Bivalve Taphonomy - GeoScienceWorld

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Bivalve Taphonomy: Testing the Effect of Life Habits on the Shell Condition of the Littleneck Clam Protothaca (Protothaca) staminea (Mollusca: Bivalvia) DARIO GUSTAVO LAZO Departamento de Ciencias Geolo´gicas, Universidad de Buenos Aires, Pabello´n II, Ciudad Universitaria, Buenos Aires 1428, Argentina, Email: [email protected]

Death assemblages generally do not enter the fossil record without taphonomic modification on the sediment surface or within the sediment. During residence in the taphonomically active zone and early diagenesis, the assemblage may acquire a taphonomic signature related to intrinsic (biology of organisms) and extrinsic factors (e.g.,

light, nutrients, oxygen, temperature, salinity, substrate composition and chemistry, and physical energy). Taphofacies analysis in fossil and recent molluscan assemblages has focused on the role of extrinsic factors, including comparisons of taphonomic trends between different depositional environments and/or depth gradients. Although extrinsic factors are extremely important in determining the quality and quantity of modifications of a given assemblage, they can not explain the taphonomic variability that results from intrinsic differences between organisms such as body plan, behavior, mode of life, and skeletal mineralogy (Kidwell and Bosence, 1991; Best and Kidwell, 2000). In particular, mode of life of benthic organisms can influence taphofacies development because, after death, infauna tend to stay buried within the sediment, while epifauna tend to remain exposed on the sediment-water interface. If the vertical tiering of the benthic fauna persists after death, the organisms will acquire a distinct taphonomic pattern according to the vertical tier they occupy. This is related to the main hypothesis tested in this paper that infauna have a different shell condition than the epifauna. This study aims to provide new evidence showing that the infaunal bivalves have a better shell condition and possibly higher fossilization potential and timeaveraging than the epifaunal bivalves. This evidence is based on a promising natural experiment found on San Juan Island (Washington, USA), where the burrowing bivalve Protothaca (Protothaca) staminea (Conrad) exhibits either the typical infaunal life habit or an epifaunal life habit in adjacent (20-m distant) habitats. This situation presents an ideal opportunity to dissociate the effects of gross shell shape and microstructure from the effects of life habit on taphonomic signature. The bivalve P. staminea (Family Veneridae, Subfamily Chioninae) has a subquadrate and inflated shell, cancellate sculpture, cream to brown external color, a finely crenulated inner shell margin, and a deep pallial sinus. The shell is aragonitic throughout and two primary shell layers are present. The outer layer has composite prismatic structure, the pallial myostracum is of crossed-lamellar structure, and the inner layer is composed of homogeneous and complex crossed-lamellar structures (Taylor et al., 1973; Hikida, 1996). Protothaca staminea has a wide geographic distribution in the eastern Pacific, from the Gulf of Alaska to Baja California Sur; it also has been reported from Japan (Coan et al., 2000). Typically, this species lives as an infaunal burrower between 5 and 15 cm depth, in soft sand or sandy mud, in bays and coves from the intertidal zone to 10 m. In Argyle Lagoon, San Juan Is-

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PALAIOS, 2004, V. 19, p. 451–459 The littleneck clam Protothaca staminea in Argyle Creek and Argyle Lagoon on San Juan Island (Washington, USA) provides an ideal opportunity to test the effect of life habits on the taphonomic signature of shells. This bivalve exhibits two different modes of life in adjacent habitats: infaunal in muds and muddy sands (Argyle Lagoon) and free epifaunal on gravels (Argyle Creek). The mode of life significantly affected the taphonomic signature of both live and dead shells. Epifaunal P. staminea exhibit more damage than infaunal shells, suggesting that the infauna has a greater fossilization potential and may be more heavily affected by time-averaging than the epifauna. Both live infauna and epifauna suffered important taphonomic modifications after death, especially on the internal surface of the shell, but infauna did not reach the high level of damage acquired by the epifauna. In Argyle Creek, taphonomic agents were more effective at the sediment-water interface than within the sediment. Because mode of life has a significant influence on processes of preservation, different taphonomic patterns in fossil bivalves do not necessarily imply different postmortem histories of shells, even when the taphonomic analysis is restricted to a single species. Some external modifications and internal shell damage cannot be regarded as unambiguously postmortem since edge and color modification, external corrasion and encrustation, and internal bioerosion can occur during the lifetime of the animal. Finally, this paper shows that a single bivalve species can exhibit more than one mode of life even within closely proximate environments. The typical mode of life is reflected in shell morphology while the secondary one is not. Thus, functional-morphology studies of extinct species can lead to incomplete interpretations of the range of a bivalve’s life habits. An integrated approach combining functional morphology, comparisons with close relatives, and lithofacies analysis can be useful in paleoecological interpretations of extinct bivalve species.

INTRODUCTION

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FIGURE 1—Location of study area in Washington State, USA. (A) Map of northwest USA and southwest Canada showing location of San Juan Island. (B) Enlargement of San Juan Island with location of Argyle Bay, Argyle Creek, Argyle Lagoon, and Friday Harbor Laboratories. (C) Detailed map of Argyle Bay, Argyle Creek, and Argyle Lagoon showing sampling areas of infaunal and epifaunal shells.

land, Washington, P. staminea is found living as an infaunal burrower in muds and sandy muds. In the adjacent Argyle Creek, however, the same species is found epifaunally, reclining on either valve among pebbles and cobbles. Because of the tidal relations (see below), the same water bathes both populations, and both the epifauna and infauna are undoubtedly recruited from the same pool of planktonic larvae. Although unusual, the epifauna seem to be equally viable as infauna, because they were collected alive and apparently healthy. The aim of this study is to analyze the shell condition of live and dead specimens of P. staminea to test differences in the taphonomic signature between modes of life. The expectation was that the shell condition varies significantly between modes of life. Other intrinsic factors, such as gross morphology, body plan, and skeletal microstructure, are the same in both modes of life because the live and dead shells came from the same species. METHODS Study Area Studied materials were collected from Argyle Creek and Lagoon on the eastern shore of San Juan Island, Washington State, USA (488 319 N, 1238 009 W; Fig. 1A, B). Argyle Bay is a relatively small protected marine embayment connected to North Bay. Argyle Creek is a narrow, curved conduit about 25 m long that connects Argyle Bay with Argyle Lagoon (Fig. 1C). Argyle Lagoon is a triangular body of water bounded by sand bars on two sides and San Juan Island on the third (Fig. 1C). The lagoon is flushed daily as

the tide rises, and then is partly emptied as the tide recedes; water is exchanged exclusively through the Creek. The water depth at low tide reaches 3 m in the center (Soto-Bussard, 1963; Tierney, 1978). Both habitats are flushed twice daily, and they share some environmental attributes, such as light, salinity, oxygen, temperature, and nutrient levels, as well as some organisms, such as mobile epifauna, free-lying oysters, infaunal bivalves, and macroalgae. The two habitats differ in sediment grain size and substrate consistency. The creek is characterized by a hard substrate composed of glacially derived pebbles and cobbles, but there are also patchy shell-gravel pavements (Shinn, 1976); fine sediments are winnowed away by the high (. 50 cm/s) currents produced as the lagoon tidally flushes through the creek. The creek includes both mobile and fixed epifaunal organisms, with mussels, barnacles, and oysters especially common. The lagoon has a soft substrate composed of organic-rich muds and muddy sands that are typical of low-energy depositional settings. Burrowing organisms clearly predominate in the lagoon. Field and Laboratory Work Infaunal specimens were collected from Argyle Lagoon at low tide. Shells and fragments were collected from a muddy sand bottom area of approximately 80 m2 (Fig. 1C). Sixty-one live infaunal individuals were hand collected and immediately identified. Dead infaunal shell material was concentrated in the field using a 12-mm mesh screen. The source sediment was obtained at a depth of at least 10 cm. Shell fractions were later identified.

MODES OF LIFE AND SHELL CONDITION IN A RECENT BIVALVE

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FIGURE 2—Taphonomic attributes used to characterize the shell condition of the bivalve Protothaca staminea. (A) External attributes: color, corrasion, bioerosion, and encrustation. (B) Internal attributes: edge modification, corrasion, bioerosion, and encrustation. All x0.75.

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Epifaunal shells and fragments were collected along the gravel bottom of Argyle Creek at water depths between 12 and 20 cm at low tide (Fig. 1C). Sixty-one live epifaunal individuals were picked from among the cobbles and gravel. Dead epifaunal shell material was collected from one shell pavement. The dead material was sieved with a 12-mm mesh and the retained fraction was identified. In the laboratory, live animals were sacrificed and shells were cleaned of soft tissues. Both dead shell samples were spread evenly on a tray and divided into six fractions. One fraction was randomly selected for subsequent analysis from each tray. Four different shell fractions were obtained: live infaunal (n 5 61), dead infaunal (n 5 88), live epifaunal (n 5 61), and dead epifaunal (n 5 77). Each specimen was scored using the following morphologic and taphonomic variables: (1) identification: sample type (epifaunal/infaunal), state (live/dead); (2) size: length, height, width; (3) attributes of the entire specimen: valves (both/right/left/undetermined), fragmentation (whole/fragment); (4) external attributes: color (unaltered, loss, discoloration), shell distribution of corrasion (central or umbonal/median/marginal), corrasion (absent/minor/major), bioerosion (absent/ present), encrustation (absent/present); (5) internal attributes: corrasion (absent/minor/major), shell location of corrasion (central or umbonal/median/marginal), edge modification (unaltered/chipped/smoothed), shell distribution of edge modification (anterior/posterior/ventral), bioerosion (absent/present), encrustation (absent/present); and (6) other attributes: predation (absent/present), repair scar (absent/present). Identification of shells was made using the key and descriptions in Coan et al. (2000). Taphonomic attributes were evaluated under a dissecting microscope. Examples of principal external and internal taphonomic attributes are presented in Figure 2. Bioerosion and encrustation were considered present if more than 2% of the shell surface was covered. The term corrasion indicates a combination of mechanical abrasion and biogeochemical corrosion (Brett and Baird, 1986). For taphonomic scoring, all collected shells were divided into the following groups: live infauna, live epifauna, dead infauna, and dead epifauna. For statistical analysis, taphonomic attributes were assigned to one of the following numerical values: (0) unaltered or absent, (1) present or minor alteration, and (2) major alteration. Frequencies and proportions of each attribute in each mentioned group were calculated, and statistical significance was evaluated using standard techniques for evaluating heterogeneity of contingency tables, based on Chi-square statistics (e.g., Zar, 1998). Comparisons made include the following: (1) live infauna versus live epifauna to determine the normal shell condition of shells, (2) live infauna versus dead infauna and live epifauna versus dead epifauna to address the main postmortem modifications, and (3) dead infauna versus dead epifauna to test differences in the taphonomy of dead shells between modes of life. All statistical analyses were performed at the Friday Harbor Laboratories using SASq v. 8.12 (SAS Institute, 2001) software and the SAS User’s guide (SAS Institute, 1989).

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FIGURE 3—Comparisons of shell alterations of Protothaca staminea live specimens by mode of life. Each bar is a proportion based on 61 specimens. Each pair of bars was tested for significant heterogeneity using Chi-square statistics; p values are shown for significant differences (*) between shell condition of infauna (I) and epifauna (E).

RESULTS Effect of Life Habits on the Taphonomy of P. staminea Effect of Life Habits on Shell Condition of Live Specimens (Live Epifauna versus Live Infauna): Statistically significant differences in shell condition of epifauna and infauna were observed for edge condition, color, corrasion, and bioerosion (Fig. 3). The finely crenulated commissural margins of infaunal specimens were unaltered or chipped, however, the epifauna showed more severe edge modifications in the form of a smoothed margin. The posterior margin showed greater alterations than the anterior or ventral margins in infauna; the three margins were not significantly different in epifauna (Fig. 4A, B). Infauna invariably showed original colors (e.g., white, gray, and brown) and markings (e.g., zigzag patterns), whereas epifauna typically showed loss of original color and/or orange, white, and green discolorations due to endolithic and epizoic algae on the shell surfaces. Both infauna and epifauna showed minor external corrasion, but the latter had a statistically higher level of corrasion. For both life habits, the central portions of the shell, particularly the umbonal sector, were the most affected (Fig. 4C, D). Bioerosion was present only in epifauna and consisted mostly of unevenly distributed circular microborings 1 mm in diameter. Level


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FIGURE 4—Distributions of edge modification (chipped or smoothed) and external and internal corrasion (minor and major) on live and dead specimens of Protothaca staminea. Each pie chart was tested for significant heterogeneity using Chi-square statistics; p values are shown for significant differences (*) between shell-margin sectors (anterior, ventral, and posterior) and shell portions (central, median, and marginal). (A), (C) Live infauna. (B), (D) Live epifauna. (E), (G),(I) Dead infauna. (F), (H), (J) Dead epifauna.

of articulation and fragmentation show no differences between life habits because all live specimens (by definition) were articulated and non-fragmented. No alteration was observed in internal surfaces (Fig. 5A), except for some bioerosion on the interiors of four epifaunal specimens where external bioerosion reached the interior shell. Only

one infaunal shell had repair scars; none showed predation marks. Taphonomic Modifications (Live Infauna versus Dead Infauna and Live Epifauna versus Dead Epifauna): Postmortem modifications were apparent for both modes of life. For infaunal shells, every taphonomic variable except ex-

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FIGURE 5—Comparisons of internal shell modifications of Protothaca staminea specimens by mode of life. Each bar is a proportion based on 61, 77, or 88 specimens. Each pair of bars was tested for significant heterogeneity using Chi-square statistics; p values are shown for significant differences (*) between shell condition of infauna (I) and epifauna (E). (A) Live individuals. (B) Dead specimens.

ternal corrasion and internal encrustation significantly increased in frequency after death (Fig. 6A). The frequency of corrasion did not change significantly, although the distribution of external corrasion on the shells differed between live and dead infauna. In live infauna, the central portions of the shells tend to be most corraded, whereas median and marginal portions are more abraded in dead specimens (Fig. 4C, G). Edge modification was essentially restricted to the posterior margin in live infauna; in dead infauna, edge modification was equally intense on all sectors of the shell (Fig. 4A, E). Epifaunal shells did not exhibit a simple pattern of postmortem modification. Only five of ten measured taphonomic variables (articulation, fragmentation, internal corrasion, internal bioerosion, and internal encrustation) statistically increased in frequency in comparison to live individuals (Fig. 6B). Unexpectedly, color and edge modifications significantly decreased in frequency. External corrasion, bioerosion, and encrustation remained constant. There was no change in the distribution of edge modification or in the distribution of external corrasion between live epifauna and dead epifauna (Fig. 4B, D, F, H). Effect of Life Habits on Shell Condition of Dead Shells (Dead Epifauna versus Dead Infauna): Dead epifauna showed a higher proportion of discoloration, external corrasion, and bioerosion than dead infauna (Fig. 7). Dead shells from the two modes of life did not differ statistically in disarticulation, number of left and right valves, fragmentation, edge modification, and external encrustation; predation marks and repair scars were undetectable in ei-

ther group. On the internal surface of shells, dead epifauna were more frequently corraded and encrusted than dead shells from infauna, while the frequency of internal bioerosion was not different statistically between habits (Fig. 5B). The distribution of internal corrasion was similar between dead epifauna and dead infauna (Fig. 4I, J). The appearance of notable internal corrasion and encrustation were the most prominent postmortem modifications in shells from either mode of life. DISCUSSION Effect of Life Habits on the Taphonomy of P. staminea Effect of Life Habits on Shell Condition of Live Specimens (Live Epifauna versus Live Infauna): As expected, the shell condition in live infauna was good and in many cases the shells were pristine. The only alterations present in live infauna were chipped margins and minor external corrasion. Internal surfaces were consistently pristine. Corrasion was preferentially located in the umbonal region and primarily affected the fine radial ornamentation developed early during growth. Contact with sediment grains during burrowing may have abraded the shell. It is unlikely that the restricted distribution of corrasion can be attributed to mechanical abrasion by physical reworking. Shell state in live epifauna was quite different from live infauna, with the former invariably more damaged and never pristine, irrespective of shell sector. Mode of life clearly influenced shell condition in live animals and three of the four significant variables (edge


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FIGURE 6—Comparisons between live and dead shells of Protothaca staminea in frequency of shell alterations. Each pair of dots was tested for significant heterogeneity using Chi-square statistics. Solid lines indicate significant differences (p , 0.05), dashed lines indicate nonsignificant differences (p . 0.05). (A) Live infauna versus dead infauna. (B) Live epifauna versus dead epifauna.

modification, external corrasion, and bioerosion) are expected to be preserved in the fossil record. These external modifications cannot be regarded as taphonomic alterations because they are produced during the life of the animal, but it is likely that they tend to diminish the fossilization potential of the epifauna. Bioeroded external surfaces were common in epifauna while internal bioerosion was infrequent. Internal borings were the result of deep external bioerosion that reached the internal surface of the shell. The internal borings were partially repaired by a thin carbonate layer produced by the mantle lobes of the bivalve. Although rare, observation of internal bioerosion produced during life is important, since internal damage observed in fossils generally is interpreted as postmortem (e.g., Best and Kidwell, 2000). This case demonstrates that bioerosion on shell interiors cannot be viewed unambiguously as postmortem, though the presence of repaired borings indicates that the animal was alive during the borer’s attack. Taphonomic Modifications (Live Infauna versus Dead Infauna and Live Epifauna versus Dead Epifauna): Shell condition in infauna drastically changed after death. Shells became fragmented, disarticulated, and extensively corroded. Bioerosion on both external and internal surfaces was observed. This indicates that in the muds and muddy sands of Argyle Lagoon, corrasion and bioerosion are significant, and probably are enhanced by bioturbation by infaunal polychaetes and other soft-bodied marine taxa.

Although the dead infauna were obtained from more than 10-cm depth in the sediment, it is clear that they remained in the taphonomically active zone (Davies et al., 1989) and possibly some fraction of the shells have spent some time at the sediment-water interface. Exposure of infaunal shells at the sediment surface would only decrease the differences between dead infauna and epifauna reported here (see below). Epifaunal shells suffered additional modifications after death, but the overall taphonomic pattern is not as clear as for the infauna. Disarticulation, fragmentation, internal corrasion, internal bioerosion, and internal encrustation increased significantly after death. Internal surfaces clearly deteriorate after death, but the external surface condition can be worse, unchanged, or even appear to improve. Live epifauna are continuously exposed and accumulate shell modifications during their lifetime. After death, a portion of the epifaunal population can stay exposed, while part can be recycled many times, resulting in death assemblages with a complex taphonomic pattern. Effect of Life Habits on Shell Condition of Dead Shells (Dead Epifauna versus Dead Infauna): Significant differences in shell condition were found between dead infauna and dead epifauna. Differences were observed in both the external and the internal surfaces, with the epifauna more damaged than the infauna. Again the infaunal bivalves seem to have a higher potential for preservation. Dead infauna came from the quiet lagoon bottom and are inter-

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FIGURE 7—Comparisons of shell alterations of Protothaca staminea dead specimens by mode of life. Each bar is a percentage based on 165 specimens. Each pair of bars was tested for significant heterogeneity using Chi-square statistics; p values are shown for significant differences (*) between shell condition of infauna (I) and epifauna (E).

preted as autochthonous, but possibly were reworked in situ. Transport of shells from and out of the lagoon is extremely unlikely; offshore waves cannot propagate up the creek (it curves) and the fetch of the creek is too short to allow wind-driven currents to carry shells. The better preservation of dead infauna suggests that taphonomic agents are more effective at the sediment-water interface than within the sediment, at least in Argyle Creek or Lagoon or both. However, the effect of burial on preservation may be complex depending on the sediment chemistry (Callender et al., 2002). Best and Kidwell (2000) also found that epifaunal bivalves exhibit significantly higher postmortem damage than infaunal bivalves, but the present results remove the potentially confounding effects of other intrinsic factors, such as shell microstructure. PALEOBIOLOGICAL IMPLICATIONS These results have two main paleobiological implications. Firstly, mode of life has a clear effect on the taphonomy of bivalves. The different taphonomic patterns recorded between infauna and epifauna are due to different modes of life. Life habits influenced the shell condition of both live specimens and dead shells and fragments, al-

though exposure of infaunal shells at the sediment surface diminished the differences in the taphonomy of infauna and epifauna. Epifauna are more highly damaged than infauna, suggesting a lower fossilization potential for epifaunal bivalves than infaunal bivalves. Mode of life clearly influences the taphonomic signature of shells and should be taken into account when evaluating the taphonomy of a fossil bivalve assemblage. Taphonomic data should be dissected by mode of life. Within the same assemblage, two bivalve species can acquire different taphonomic patterns simply because they have different modes of life. Because differences in taphonomic features do not necessarily indicate different postmortem histories of shells, paleoenvironmental parameters inferred from comparative taphonomic analyses could be biased. Additionally, some attributes commonly regarded as postmortem—namely external corrasion, edge modification, external bioerosion, and internal bioerosion—can be acquired during life, especially in epifauna. This demonstrates that even internal damage cannot be regarded unambiguously as postmortem. Secondly, this paper shows a natural situation where a bivalve can acquire more than one mode of life, namely infaunal in soft sediments, where it can burrow, and epifaunal on hard substrates, where burrowing is not possible.


This situation is not unique. In fact, in the Argyle Creek and Argyle Lagoon system, a second burrowing bivalve, Venerupis philippinarum Adams and Reeves (Order Veneroida, Family Veneridae), occurs both infaunally (in the lagoonal soft substrates) and epifaunally (over gravels in Argyle Creek). There is another example in the North Sea, where a population of the common cockle Ceratodesma edule Linnaeus (Order Veneroida, Family Cardiidae) is prevented from burrowing into the sediment by the coherent byssal mesh of aggregates of Mytilus edulis Gray (Wehrmann, 2003). This is in contrast with the simplistic view of many paleoecologists and taphonomists, who typically classify a given bivalve species to a single mode of life. Functional-morphologic studies of extinct bivalve species can lead to incomplete interpretations of modes of life. Shell morphology, comparisons with Recent close relatives, and field observations (e.g., life position and lithofacies bearing bivalves) may be a more effective approach to the reconstruction of the mode of life. CONCLUSIONS (1) Modes of life of Protothaca staminea significantly affected the taphonomic signature of live and dead shells. Epifaunal shells are more damaged than infaunal shells. (2) Infaunal bivalves apparently have a higher fossilization potential and possibly more time-averaging than epifaunal bivalves. (3) Taphonomic data should be dissected by modes of life, at least in preliminary analyses, to see of there are differences between epifaunal and infaunal bivalves. (4) Internal shell damage cannot be regarded as postmortem by default. Edge modification and internal bioerosion can be produced during an animal’s lifetime. (5) A single bivalve species can have more than one mode of life even within the same environment. The typical mode of life is reflected in shell morphology. The secondary one is not reflected in shell morphology. An integrated approach among functional morphology, comparisons with close relatives, and lithofacies analysis can be useful in paleoecological interpretations of extinct bivalve species. ACKNOWLEDGEMENTS The author thanks Michael LaBarbera and Michal Kowalewski for their assistance and encouragement during the Taphonomy course 2002 at Friday Harbor Laboratories (University of Washington), and for careful revision of the various versions of the manuscript. Thomas Rothfus and classmates of the course are thanked for their help. Susan Kidwell recommended me for an application to the Taphonomy course. This work is based on an original idea of Michal Kowalewski, for which the author is grateful. Michael LaBarbera helped with statistical analysis of morphometric data and had infinite patience during the correction process. This paper benefited from the critical

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comments and suggestions of Karla Parsons-Hubbard, Thomas Olszewski, Eric Powell, and an anonymous reviewer. Friday Harbor Laboratories and Marı´a Beatriz Aguirre-Urreta (University of Buenos Aires) generously provided financial aid to travel and stay in the USA. REFERENCES BEST, M.M.R., and KIDWELL, S.M., 2000, Bivalve taphonomy in tropical mixed siliciclastic-carbonate settings; II, Effect of bivalve life habits and shell types: Paleobiology, v. 26, p. 103–115. BRETT, C.E., and BAIRD, G.C., 1986, Comparative taphonomy: a key to paleoenvironmental interpretation based on fossil preservation: PALAIOS, v. 1, p. 207–227. CALLENDER, W.J., STAFF, G.M., PARSONS-HUBBARD, K.M., POWELL, E.N., ROWE, G.T., WALKER, S.E., BRETT, C.E., RAYMOND, A., CARLSON, D.D., WHITE, S., and HEISE, E.A., 2002, Taphonomic trends along a forereef slope: Lee Stocking Island, Bahamas. I. Location and water depth: PALAIOS, v. 17, p. 50–65. COAN, E.V., SCOTT, P.V., and BERNARD, F.R., 2000, Bivalve seashells of Western North America. Marine Bivalve Mollusks from Arctic Alaska to Baja California: Santa Barbara Museum of Natural History Monographs Number 2, Studies in Biodiversity Number 2, 744 p. DAVIES, J.D., POWELL, E.N., and STANTON, R.J., JR., 1989, Relative rates of shell dissolution and net sediment accumulation—a commentary: can shell beds form by the gradual accumulation of biogenic debris on the sea floor?: Lethaia, v. 22, p. 207–212. HIKIDA, Y., 1996, Shell structure and its differentiation in the Veneridae (Bivalvia): The Journal of the Geological Society of Japan, v. 102, p. 847–865. (In Japanese with abstract and figure captions in English) KIDWELL, S.M., and BOSENCE, D.W., 1991, Taphonomy and time-averaging of marine shelly faunas: in Allison, P.A., and Briggs, D.E.G., eds., Taphonomy: Releasing the Data Locked in the Fossil Record: Topics in Geobiology 9, Plenum Press, London, p. 115– 209. SAS INSTITUTE, 1989, SAS/STAT User’s guide, Version 6, 4th ed.: SAS Institute, Cary, North Carolina, v. 1, 943 p., v. 2, 846 p. SAS INSTITUTE, INC., 2001, SAS Version 8.12: SAS Institute, Cary, North Carolina. SHINN, G., 1976, Ostracoda of Argyle Lagoon and North Bay, San Juan Island, Washington: Unpublished manuscript, Friday Harbor Class Papers, Marine Algology and Marine Zoology, Botany 445 and Zoology 430, Friday Harbor Laboratories., 5 p. SOTO-BUSSARD, S., 1963, Distribution and abundance of natural populations of barnacles in Argyle Creek: Unpublished manuscript, Friday Harbor Class Papers, Advanced Invertebrate Zoology, Zoology 533b, Friday Harbor Laboratories, 11 p. TAYLOR, J.D., KENNEDY, W.J., and HALL, A., 1973, The shell structure and mineralogy of the Bivalvia II. Lucinacea-Clavagellacea. Conclusions: Bulletin of the British Museum of Natural History, Zoology, v. 22, p. 255–294. TIERNEY, C., 1978, A study of the community on large rocks in Argyle stream: Unpublished manuscript, Friday Harbor Class Papers, Marine Algology and Marine Zoology, Zoology 430 and Botany 445, Friday Harbor Laboratories, 16 p. WEHRMANN, A. 2003, Biogenic and taphonomic processes affecting the development of shell assemblages: an actuopaleontological case study from mussel banks on North Sea tidal flats: Facies, v. 49, p. 19–30. ZAR, J.H., 1998, Biostatistical Analysis, 4th Ed: Prentice-Hall, Upper Saddle River, 929 pp.

ACCEPTED MAY 8, 2004