little relationship to any attribute. Discussion Sediment texture

Sediment texture varied considerably with depth in both cores. The change from muddy sand/sandy mud to clean sand probably marks the time when the bay shoaled enough for wave action to prevent mud deposition. At the same time, the gradual transition towards dominance by an oligohaline fauna suggests more frequent periods of low salinity, possibly due to the nearing of the Aransas River mouth which today is less than 3 km distant. Sedimentological and biological changes over

221

most of the cores' lengths probably represent natural phenomena. Much of each core predates significant human influence in the area; even today the region is sparsely settled. However, increased mud content near the surface may indicate the lowered water energy associated with construction of a causeway and bridge just down estuary from our collection site. Fining of sediment began about 100 yr ago, judging from sedimentation rates; in the correct time frame for bridge construction based on Refugio County records. The possibility that the dramatic decline in faunal abundance at this point originates in the reduced circulation caused by bridge construction cannot be discounted; accordingly, the upper 30-40 cm of each core may have been affected by anthropogenic processes. Shellfishing and shrimping activities, so pervasive in some areas (Frey et al., 1987; Aitken et al., 1988), did not occur at our site, however. Despite the expectation that significant variation in sediment texture should affect species composition and taphonomic attributes, not a single parameter was so associated. In fact, the primary changes in community attributes all occurred completely independently of changes in sediment texture and no discernible impact on fragmentation rate, frequency of evidence of dissolution, or size frequency could be found. (Although the oligohaline fauna reached highest abundances in the sandiest sediments, no significant relationship existed over the entire core.) The absence of a correlation between sediment texture and any biological attribute can be explained by the fact that most of the taxa are not particularly substrate-specific (Parker, 1959). Additionally, we infer that sedimentation rate had little effect on sediment texture because the result of changing sedimentation rate, the dilution or concentration of carbonate [Powell et al., 1989b, see also Brush (1989)], should result in covariance of biological attributes and sediment texture if changes in sedimentation rate were an overriding influence. Powell et al. (1989b) and Cummins et al. (1986b) did not observe a significant effect of sediment texture on dissolution capacity (the amount of carbonate potentially dissoluble based upon an estimate of acid production from the rate of

222

decomposition of organic matter), nor did we. The absence of a correlation between sediment texture and any taphonomic attribute indicates that taphonomic capacity is a conservative feature of this environment. Taphonomic capacity probably is more a function of the amount and lability of the organic matter added (Reaves, 1986; Henrichs and Doyle, 1986) and the physical environment, and these are likely to be conservative environmental features.

Taphonomic attributes No evidence exists that taphonomy affected biological attributes differentially. The proportion of the assemblage that was fragmented, the frequency of dissolved shell surfaces, and the size-frequency distribution of whole shells did not covary consistently with any biological attribute nor amongst themselves. The only exception was a significant relationship between biomass and fragmentation frequency wherein biomass tended to be higher in horizons where the proportion of the assemblage (not the number) that was whole was greater. (Of course the number of whole shells might be expected to covary with biomass, hence our use of the ratio.) Biomass at time of death can be calculated from whole shells and the few fragments that are complete enough to provide the necessary dimensions. Thus biomass is strongly dependent on the number of individuals originally present, the residue of whole shells left after taphonomy has taken its toll, the extent to which the different species are large or small, and the size-frequency of the species. Biomass, then, could be a product of both the structure of the living community and the taphonomic milieu. Large shells, being robust, should fragment less often. This might imply that times when large shells were added due to biological processes produced fewer fragments and accordingly larger biomass. In our cores, both large and small shells (< >3.0 mm) (Figs.19 and 20) covafled with the fraction of the assemblage that was whole, however. Consequently, it is the number of fragments and the number of whole shells that do not covary. Size is relatively unimportant. The data suggest that periods when fragmenta-

E.N. POWELL ET AL.

tion rate was lower produced higher biomass more frequently than periods when the total number of shells preserved was highest. Which environmental parameters would conspire to encourage the preservation of whole shells regardless of size is unclear. More rapid burial might produce this effect or a change in sedimentation rate. Significant changes in sedimentation rate are unlikely, as documented earlier. Local increases in burial rate could preserve whole shells before they were fragmented. On occasions when an increased rate of burial occurred soon after the addition of a large pulse, many shells of all sizes might have been buried. In addition, the size frequency would be enriched with older, larger shells because these, having lower decay rates than small shells (Powell et al., 1986), would have accumulated over a longer period of time and be proportionately more abundant. In any event, fragments and whole shells are distinctive populations in both cores, as observed elsewhere (Davies et al., 1989; Staff and Powell, 1990), and biomass variation would not appear to be solely a function of community dynamics, although certainly more so than numerical abundance.

Causes of temporal variations Substantial changes occurred in biological attributes on two scales, tens of centimeters and 1-3 cm. Most biological attributes varied similarly on both scales and were quasi-cyclic as evidenced by the flip-flop between oligohaline and euhaline/ hypersaline taxa. The two extremes, the oligohaline fauna and the euhaline/hypersaline fauna, did not separate consistently; accordingly, in some cases, the change from oligohaline to euhaline/hypersaline conditions was not resolved at the cm scale of sampling or time averaging resulted in some variations becoming unrecognizable. Certainly, however, many such changes could be resolved on a 1 cm scale so that time averaging was normally significant only on temporal scales of less than 10yr. Powell et al. (1989a) reached the same conclusion by directly measuring time-since-death of shells obtained from cores taken in the same area. Covariance among biological attributes might

223

P R E S E R V A T I O N O F M O L L U S C A IN C O P A N O BAY, T E X A S

0

==

15

20 l

i

40 I

=

60 I

,

10 Individuals_<- 3 ra~" _ _ _ ~

1~[~[~ A

i

10

80 I

I

~

"I0 -> 3 mm

Individuals

"6

i-eJ J~ E -t

10

5

z

0

20

40

Depth

60

80

100

Fig. 19. The number of small (~<3 mm) whole shells and the number of large (>-3 mm) whole shells with depth-in-core, core 1. 20 I

8

Individuals

40 I

60 I

80 I

10£ i

6o

8o

lo0

~< 3 m m

0

o

e-

Individuals

>- 3

mm

,4

i

E z

2

0

---

o

" A

_A

. . . . .

i .

20

.

.

.

.

.

T

40

Depth

Fig.20. The number of small (~< 3 mm) whole shells and the number of large (>" 3 mm) whole shells with depth-in-core, core 2.

be produced by factors involved in preservation such as variations in sedimentation rate, sediment texture, rate of burial, or taphonomic processes or they might have been controlled by biological or environmental factors affecting community attributes such as production and species composition. We discount taphonomic loss. Taphonomic processes showed nonrandom patterns, but a consistent 1-3 cm signal was not apparent. No relationship existed between taphonomic attributes and biological attributes. Hence, factors controlling taphonomy, although distinctive in their temporal nature, would appear to be both more aperiodic and not coincident with biological or sedimentological processes. Changes in burial rate can produce variations in shell carbonate content (used here to denote the

weight rather than the number of shells present). An increase in burial rate, for example, could explain the increased shell carbonate content at about 40 cm. Reworking and burial usually produces local shell carbonate enrichment overlain by a zone deficient in shells, from which the shells were stripped out to supply the concentration below. The central part of both cores have this appearance. Several factors suggest that physical burial was not important in effecting changes in shell carbonate content, however. (1) Neither taphonomic nor sedimentary textural changes coincided with changes in shell content; (2) Only the oligohaline fauna was affected. Both oligohaline and euhaline/hypersaline fauna should have been if burial had been important. Bioturbation may also concentrate shells. Bid-

224

turbators at this site included several infaunal bivalves and a host of deposit-feeding worms, as described by Staff et al. (1986). Counting the razor clams, the bioturbate layer extended to about 60cm. Deposit-feeding worms were commonly encountered to about 20 cm. Although bioturbation probably reoriented shells, several evidences suggest that bioturbation was not instrumental in determining the downcore variations in shell content. (1) Shell-filled burrows were rarely observed. Most shelly sections were oriented horizontally and extended across the entire core. Most of the deposit-feeding worms were too small to physically move shells except by undercutting; (2) Discrete shelly horizons were present within as well as below the bioturbate zone, despite the activity of bioturbators. The scale of bioturbation does not fit the 1 to 3 cm scale of variation in shell carbonate content; (3) No evidence for substantial time averaging exists in the upper 70 cm at this site (Powell et al., 1989a); (4) Glass beads of appropriate size left out on the sediment surface remained within 1 cm of the sediment surface after nearly 1 yr. Does the evidence support variations in sedimentation rate on the large scale or on the local scale? To explain changes in shell carbonate content, sedimentation rate would have to vary by more than a factor of i0 over the site's history. A change of this magnitude should have been apparent in several other measured parameters. Measurements of time-since-death of shells from this site suggest little change in sedimentation rate (Powell et al., 1989a), however. Shell age increases steadily throughout the upper 70 cm of the sedimentary column where significant changes in sediment texture and shell abundance occur. Sediment textural changes did not coincide with changes in biological attributes either. Moreover, variations in abundance often affected the oligohaline but not the euhaline/hypersaline fauna. Lower sedimentation rates, which might result in higher shell concentrations at a given production rate for example, would have concentrated all of the fauna, not just the oligohaline species. The single exception, lower shell carbonate content near the top of core 2, probably does indicate a, local, temporary increase in sedimentation rate, but below this

E. N. P O W E L L E T AL.

horizon concordant changes occurred in both cores, so that variations in sedimentation rate were likely unimportant overall. Hence, comparison of biological, taphonomic and sedimentological attributes can help distinguish between changes in shell carbonate content due to changes in sedimentation rate, or rates of burial, taphonomy or shell production. The evidence points to variations in community productivity as the primary factor controlling shell carbonate content at this site.

Persistence of attributes Interestingly, all biological parameters described similar changes in the short-term (1-3 cm) and long-term (tens of cm). The former suggests a 1-3 cm cyclic phenomenon, a time scale in the range of 3-10 yr. Cycles of this order are well known, E1 Nifio being a good example, and do affect this area of the Gulf of Mexico (Powell and Cummins, 1985; Powell et al., in press). The suggestion has been made that the persistence of biological attributes in community structure can be ranked: trophic structure>species composition > numerical abundance (see review in Staff et al., 1986). Productivity, and consequently biomass-at-death, should then be conservative (Beukema et al., 1978; Staff et al., 1985, 1986; Staff and Powell, 1988), but production of shell carbonate might not be because, as the time span required for adults to be produced lengthens, proportionately more shelled biota should be present (Staff et al., 1985). Shelled biota tend to be relatively large and long-lived, whereas most community productivity tends to be in the juvenile age span and in smaller taxa (e.g. Zaika, 1970; Warwick and Price, 1975; Warwick et al., 1979). One of the striking aspects of Copano Bay cores is the persistence of species composition (as opposed to species richness). Few taxa were confined to any one discrete depth zone; the same taxa came and went as environmental conditions changed. So, on a large scale, species composition changed relatively little whereas substantial changes in productivity, judging from changes in shell carbonate content and numerical abundance, occurred. Some of the variations in carbonate

225

PRESERVATION OF MOLLUSCA IN COPANO BAY, TEXAS

content probably can be attributed to human influence, but most certainly occurred prior to modernization of the area. On a long enough time scale, then, species composition may be more persistent than trophic structure, as measured by biomass [trophic tiers are always skewed towards filter feeders in fossil assemblages as is true in this case - - Staff et al. (1985)] [Hofmann (1978) presented an alternative view over much longer time scales]. Of course, this species composition represents the time-averaged sum of all short time-scale variations (Staff and Powell, 1988), akin to the ecological concept of beta diversity (Ricklets, 1990; see also Springer and Miller, 1990), so that the term persistence, as applied in palaeoecology and used here, is not equivalent to the ecological concept. Above the 1-3 cm scale, then, changes in species composition occurred more slowly and were of less significance than marked changes in productivity, shell carbonate content, and numerical abundance. The fraction of the community that was infaunal was also very stable as was the taphonomic milieu. Hence, taphonomy, habitat guilds, and species composition appear to be more persistent than biomass and considerably more persistent than numerical abundance. This consistency permits long- and medium-term changes in community trophic history to be observed. Copano Bay is probably a typical bay, hence the same conclusions should apply to a broad selection of shallow water, estuarine habitats.

Size frequency and preservation Small and large shells differed in many respects. The preservation of small shells can be distinguished from that of large ones in several ways. Preservation of small shells may be solely controlled by taphonomic processes because their decay rate is so high. Small shells are preserved only when burial events occur shortly after death and record events, potentially unusual events, occurring over short time scales. Numerical abundance of small shells should be more dependent on the frequency of burial events than biological processes. The number of large shells preserved probably depends on a combination of taphonomic

processes and biological productivity which controls the source content (Cummins et al., 1986c). Large shells take longer to be formed and, on the average, remain for longer times in the nearsurface sediment (Powell et al., 1986). Hence, any burial event integrates a longer time scale in preserving large shells than smaller ones and biological processes can exert a greater influence on the number preserved. The abundance of large shells, then, should be controlled by both biology and taphonomy and, interestingly, large shells covaried with the ratio of whole shells to fragments less significantly than did small shells, as expected from this fact. The presence of adult, large shells particularly records long-term conditions and, at Copano Bay, these individuals indicate substantial variation in community productivity over the site's history. Tagelus plebeius died in place at depths of at least 60 cm. Hence the depth of final burial for the entire assemblage must be deeper than 60 cm in this area. The absence of downcore trends in taphonomic signature and the discrete preservation of community changes on a l cm scale at shallow depths in the core suggests that burial processes extended to only a few cm and that taphonomic loss was limited to the upper ~ 1 cm. The requirement that most large shells be preserved and the difficulty of rapidly burying large shells deeply in this area also suggests that taphonomic processes were restricted to ~< 1 cm in depth and that burial processes on this scale occurred frequently, as indeed they do (Cummins et al., 1986a). The single exception is the fraction of shells that were whole which declined below about 20 cm depth in one core. So, if the taphonomically-active zone (TAZ) is defined solely on preservational processes, then the TAZ was certainly in the top 20 cm at this site and probably much nearer the surface than 20 cm; many cm above the depth of final burial in the sedimentary column as defined by infaunal addition. For strictly the epifaunal component, the depth of final burial was probably within 10 cm of the sedimentary surface and more nearly equivalent to the TAZ. Conclusions

Certainly below the top l O cm of the sedimentary column, variations in shell carbonate content with

226

depth could be attributed to real variations in shell addition rate, probably variations in community productivity of the shelled biota. Nearly all biological attributes demonstrated the same basic depthdependent variations on large and small temporal scales. Certainly, the range of variation was larger in some, like biomass, than others, like species richness and certain portions of the sedimentary record showed more intense changes in some biological attributes than did others. Nevertheless, viewed over the entire sedimentary column, all biological attributes varied concordantly to a significant degree. These variations could not be explained by any taphonomic process, burial or sedimentation rate. Accordingly, the history of this community was, on the average, characterized by highs and lows in productivity of shelled biota; the highs being associated with highs in species richness, biomass and numerical abundance, and the record of these events was preserved. But, the factors controlling small-scale changes in abundance and biomass were, to some extent, distinct from the environmental factors controlling salinity; the oligohaline fauna was not associated with biomass and abundance highs more frequently than any other, for example. One must recognize that the processes initially responsible for shell preservation usually operate on shorter time scales than the time scales normally responsible for shell carbonate variation in the sedimentary record; those variations in shell content permanently interred below the depth of final burial normally represent longer-time scale phenomena having little to do with initial shell preservation (or catastrophic, but rare short-term events, like 100-yr storms). Although the similarities in the temporal variability of the biological attributes was clear, relative persistence certainly varied among them. Species composition and the number of infauna were relatively stable, species richness somewhat less so, biomass and numerical abundance still less and the number and biomass of epifauna (particularly the oligohaline component) more variable still. Large scale changes over 10's of cm were primarily produced by variations in the epifaunal component, to a great extent the oligohaline epi-

Z . N . POWELL ET AL.

faunal component. Small scale quasi-cyclic variations affected all attributes more or less equally. To explain the shell content of Copano Bay sediments requires the preservation of nearly all shell carbonate produced, much of which resides in the relatively large-shelled biota which retain important evidence of changes in the community's history in this area. Since most individuals are not preserved (Cummins et al., 1986b), most large individuals must be. Accordingly, and from the evidence presented here and elsewhere (e.g. Powell et al., 1989a) on the limited extent of time averaging, most of the sedimentary column must be below the taphonomically active zone, despite bioturbation, storms and infaunal bivalves disturbing sedimentary horizons [from Denne et al. (1989) and Powell et al. (1989b) this may be frequently the case]. Variations in shell carbonate content were, in the main, produced by variations in shell carbonate supply. Miller (1990) emphasized the importance of the "supply-side" determinants of sediment shelliness, as do we. Consequently, most of the community history is preserved and the effects of time averaging are minimized. The data reemphasize the importance of large individuals and biomass in paleontologic reconstruction and suggest that substantial evidence of changes in community productivity, which in paleontologic usage, must be shell carbonate productivity, is preserved. The productivity of benthic ecology, of course, may not be as well preserved since much of that productivity resides in juveniles and non-preservable biota and the relationship between carbonate addition rate and community productivity is unclear. Like the term persistence, productivity in paleoecological usage differs considerably from its ecological counterpart. Nevertheless, the production of large individuals, especially adults, is an important measure of community history - - it corresponds to a large fraction of the energy flow through the community when scaled t o the default measure in paleontology, animal life spans (Powell and Stanton, 1985) - and that record is surprisingly well preserved.

Acknowledgments We thank D. Davies for help in core collection and analysis. This study was funded by NSF grants

P R E S E R V A T I O N O F M O L L U S C A IN C O P A N O BAY, I ' E X A S

EAR-8506043 and EAR-8803663 to E.P. and R.S. and computer funds from the TAMU College of Geosciences and Maritime Studies. We appreciate this support. References Aitken, A. E., Risk, M. J. and Howard, J. D., 1988. Animalsediment relationships on a subarctic intertidal flat, Pangnirtung Fiord, Baffin Island, Canada. J. Sediment. Petrol., 58: 969 978. Berger, W. H., Ekdale, A. A. and Bryant, P. P., 1979. Selective preservation of burrows in deep-sea carbonates. Mar. Geol., 32: 205-230. Beukema, J. J., De Bruin, W. and Jansen, J. J. M., 1978. Biomass and species richness of the macrobenthic animals living on the tidal flats of the Dutch Wadden Sea: long term changes during a period with mild winters. Neth. J. Sea Res., 12: 58-77. Brush, G. S., 1989. Rates and patterns of estuarine sediment accumulation. Limnol. Oceanogr., 34: 1235-1246. Cummins, H., Powell, E. N., Newton, H. J., Stanton Jr., R. J. and Staff', G., 1986a. Assessing transportation by the covariance of species with comments on contagious and random distributions. Lethaia, 19:1 22. Cummins, H., Powell, E. N., Stanton Jr., R. J. and Staff, G., 1986b. The rate of taphonomic loss in modern benthic habitats: how much of the potentially preservable community is preserved? Palaeogeogr. Palaeoclimatol. Palaeoecol.. 52: 291 320. Cummins, H., Powell, E. N., Stanton Jr., R. J. and Staff, G., 1986c. The size-frequency distribution in palaeoecology: the effects of taphonomic processes during formation of death assemblages in Texas bays. Palaeontology, 29:495 518. Davies, D. J., Powell, E. N. and Stanton Jr., R. J., 1989. Taphonomic signature as a function of environmental process: shells and shell beds in a hurricane-influenced inlet on the Texas coast. Palaeogeogr., Palaeoclimatol., Palaeoecol., 72:317 356. Davies, D. J., Staff, G. M., Callender, W. R. and Powell, E. N., 1990. Description of a quantitative approach to taphonomy and taphofacies analysis: all dead things are not created equal. In: W. Miller Ill (Editor), Paleocommunity temporal dynamics: the long-term development of multispecies assemblages. Spec. PuN. Paleontol. Soc., 5: 328-350. Denne, R. A. and Gupta, B. K., 1989. Effects of taphonomy and habitat on the record of benthic foraminifera in modern sediments. Palaios, 4: 414-423. Frey, R. W., Hong, J.-S., Howard, J. D., Park, B. K. and Ham S. J., 1987. Zonation of benthos on a macrotidal flat, Inchon, Korea. Senckenberg. Marit., 19:295 329. Henrichs, S. M. and Doyle, A. P., 1986. Decomposition of :4C-labeled organic substances in marine sediments. Limnol. Oceanogr., 31:765 778. Hofmann, A., 1978. System concepts and the evolution of benthic communities. Lethaia, 11: 179-183. Kidwell, S. M., 1986. Models for fossil concentrations: paleobiologic implications. Paleobiology, 12: 6--24.

227 Koch, C. F. and Sohl, N. F., 1983. Preservational effects in pateoecological studies: Cretaceous mollusc examples. Paleobiology, 9: 26-34. Milenkovi6, B. S., 1989. Verification of the hypothesis regarding the random nature of fluctuations of geological properties. J. Geol., 97:109-116. Miller III, W.. 1990. Hierarchy, individuality and paleoecosystems, In: W. Miller lII, (Editor), Paleocommunity temporal dynamics: the long-term development of multispecies assemblages. Spec. PuN. Paleontol. Soc., 5:31 47. Parker, R. H., 1955. Changes in the invertebrate fauna, apparently attributable to salinity changes, in the bays of central Texas. J. Paleontol., 29: 193-211. Parker, R. H., 1959. Macro-invertebrate assemblages of central Texas coastal bays and Laguna Madre. Bull. Am. Assoc. Pet. Geol., 43:2100 2166, Powell, E. N. and Cummins, H. C., 1985. Are molluscan maximum life spans dominated by long-term cycles in benthic communities? Oecologia (Berl.), 67:177-182. Powell, E. N. and Stanton Jr, R. J., 1985. Estimating biomass and energy flow of molluscs in paleo-communities. Palaeontology, 28:1 34. Powell, E. N., Stanton, R. J., Cummins. H. and Staff, G., 1982. Temporal fluctuations in bay environments - - the death assemblage as a key to the past. In: J. R. Davis (Editor), Proceedings of the Symposium on Recent Benthological Investigations in Texas and adjacent states. Texas Academy of Science, Austin, Tex., pp. 203-232. Powell, E. N., Stanton Jr., R. J., Davies, D. and Logan, A., 1986. Effect of a large larval settlement and catastrophic mortality on the ecologic record of the community in the death assemblage. Estuarine Coastal Shelf Sci., 23: 513-525. Powell, E. N., Logan, A., Stanton Jr., R. J., Davies, D. J. and Hare, P. E., 1989a. Estimating time-since-death from the free amino acid content of the mollusc shell: a measure of time averaging in modern death assemblages'? description of the technique. Palaios, 4: 16-31. Powell, E. N., Staff, G. M., Davies, D. J. and Callender, W. R., 1989b. Macrobenthic death assemblages in modern marine environments: formation, interpretation and application. Crit. Rev. Aquat. Sci., 1:555 589. Powell, E. N., Gauthier, J. D., Wilson, E. A., Nelson, A., Fay, R. R. and Brooks, J. M., in press. Oyster disease and climate change. Are yearly changes in Perkinsus marinus parasitism in oysters (Crassostrea virginica) controlled by climatic cycles in the Gulf of Mexico? Mar. Ecol. (Pubbl. St. Zool. Napoli I) Reaves, C. M., 1986. Organic matter metabolizability and calcium carbonate dissolution in nearshore marine muds. J. Sediment. Petrol., 56:486 494. Ricklets, R. E., I990. Long-term development of biological communities. In: W. Miller IlI (Editor), Paleocommunity temporal dynamics: the long-term development of multispecies assemblages. Spec. PuN. Paleontol. Soc., 5: 1-12. Shepard, F. P. and Moore, D. G., 1960. Bays of central Texas coast. In: F. P. Shepard, F. B. Phleger and T. H. Van Andel (Editors), Recent sediments, northwest Gulf of Mexico, Am. Assoc. Pet. Geol., Tulsa, Okla., pp. 117-152. Springer, D. A. and Miller, A. 1., 1990. Levels of spatial variability: the "'community" problem. In: W. Miller 1II

228 (Editor), Paleocommunity temporal dynamics: the long-term development ofmultispecies assemblages. Spec. Publ. Paleontol. Soc., 5: 13-20. Staff, G. M. and Powell, E. N., 1988. The paleoecological significance of diversity: the effect of time averaging and differential preservation on macroinvertebrate species richness in death assemblages. Palaeogeogr. Palaeoclimatol. Palaeoecol., 63: 73-89. Staff, G. M. and Powell, E. N., 1990. Local variability of taphonomic attributes in a parautochthonous assemblage: can taphonomic signature distinguish a heterogeneous environment? J. Paleontol., 64: 648-658. Staff, G., Powell, E. N., Stanton Jr., R. J. and Cummins, H., 1985. Biomass: is it a useful tool in paleocommunity reconstruction? Lethaia, 18: 209-232. Staff, G., Stanton Jr., R. J., Powell, E. N. and Cummins, H.,

E.N. POWELLET AL. 1986. Time-averaging, taphonomy and their impact on paleocommunity reconstruction: death assemblages in Texas bays. Geol. Soc. Am. Bull., 97: 428-443. Waldron, J. W. F., 1987. A statistical test for significance of thinning- and thickening-upwards cycles in turbidites. Sediment. Geol., 54: 137-146. Warwick, P~. M. and Price, R., 1975. Macrofauna production in an estuarine mud-fiat. J. Mar. Biol. Assoc. U.K., 55: 1-18. Warwick, R. M., Joint, I. R. and Radford, P. J., 1979. Secondary production of the benthos in an estuarine environment. In: R. L. Jeffries and A. J. Davy (Editors), Ecological Processes in Coastal Environments. Blackwell, Oxford, pp. 429-450. Zaika, V. Y., 1970. Relationship between the productivity of marine mollusks and their life-span. Oceanology, I0: 547-552.