Community replacement of a Pleistocene CrepiduZu biostrome WILLIAM MILLER. 111 AND J. R. DuBAR Miller, William, I11 & DuBar, J. R. 1988 01 15: Community replacement of a Pleistocene Crepidula biostrome. Lethaia, Vol. 21, pp. 67-78. Oslo. ISSN 0024-1164. A sequence of fossil associations preserved in early Pleistocene mudstone, exposed on the Neuse River, eastern North Carolina, illustrates community replacement by species turnover, in this case involving addition of new species with rank demotions of formerly important community members. Here, a Crepidula biostrome is replaced by more diverse fossil associations dominated by oysters and infaunal bivalves. Biostrome deposits represent an extensive Crepidula snail bank that occupied a subtropical bay environment with low turbidity, near-normal marine salinity, and temporally stable water currents and food supplies. Because of local shoaling, extensive substrate coverage by these sessile epifaunal gastropods was gradually disrupted. A patchier subtidal environment, with more vaned habitats for benthic organ- isms, succeeded the snail bank, thus giving rise to species-rich Ostrea clump and Anadara-Noetia associations containing only a few remaining Crepidula aggregations. 0 Community replacement, species turnover, addition with rank demotions. autochthonous fossils, subtropical marine bay. William Miller, Ill, Department of Geology, Humboldt State University,Arcata, California 95521, U. S.A.; Jules R. DuBar, University of Texas, Bureau of Economic Geology, Austin, Texas 78712-7508, U.S.A.; 20th February, 1987. The analysis of benthic paleocommunities has been frequently repeated in marine paleoecology without deriving new insights into community structure and development. An example of this is the interpretation of local faunal transitions observed in vertical sequences of autochthonous fossils. Where obvious signs of environmentally forced change cannot be seen, or where thin stratigraphic intervals are studied, many paleo- ecologists have invoked short-term autogenic processes to explain such transitions. Temporal scope of most paleontologic sample sets and mix- ing of microstratigraphic levels by the activities of burrowing organisms, however, make the reso- lution of many patterns compiled in ecologic time problematic (Fiirsich 1978; Schindel 1980). It seems unlikely that patterns resulting from eco- logic succession and allogeniccommunity response (see Miller 1986a: Table 1) will be preserved in soft bottom marine environments except under special circumstances [e.g. during pulses of accel- erated sedimentation (Parsons et al. 1986); on localized hard substrata (Bailey & Tedesco 1986; Kauffman 1986)l. On the other hand, where sedi- mentologic and geochemical signals are easy to detect, the reason for long-term faunal transition is perceived as obvious: local benthos either con- formed to environmental changes in some way or were supplanted by organisms that could exploit new conditions. Such clear cut examples of environmentally forced transition are often regarded as sufficiently self-evident patterns, of corroborative use in facies interpretations but not a potential source of new ideas concerning long- term development of benthic communities. After all, if it isn't evolution and it can't be used to vindicate a popular ecologic notion of community organization, of what use is it? The two approaches - clumsy transposition of ecologic theory and benign neglect - are gradually giving way to carefully scaled studies that interpret more effectively the patterns of local faunal change (e.g. Wilson 1985; Bailey & Tedesco 1986; Miller & Metelman Alvis 1986). We describe an example of such a faunal tran- sition as an illustration of community replace- ment, the abrupt to gradual substitution of one species assembly for another owing to habitat alteration (Miller 1986a). The transition involves a remarkable exposure of a Crepidula biostrome conformably overlain by a shell bed that is much richer in species, especially infaunal bivalves. The fossils occur in a 3-metre-high mudstone bank within the early Pleistocene James City Forma-
Transcript
Community replacement of a Pleistocene CrepiduZu biostrome WILLIAM MILLER. 111 AND J . R. DuBAR
Miller, William, I11 & DuBar, J. R. 1988 01 15: Community replacement of a Pleistocene Crepidula biostrome. Lethaia, Vol. 21, pp. 67-78. Oslo. ISSN 0024-1164.
A sequence of fossil associations preserved in early Pleistocene mudstone, exposed on the Neuse River, eastern North Carolina, illustrates community replacement by species turnover, in this case involving addition of new species with rank demotions of formerly important community members. Here, a Crepidula biostrome is replaced by more diverse fossil associations dominated by oysters and infaunal bivalves. Biostrome deposits represent an extensive Crepidula snail bank that occupied a subtropical bay environment with low turbidity, near-normal marine salinity, and temporally stable water currents and food supplies. Because of local shoaling, extensive substrate coverage by these sessile epifaunal gastropods was gradually disrupted. A patchier subtidal environment, with more vaned habitats for benthic organ- isms, succeeded the snail bank, thus giving rise to species-rich Ostrea clump and Anadara-Noetia associations containing only a few remaining Crepidula aggregations. 0 Community replacement, species turnover, addition with rank demotions. autochthonous fossils, subtropical marine bay.
William Miller, I l l , Department of Geology, Humboldt State University, Arcata, California 95521, U. S.A.; Jules R. DuBar, University of Texas, Bureau of Economic Geology, Austin, Texas 78712-7508, U.S.A.; 20th February, 1987.
The analysis of benthic paleocommunities has been frequently repeated in marine paleoecology without deriving new insights into community structure and development. An example of this is the interpretation of local faunal transitions observed in vertical sequences of autochthonous fossils. Where obvious signs of environmentally forced change cannot be seen, or where thin stratigraphic intervals are studied, many paleo- ecologists have invoked short-term autogenic processes to explain such transitions. Temporal scope of most paleontologic sample sets and mix- ing of microstratigraphic levels by the activities of burrowing organisms, however, make the reso- lution of many patterns compiled in ecologic time problematic (Fiirsich 1978; Schindel 1980). It seems unlikely that patterns resulting from eco- logic succession and allogeniccommunity response (see Miller 1986a: Table 1) will be preserved in soft bottom marine environments except under special circumstances [e.g. during pulses of accel- erated sedimentation (Parsons et al. 1986); on localized hard substrata (Bailey & Tedesco 1986; Kauffman 1986)l. On the other hand, where sedi- mentologic and geochemical signals are easy to detect, the reason for long-term faunal transition is perceived as obvious: local benthos either con-
formed to environmental changes in some way or were supplanted by organisms that could exploit new conditions. Such clear cut examples of environmentally forced transition are often regarded as sufficiently self-evident patterns, of corroborative use in facies interpretations but not a potential source of new ideas concerning long- term development of benthic communities. After all, if it isn't evolution and it can't be used to vindicate a popular ecologic notion of community organization, of what use is it? The two approaches - clumsy transposition of ecologic theory and benign neglect - are gradually giving way to carefully scaled studies that interpret more effectively the patterns of local faunal change (e.g. Wilson 1985; Bailey & Tedesco 1986; Miller & Metelman Alvis 1986).
We describe an example of such a faunal tran- sition as an illustration of community replace- ment, the abrupt to gradual substitution of one species assembly for another owing to habitat alteration (Miller 1986a). The transition involves a remarkable exposure of a Crepidula biostrome conformably overlain by a shell bed that is much richer in species, especially infaunal bivalves. The fossils occur in a 3-metre-high mudstone bank within the early Pleistocene James City Forma-
68 William Miller, III and J . R. DuBar LETHAIA 21 (1988)
Hoffman & Narkiewicz (1977), M. E. Johnson (1977), and Rollins et al. (1979).
Miller (1986a, b) suggested that community replacement proceeds in one of two general ways, with reference to a particular area of seafloor: (1) by species-abundance reorganization, or the reshuffling of numerical and biomass rank order of organisms during slight, gradual alterations of environment; and (2) by species turnover, when environmental tolerance limits of organisms are overstepped resulting in local extinctions and invasions of species new to the area. It is thought that the pathways operate together to produce sequences of slightly to sometimes sharply dif- ferent fossil associations in suitable geologic sec- tions, with one or the other predominating.
Fig. 1. Location of study site. Dashed line encloses area of Crepidula biostrome.
tion, on the Neuse River in eastern North Caro- lina (Fig. l). Decline of the biostrome and replacement by a bivalve-rich community was caused by subtle environmental changes over many generations of benthic organisms, involved two different communities, and is therefore attributable to community replacement, instead of succession or environmentally driven suc- cession-like processes. The James City sequence is described to illustrate one of several possible replacement patterns that we think are common in nature.
Theoretical background Sequences of benthic fossil associations are sen- sitive records of environmental changes when pre- served in depositional settings in which hydraulic transport, condensation and dissolution of skele- tons are insignificant. Important environmental qualities affecting the distribution of benthic organisms include salinity, turbidity, oxygen ten- sion, turbulence level, moisture, temperature, and physical and chemical properties of substrata. Gradual to rapid change in one or more of these, or in their short-term temporal variation, may result in community replacement. Community replacement sequences may reflect local changes in bottom conditions in a limited part of a basin, or the basinwide (shelfwide) changes accompanying major episodes of climate or seascape disruptibn. The importance and distinctiveness of replace- ment in the fossil record were first pointed out by
Species-abundance reorganization. - Reorgan- ization of relative abundance in gradually chang- ing environments involves processes such as: (1) oscillation in number or biomass of dominant species; (2) exchanges in species order (rank) of subdominant taxa; (3) gradual demotions of once important taxa to lower rank; (4) gradual pro- motions; and (5) occasional rapid promotions or demotions of rare species. Also, some rare taxa may be added or deleted without attrition of the more abundant organisms. Population-level mechanisms involved might include reactions to changes in seasonality, trophic resources, pres- sure from competitors or predators, recruitment dynamics, and in the importance of obligatory ecologic associations (e.g. mutualism, parasitism) over many generations. This pathway has been described with examples in previous papers (Miller 1986a, c; Miller & Metelman Alvis 1986).
Species turnover. - The second pathway of replacement is by species turnover, in which the environmental tolerance limits of organisms are closely approached or exceeded, and behavioral adjustments and replenishment by propagules are unsuccessful in maintaining populations on a site. This type of replacement has not been previously described in detail. We propose four different models of transition by species turnover (Fig. 2): (1) catastrophic turnover when a resident com- munity is quickly eliminated and replaced by a wholly different assembly of species (e.g. rapid abandonment of a freshwater stream and sub- aerial, exposure of its channel floor, followed by invasion of terrestrial plants and animals from the surrounding floodplain and uplands); (2) simple
LETHAlA 21 (1988) Community replacement 69
A
C
B
D
- L o n g - term relative abundance-
Fig. 2. Four models of community replacement by species turnover. 0 A. Catastrophic turnover of species during rapid environmental change. O B . En echelon turnover caused by gradual environmental change or varied species-specific reac- tions to change. 0 C. Deletion without replacement leaving only organisms tolerant of new environment. 0 D. Addition of species with demotion in rank of previously important com- munity components. Bars with diagonal stripes represent species of previous communities; chop-marks indicate replace- ment species. Bar widths represent idealized long-term abun- dance. Symbols: pe- previous environment, ed- environmental discontinuity, tr - gradual environmental transition, se - sub- sequent environment.
attrition in which species relinquish their hold on an area in an en echelon pattern through time as populations react differentially to changing environmental factors (e.g. gradual substrate alteration from sand to mud in coastal bays; R. G. Johnson 1972:15&158); (3) deletion without replacement in which many community members disappear without substitution by species new to the area, leaving only eurytopic or physiologically resilient organisms to cope with the new environ- ment (e.g. aggradation of a subtidal sandbar into intertidal zone of a coastal estuary, leaving only a few species of deep-burrowing bivalves and polychaete worms as resident macrobenthos); and
(4) addition with rank demotions occurring either when environmental changes cause invasions of outsiders readjusting their geographic ranges or when ecotones between adjacent communities collapse, enriching the revamped community in species but leading to reduction in abundance of formerly important taxa (e.g. Pleistocene community reassortments following climatically forced geographic displacements of terrestrial plants and vertebrates).
How do populations 'experience' community replacement? Usually communities do not react to environmental alteration as units, but become disorganized and are eventually replaced owing to differing population-specific responses to change (Davis 1986). Only in catastrophic turnover are tolerance limits of all species simultaneously sur- passed. Variations among species in environ- mental tracking patterns and reaction lag times cause the en echelon attrition, deletion without replacement, or addition with demotion patterns resolvable at the community level (Fig. 2).
Recent long-term field experiments involving artificial acidification of a small Canadian lake (Schindler et al. 1985) allow conceptualization of the ways resident populations experience habitat alterations that drive replacement. Depending on rate and intensity of environmental changes, these reactions would include:
(1) Catastrophic environmental alteration, causing immediate reactions such as widespread physical trauma to individual organisms, chemical toxicity, rapid physiologic degeneration and death to all age classes, evacuation by large motile organisms, and biologically mediated extermination (e.g. competitive displacement or eradication through over-predation in a single prey generation as a result of newly introduced organisms).
( 2 ) Environmental changes over a few generations, leading to reduced condition of individuals and decline of populations by repeatedlsustained physical insult or change in chemical environ- ment, recruitment failure over the short-term (because of death of locally produced propagules, lack of reproduction, or colonization failure by foreign propagules), abrupt decline in trophic resources, interruption of biogeochemical cycles, rapid competitive exclusion, elimination by a newly-introduced predator, emigration of motile species and synergistic effects (e.g. fatal epidemic disease following reduction in condition of indi-
70 William Miller, III and J . R. DuBar LETHAIA 21 (1988)
viduals within a local population, with no sub- sequent successful recruitment).
( 3 ) Long-term environmental degradation, caus- ing reduction in success of propagules from both proximal and distal sources, gradual curtailment of trophic resources, alteration or elimination of biogeochemical cycles over long time spans, grad- ual exclusion by asymmetric competition, intro- ductions and gradual expansion of new predator
species, shifts in population centers and syn- ergistic phenomena involving gradual decline of populations followed by extirpation in isolated surviving patches. Owing to temporal conden- sation, we expect that most autochthonous fossil transitions would reflect conformational changes occurring over many generations, and that rapid reactions would be preserved in local sections only under special biologic and sedimentologic conditions (Fursich 1978; Staff et al. 1986).
Fig. 3. Exposure at Johnson Point. 0 A. Mudstone bank containing biostrome (vertical part of ruler, 0.5 rn long). 0 B. Crepidula fornicara stacks within biostrome, rotated but with shells still attached (pencil, 16cm long).
LETHAIA 21 (1988) Community replacement 71
Community replacement in the James City Formation DuBar & Solliday (1963) recognized two for- mations in river-bluff exposures within our study area: (1) an upper, lithologically varied unit of Pleistocene age they termed ‘Flamer Beach For- mation’, and (2) a lower fine-grained fossiliferous unit named the ‘James City Formation’, which they regarded as possibly Pliocene in age. They established the James City type section 3 km upstream (northwest) from Johnson Point in a river-bank outcrop near the village of Grantham (Fig. 1). More recently DuBar et al. (1974) deter- mined that James City deposits actually are early Pleistocene, based on mollusks and forami- niferids, and proposed correlations with the Wac- camaw Formation in southeastern North Carolina-northeastern South Carolina and the Caloosahatchee Formation in Florida. Support for this age revision has come from regional bio- stratigraphic and geochronometric studies (Blackwelder 1981a; McCartan et al. 1982). Using paleontologic and paleomagnetic evidence, Cronin et al. (1984) now question exact time equivalence with the Waccamaw and estimate the age of James City deposits as 1.3 to 0.7Ma. Blackwelder (1981b: Fig. 2) attributed this depo- sitional cycle in the Atlantic Coastal Plain to a high-stand of sea level (to +25 m MSL) associated with an early Pleistocene warm interval in the Antarctic bracketed by pulses of glaciation recog- nized in South America.
Fig. 4. Stratigraphic section at Johnson Point showing levels where samples were collected. Sample numbers same as in Table 1. Community replacement sequence summarized in column at right. 0 A. Crepidula biostrome. 0 B. Osrrea clump. 0 C. Anadara-Noetia soft bottom association.
Methods The Crepidula biostrome was first described by DuBar et al. (1974:lll). It extends for approxi- mately 150m in a steep-sided mudstone bank between the end of County Road 1164 and the tip of Johnson Point, on the south shore of the Neuse River (Figs. 1, 3). We sampled the bio- strome and overlying bivalve-rich shell bed by surface picking, and by collecting bulk samples of each type of bedding unit at one site where material was newly exposed and unweathered (Fig. 4). Each one-liter sample was washed on a sieve with 2 mm openings; fossils separated from matrix were identified to species if possible, coun- ted, examined for epi- and endobionts, and the dominant species were measured to estimate their size-frequency. More than 17,400 specimens belonging to approximately 80 species, mostly of small mollusks, were recovered in this way (a complete inventory is given in Miller in press). Specimen counts were corrected to reflect original numbers of skeletonized organisms (e.g. 1 gas- tropod apical fragment = 1 individual, 1 bivalve beak fragment = 4 individual, 1 balanid side plate = Q individual, etc.). Species diversity of noncolonial organisms was calculated by sub- stitution in the Shannon-Weaver equation, H‘ = - ZPi (In P i ) , where Pi is proportion of the ith species and In is the natural logarithm. Substrate-niche and feeding categories were determined for extant taxa using the literature on modern benthic organisms from the Western
72 William Miller, I l l and J . R. DuBar LETHAIA 21 (1988)
Table I . Most important taxonomic components in Johnson Point fossil associations. Only species making up 20.5% of individuals in an association are listed. Rank based on biovolume estimated using numbers and sizes of articulated/complete (or restored) shells. Symbols have the following meanings: Organism Type - G , gastropod; B, bivalve; C, cirriped; M , malacostracan. Substrate- niche - EP, epifaunal; IN, infaunal; VAG, vagrant. Feeding behavior - SF, suspension feeder; DF, deposit feeder; C, carnivore; P, ectoparisite; S, scavenger; (NO), no data. Species represented mainly by juvenile specimens indicated with'. H' = Shannon- Weaver diversity index (see Methods). Sample numbers refer to Fig. 4.
Crepidula fornicata Boonea seminuda Bolanus spp. N U C U ~ ~ M acuta Ostrea sculpturata Anadara aequicostata Anachis lafresnayi var. A . obesa Nassarius albus H' = 1.66
Atlantic Ocean (Bird 1970; Stanley 1970; Andrews 1977; Gosner 1978); for extinct species, comparison with closest living cogeneric relative was used. Ecologic properties such as the diversity index were used to assess changes in fossil associ- ation composition, not to estimate their exact values in the original living communities.
Fossil associations Fossil associations are defined here as the skele- tal remains derived from a single original community. We regard communities as func- tionally related aggregations of organisms that share similar environmental requirements and/or belong to obligatory ecologic associations (e.g. parasite-host interaction). Three different fossil associations are represented in the gohnson Point exposure: (1) a Crepidula biostrome assokiation; (2) an Ostrea clump association; and (3) an Anad
dara-Noetia soft bottom association (Fig. 4; Table 1). These were recognized by noting changes in taxonomic composition, numerical and estimated biovolume rank, species diversity ( H ' ) , and descriptive trophic structure.
CREPIDULA biostrome. - The lower half of the river-bank exposure consists of three to six mudstone/shelly layer cycles. Shelly intervals are made largely of the slipper snail, Crepidula for- nicata, a sessile epifaunal suspension-feeding snail. These snails are protandrous hermaphro- dites, occumng in stacks with the larger, older female (or sexually transitional) individuals living beneath smaller, younger males (Fig. 3B; see Fretter & Graham 1962). Found with Crepidula are large numbers of its ecologic associates: Boonea seminuda (minute ectoparasitic snail; Robertson 1978), balanid barnacles (Zullo & Miller 1986) and varied cheilostome bryozoans.
74 William Miller, III and J. R. DuBar
Sheily layers within the biostrome average 7.4 cm thick [range (r) = 4-12cm; number of layers examined (n) = 201. The layers could be traced only a few metres laterally before merging with another shelly layer, splitting in two, or blending into surrounding mudstone. Both the shelly layer and mudstone interbed that we sampled con- tained the same fossils (Table 1). Species richness was slightly lower in the shelly layer (44 species) compared to the mudstone interbed (50). Non- colonial species diversity, however, was not sig- nificantly different between the two biostrome samples (t-test with t, = 1.13, p = 0.13). The Crepidula-rich layers all grade laterally into mud- stone to claystone to the NW and SE. A large rectangular platform of land west of the exposure may correspond to the main body of the biostrome. If an average thickness of l m is assumed and the Crepidula-rich layers extend at least 100 m landward of the river bank, the bio- strome has a volume of roughly 15,000m3. At maximum development it may have covered as much as 10.000 m2 of seafloor.
OSTREA clumps. - The Crepidula layers are suc- ceeded in <50 cm of vertical section by a different fossil association containing clumps of the extinct marine oyster, Ostrea sculpturata. Oysters were in living positions or were slightly disturbed with valves still articulated. In some places in the exposure the Ostrea clump association is absent and the biostrome is succeeded directly by the Anadara-Noetia association. Crepidula fornicata, B. seminuda, and balanids continue to dominate in numbers, but 0. sculpturata and an extinct infaunal suspension-feeding bivalve, Anadara aequicostata, become dominant in terms of bio- volume with many large individuals (Table 1). Crepidula-rich layers disappear abruptly at this level, and both species richness (57 species) and diversity (H' = 2.15) increase. This pattern reflects decline of the biostrome and introduction of a more varied community containing abundant oysters, infaunal bivalves, and carnivorous/scav- enging gastropods.
ANADARA-NOETIA association. - The most spe- cies-rich (61 species) and diverse (H' = 2.66) sample was collected in the upper part of the exposure (Fig. 4; Table 1). Species diversity of noncolonial organisms was significantly higher compared to the biostrome shelly layer sample (t- test with t, = 29, p = 0). In the long species list
LETHAIA 21 (1988)
from the Anadara-Noetia association C. fornicata again was numerically dominant, but occurred as smaller shells in rare, isolated stacks. Proportion of noncolonial epifaunal suspension-feeders in general slipped from 58.5% in the biostrome to 38.9% in this association (Table 1). Bivalves usually were preserved as disturbed shells with valves articulated. Fossils were concentrated in shelly layers averaging 7.4 cm thick (r = 4-18 cm; n = 12) separated by mudstone to slightly coarser siltstone interbeds averaging 11.6 cm thick ( r = 7-18cm; n = 7). This pattern of shelly layer/ mudstone couplets was interrupted near the top of the bank by lenses of disarticulated bivalves.
Population patterns Changes in size-frequency distributions of the two key primary consumers in the replacement sequence, Crepidula fornicata and Anadara aequi- costata, were compared to assess faunal transition at the population level (Figs. 5,6). Large numbers of C. fornicata that grew to large shell sizes (4.5 cm in length) characterize population struc- ture in the biostrome, whereas fewer, smaller individuals were typical of the Anadara-Noetia association. The opposite pattern is shown by A. aequicostata: only a few individuals occurred in the biostrome, but the species was abundant and shells were much larger (4.4cm in length) in
290
250
200
150
loo
w
C
I 1501 Crepidula
n = 460 50
2 8 14 20 26
length (mm)
n 4311 A
2 8 14 20 26 32 38 44
B
n
Fig. 5. Size-frequency distribution of Crepidulufornicuru. 0 A. Sample from shelly layer in Crepidufu biostrome (JP-I). 0 B. Sample from Anudaru-Noetiu association (JP-4).
LETHAIA 21 (1988) Community replacement 75
180-
Anadara
2 8 14 20 26 32 38 length ( m m )
501
n=134 10
2 8 14 28 32 38
Fig. 6. Size-frequency distribution of Anadara aequicostata. 0 A. Sample from shelly layer in Crepidula biostrome (JP-1). 0 B. Sample from Anadara-Noetia association (JP-4).
the Anadara-Noetia association. One similarity between both sets of size-frequency distributions is the high rates of mortality indicated for ju- veniles and older adults, with relatively lower rates for intermediate growth stages (see Richards & Bambach 1975). Another similarity is that when each species was a dominant, it clearly paid for the privilege with high juvenile mortality. Thus, replacement is reflected in long-term population patterns as well as in the whole-faunal survey (for an example of intraspecific population changes in a replacement sequence see Miller 1986c: Figs. 8, 9). Changes in preserved population structure mirror the disappearance of the biostrome and introduction of a bivalve-dominated community.
Interpretation
Fossil associations were derived from a Crepidula snail bank that was replaced by a more diverse soft bottom community consisting of many different infaunal bivalves, predatory and scavenging snails, and small clumps of oysters (Fig. 7). Both communities inhabited a muddy-bottom sub- tropical marine bay (DuBar et al. 1974; Black- welder 1981b). We envision the snail bank to have been a broad, low mound or blanket made of millions of C. fornicata shell stacks, with many open patches (represented by sparsely fossili- ferous mudstone interbeds) of thet kind produced by localized physical disturbances (Sou$a 1485; Connell & Keough 1985). Little is known con-
cerning physical and biological conditions leading to the development of Crepidula banks of this size. A long interval with strong water currents conveying food, low to moderate turbidity, near- normal marine salinity, and a disturbance regime consisting of minor, possibly regular episodes of patch formation were probably important factors in development and persistence of the bank (see J. K. Johnson 1972; Hoagland 1979; Loomis & Van Nieuwenhuyze 1985). Similar extensive build-ups of C. fornicata stacks are found today in the bays of Long Island, off the coast of =ode Island, and at Martha’s Vineyard, all in the north- eastern United States (E. Hoagland in litt.).
The same sustained, directional changes in seafloor environment that caused decline of the snail bank also were conducive to establishment of the subsequent, more diverse community. Ostrea clump and Anadara-Noetia associations represent a broader array of habitats for benthic animals, and suggest greater environmental (and temporal?) heterogeneity. Changing factors alter- ing snail bank habitats probably included local shallowing of the bay (indicated by slight coar- sening of muddy matrix moving upsection and by lenses of disarticulated bivalves at top of ex- posure) leading to altered current patterns and episodes of increased turbulence and turbidity from waves (limiting extensive substrate coverage by Crepidula). Changes in aerial extent, intensity and frequency of physical disturbance could have yielded a patchier habitat with more varied oppor- tunities for benthos. This is comparable to the maintenance of high diversity and species richness via intermediate levels of disturbance in modern forests, coral reefs and intertidal boulder fields (Connell 1978; Denslow 1985; Sousa 1985). Reduction in dominance of one or a few species is known to free resources for less competitive species, while increasing environmental het- erogeneity creates habitats for more varied kinds of organisms. Considering transition of the bio- strome to more diverse associations takes place within 50 cm of continuous section and that depo- sitional setting was a marine embayment, decline and disappearance of the snail bank probably took place in the order of 10 to lO*yr (Schindel 1980; Sadler 1981).
Conclusions Although the exact forcing factors that caused community replacement in the James City For-
76 William Miller, III and J . R. DuBar LETHAlA 21 (1988)
Fig. 7 . Community reconstructions. 0 A. Crepidula snail bank: 1 - Crepidula fornicata, 2 - Balanus spp., 3 - Boonea seminuda, 4 - Anadara aequicostata, 5 - Nuculana acuta, 6 - Ostrea sculpturata.; 0 B. Subsequent bivalve-rich community: 1 - Anadara aequicostata, 2 - Noetia limula, 3 - Mercenaria permagna, 4 - a t r e a sculpturata, 5 - Crepidda fornicata, 6 - Balanw SQQ., I - Boonea seminuda, 8 - Anachis spp., 9 - Abra aequalis, 10 - Nuculana acuta, 11 - brachyuran crab.
LETHAIA 21 (1988) Community replacement 77
mation have been resolved only roughly, we may yet describe the pathway of ecosystem transition. A subtle, long-term environmental change caused disruption of the snail bank and replacement by a more diverse community. Transition resulted from apparent proliferation of habitats and reduction in dominance by Crepidula. Although the snail bank fauna was reduced in importance, it was not entirely eliminated. At the same time, species richness increased by nearly 40% among the preservable components and previously uncommon Ostrea and Anadara rose to domin- ance. This is the pathway predicted by our model of addition with rank demotions shown in Fig. 2D. The James City transition resembles ‘community mixing’ described by Rollins & Donahue (1975:263), which was postulated for benthic com- munities undergoing reassortment during marine regressions.
Analysis of modern communities and inter- pretation of autochthonous fossil sequences like the one described here obviously are not the same. This is not only because of taphonomic distortion and time-averaging (Staff et al. 1986), but also because of the time scale of community processes that yield preservable patterns in the local fossil record (Miller 1986a). It is important to realize that what we see in many fossil tran- sitions is long-term change in the underlying environmental control of ecosystem composition and distribution. Ecologists are beginning to focus attention on these ‘ultimate’ or ‘regional’ environ- mental controls to understand relationships between modern-day diversity patterns and long- term trends in resource flux, spatial heterogen- eity, disturbance regimes, and climate cycles (Ricklefs 1987). By discovering generalizations about community replacement, paleoecologists will contribute substantially to understanding these linkages.
Acknowledgements. -We thank A. Cheetham for identifications of bryozoans, V. Zullo for identification of barnacles, and E. Hoagland for information on modern populations of Crepidula fornicata and comments on the manuscript. N. Walters sorted and measured shells; C. Armstrong typed drafts of the manu- script. Fieldwork by Miller was supported by Geological Society of America and Sigma Xi (Tulane University Chapter). Critical review by H. Rollins improved the quality of the final version of this paper.
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