Human Ecology of Shellfish Exploitation at a Prehistoric Fishing-Farming Village on the Pacific...

Journal of Island & Coastal Archaeology, 9:183–202, 2014Copyright © 2014 Taylor & Francis Group, LLCISSN: 1556-4894 print / 1556-1828 onlineDOI: 10.1080/15564894.2014.881935

Human Ecologyof Shellfish Exploitationat a PrehistoricFishing-Farming Villageon the Pacific Coastof MexicoCarley B. Smith,1 Claire E. Ebert,2 and Douglas J. Kennett21Department of Anthropology, University of Oregon, Eugene, Oregon, USA2Department of Anthropology, The Pennsylvania State University, University

Park, Pennsylvania, USA

ABSTRACT

Shellfish remains excavated from an early agricultural village on thePacific Coast of Mexico (Guerrero) indicate a dietary shift from lo-cally obtained estuarine shellfish (1400–1100 BC) to a greater diversityof mollusks collected from more distant marine environments (900–500 BC). The timing of this shift suggests that it occurred as humanpopulations increased and impacted the availability of local estuarineresources. We argue that this prompted the incorporation of a morediverse array of shellfish species harvested at greater distances or ob-tained via trade, possibly with the use of boats to transport shellfish andother resources back to the village.

Keywords behavioral ecology, formative period, historical ecology, maritime adaptations,Mesoamerica, mollusks, optimal foraging theory

INTRODUCTION

Ancient shell middens along the coasts ofthe world’s oceans indicate the importanceof shellfish exploitation for anatomically

Received 19 June 2013; accepted 10 December 2013.Address correspondence to Douglas J. Kennett, Department of Anthropology, The Pennsylvania StateUniversity, 409 Carpenter Building, University Park, PA 16802, USA. E-mail: [email protected] versions of one or more of the figures in the article can be found online athttp://www.tandfonline.com/uica.

modern humans as they expanded out ofAfrica over 100,000 years ago (Erlandson2001). Most shellfish are sessile, distributedin aggregated clumps, and available yearround. Little or no specialized technology is

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Carley B. Smith et al.

required to collect mollusks and people ofall ages (men and women, young and old)can collect them easily (Bird et al. 2002; Er-landson 1988; Glassow and Wilcoxon 1988;Rick and Erlandson 2009; Yesner 1980). Be-cause of these characteristics, shellfish are aconvenient and plentiful source of proteinand often a dietary staple in coastal envi-ronments. During the Holocene shellfish andothermarineresourcesprovidedastablesub-sistence base for growing populations andthe emergence of complex hunter-gatherersocieties in some regions (e.g., California,Kennett 2005; Rick et al. 2005; NorthwestCoast, Ames and Maschner 1999). People liv-ing in areas of high productivity like coastaland estuarine regions were often slowerto implement and commit to agriculture asthey had access to a wide variety of aquaticprey such as shellfish and fish (Piperno2006).

The characteristics that make shellfishan attractive economic resource also makethem particularly susceptible to predation(Erlandson and Rick 2008). The differen-tial impacts of human predation on shell-fish species depend on species’ habitats, ge-ographic distribution, available refugia, andlife history characteristics as well as hu-man preference for particular species andtechnologies used (Claassen 1986; Erland-son et al. 2011; Mannino and Thomas 2002;Thacker 2011). Heavy predation and long-term human selection of individuals cancausepopulationstructures tochangesothatthe average size of available individuals in anarea decreases over time along with alter-ations in species composition (Braje et al.2007; Erlandson et al. 2011; Mannino andThomas 2002; Milner et al. 2007; Rick andErlandson 2009). Because of the ease of pro-curement, clumped shellfish that are foundin shell-beds (like oysters and mussels) orthose found on the surface (gastropods, oys-ters, mussels) are more prone to populationdecline due to predation compared to shell-fish with more elusive behavioral attributes(e.g., burrowing taxa; de Boer et al. 2000).Larger bodied solitary species are also proneto decline in number. An increase in di-versity in species preyed upon, changes inage and size profiles, as well as shifts to

focusonalternative shellfishspecies throughtime can signify human-caused resource de-pression sometimes influenced by naturalprocesses (de Boer et al. 2000; Erlandsonand Rick 2008; Erlandson et al. 2004;Klein et al. 2004; Mannino and Thomas2002).

Here we explore the effects of humanpredationonshellfishpopulationswithin thevicinity of the early agricultural village of LaZanja, located in the Acapulco Bay region ofGuerrero, Mexico. Estuaries, sandy beaches,and rocky shorelines supported a remark-able diversity of molluskan species duringthe Middle and Late Holocene (Kennettet al. 2008). Shellfish populations have beenstrongly and negatively impacted during thelast century with the expansion of Acapulcoas a major urban center and tourist destina-tion. Our study provides an environmentalbaseline for comparison with the contem-porary distribution and diversity of shellfishtaxa in this region of Mexico. This workalso provides a glimpse at changing shellfishharvesting strategies as expanding agricul-tural populations impacted the more pristinecoastal environments of Mexico. Using opti-mality models from behavioral ecology (opti-mal foraging theory[OFT])wepredictedthatresource depression at this settlement loca-tion would lead to expanding diet breadthand the acquisition of resources from moredistant locations via long-range forays ortrade. One of the unintended consequencesof an increasing commitment to agricultureis population expansion and a host of asso-ciated environmental impacts (Hooke 2000)that effect the distribution and abundance ofnon-agricultural subsistence resources. Thedeposits at La Zanja span this critical intervalof early agricultural commitment and expan-sion. Changing shellfish harvesting strategieswithin this context that are generally consis-tent with OFT predictions.

BACKGROUND

La Zanja is located 2 km from the PacificCoast, south of Acapulco, at the confluenceof the Sabana River and the Tres Palos La-goon (see Figure 1). The site is located

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Human Ecology of Shellfish Exploitation in Mexico

Figure 1. Map of coastal Guerrero, Mexico showing the location of Las Zanja relative to other sitesin the region.

at the edge of the northeastern end of theestuary, on a narrow coastal plain (2.5–13 km wide) between the Sierra Madredel Sur and the Pacific Ocean. Stratified ar-chaeological deposits in this low earthenmound site span the Early (∼1400–1100 BC)and Middle Formative (∼900–500 BC) pe-riods of Mesoamerican prehistory (see be-low). The Formative Period (∼1800 BC–AD300) followed a trajectory established duringthe Archaic Period (∼8000–1800 BC; Ken-nett 2012) of increasing reliance upon do-mesticated plants complemented by manywild foods (Flannery 1973, 1986; Kennettand Voorhies 1996; Kennett et al. 2006;Piperno 2006; Smith 2001; Voorhies 1996).Greater commitment to agricultural produc-tion required more intensive use of land sur-rounding central places, which altered habi-tats and gradually changed the relationship

between human and local coastal andterrestrial environments (Doolittle 1990;Lentz 2000; Piperno 2006). New rela-tionships were characterized by popula-tion growth, greater sedentism, environ-mental and social circumscription, and theemergence of social hierarchies that ulti-mately led to the first state institutionsin Mesoamerica (Clark and Blake 1994; Ken-nett et al. 2006; Rosenswig 2009; Webster2002).

Early excavations in the Acapulco regionby Charles Brush between 1959 and 1960identified a number of sites along the Sa-bana River and around Acapulco Bay (Brush1969). Manzanilla Lopez et al. (1991; Man-zanilla Lopez 2000) have greatly expandedour working knowledge of the prehistory inthis region and reassessed the regional ce-ramic typology/chronology based on new

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excavations and Brush’s original collections.Puerto Marques is the earliest site in the re-gion, dating to the Late Archaic (∼3500 BC)and was occupied through the Classic Period(∼AD 800; Brush 1969; Kennett et al. 2008;Manzanilla Lopez 2000; Manzanilla Lopezet al. 1991). Puerto Marques is located di-rectly on the coast and the faunal assemblageof this site is dominated by species frommarine ecosystems in all periods. A deeplyburied shell mound at Puerto Marques wasdated to the Late Archaic (3500–2000 BC);this mound shows the importance of coastalresources to early foragers in this area (Brush1969; Kennett et al. 2008). La Zanja was es-tablished within a lagoonal/mangrove set-ting farther from the coast during the laterpart of the Early Formative (∼1400 BC). Theproximity of the two sites (∼5 km) and theearlier founding of Puerto Marques suggestthat La Zanja was originally settled by a groupof people from Puerto Marques.

Laguna Tres Palos and the bay at PuertoMarques were probably linked by a man-grove swamp during the early and middleHolocene but are now separated by a bar-rier beach (Kennett et al. 2004, 2008). Con-ditions in the modern lagoon change sea-sonally with the discharge of the La SabanaRiver and changing tidal influences as theinlet opens and closes to the Pacific Ocean(Kennett et al. 2004). The coastal lagoonsand rivers provided access by watercraftto many areas along the coastline and sea-sonal inundation of the lower reaches of thecoastal plain offered a form of natural irri-gation that could have facilitated agricultureduring the Formative Period. In this way LaZanja’s location allowed for strategic use ofa wide variety of resource patches with dif-ferent prey within a 5-km radius of the site.The coastal plain and piedmont areas sur-rounding the site provided access to terres-trial mammals and the adjacent marine habi-tat supplied fish, shellfish, and sea turtles.A mangrove estuary along which the siteis located offered additional local resourcessuch as birds, lagoon fish, gastropods, andbivalves that required little time and effortto acquire. Aquatic invertebrates were alsoavailable from the intertidal mudflat near thesite. A decrease in overall energetic returns

from wild fauna over time at La Zanja proba-bly coincided with a greater dependence ondomestic plant foods, which would be com-plemented by protein from shellfish. Agricul-ture and high populations would have had aperceptible impact on local resources thatmay have necessitated forager-fisher-farmersto travel farther into different habitats to pro-cure resources. Lagoon systems around thesite could have provided efficient travel tomore distant resource patches and the trans-port of these resources back to La Zanja viawatercraft.

THEORETICAL FRAMEWORK

Human behavioral ecology (HBE) providesa set of models that researchers can useto explore complex ecological interactionswithin changing social and natural environ-ments (Broughton and O’Connell 1999; Ka-plan and Hill 1992; Kennett 2005; Kennettet al. 2009; Winterhalder and Kennett 2006;Winterhalder et al. 2010; Winterhalder andSmith 2000). Within the broader HBE frame-work, OFT provides models to evaluate andexamine prehistoric subsistence strategies.Costs and benefits of different foraging be-haviors are evaluated given a set of environ-mental parameters and the models assumethat humans tend to make economically ra-tional decisions that optimize energetic re-turns while minimizing investment costs andrisk (Broughton and O’Connell 1999; Kaplanand Hill 1992; Smith 1983; Stephens andKrebs 1986). In OFT, benefits are estimatedby caloric energetic returns of different preyitems with costs including time spent inmanufacturing tools, search, pursuit, andprocessing.

We use the prey choice model andthe Marginal Value Theorem here to ex-plore shellfish harvesting strategies at LaZanja through the Early and Middle For-mative Periods and the expansion of dietbreadth through time. With long-term inten-sive use of a relatively homogenous local en-vironment, usually associated with increas-ing populations and targeting of high-rankedspecies, the prey choice model predictspredation pressure that results in declining

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abundance of higher ranked large species(Broughton and O’Connell 1999; Kaplan andHill 1992; Smith 1983). With resource de-pression, encounter rates with high-rankedfoods decrease until foraging return rates de-crease enough that foragers expand their di-ets to incorporate lower ranked prey itemsand the optimal diet becomes more diverse(Broughton and O’Connell 1999; Hawkesand O’Connell 1992; Kennett et al. 2006).The Marginal Value Theorem makes predic-tions about how long an individual will stayin a given resource patch before moving onto exploit another (Charnov 1967). Individ-uals will widen their foraging range as localfood sources are impacted, and may focus onhigher rankedprey tocompensate forenergyexpended in travel until a diet that optimizesenergetic returns is reached.

The prey choice model assumes homo-geneity of prey distribution within the en-vironment, but a division of the foragingarea into resource patches allows applica-tion of measures of evenness and diver-sity within patches that are sensitive to re-sourcedepressionandwideningdietbreadth(Nagaoka 2002). A decrease in encounterratescouldprompt individuals towidentheirsearch area or to target different patches.However, at a certain point, if high-rankeditems become so depleted that the increasein search time is not met with sufficient re-turns, more low-ranked local resources maybeaddedto increasedietbreadth(Broughtonand O’Connell 1999; Hawkes and O’Connell1992; Hawkes et al. 1997). In this way, anincrease in diversity indices can also signala decrease in foraging returns. Hawkes andO’Connell (1992) point out that where thediet is relatively narrow, foragers will typi-cally target high-ranked prey to justify costs,with high search times relative to handlingtime. They go on to say, “Conversely, wherediet isbroadandhandlingrepresents thebulkof foraging effort, improvements in handlingefficiencywouldhave large effects” (Hawkesand O’Connell 1992:64). As targeted preyspecies decline and high populations pre-vent the expansion of sedentary groups intoless impacted environments, reliance on in-tensive food production with high handlingcosts may increase.

Within the La Zanja faunal assemblage,the highest ranked prey were marine tur-tles that were hunted locally while nestingon beaches and in the estuarine zone (Smithet al. 2007). Other high-ranked mammalsfound in the assemblage include deer andpeccary; the energetic returns for thesespecies may have been moderated by in-creased travel and search time in interiorlocations. Estuarine fish and shellfish mayhave individually represented a lower pro-tein/energycontributionthanlargeranimals,but were the easiest prey to acquire asthey were present in abundance in the la-goon near the settlement. There is a trade-off between energy capture and subsistencerisk and the reliability of shellfish may haveserved the foraging goals of men, women,and children differently (Zeanah 2004). Thehabitats of marine fish and shellfish wouldrequire more time spent in travel to acquirethan estuarine varieties, but were probablya more reliable and consistent food sourcethan many of the larger animals. Madsen andSchmitt (1998) emphasize that prey itemssuch as insects and shellfish can be collectedin mass and that their dietary ranking is den-sity dependent. They point out that whensmall animals are available in large amountsthey become a high-ranked item and can dis-place larger prey in optimal diets. Many ofthe shellfish species in the La Zanja assem-blage occur in clumps and were likely cap-tured with mass harvesting techniques (e.g.,rakes) and thus contributed to larger ener-getic returns compared with solitary animalsof the same size.

METHODS

The La Zanja site is an earthen mound 3.2 mhigh with an area of 40,000 m2. In 2001we excavated a 2 × 2 m unit at the high-est point in the mound down to a depth of3.6 m in order to gain a complete view of For-mative period occupation (20 cm levels; seeFigure 2). Previous excavations at the site byBrush (1969), along with our own system-atic auger tests, indicate that the Early andMiddle Formative Period deposits are onlyrepresented in the center of the site. The


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Figure 2. North profile of excavated unit at La Zanja showing Early and Middle Formative Periodstratigraphic units and associated AMS 14C dates.

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upper Middle Formative Period deposit ex-tends more widely and is indicative of pop-ulation expansion after the settlement wasinitially established in the Early Formative.The 2 × 2 m unit was placed in the centerof the site to sample the stratigraphic su-perposition of Early and Middle FormativePeriod deposits. The study was exploratorydue to budgetary constraints and additionalwork will be required to determine thedegree of spatial heterogeneity of thesedeposits.

Pumps were used to excavate below thewater table (∼108 cm beneath the surface;Kennett et al. 2004). Stratigraphy was essen-tially horizontal and there were two majorcultural components represented by uniqueartifact assemblages dating to the Early (360–240 cmbd) and Middle Formative (200–80cmbd) Periods. Seven AMS 14C dates werecalibrated and modeled stratigraphically inOxCal 4.2 to establish the chronology for thesite (Figure 3; Bronk Ramsey 2013; Kennettet al. 2011). Table 1 provides the 2-σ cali-brated ranges for each date and the modeledage for each phase (or period). Dates wereplaced in an ordered sequence and modeledwithin two phases, Early and Middle Forma-tive, based on stratigraphic association. Thespan of time representedbyeachdatedeventfor the Early Formative phase is estimatedto be between 24 and 344 years (∼1400–1100 BC), with a total maximum durationfor the phase of 846 years. The span of thedated events for the Middle Formative phaseis between 53 and 354 years (∼900–550 BC),with the total maximum duration estimatedat 796 years. While the maximum durationof occupation at the site for the Early Forma-tive is approximately 120 years longer thanthe Middle Formative, average durations foreach phase (423 and 420 years, respectively)suggest comparable occupations of compa-rable duration. The Formative period occu-pation at La Zanja is contemporary with thegrowth of settlements on the Gulf Coast (SanLorenzo; Cyphers 1996; Cyphers et al. 2007–2008), Oaxaca (San Jose Mogote; Flanneryand Marcus 2000), and the Soconusco (Pasode la Amada; Clark and Blake 1994).

Sediments from each 20-cm level of theunitwere screened through5-mmmeshwith

anadditional104-liter sample fromeach levelpassed through a 3-mm screen (Kennett et al.2004). Shells in the assemblage were frag-mented from processing, but large enoughthat the 5-mm sample considered here is rep-resentative. Field assessment of the 3-mmresidue indicate that shellfish species smallerthan 5 mm are rare in the assemblage. Lev-els analyzed are of equal volume (5.6 m3)for both periods and the duration of occupa-tion is similar so we argue that the samplesare directly comparable. Poteate and Fitz-patrick (2013) have suggested that samplingless than 16% of zooarchaeological remainsof a study population may produce resultsthatarenotstatisticallysignificantandarenotrepresentative. Though the assemblage ana-lyzed represents a subsample of the greaterpopulation at the site, analysis of smallerunits (e.g., 50 × 50 cm) are not uncommonfor coastal settings. Shellfish are abundant inthe excavation at La Zanja, and the overallzooarchaeological assemblage is closely mir-rored by the diverse assemblage excavated atthe nearby site of Puerto Marques. Previousresearch from both sites shows a trend of hu-man predation that resulted in the reducedavailability of high-ranked sea turtles fromthe Early to Middle Formative periods (Smithet al. 2007). Similar trends observed in theshellfish assemblage at La Zanja are likely tobe representative of long-term changes in hu-man foraging patterns that led to an increas-ing diet breadth. Modern trash was mixedinto the upper 80 cm of the deposit and therewas a single mixed level (200–220 cm) be-tween the Early and Middle Formative Periodcomponents. These mixed deposits were ex-cluded from our analysis.

Shellfish remains were identified us-ing a comparative collection developedby Barbara Voorhies and Douglas Kennett(Table 2). We used the Number of Identi-fiedSpecimens(NISP)andMinimumnumberof Individuals (MNI) to quantify taxonomicabundances for mollusk species. MNI andNISP are common zooarchaeological meth-ods that are often highly correlated. MNI pro-vides researchers with a close approxima-tion of the number of individuals belongingto a study population (Classen 1998; Reitzand Wing 1999).


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#OS-40961 - RC 20

#OS-40963 - RC 5

#OS-40962 - RC 1

#OS-40865 - RC 2

#OS-40964 - RC 7

#OS-40963 - RC 4

#OS-41342 - RC 15

Modelled date (BC/AD)

0 500 1000 1500Calendar Years

Duration of Middle Formative Occupation

Duration of Early Formative Occupation

2500 2000 1500 1000 500 1BC/1AD 500

Earliest La Zanja Boundary

Latest La Zanja BoundaryM

iddl

e Fo

rmat

ive

Phas

eEa

rly F

orm

ativ

e Ph

ase

Boundary - Transition

Figure 3. Modeled calibrations for AMS 14C dates at La Zanja.

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Table 1. AMS 14C dates from La Zanja used in Bayesian chronological model.

Sequence/phase OS # Sample 14C age (BP) Modelled 2-σ cal range

Boundary 784–266 BC

Middle Formative period

41343 RC 15 2510 ± 30 793–551 BC

40963 RC 4 2520 ± 35 797–552 BC

40964 RC 7 2720 ± 40 936–803 BC

40865 RC 2 2540 ± 40 805–551 BC


Early Formative period

40962 RC 1 2890 ± 30 1208–1001 BC

41342 RC 5 3010 ± 40 1382–1116 BC

40961 RC 20 3030 ± 40 1386–1126 BC


Indices were calculated for richness, di-versity and evenness in the Early and Mid-dle Formative Period shellfish assemblages.Richness was determined as a raw countof taxa present in the Early and Middle For-mative assemblages. It is simply a measureof the number of shellfish taxa representedin the sub-assemblages being compared andis a product of the overall species diversity,prehistoric subsistence and processing deci-sions, and variations in sample size and shellpreservation. Diversity was calculated usingthe Shannon-Weaver Index and takes into ac-count number of species present and howevenly the numbers (MNI) are distributedamong taxa (Reitz and Wing 1999:104–106).This accounts for the overall dietary contri-bution of each species rather than simplepresence or absence. Evenness or equitabil-ity was calculated by dividing the Shannon-Weaver function by the number of species inthecommunity. Itprovidesameasureofhowevenly distributed the abundance of taxa arein the assemblage with 1 indicating evendistribution and lower values indicating thedominanceof1ormorespecies.This index istherefore sensitive to dietary shifts from sin-gle high-ranked prey to a greater diversity oflow-ranked prey, or vice versa, through time.

RESULTS

Shellfish taxa were compared using MNIwith most species divided into habitats(Table 2, Figure 4A). A total of 4702 shellfish(MNI) from La Zanja were identified fromboth Early and Middle Formative strata. Inthe Early Formative, 2,102 total shellfish(MNI) were recovered and 2,091 wereidentified to the species level (23 distinctspecies). Another 11 MNI were identifiedto the genus level (two additional generaidentified). In the Middle Formative, shell-fish remains increase to 2,600 MNI andrepresent a much more diverse collectionof taxa with 2,562 MNI identified to 31different species, 35 MNI identified to 5genera, 2 MNI identified to family, and 1MNI identified only as an unknown taxa ofbarnacle. Ten species dominate the La Zanjashellfish assemblage: Chione californiensis,Donax punctatostriatus, Megapitariaauranitiaca, Noetia reversa, Ostrea pal-mula, Tagelus affinis, Tivela hians, Mytellastrigata, Oliva incrassata, and Theodoxusluteofasciata. Together these species con-stitute 96% of the Early Formative shellfishassemblage and 95% of shellfish remains inthe Middle Formative (Figure 4B).


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Table 2. La Zanja shellfish identified taxa with MNI, preferred habitat, and average shell size.

Species

EarlyFor-

mative

MiddleForma-

tiveCommon

name Habitat

Typicalshell

length(mm) Reference

Bivalves

Marine

Anadara

formosa

0 1 Ark Clam Marine offshore

11–82 m,

attached to

rocks

121 Keen 1971

Anadara

multi-

costata

7 12 Many-ribbed

Ark

Sandbars

accessible at

low tide

59 Keen 1971

Antigona

multi-

costata

1 2 Sand and

among rocks

at low tide

Unknown Voorhies,

personal

communi-

cation

2010

Chama

mexicana

0 3 Intertidal 63–101 Keen 1971

Chione cali-

forniensis

10 64 California

Venus

Intertidal

mudflats

68 Keen 1971

Diplodonta

sericata

2 2 Flat Diplodon Intertidal sand

beaches and

flats to 10 m

22 (height) Parker 1964

Donax punc-

tatostriatus

5 952 Marine

intertidal to

5m

45 Voorhies,

personal

communi-

cation

2010

Euvola sp. 0 1 Marine

Glycymeris

gigantea

0 3 Offshore or

intertidal

∼70 Keen 1971

Lyropecten

subno-

dosus

1 0 Pacific Lion’s

Paw

Marine offshore 165 Keen 1971

Megapitaria

aurantiaca

92 69 Golden

Callista

Tidal flats 112 Keen 1971

Noetia

reversa

4 26 Intertidal and

sub-tidal

73 m

37 Voorhies,

personal

communi-

cation

2010

(Continued on next page)

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Table 2. La Zanja shellfish identified taxa with MNI, preferred habitat, and average shell size.(Continued)

Species

EarlyFor-

mative

MiddleForma-

tiveCommon

name Habitat

Typicalshell


Ostrea

palmula

78 53 California

Oyster

Attached to

mangrove

roots or

rocks, usually

in areas

exposed to

surf up to 7 m

76 Keen 1971

Pinnidae sp. 0 2 Pen shells Mud and gravel

in quiet bays

220< Keen 1971

Pododesmus

macroschisma

1 0 Alaska Jingle Intertidal and

sub-tidal to

90 m

67 Coan et al.

2000

Tagelus

affinis

10 87 Neighbor

Tagelus

Mudflats or

offshore to

73 m

55 Keen 1971

Tivela hians 2 67 Marine,

near-shore

sand

50 Voorhies,

personal

communi-

cation

2010

Trachy-

cardium

consors

4 1 Tidal flats and

sub-tidal to

45 m

60 Keen 1971

Tucetona

multi-

costata

0 2 Marine offshore

to 90 m

35 Keen 1971

Undulostrea

megodon

0 17 Low to deep

(110 m)

rocky

intertidal

76 Keen 1971

Estuarine

Chione

subrugosa

1 6 Semi-rough

Chione

Lagoons and

mudflats

42 Keen 1971

Mytella

strigata

100 132 Mangrove

mussel

Common in

mud near

mangroves.

58 Abbot and

Dance

1982



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Species

EarlyFor-

mative

MiddleForma-

tiveCommon

name Habitat

Typicalshell


Polymesoda

sp.

4 6 Brackish to

freshwater,

not sandy

beach

30–60 Keen 1971

Gastropods

Marine

Cerithium

stercusmus-

carum

20 18 Cerith snails Intertidal rocky

shores

25 Parker 1964

Cymatium

wiegmanni

5 2 Deep water on

rocks; used

by hermit

crabs

76

Hexaplex

erythrosto-

mus

3 8 Pink Murex Intertidal; feed

on

Megaptaria

squalida

101 Keen 1971

Natica

chemnitzii

0 7 Intertidal

mudflats and

sand

33 Keen 1971

Northia

northiae

0 1 Offshore 45 Voorhies,

personal

communi-

cation

2010

Oliva

incrassata

31 151 Olivella Snail Outer edge of

sandspits at

very low

tides

55 Keen 1971

Semicassis

sp.

0 1 Sand at low tide 50 Keen 1971

Stramonita

biserialis

3 2 Rocky intertidal 75 Keen 1971

Strombus

galeatus

0 2 Cortez Conch Rocky areas at

low tide and

close

offshore

190 Keen 1971


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Species

EarlyFor-

mative

MiddleForma-

tiveCommon

name Habitat

Typicalshell


Tripsycha sp. 0 4 Worm Snails Marine. 80 (length

of coil); 10

(diam. of

opening)

Keen 1971

Turritella

leucostoma

0 2 Marine, up to

depths of

40 m mud

and sand flats

115

(length);

20 (diam.)

Keen 1971

Estuarine

Cerithidea

mazatlanica

17 10 Incidental?

Tidal flats,

mudflats, and

marshes

27 Keen 1971

Theodoxus

luteofasciata

1,690 860 Margins of

mangrove

swamps and

mudflats

12 Keen 1971

Marine/Estuarine

Melongena

patula

4 1 Pacific Crown

Conch

Sand, mudflats,

intertidal,

and estuaries

117 Keen 1971

Arthropods

Marine

Megabalanus

sp.

7 23 Barnacle Intertidal

marine on

rocks

Unknown

barnacle

0 1 Barnacle Intertidal

marine on

rocks

Total MNI 2,102 2,600

Total taxa 25 38

We identified the habitat for each mol-lusk taxa based on Keen’s (1971) designa-tions for western American tropical shellfish(Table 2, Figure 4A). Seven of the ten mostcommon species are marine clams that are

found in intertidal sandy beach or mudflathabitats. Most of the clam species in the as-semblage are small, with shell sizes that av-erage less than 70 mm. Megapitaria aurani-tiaca is larger with an average shell width


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n=1812

n=286

0

200

400

600

800

1000

1200

1400

1600

1800

2000

MN

I

Early Formative Middle Formative

n=1013

n=1586

Estuarine

Marine

10 592

4 3178

10 2

100

1690

64

952

6926

151

53 87 67132

860

0

200

400

600

800

1000

1200

1400

1600

1800

Chione californ

iensis

D . puncta

tostriatu

s

M. a

urantia

ca

Noetia re

vers

a

O liva in

crass

ata

Ostrea palm

ula

Tagelus a

ffinis

Tivela hians

Mytella

strig

ata

Theodoxus lu teofa

sciata

Early Formative

Middle Formative

MN

I

Est

uarin

e

Mar

ine

A

B

Figure 4. A. La Zanja Early and Middle Formative Period shellfish species divided by habitat in MNI.(4B). La Zanja ten most common shellfish species Early and Middle Formative Period MNI.

of about 112 mm. Chione californiensis isfound in intertidalmudflats.TheclamDonaxpunctatostriatus is found in intertidal andsubtidal areas up to 5 m. Megapitaria au-

ranitiaca, the Golden Callista clam, is foundin tidal flats. The preferred habitat for Noe-tia reversa is intertidal areas and it can alsobe found in subtidal habitats up to 73 m in

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depth. Tagelus affinis, the Neighbor Tagelusis found in mudflats or offshore sand to 73 m.The clam Tivela hians, is found in near-shoresand. Another bivalve species that is com-mon at La Zanja is the California oyster, Os-trea palmula, which attaches to mangroveroots and rocks typically in areas of tidalreach up to depths of 7 m. The Olivella snail,Oliva incrassata is also typically found inmarine, intertidal sandy beaches, commonlyfound on the surface at low tides. These ma-rine taxa were harvested either to the westor southwest of La Zanja along the barrierbeach or northwest in Puerto Marques. Bothof these areas were easily accessed by water-craft from La Zanja. The dominant estuarineshellfish taxa found in the assemblage (Theo-doxus luteofasciata and Mytella strigata)are both found in muddy lagoon sedimentsadjacent to the site.

Locally available estuarine taxa domi-nate the Early Formative Period assemblage(Table 2, Figure 4A; N = 1812, MNI). Onlya small number (N = 286, MNI) were frommarine habitats and the dominant estuarinespecies was a small (12-mm shell) brack-ish water snail (Theodoxus luteofasciata;N = 1690, MNI or 80% of the assemblage).There was an overall increase in shellfish ex-ploitation during the Middle Formative Pe-riod (N = 2600, MNI) and the number ofmarine taxa jumped dramatically (1,586 of2,600 total MNI). Richness, diversity, andevenness in the mollusk assemblage all in-creased from the Early to Middle FormativePeriod (Figure 5). Theodoxus declined by al-most half, but the snail still represents 33%of Middle Formative shellfish assemblage.Donax punctatostriatus, a small marineclam species, dominates the Middle Forma-tive assemblage (N =952; 37%). Marine clamspecies increase overall in the Middle For-mative Period deposits and all clam speciesfrom marine intertidal or mudflat habitatsrepresent over 50% of the total assemblage.This suggests a transition through time frommore local, estuarine snail gathering to awider foraging area that included the collec-tion of clams from marine habitats that weretransported back to La Zanja from beachenvironments.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Early Formative Middle Formative

0.41

0.81

0.29

0.52

Diversity

Evenness

Div

ersi

ty a

nd

Eve

nn

ess

Ind

ices

Figure 5. Diversity and evenness values ofshellfish assemblage at La Zanjafrom the Early to Middle FormativePeriod.

DISCUSSION

The shellfish assemblage at La Zanja is tax-onomically rich and composed of mollusksfrom different types of habitats. There arechanges in the shellfish assemblage throughthe Formative Period that we interpret aseconomic reorganization as populations ex-panded at the site and became more de-pendent upon maize-based food production(Kennett et al. 2008). This may have resultedfrom the depression of locally available re-sources or increases in the economical via-bility of mollusks from more distant marinehabitats because of improved watercraft. Al-though the sample is small it represents afirstglimpseof subsistencechangesbetweenthe Early and Middle Formative Periods andprovides a starting point for testing these hy-potheses in the future with additional workat the site.

Based on the available data we arguethat decreases in the number of estuar-ine shellfish in the Middle Formative atLa Zanja indicate local depletion resultingfrom population-dependent increases in hu-man predation. A marine species that wasminimally present in the Early Formativeassemblage, Donax punctatostriatus, be-came the most common species in the Mid-dle Formative. This occurred as the mostdominant Early Formative estuarine species


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(Theodoxus luteofasciata) was reduced toover half its former abundance. The col-lection of this marine species at more dis-tant locations may have been enhanced bythe availability of watercraft. There was anincreased focus in the Middle Formativeon bivalve clam species from sandy andmudflat intertidal zones. The two mostcommon shellfish species in the Early For-mative, the estuarine gastropod Theodoxusluteofasciata and the brackish water musselMytella strigata, are both perching speciesthat would be easily located and collectedin large numbers from the surface of sand,rocks, or mangrove roots. These specieswould require little skill or time to find andgather compared with the burrowing clamspecies that make up over 50% of the MiddleFormative shellfish assemblage.

Small species such as aquatic gastropodsindividually represent a small energeticreturn, but require little skill to collect inlarge quantities. These gastropods were alsoeasy to process by crushing in large quanti-ties or boiling, as demonstrated ethnograph-ically and archaeologically in other areas(Raab 1992; Voorhies 2004). Although themollusk assemblage is well preserved thesegastropods all show evidence for crackingfor meat extraction. Most of the marine shell-fish species evident in the Middle Forma-tive deposits were mudflat or intertidal clamsand have larger meat packages than the es-tuarine snail. Large tidal flat bivalves requiremore expertise and time to locate and har-vest because they burrow in the sand (Raab1992), although technologies such as rakesmay have been used to reduce time andeffort. Clam meat can be dried and storedfor long periods of time. The handling andtransportationcostsof acquiringmarinemol-lusks from more distant locations places lim-its on the taxa pursued and how they wereprocessed (Bird and Bliege Bird 1997). Ifwatercraft were more limited in the EarlyFormative Period, then extensive field pro-cessing (e.g., removal of shells, drying; Birdand Bliege Bird 1997) would have beennecessary to generate viable load sizes ex-ceeding the energy expended in transportbiasing the assemblage towards underrep-resentation of shellfish remains (Bird et al.

2002). The marine mollusks in the MiddleFormativeassemblage,ontheotherhand,arethe largestavailablealongthecoastandlargerthan locally available snails. This pattern isconsistent with the prey choice model pre-diction that larger species will be targeted astransport costs increase. However, the dom-inance of marine mollusks at La Zanja frommore distant locations during the Middle For-mative points to changes in transportationcosts consistent with the availability of moreviable watercraft (see below).

The overall quantity of shellfish in-creased in the assemblage from the EarlyFormative (MNI = 2098) to the Middle For-mative (MNI = 2599), perhaps indicating ahigher level of predation pressure as humanpopulations rose in the village (Table 2, Fig-ure 4A). Shellfish increased in diversity andevenness during the Middle Formative (Fig-ure 5) suggesting expanding diet breath inthe face of population-dependent resourcedepression locally. Larger shellfish were alsocollected at more distant locations and trans-ported back to La Zanja. This suggests thatpeople were searching farther and longer forprey during the Middle Formative. If water-craft were used more in the Middle Forma-tive, this could increase foraging efficiency,as these early farmers could cover moreground and easily return with materials frommore distant resource patches (Gomez et al.2011;Thomas2008).Arise in theuseofboatsmight negate the importance of removinglow-utility parts (shell) prior to transport ofshellfish back to central places (Ames 2002).More marine shell in the Middle Formativeassemblage is expected if boats were usedfor the transport of goods more frequentlythan in the Early Formative. However, if ma-rine shellfish were transported primarily onfoot in both periods they may be underrepre-sented in both assemblages and more impor-tant dietary constituents than faunal remainsindicate (Bird et al. 2002).

There isevidence for increasedmaritimetrade along the Pacific Coast during the lat-est Early Formative and earliest Middle For-mative Periods (Gomez et al. 2011). There isalso evidence for the development of moreintensive trade networks with the Gulf CoastOlmec either over land or across the Isthmus

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of Tehuantepec and up and down the Pa-cific Coast. Lagoons and coastal river systemsserved as important flat-water conduits forboats carrying pottery, obsidian, and othervaluable resources. White slipped and in-cised pottery became more prevalent in theassemblage at La Zanja during the MiddleFormative along with other status items sug-gesting linkages/contact with the Gulf CoastOlmec (Blomster et al. 2005; Neff 2006). It islikely that watercraft were commonly usedat this time both for the transport of long-distance trade goods and for local travel. Iftransportation via watercraft became morecommon in the Middle Formative, there mayhave been less incentive to process mollusksbefore transport, consistent with the higherincidence of low-utility shell from marine in-vertebrates in the La Zanja Middle Formativecomponent. As populations increased dur-ing the Middle Formative, the use of water-craft might have served as a cost-reductionstrategy in the face of resource depression,since travel costs to more distant patcheswould be reduced. It is also possible that ma-rine mollusks were obtained via trade withcoastal communities. Agricultural productiv-ity was higher on the more expansive allu-vial portions of the coast compared with bar-rier beach locations and maize, beans, andsquash could have been exchanged for ma-rine foods including shellfish. Trade and theproduction of trade items to exchange forfood provides another mechanism for ex-panding dietary breadth (Kennett 2005). Re-gardless, changes in the La Zanja shellfish as-semblage through time are generally consis-tentwiththepreychoicemodelandMarginalValue Theorem for dietary expansion in theface of population-dependent resource de-pression.

CONCLUSION

The Formative Period occupants of La Zanjacombined the cultivation of domesticatedplants with a heavy reliance upon coastaland estuarine resources, largely shellfish andfish. Foraging strategies changed and dietbreadth expanded to incorporate a larger ar-ray of shellfish taxa as people became more

reliant on maize-based food production andvillage populations increased during the Mid-dle Formative Period. Mollusks were also col-lected from more distant beach habitats andtransported back to La Zanja. This parallelsan increased reliance upon boats for tradeevident along with coasts of Mesoamericaduring the Formative Period (Gomez et al.2011). The predictions of OFT models areconsistent with many of the patterns seenin the shellfish assemblage at La Zanja wherethe increase inshellfishcatchmentarea in theMiddle Formative seems to occur at a time ofpopulation expansion and increasing humanenvironmental impacts with a growing com-mitment to maize agriculture.

While our examination of shellfish ex-ploitation and expanding diet breadth at LaZanja predict a more generalized diet anda movement of foragers into more distanthabitats, with decreasing encounter rateswith local high-ranked prey (Broughton andBayham 2003; Broughton and O’Connell1999; Hawkes and O’Connell 1992), parallelshifts in diet breadth might also be exploredthrough changes in other aquatic taxa (fish,turtles, etc.). Our previous work suggests thelocal depression of nesting sea turtle popu-lations at this and other locations throughtime (Smith et al. 2007) that is consistentwith expanding human populations region-ally. Future work will focus on analysis offish taxa and other faunal classes (e.g., largemammals) in order to rank prey species andexamine changes in foraging patterns thatmay have existed at La Zanja. Through an ex-amination of a larger zooarchaeological as-semblage, optimal foraging models from be-havioral ecology provide an effective tool forexploring the ecological effects of popula-tion growth and human predation withincoastal settings.

ACKNOWLEDGEMENTS

We thank Madonna Moss, Jon Erlandson,Scott Fitzpatrick, and two anonymous re-viewers for thoughtful comments and sug-gestions that improved the article greatly.


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FUNDING

This work was funded by the National Sci-ence Foundation (BCS- 0211215, Kennett).

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