Taphonomic processes in modern freshwater molluscan death assemblages: Implications for the freshwater fossil record

Palaeogeography. Palaeoclimatology, Palueoecology, 108 (1994): 55-73

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Elsevier Science B.V., Amsterdam

Taphonomic processes in modern freshwater molluscan death assemblages: Implications for the freshwater fossil record Robert Hays Cummins School o[Interdisciplinary Studies, Miami University, O.,¢fbrd, Ohio 45056, USA (Received October 12, 1992; revised and accepted July 22, 1993)

ABSTRACT Cummins, R.H., 1994. Taphonomic processes in modern freshwater molluscan death assemblages: Implications for the freshwater fossil record. Palaeogeogr., Palaeoclimatol., Palaeoecol., 108:55 73. Unionid life and death assemblages in several streams and reservoirs in east-central Ohio were investigated to examine taphonomic processes in freshwater environments. Twenty-six species were collected. As in most marine environments, rnost living molluscan species were represented in the death assemblage. Fourteen species were found in both the life and death assemblage at at least one study site, one species was found only in the living community, and one species was found live at one site, dead at another Ten species were found only in the death assemblage. With one exception, the time-aw~'rageddeath assemblage was more species rich than the living molluscan community based on single-sample census data. The death assemblage preserves the rank orders (live/dead) of abundance and biomass of the preservable molluscan components in some environments but not iq those the most environmentally disturbed. Unionid live/dead fidelity is high and compares favorably with live/dead fidelity in marine and estuarine environments. In nine of thirteen within-study site comparisons of life and death assemblages, there were no significantsize differencesbetween the single-censuslife and time-averaged death assemblages. These results are in contrast to estuarine assemblages along the Texas coast where 13 of 15 comparisons of the death assemblage and the estimated mortality from the living community indicated little similarity between life and death assemblage size distributions. It is likely that death assemblage formation and associated taphonomic processes, coupled with the unique lifecycle of unionid molluscs, are distinctive in freshwater environments. Recognition of the unique taphonomic characteristics of unionids should prove useful in freshwater paleocommunity analyses.

Introduction Most c o m p a r a t i v e analyses of living aquatic c o m m u n i t i e s a n d death assemblages have been c o n d u c t e d in m a r i n e e n v i r o n m e n t s such as estuaries, lagoons, tidal flats, a n d bays (e.g. Cadee, 1968, 1982; Lawrence, 1968; W a r m e et al., 1976; Peterson, 1976; C u m m i n s et al., 1986 a - c ; Frey, 1987; Powell et al., 1986; Staff et al., 1985, 1986; Ffirsich a n d Flessa, 1987; Miller, 1988; Davies et al., 1989; Meldahl a n d Flessa, 1990; Miller a n d C u m m i n s , 1990; Russell, 1991) a n d the c o n t i n e n t a l shelf (Bosence, 1979; C a r t h e w a n d Bosence, 1986; Staff a n d Powell, 1988; Callender et al., 1990). These studies provide valuable insights into the initial steps of fossil assemblage f o r m a t i o n a n d the effects o f t a p h o n o m i c processes on n u m e r i c a l 0031-0182/94/$07.00 © 1994 SSD1 0031-0182(93)E0128-G

a b u n d a n c e , diversity, p o p u l a t i o n dynamics, transport, t a x o n d o m i n a n c e , a n d c o m m u n i t y biomass. Kidwell (1986), a n d others, quite logically questioned whether t a p h o n o m i c processes are equivalent from one aquatic e n v i r o n m e n t to the next. In Kidwell a n d Bosence's (1991) meta-analysis of a wide variety of data sets from m a r i n e environments, they f o u n d an a p p a r e n t lack of strong t a p h o n o m i c distinctiveness a m o n g m a r i n e a n d estuarine e n v i r o n m e n t s when using a series of s t a n d a r d i z e d metrics to e x a m i n e fidelity of life a n d death assemblages. Freshwater systems have received far less a t t e n t i o n than their marine counterparts in m o d e r n t a p h o n o m i c studies. With some exceptions (e.g. Pip, 1988; Cohen, 1989), little is k n o w n of freshwater death assemblage f o r m a t i o n . The freshwater fossil record is often charac-

ElsevierScience B.V. All rights reserved.

56

terized by low diversity compared to the marine record due, in part, to poorer preservation of organisms as well as the ephemeral nature of many freshwater habitats (Gray, 1988; Taylor, 1988). However, the freshwater record still provides essential indicators of paleoecological and paleoclimatological environments (e.g. LaRocque, 1967, 1968, 1970; Cvancara, 1976; Miller et al., 1979; Good, 1987; Hanley and Flores, 1987; Wilson, 1988) and has also been used in the analysis of species composition changes resulting from human impact (Parmalee et al., 1980, 1982; Taylor and Spurlock, 1982; Theler, 1987; Neck, 1990). The purpose of this study is to add to initial understandings of taphonomic processes in freshwater environments to discern how freshwater death assemblages form. How do marine and freshwater taphonomic patterns compare? Unionid life and death assemblages were collected from a variety of streams and reservoirs in east-central Ohio to determine the similarities and differences between the unionid living community and the time-averaged death assemblage. Issues which I will address include live/dead comparisons of: (1) species richness and similarity; (2) fidelity comparisons with marine environments; (3) size frequency distributions, and (4) biomass and numerical abundance.

The unionidfossil record Unionids are one of the most important pelecypod groups in the freshwater fossil record--their history extends to the Paleozoic (Henderson, 1935; Gray, 1988) and they are significant members of benthic freshwater systems from many localities and stratigraphic sequences around the world. Many living families of unionids were present during the Cretaceous and they are important in zoogeographic reconstructions • (Taylor, 1988). Numerous unionid fossils occur in freshwater deposits in the Upper Cretaceous and Tertiary in western North America (Henderson, 1935; Good, 1987; Taylor, 1988). Unionids are also important Pleistocene fossils (LaRocque, 1967). About half of freshwater clams are grouped together as unionids (Class Pelecypoda, Superfamiliy Unionacea) and about 200 species of unionids

R.H. CUMMINS

occur in the United States (Miller et al., 1987). These molluscs are frequently the biomass dominants of modern freshwater benthic communities (Golightly and Kosinski, 1981; Hanson et al., 1988). Unionids have a weak hinge and a highorganic nacreous aragonitic shell that favors taphonomic destruction, but many species have robust and large shells which may increase the chance of preservation.

The unique natural history of unionids Unionids have a complex, specialized life-cycle (Stearns, 1976) that is dramatically different from other molluscs (Fuller, 1974). Their unique lifecycle may impact interpretations of aspects of their taphonomy particularly when examining death and fossil assemblage size frequency distributions. Many marine lamellibranchs produce planktonic veliger larvae (Miller et al., 1987). In contrast, most unionids produce parasitic glochidia larvae that attach to a suitable vertebrate host such as a fish or salamander. Host specificity can be a "weaklink" in unionid reproductive strategy in that reproductive success is dependent upon the availability of the host species. Thus, reproduction can be infrequent. The glochidium, upon maturation, drops from the host onto the substrate where it develops into a juvenile unionid (Pearse et al., 1987). Site selection considerations

Freshwater molluscan fossils are common in sandstones, conglomerates and shales of river and lake deposits in North America (Henderson, 1935; Feth, 1964). In this study, sampling sites were streams and reservoirs. Though artificial, reservoirs provide a tranquil setting that may be analogous to oxbow and lateral levee lakes in the flood plains of mature rivers or other natural impoundments represented in the fossil record (Edwards, 1978). Many unionid species thrive in reservoirs and, by virtue of their abundance, reservoir sites can provide important data on taphonomic processes in freshwater systems. Another important consideration is the bedrock lithology of the study area given the possibility of

I A P H O N O M I C PROUF~SSES IN M O D E R N F R E S H W A T E R M O L L U S C A N DEATH ASSEMBLAGES

5"7

acre system, more like a large river than a reservoir, about 6 m deep and slightly wider than the 50 m wide Pleistocene stream-bed it now covers. It is slow moving and turbid with water residence times of only 3 days. Seneca Lake reservoir is a much wider, 3550 acre lobe-shaped system about 15 m deep. It was built during the 1930's and has a water residence time of 116 days. Salt Fork reservoir, built thirty years later than Wills Creek and Seneca Lake in 1968, is a large 2952 acre lake similar to Seneca Lake (Corps of Engineers, Huntington District, Open File Report and Ohio Department of Natural Resources, pers. comm.). Four streams (Seneca Fork, Wills Creek, Walhonding River, and Sugar Tree Fork) were sampled (Fig. 1 and Table 1). Seneca Fork is a major Pleistocene tributary of Wills (?reek and is fed by the outflow from Seneca Lake reservoir, while Wills Creek, a Pleistocene tributary of the Muskingum River, is fed by the continual release of water from Wills Creek reservoir. The

taphonomic loss due to shell dissolution. Abiotic buffering capacity of freshwater systems is primarily a function of the underlying bedrock (Drever, 1982). The bedrock in this study consists of alternating Pennsylvanian strata of sandstone, limestone, shale, and coal seams. Thus, the streams and reservoirs are not as buffered as Ca-rich seawater or those streams in southwestern Ohio where the Ordovician bedrock is primarily limestone (Green et al., 1985). Overlying waters were severely undersaturated with aragonite (Table 1). Taphonomic processes may be different in other settings with different water chemistries. Methods

Sampling methodology Three reservoir locations (Wills Creek, Seneca Lake, and Salt Fork) were sampled (Fig. 1 and Table I). Wills Creek reservoir is a narrow 900 TABLE 1

Physical and chemical parameters of the streams and reservoirs. Chemical data obtained from the Corps of Engineers, Huntington, West Virginia (Open file: available data covers the time period of 1974 1989 for Seneca Fork, Wills Creek and Wills Creek reservoir: 1975 I981 for Seneca Lake and 1974 1975 for the Walhonding River) and Ohio Department of Natural Resources (pers. comm.: Salt Fork reserw3ir average pH) - refers to missing data Salt Fork reservoir

Seneca Lake reservoir

Wills Creek reservoir

Seneca Fork

Wills Creek

Walhonding River

Sugar Tree Fork

Age/construction date

1960's

1930's

Pleistocene

Pleistocene

Pleistocene

Pleistocene

Water residence time Surface area/stream width Physical setting Area sampled Sampling depth range Dominant substrate

2952 acres tranquil

116 days 3550 acres tranquil 236 m 2 5 7m mud

1930's/ Pleistocene stream bed 3 days 900 acres tranquil 247 m z 0.5-6 m mud

12 m energetic 130 m 2 0.5 2.5 m rock/mud

20 m energetic 190 m 2 0.5-4 m rock/mud

100 m energetic 225 m 2 0.5 3 m rock,mud

4 m energetic 106 m 2 0.5 m rock,'mud

7.7 6.3 39.5 12.3 106.7

7.7 6.5 7 10.5 53.3

Physical parameters

243 m 2

0.5-7 m mud

Chemical parameters Mean pH Minimum pH Mean dissolved CaZ+(mg/l) Mean dissolved Mg 2 +(mg/1) Mean HCO3 Akalinity (mg~l) Aragonite saturation index Minimum pH Mean pH

7.4 -

7.7 6.4 36.8 9.6 213.7

0.04 0.83

7.4 6.5 8 79.9

0.004 0.033

0.02 0.44

0.002 0.039

7.4 7.1 43.6 164.2

0.19 0.65

58

R.H. CUMMINS

Columbus

"-

/

N

Kilometers

0 H~ 0 ~

1

Fig. Location map of the study sites in east-central Ohio. Numbers refer to sampling locations. / = S e n e c a Lake Reservoir; 2 = Seneca Fork; 3 = Sugar Tree Fork; 4 = Salt Fork Reservoir; 5 = Wills Creek Reservoir; 6 = Wills Creek and 7= Walhonding River.

Walhonding River, a large Pleistocene system which combines with the Tuscarawas River to form the Muskingum River, was also sampled in addition to a small creek, Sugar Tree Fork, which feeds into Salt Fork reservoir. Unionid species were rare as compared with molluscan abundance in marine settings (Staff et al., 1986). Numbers in the death assemblage ranged from 0.02-1.18 individuals per m 2. Due to

the low numbers, box core sampling techniques were ineffective, and so line transect and quadrats were employed to collect live and dead unionids. Individual site sampling area ranged from 106 m 2 at Sugar Tree Fork to 247 m 2 at Wills Creek Reservoir (see Table 1). Near exhaustive sampling methods were employed. Total bottom area sampled at each site was always more than the sampling area necessary to reach the live and dead species area curve plateau (Cairns and Pratt, 1986), and sampling effort exceeded that of many ecologic studies ofunionids (Downing and Downing, 1992). In all, over 1300 m 2 of reservoir and stream bottom were sampled using SCUBA, 0.25 m 2 random quadrats, and a modified line-transect method (50 cm beyond both sides of the transect line and 5 m in length per line-transect) in water depths ranging from 0.5 m to 7 m. At each site, approximately 95% percent of the bottom was sampled using the line transect method (5 m x 1 m per transect) and the remainder was sampled using the 0.25 m 2 quadrats. Divers meticulously collected molluscs by hand on and within the substrate. Each transect line was separately sampled by two divers to ensure the collection of all live and dead specimens present. As many distinct physical and sedimentologic environments as possible were sampled at each study site in shallow nearshore and deeper offshore locations (Table I). Sedimentologic environments varied from mud-dominated sediments in the reservoirs and deeper pools of the river systems to pebbly rock dominated substrates, mixed with mud, in the shallower sections of the river systems.

Ecological parameters All unionids were identified, categorized as right or left valve, whole shell or fragment, and measured (maximum anterior-posterior lengths). To determine the minimum number of whole individuals at each site, articulated valves and disarticulated right/left valves of the same size were counted as one individual. Whole shells and shell fragments (with umbo present) were used to determine the abundance of total dead in the death assemblage. Taxonomic identification followed Clarke (1973), Taylor (1980) and Watters (1988) and was con-

IAPHONOMI(

P R O ( ' E S S E S IN M O D E R N F R E S H W A T E R M O L L U S C \ N DI~ATH A S S E M B L A G E S

firmed by specialists at the Museum of Zoology, Ohio State University. Measures commonly used in livedead ecologic and paleontologic studies were calculated including species richness, the index of similarity and other measurements of live/dead fidelity, size frequency distributions, relative abundance, and biomass. No other organisms were included in these calculations.

Species richness and index of similariO, Species richness was calculated to compare the unionid life assemblage and time-averaged death assemblage within each habitat. The Margalef's index of species richness minimizes the effect of sample size bias (Odum, 1971; Dodd and Stanton, 1990) using the formula: D = S - l / 0 n N) where D = species richness value, S = number of species, and N = number of individuals collected. The index of similarity was used to compare species composition of unionid life and death assemblages in terms of the number of species present and the number of species in common using the formula: S=2C/(A + B) where S = index of similarity, A = number of species in death assemblage, B = n u m b e r of species in life assemblage, and C = number of species common to both the life and death assemblage (Odum, 1971). Higher values (ranging from 0.0-1.0) indicate more similar species compositions.

Live/deadfi&~lity comparisons with marine and estuarine enviromnents The question of fidelity of death assemblages to live shelly faunas was examined in the metaanalysis of taphonomy and time-averaging in estuarine and marine settings (Kidwell and Bosence, 1991). To better compare freshwater systems with their marine and estuarine counterparts, the same % metrics formulas were used here where (ND is the number of species found live only, (ND) iS the number of species found dead only and (Ns) is the number of species found both live and dead. While their marine study included all molluscan fauna, here only unionid molluscs were part of the calculations. Still, these freshwater/marine % metrics comparisons are the first ever done and provide

5q

important data on unionid live/dead fidelity and preservability as compared with marine settings. Three comparisons they addressed include: (1) "What percentage of shelly species found live arc also found dead at the same site'?" [i.e. (Ns x 100)/(NL + Ns)]; (2) "What percentage of species found dead are also found alive'?" [i.e. (N s × 100)/(ND + Ns~]; and (3)"What percentage of dead individuals are from species found alive?" [i.e. (dead individuals of Ns × 100)/(dead individuals of ND + Ns~].

Size J)'equenO" distributions The Kolmogorov-Smirnov two-sample, twosided test was used to compare size distributions of the unionid life (single-sample census data) and death assemblage (Daniel, 1978). Chi-square tests were not appropriate here because of the low numbers of individuals present in many ot" the live/dead size classes. Cummins et al., (1986a) estimated molluscan growth and mortality rate from data on the timeaveraged living community along the Texas coast. We estimated mortality by determining the size classes of living individuals at two sampling periods. The difference between the number collected from one sampling period and the number predicted from the previous period, if no mortality occurred, yielded an estimate of mortality. Here, mortality of living unionids could not be estimated because only single-sample census data was available for the living community. Size con> parisons were made between the time-averaged death assemblage and a "'snapshot" of the living community. To obtain mortality rates from this community, sampling would have to extend over many years in these long-lived molluscs because recruitment is sporadic. This is in stark contrast with many estuarine molluscs which frequently experience wave after wave of recruitment, and fresh input into the death assemblage due to mortality, each year (Cummins et al.. 1986a).

Relative abundance and hiomass determinations Biomass estimates were calculated from the relation between biomass and shell dimensions (Powell

R.H. CUMMINS

60

and Stanton, 1985). For bivalves, the equation is: loglo Biomass (g)=0.9576 logxo L 3 - 4.8939 where L is the maximum anterior-posterior length in mm. Separate rank order comparisons were made for live/dead abundance and live/dead biomass using the one-sided, Spearman Rank correlation. The null hypothesis is that rank orders of live/dead are independent. The alternative hypothesis is that rank orders of live/dead are correlated. Sugar Tree Fork and Salt Fork reservoir were not included in within habitat live/dead analyses because there were less than four species in both the life and death assemblages prohibiting Spearman rank comparisons. Results

Twenty-six unionid species were collected from the seven study sites. Abundances of live and whole shell abundances of dead, shell size, minimum and maximum size, and total dead (whole shells + fragments) are shown in Appendix A. Fourteen species were found in both the life and death assemblage at at least one study site, one species, Quadrula quadrula (Raf., 1820) was found only in the living community, and another, Tritogonia verrucosa (Raf., 1820) was found live at one site, dead at another. Ten species were found only in the death assemblage (Table 2). Reservoirs accounted for ,~ 21% of the number of individuals (whole shells + fragments) in the death assemblage and ~10% of the life assemblage. Species richness and similarity indices are shown in Table 3.

Live/dead fidelity comparisons with marine and estuarine environments Live/dead fidelity results are summarized in Tables 4 and 5. Death assemblage fidelities range from 50 to 100% in reservoirs and from 83 to 100% in streams when the question "What percentage of shelly species found live are also found dead at the same site?"; 40-100% in reservoirs and 50-58% in streams when the question "What percentage of species found dead are also found alive?"; and 85-100% in reservoirs and 58-97%

in streams when the question "What percentage of dead individuals are from species found alive?"

Size frequency distribution comparisons between the living and dead In nine of thirteen within-study site species comparisons of single-census life and death assemblages, no significant differences between the size distributions were found using the KolmogorovSmirnov two-sample, two-sided test (Table6). When comparing size distributions of life and death assemblages of species when all sites were combined, only the size distribution for Q. pustulosa pustulosa (Lea, 1831) was significantly different. Size frequency distributions (live/dead) for A. plicata plicata (Say, 1817) are shown for Seneca Lake reservoir, Seneca Fork, Walhonding River, and all study sites combined (Fig. 2). Other species' (live/dead) size frequency distributions are shown for L. radiata luteola (Lain., 1819), L. complanata (Barnes, 1823), and A. ligamentina carinata (Barnes, 1823) and E tiara (Raf., 1820) [Figs. 3 and 4].

Relative abundance and biomass determinations Spearman rank live/dead comparisons of relative abundance and biomass are shown in Table 7. Rank orders of relative abundance and biomass are shown in Figs. 5 and 6. Individual sites' life and death assemblages were significantly correlated (P~<0.05) in 3 of 5 biomass and 3 of 5 relative abundance comparisons of live and dead individuals. When unionid species' live/dead relative abundance and biomass from all study sites were combined and compared for the entire drainage area, rank order comparisons were correlated for relative abundance and biomass. Discussion

Water chemistry The taphonomic destruction of unionid shell material may have an important impact on paleontologic analyses including live/dead fidelity, size

IAPHONOMIC PR(}{'ESSES IN MODERN FRESHWATERMOLLUSCANDEATH ASSEMBLAGES

61

TABLE 2 A listing of unionid species collected live and dead, dead only, and live only. Numbers refer to sampling locations illustraled in Fig. 1

Species

Live and dead

Actinonaias l(gamentina carinata Amhlema p. plicata Anodonla g. grandis Anodonta imbecillis Cychmaias mberculata Elliptio dilatata Epioblasma triquetra Fusconaia flava Lampsilis radiata luteola Lantpsilis ventricosa Lasmigona complanata Lusmi~zona costata Leptodea .fragilis Lig,umia recta Ohovaria suhrotunda Plethobasus cyphyus Pleurohema clava Pleurohema sin toxia Potamilus alatus' Ptychobranchus fasciolaris Quadrula cylindrica Quadrula p. pustulosa Quadrula quadrula Strophitus u. undulatus Toxolasma parvus Trilogonia verrucosa

7 1,2,5,6,7 2,4,5,6 4,5 7

TABLE 3 Live an(] dead similarity and species richness Study site

Index of similarity

I. Seneca Lake reservoir 0.57 2. Seneca Fork 0.67 3. Sugar Tree Fork 0.67 4. Sail Fork resevoir 0.67 5. Wills Creek reservoir 0.83 6. Wills Creek 0.70

7. Walhonding River 0.69

Species richness (live) (dead) (live) (dead) (live) (dead) (live) (dead) (live) (dead) (live) (dead)

0.37 0.93 0.99 1.59 0.00 0.91 1.44 0.62 1.28 1.63 2.04 2.54

(live)

1.87

(dead) 3.15

Dead only

Live only

4 3 1 2,7 6

2,5,6,7 1 2,3 6,7 7 2,5,6,7

7 7 -

5 2,6 I 2,6,7 6 7 5 7 7

4

-

6 7 7 6 7

7 6 2,6 1 7

2

frequency distributions, numerical abundance, and biomass estimates. Unlike seawater that is carbonate buffered and maintains an alkaline pH, some freshwater systems are undersaturated with respect to carbonates: the preservation potential of skeletal material is highly variable in freshwater environments (Canfield and Raiswell, 1991). Sulfate reduction is much less significant than in marine sediments and shell preservation is more dependent upon initial water composition, methanogenesis, and iron reduction (Canfield and Raiswell, 1991). Over 45% of the aragonitic unionid shells in the death assemblage showed macroscopic effects of severe dissolution: the umbos were frequently pitted and etched. Even live unionids commonly had shells altered by pitting and elching in the umbo region. One way to determine the importance of shell dissolution in these environments is to calculate aragonite saturation indices. The formula for the

62

R.H. CUMMINS

TABLE 4 Fidelity of unionid assemblages: presence-absence data Location

What % species found live are also found dead at the same site?

What % of species found dead are also found live?

What % of dead individuals are from species found alive?

Seneca Lake reservoir Seneca Fork Sugar Tree Fork Salt Fork reservoir Wills Creek reservoir Wills Creek Walhonding River All study sites combined

100 83 I00 50 100 88 91 94

40 56 50 100 71 58 56 60

85 97 67 100 95 58 81 80

TABLE 5 Comparison of fidelity of unionid assemblages and molluscan fauna from marine settings. Data from estuarine and marine settings is from Kidwell and Bosence (1991) Setting

What % of species found live are also found dead at the same site?

What % of species found dead are also found live at the same site?

What % of dead individuals are from species found alive at the same facies/site? (facies as used by Kidwell and Bosence, 1991; site, this study)?

Mean

Range

Mean

Range

Mean

Range

83 95 84

45-100 82-100 54-97

54 33 45

27-100 10-58 38-64

90 89 70

79-100 77-100 6-97

83 91

50-100 83-100

70 55

40-100 50-58

93 76

85-100 58-97

Marine

Intertidal Coastal Open marine Freshwater

Reservoirs Streams

a r a g o n i t e s a t u r a t i o n index (Drever, 1982) is:

S I = a C a 2 +'K2"aHCO3

Ko

"10

pn

where S I is the s a t u r a t i o n index o f a r a g o n i t e (aragonite dissolves when S I < 1), a is the activity o f C a / + a n d HCO3, K c is the solubility p r o d u c t c o n s t a n t o f CaCO3, a n d K2 is the second dissocia t i o n c o n s t a n t o f c a r b o n i c acid. A r a g o n i t e saturation indices were well below 1 at m i n i m u m a n d m e a n p H values (Table 1). There can be little d o u b t

that shell material exposed o n these lake or river b o t t o m s is subject to dissolution.

Species richness and index o f similarity W a l h o n d i n g River a n d Wills Creek, the older a n d larger river systems, had the highest species richness values in the life a n d death assemblage. W i t h one exception, the time-averaged u n i o n i d death assemblage was more species rich t h a n the living c o m m u n i t y . A t Salt F o r k reservoir (built in

TAPHONOMIC PROCESSES IN MODERN FRESHWATER MOLLUSCAN DEATH ASSEMBLAGES

63

TABLE 6 Size comparisons utilizing the Kolmogorov-Smirnov two-sample, two-sided test for species with individuals in both the life and death assemblage Species

Site

N(live/dead)

Max. difference

A. grandis

Seneca Fork All sites Walhonding River Seneca Fork Seneca Lake reservoir Walhonding River Wills Creek Wills Creek reservoir All sites Wills Creek reservoir Seneca Fork All sites Walhonding River All sites Seneca Fork Walhonding River All sites Wills Creek All sites

6/7 15/15 41/63 73/55 13/44 53/59 7,,'6 7/l 1 155/175 7:'7 8,'7 20/25 86/17 92/46 45/45 7/7 67/74 6/19 13/19

0.524 0.267 0.462 b 0.345 b 0.287 0.429 b 0.286 0.442 0.111 0.429 0.446 0.330 0.224 0.207 0.222 0.429 0.180 0.8338 0.538 a

A. ligamentina A. plicata

Eflava

L. complanata L. radiata

Q. pustulosa

a0.01 < P < 0.05. b0.001 < P<0.01.

1968), unionids are gradually colonizing bottom habitat which was previously unavailable prior to reservoir construction. As the reservoir ages, the time-averaged death assemblage will likely become more species rich than the life assemblage (as judged by comparisons of the time-averaged death assemblage and ecological "snapshots" of the living community in future sampling efforts). The study site having the lowest live/dead similarity was Seneca Lake reservoir (S=0.57). Wills Creek reservoir had the highest live/dead similarity (S = 0.83). Live/ dead fidelity comparisons with marine and estuarine environments

Like shelly fauna in marine environments, these freshwater unionid assemblages have high live/ dead fidelity despite the likelihood of shell dissolution in the death assemblage (Tables 4 and 5). This bodes well for paleontological analyses. When addressing the questions "What percentage of shelly species found live are also found dead at

the same site?" and "What percentage of species found dead are also found aliveZ'" percentages compare favorably with marine and estuarine environments. While overall percentages are lower for the latter question, less fidelity between the live and dead than in the preceding question may be due to environmental perturbation and/or undersampling of the current living community (Kidwell and Bosence, 1991). The distinction is critical in environmental reconstruction. These freshwater systems also compare favorably when addressing the question"What percentage of dead individuals are from species found alive?" At the Walhonding River, eighteen unionid species were in the death assemblage and eleven in the life assemblage. Yet, over 81% of the individual dead were from species found alive. The use of death assemblages in studying environmental change in freshwater systems has great potential because of the high live/dead fidelity present in undisturbed systems provided adequate sampling of the living community has taken place. Environmental change can be recognized by shifts

64

R.H. C U M M I N S

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0-2

2-4

4-6

6 - 1 ) 0-10 1 0 - 1 2 1 2 - 1 4 1 4 - 1 6 t 6 - 1 8 1 8 - 2 0 Size(ca)

Fig. 2. Live/dead size frequency distributions for A. plicata at Seneca Lake, Seneca Fork, and the Walhonding River. Juvenile recruitment in the life assemblage of A. plicata is shown in the Walhonding River and Seneca Fork populations. Even with juvenile recruitment, the life assemblage is dominated by large, mature individuals. The death assemblage at Seneca Fork is negativelyskewed and bimodal.

in live/dead fidelity. For instance, at Wills Creek, lower fidelity (58% of dead individuals are from species found alive) is probably due to environmental change resulting in local extinctions.

Life assemblage size frequency distributions Recruitment success and failure have an important influence on the shape of size frequency distributions of the living community. Those populations experiencing recent recruitment are often positively-skewed or bimodal, while those not

having juvenile recruitment for several years are typically negatively-skewed as cohorts grow to maturity (unionids: Hanson et al., 1988; Miller and Payne, 1988; other molluscs: Shimoyama, t985; see Dodd and Stanton, 1990 and Kidwell and Bosence, 1991). In this study, all of the size frequency distributions of the unionid life assemblage were negatively-skewed. Only two species, A. plieata and Q. pustulosa showed any indication of juvenile recruitment. A. plicata's juvenile representatives were found primarily in the Walhonding River and Seneca Fork populations (Fig. 2).

1 A P H O N O M I C PROCESSES IN M O D E R N F R E S H W A T E R M O L L U S C A N D E A T H A S S E M B L A G E S

65

50

L.

v

~2

L. radlala

radlala 40

Fork

Seneca

All Study

Slits

30" •

•

Live

Live

[] n e ~ 30.

"~

20'

"~

20

0 0-2

2-4

4-6

5-8

0-I0

10-12

12-14

14-16

0"2

2-4

4-6

6-8

Size(ca)

8-I0

I0-12

12-14

14-16

Size(ca)

Fig. 3. Live/deadL size frequency distributions for L. radiata collected from Seneca Fork and all sites combined.

30'

20"

A.

F..fT~ va All Study

Sites

llgamentlna

cmrlnata

Walhondln$

River

• Live I~D~a

• Live [] Oe.d

i0 ¸ o

10

Z

0-2

2-4

4-6

6-8

B-tO

0-2

2-4

4-6

Slze(cm) 40.

L. o

Sites

30'

• Live El D,~

,t

~

coraplanala

All Study

20'

o

Z

0-2

2-4

4-6

6-8

8-10 I0-1212-1414-1616-1818-2020-22

SIze(cm)

Fig. 4. Live/dead size frequency distributions are shown for E flava, A. ligamentina and L. complanata.

6-8

8-10

$1ze(cm)

tO-t2

12-14

14-16

66

RH. CUMMINS

TABLE 7 A Spearman rank correlation test was used to compare numerical abundance (live/dead) and biomass estimates (live/dead). H o= Live and dead rank order of abundance are independent; H1 = Live and dead rank order of abundance are directly related. N= number of live/dead species comparisons Site

N

Abundance Rho values (Live/Dead)

Biomass Rho values (Live/Dead)

1. Seneca Lake reservoir 2. Seneca Fork 5. Wills Creek reservoir 6. Wills Creek 7. Walhonding River All Sites

5 10 7 13 19 25

0.89a 0.57~ 0.67 -0.06 0.39a 0.64c

0.89a 0.61 a 0.42 -0.08 0.42a 0.49b

a0.025 < P ~<0.05. b0.005 ( P< 0.01. ~P < 0.001.

A. plicata

Aplicata L. radiata A. ligameatilm L.compla~ta L. fragilis

L. compla~ta

m l F.flava i

L. radlam

m mm mm

A. ligamentina T. verrucosa F. flav a A. gratidis

Living Community Total Taxa= 15 N=454 Individuals

Q. pustulosa Q. quadrula A. imbeciUis

m

S. undulatus C atberculam O. subroUmda L. ventricosa P. alatus

o

,.O Percentage

2'o of

i

3o

Individuals

r 40

Collected

Q. pustulosa L. costata A. 8~andis L. ventricosa A. imbecillis T. verrucosa P. fasciolaris C. mberculata P. cyphy~ E. dilatata 5. undulat~ P. alams O. subromnda E. triquetra P. sintoxia T. parvus Q. cylindrica P. clara L. recta

,m

m i m

Death Assemblage Total Taxa= 25 N=574 Individuals l " " "

,.O Percentage

2.O of

3.O

Individuals

g0 Collected

Fig. 5. Numerical abundance of species in the unionid life and death assemblage for all sites combined. Rankings are based upon the percentage of the total number of specimens represented by each species.

It is possible that I did n o t collect all o f the juveniles that were present. P e r h a p s either the e n v i r o n m e n t w h e r e juveniles live was ecologically distinct f r o m t h e adults (Cad6e, 1982) or the s a m p l i n g m e t h o d s were n o t effective at collecting u n i o n i d s smaller t h a n 1 c m ( H a n s o n et al., 1988). H o w e v e r , all c o n c e i v a b l e s e d i m e n t o l o g i c a nd physical e n v i r o n m e n t s were s a m p l e d at each study site

including energetic an d t r a n q u i l settings, shallow n e a r s h o r e a n d offshore habitats, an d fine a nd course g r ai n ed sediments. F e w j u v e n i l e u n i o n i d s were f o u n d b u t specimens as small as 4 m m o f Corbiculafluminea, the Asiatic clam, were collected at m a n y locations. I f j u v e n i l e u n i o n i d s were present in these e n v i r o n m e n t s , I a m co n f i d en t m o s t w o u l d have been collected.

IAPHONOMIC

PROCESSES

IN MODERN

FRESHWATER

MOLLUSCAN

DEATH

L.con~lanata

A4Micata L.complanata A. ligarnentlna L. radlata L. fragilis L. costata T. verrucosa L. ventricosa A. grandis F. tiara A. imbecillis C. tuberculata P. alatus Q. pustulosa P. cyphyus P. fasciolaris E. dilatata L. recta S. undulatus P. sintoxia Q. cylindrica E. triquetra O. subrotunda P. clara T. parvus

A.plicata A. ligamentina L. radiata A, grandis

/ /

T. verru~osa F. tiara Q quadtula P, alatus

Living Community Total Taxa=15

Biomass

Q. pustulosa C. tuberculata S, undulatus L. ventricosa A. imbecillis O. subrotunda 10

Percentage

20

of

30

Total

40

Biomass

67

ASSEMBLAGES

I I I II I Death Total

Assemblage Taxa=25

Biomass

i

i

,

i

I0

20

3O

40

Percentage

of

Total

Biomass

Fig. 6. Biomass estimates for the unionid life and death assemblage for all sites combined. Rankings are based upon the percentage of the total biomass represented by each species.

Death assemblage size frequency distributions Size frequency distributions of the unionid death assemblage were negatively-skewed or negatively skewed and bimodal (Figs. 2-4). The timeaveraged unionid death assemblage contained no juveniles at any of the study sites. The absence of juveniles in the death assemblage might be explained by unusually high juvenile survivorship or taphonomic destruction of the smallest size classes. In most aquatic invertebrates, survivorship of juveniles is low (Powell et al., 1984). Unionids have a wide range of potential enemies including insect larvae, leeches, crustaceans, fish, turtles, birds, and mammals (Clarke, 1986). Yet the record in the death assemblage would imply that all juveniles grow to adult size. This is unlikely. In a study of unionid reproduction and survivorship in a Canadian lake, Jansen and Hanson (1991) found that the mortality rate from the time of excystment (glochidia drops from fish host to the substrate) to the age of sexual maturity (age 5) is approximately 82 91%. The shape of size frequency distributions of the time-averaged unionid death assemblage may be a function of the intensity of taphonomic loss, particularly destruction of the smallest size classes. A

shell whose dimensions are 15 × 10cm has approximately 10 x the shell surface area, and is also much more massive, than an individual 4 x 4cm. Given equal taphonomic processes in these aragonitic poor waters, it is likely that smaller shells would be destroyed more rapidly than larger ones. This can result in negatively-skewed, timeaveraged death assemblage size distributions.

Size frequency distribution comparisons between the living and dead Of the four comparisons with significant differences between the live and dead size classes, A. plicata accounted for two of the significant results. If the juveniles of A. plicata are removed from the life assemblage size frequency distributions, the live/dead size frequencies are remarkably similar (see Fig. 2). The similarity in live/dead size frequency shape may be typical of what one would expect in unionid life and death assemblages: live/dead equivalence is the norm except during periods of juvenile recruitment. In other studies of unionids, juveniles were present in low numbers or absent from the living community (James, 1985; Tevesz et al., 1985; Clarke, 1986; Miller et al., 1987). Even in unionid beds, which routinely support a high diversity of unionid species, annual

68

recruitment is not guaranteed. In successful recruitment years, the living unionid community, usually dominated by one or two species, may contain an abundance of juveniles (Miller and Payne, 1988). As those juveniles mature into adults and the length of time increases between recruitment events, the living community becomes dominated by large, adult individuals representative of many distinct cohorts. In the death assemblage, negatively-skewed size frequency distributions are probably the norm. Larger, more robust shells may be preferentially preserved whereas the smallest shells are lost due to dissolution. And, because the study sites may have been located in borderline habitats for many of the twenty-six species collected, more years than usual might pass between successful recruitment events. This would reinforce the occurrence of negatively-skewed size frequency distributions in the living community which match those in the death assemblage.

Significance of live/dead size frequency distribution similarity These results are in marked contrast to estuarine, molluscan life-death assemblages along the Texas coast where 13 of 15 comparisons of the death assemblage and the estimated mortality from the living community indicated little similarity between life and death assemblage size frequency distributions (Cummins et al., 1986a). It is likely that death assemblage formation and associated taphonomic processes, coupled with the unique life-cycle of unionid molluscs, are distinctive in freshwater environments. However, it is difficult to make unequivocal statements as to the causes of live/ dead size frequency similarity in unionids. Is similarity due to a combination of the unique reproductive strategy of members of the living community ---making juveniles rare in the living community much of the time - - a n d taphonomic 10ss of smaller size classes in the death assemblage? Or, are these results more controlled by disturbance of the living community making recruitment less frequent? Suggestions for further study include comparing unionid live/dead along a gradient of disturbance and in a variety of water-sediment

R.H. CUMMINS

chemistry and buffering settings to see how these findings compare.

Relative abundance and biomass Numerical abundance of species is a frequently used parameter in ecological and paleontological analyses in studies comparing relative abundance of life and death assemblages in marine environments. This technique is powerful when comparing shelled fauna (Kidwell and Bosence, 1991). However, rarely does the relative abundance of the life assemblage equal that of the death assemblage, when soft-bodied organisms are also considered, because in many marine communities, the vast majority of benthic organisms are soft-bodied and are not preserved. Because of the uncertainty inherent in interpreting relative abundance data when most of the original community is not preserved, some researchers (Powell and Stanton, 1985) have suggested that biomass can be a more important paleontological measurement than relative abundance. Molluscs are frequently the biomass dominants of benthic communities and biomass, as estimated by the volume of skeletal remains, is less affected by taphonomic loss than is numerical abundance (Staff et al., 1985, 1986; see Kidwell and Bosence, 1991). In comparisons of live/dead relative abundance and live/dead biomass in these freshwater environments, the death assemblage preserves the rank orders of abundance and biomass of the preservable molluscan components in some instances and not in others. There is outstanding live/dead fidelity of relative abundance and biomass at the least disturbed sites. This finding compares favorably with marine environments (Kidwell and Bosence, 1991) and could prove useful to ecologists attempting to document long-term environmental change. Wills Creek reservoir and Wills Creek were the most obviously disturbed environments in the study and showed little live/dead similarity in biomass and relative abundance. At Wills Creek reservoir, the rank order of relative abundance and biomass was dissimilar despite the high index of similarity (S=0.83) between species composition of the life and death assemblage. Differences between the live/dead rank order of

T A P H O N O M I C PROCESSES IN M O D E R N FRESHWATER M O L L U S C A N DEATH ASSEMBLAGES

relative abundance and biomass at Wills Creek reservoir and Wills Creek may be the result of several processes. First, the aquatic environment has been degraded over time. Habitat alteration is likely due to agricultural sediment run-off, strip mining, and the construction of dams. Because of the unique life-cycle of unionids, any factor that disrupts the glochidia "host" life-cycle also inevitably impacts unionid population structure and species composition. Natural disturbance is often episodic and can occur on time scales ranging from annual to a 100 years or more (Minshall, 1988). Disturbance, whether due to floods, alteration of watersheds due to fire, or changes in sediment load, is critical in structuring freshwater communities. The preservation of extant freshwater death assemblages, as well as freshwater fossil communities, may be partially controlled by such disturbance pulses. The present unionid living species "mix" at Wills Creek and Wills Creek reservoir is currently distinctive, in terms of live/dead numerical abundance and biomass probably because of recent disturbance. If these systems were less perturbed, live/dead unionid relative abundance and biomass might be more compatible. Second, this study represented no more than a "'snapshot" of the living community at each study site. Repetitive sampling over many years might reduce the differences between the living community and the time-averaged death assemblage (Kidwell and Bosence, 1991). Third, transportation of shells into each study site, particularly in larger streams, may play a very important role in thoroughly mixing the death assemblage, whose constituents may have originally died far removed from where they were collected. This may enhance the differences between the living and dead.

Conclusions Understanding the initial steps in unionid freshwater death assemblage formation and taphonomy

(~L)

should prove useful in interpreting the unionid fossil record. Comparisons of live/dead assemblages using % metrics formulae, relative abundance and biomass should also be of interest to ecologists attempting to document long-term environmental change in freshwater environments. While death assemblage formation is similar in certain respects to that of marine molluscs, an important distinction is in the comparison of live/dead size distributions. In many instances, size distributions of live/dead assemblages were similar in shape. Possible explanations for this similarity include a combination of taphonomic loss ofjuvcniles in the death assemblage and the infrequency of recruitment in the life assemblage. Despite the likelihood of intense taphonomic pressure on the unionid death assemblage due to dissolution, there are several general findings that unionids share, in part, with estuarine and marine molluscs: (1) There is high fidelity of live/dead using % metrics formulae which compares favorably with marine environments. Changes in fidelity can be used as indicators of environmental change in modern environments. (2) The time-averaged death assemblage in most instances was more species rich than the living community. (3) Rank orders of molluscan biomass (live/dead) and relative abundance (live/dead) were similar except at the most environmentally disturbed sites.

Acknowledgments I thank C. Martin, T. Danneman, D. Van Tassel, and L. Normansell for their assistance in the field. Special thanks to S. Kidwell, A. Cohen, A.I. Miller, C. Wolfe, C. Myers, and an anonymous reviewer who provided helpful criticisms. Funding for this project was provided by the Ohio Board of Regents Research Challenge Program.

70

R.H. CUMMINS

APPENDIX

A--The

number

o f live a n d d e a d ( w h o l e shells o n l y ) , m e a n size ( c m ) , s t a n d a r d

sizes ( c m ) c o l l e c t e d , t h e t o t a l n u m b e r Live

1. S e n e c a

Lake

Mean

deviation, the minimum

and maximum

o f d e a d ( w h o l e s h e l l s + f r a g m e n t s ) a n d t o t a l s a m p l i n g a r e a a t all s t u d y sites. Std.

Min.

Max.

dev.

size

size

Dead

Mean

Std.

Min.

Max.

Total

dev.

size

size

dead

46

Reservoir

( A r e a = 2 3 6 m 2)

A. A. L. L. T.

plicata imbecillis radiata complanata parvus

2. Seneca

(Area=

13

10.60

0

-

2

12.30

0 0

-

1.14

8.70

12.30

44

9.89

1.63

6.20

12.50

7

8.60

0.87

7.70

10.00

9

12

10.61

1.72

7.60

14.00

17

-

-

-

-

0.42

12.00

12.60

-

-

-

1

12.00

1

-

-

-

2

3.71

0.14

3.60

3.80

2

-

Fork

130 m 2)

A. plicata A. grandis E. dilatata

73

10.75

1.40

5.40

13.80

55

11.78

2.87

6.20

19.00

60

6

11.28

0.94

9.90

12.30

7

10.70

0.57

9.60

11.30

7

0

-

-

1

-

-

Eflava L. radiata L. ventricosa L. complanata L. costata S. undulatus T. verrucosa

8 45

8.34 10,99

0.67

7.70

0.96

7.80

0

-

-

2

15,95

2.90

13.90

0 0

-

-

-

-

-

12.00

1.21

3. Sugar

Tree

19

-

9.40

9.30 12.40

7 45

7.84

0.87

8.70

I

9.30

7

14.30

68

6.80

10.92

1.67

-

1

-

-

-

13,70

I

18.00

7

17.91

2.77

13.70

21,20

8

12,80

1

I

-

-

-

1

-

-

14.50

0

-

-

0.64

6.70

-

8.00 -

I 0

Fork

( A r e a = 106 m 2)

A. grandis L. radiata 4. Salt

Fork

0 12

-

-

-

-

1

10.22

0.89

9.10

12.10

2

10.05

-

-

8.50

1

9.60

10.50

2

8.60

13.21

4

Reservoir

( A r e a = 243 m 2)

A. A. A. L.

plicata grandis imbecillis complanata

5. Wills

Creek

2

10.25

1.49

9.20

11.30

0

.

4

11.56

2.20

8.98

13.60

4

10.83

.

1 1

-

-

-

9.30

1

-

-

-

15.25

0

.

.

0

2.11 -

-

-

-

6.90 -

1 0

Reservoir

( A r e a = 2 4 7 m 2)

A. plicata A. grandis A. imbecillis E tiara L. complanata L. radiata O. subrotunda 6. Wills

Creek

(Area=

190 m 2)

A. plicata A. grandis E. triquetra F. f l a v a L. radiata L. ventricosa L. complanata

7

12.16

1.93

9.40

13.90

11

10.83

2.05

3

9.03

1.88

7.80

11.20

1

-

-

-

7.80

14.00 7.60

11 1

4

8.05

0.91

7.40

9.40

1

-

-

-

6.80

1

7 2

8.22 13.73

0.49

9.40

13.90

7

8.41

0.75

7.00

9.20

7

0.55

13.20

14.30

16

13.25

1.65

9.10

16.00

18

0 0

-

-

-

-

1

-

-

-

12.10

1

-

-

-

1

-

-

-

5.80

1

7

8.80

2.82

5.00

12.30

6

9.85

1.89

7.50

12.80

6

2

10.80

0.57

10.40

11.20

2

9.90

4.67

6.60

13.20

2

0

-

-

-

-

3

1 0 1

6.17

-

0.67

5.60

2

7.35

0.07

7.30

7.40

2

6.90

2

7.80

1.41

6.80

8.80

2

-

-

12.80

7

11.49

2.19

6.80

13.10

7

-

-

-

7

11.46

0.96

10.60

13.60

7

-

-

10.40

5

16.26

3.35

13.80

22.00

5

T A P H ( ) N O M I C P R O C E S S E S IN M O D E R N F R E S H W A T E R M O L L U S C A N D E A T H A S S E M B L A G E S

71

A P P E N D I X A--(continued) Live

L. costala L. fi'agili.v P. alatu.s Q. pustulosa Q. quadrula S. undulatus

0 0 1 6 10 0

Mean

4.35 9.32

Std. dev. 0.79 0.71 -

Min. size

3.60 7.90

Max. size

17.00 5.90 10.10

Dead

I 21 3 19 0 1

Mean

11.98 12.77 6.67

Std. dev.

Min. size

Max. size

Total dead

1.49 1.86 0.86

9.10 II .50 5.30

10.00 14.30 14.90 8.10

I 21 3 19 0 [

10.30

7. Walhonding River (Area = 225 m 2) A. ligamentina 41 A. plicata 53 C. tuherculata 4 E. dilatata 0 F tiara 2 L. radiata 7 L. ventrwosa 2 L. complanata 86 L. costata 0 L. recta 0 O. suhrotunda 2 P. cyphyus 2 P. clara 0 P. sinto.via 0 P. jasciolaris 0 Q. cylindrica 0 Q. pustulosa 7 S. undulatus 6 T, verrucosa 0

13.18 12.34 I 1.70

0.95 2.79 0.58 -

10.34

1.60

15.42

2.30

6.10 12.50

0.28 1.70

11.70 3.65 11.20 8.00 9.40 5.90 11.30

15.80 17.20 12.20 12.40 12.40 21.00 6.30 13.70 -

-

8.47 8.49

-

0.16 1.45

8.20 6.90 -

8.70 10.20 -

References Bosence, D.W.J., 1979. Live and dead corals from coralline algal gravels, Co. Galway. Palaeontotogy, 22: 449-478. Cad6e, G.C., 1968. Molluscan biocoenoses and thanatocoenoses in the Ria de Rosa, Galacia, Spain. Zool. Verh. Rijksmus. Nat. Hist. Leiden, 95: 1-121. Cadde, G.C., 1982. Low juvenile mortality in fossil brachiopods, some comments. Interne Versl. NIOZ, Texel, pp. 1-29 Cairns Jr. J., and Pratt, J.R,, 1986. Developing a sampling strategy. In: B.G. Isom (Editor), Rationale for Sampling and Interpretation of Ecological Data in the Assessment of Freshwater Ecosystems. Am. Soc. Test. Mater. Spec. Tech. Publ., 894:168-186. Callender, W.R., Staff, G.M., Powell, E.N. and MacDonald, I.R., 1990. Gulf of Mexico hydrocarbon seep communities V. Biofacies and shell orientation of autochthonous shell beds below storm wave base. Palaios, 5: 2-14. Canfield, D.E. and Raiswell, R., 1991. Carbonate precipitation and dissolution: Its relevance to fossil preservation. In: P.A. Allison and D.E.G. Briggs (Editors), Taphonomy: Releasing the Data Locked in the Fossil Record (Topics Geobiol., 9). Plenum Press, New York, pp. 412-455. Carthew, R. and Bosence, D., 1986. Community preservation in recent shell-gravels. Paleontology, 29(2): 243-268.

63 59 5 4 9 7 6 17 16 1 2 5 1 2 7 1 0 3 8

12.06 11.51 10.27 9.08 6.80 10.63 12.63 14.98 12.68

9,80 7.80 9,30 6.90 4.80 9.8(t 12.30 9.80 9.20

5.87 10.40

1.20 1.46 0.56 2.31 1.31 0.65 0.52 3.25 1.46 0.75 0.56

9.30 8.24

0.99 1.05

8.60 7.10

7.78 15.53

0.86 1.90

6.90 13.50

5.34 9.7(I

15.50 14.50 11.00 12.20 9.00 11.60 I3.30 20.10 14.60 15.20 6.4(t 11.00 7.10 10.00 10.00 10.1(~ 8.711 19.50

63 63 5 4 9 7 6 17 17 [ 2 5 1 "~ 7 [ 0 3 8

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