Auszug aus:

Roland de Beauclair, Susanne Münzel und Hannes Napierala (Hrsg.)

Knochen pflastern ihren Weg Festschrift

für Margarethe und Hans-Peter Uerpmann

BioArchaeologica 5 Verlag Marie Leidorf GmbH . Rahden/Westf.

2009

Canan Çakırlar

Problems in determining the chain of production in shell “objects”

Observations on shell assemblages from coastal sites in the Eastern Mediterranean

Von Seite 45 bis 50

ISSN: 1611-356XISBN-13: 978-3-86757-952-0ISBN-10: 3-86757-952-0

Introduction

Mollusk shells of marine, freshwater or terrestrial origin are

regularly represented in archaeological deposits. Despite

their ubiquitous presence, this category of organic mate-

rial constitutes the find group that is least exploited for

purposes of palaeoenvironmental and palaeoeconomical

interpretation. This situation is notably evident in archae-

ological schools dealing with late-prehistoric and historic

periods of the Old World. In contrast, archaeological shell

recovered in a state other than its natural form frequently

attracts researchers’ attention. Modified shells are often un-

critically considered as artifacts that represent human cog-

nition, beliefs, symbolism, technology, and tool utilization.

The deficiency of criticism in approaching modified

mollusk shells is caused by two factors. One of them is the

pressure an archaeologist feels about finding direct evi-

dence of past human activity. This pressure is caused both

by scientific and public expectations, and is a tradition in-

herited from the early days of archaeology when the state

Problems in determining the chain of production in shell “objects” Observations on shell assemblages from coastal sites in the Eastern Mediterranean

Canan Çakırlar

AbstractArchaeological mollusk shells can be recovered in a variety of forms. Shell morphology of aquatic mollusks can be altered by natural agents during the organism’s lifetime, after its death, and after its deposition. They can be alternatively or coevally modified by human activity. This paper criticizes the lack of detailed analyses in the interpretation of modified archaeological shells, which often leads to the representation of false or manipulated evidence for anthropogenic activity. Modes of shell modification are exemplified by using modern and archaeological finds from the Eastern Mediterranean. The need for conducting experimental and taphonomic research in order to accurately unfold the processes that were in play in shell modification is emphasized. Keywords: Mollusks, shell modification, taphonomy, chain-of-production, diagenesis, ecology

ZusammenfassungArchäologische Molluskenschalen können in verschiedenen Formen gefunden werden. Die Schalenmorphologie von aquatischen Mollusken kann durch natürliche Prozesse während der Lebenszeit des Organismus, nach seinem Tod und nach seiner Ablagerung verändert werden. Alternativ dazu oder zusätzlich können sie durch menschliches Zutun modifiziert werden. Dieser Beitrag kritisiert den Mangel an detaillierten Analysen bei der Interpretation von modifizierten archäologischen Molluskenresten, der häufig zur falschen oder manipulierten Darstellungen von Belegen für anthropogene Aktivität führt. Verschiedene Arten von Modifikationen an Molluskenschalen werden anhand moderner und archäologischer Funde aus dem östlichen Mittelmeer illustriert. Abschließend wird die Notwendigkeit von experimenteller und taphonomischer Forschung betont, um die Prozesse genau aufzuzeigen, die bei der Modifikation von Molluskenresten eine Rolle spielten.Schlüsselwörter: Mollusken, Modifikation, Taphonomie, Produktionskette, Diagenese, Ökologie

46 Canan Çakırlar

of art was about collecting artifacts of beauty or price. As

a consequence, sometimes the evidence is created, caus-

ing different levels of harm to the scientific discipline. The

second factor behind this lack of criticism when handling

modified shell remains is the distance some archaeological

circles insist on keeping from natural sciences. This practice

is also rooted in the complicated avenues of history of the

science. One outcome of this is the unconscious interpreta-

tion of virtually all modified shells as end-products of de-

liberate human activity. What are at stake here are accurate

descriptions of human behavior, which takes the form of ty-

pologies, explanations of the chain of production, and use,

when the focal point is the discussion of “osseous artifacts”.

The purpose of this paper is to promote the caution

that must be taken in identifying the modes of modifica-

tion on shell finds and to evoke skepticism in evaluating

the shell finds that have been published as shell artifacts.

Attention will be drawn to natural agents that modify shell

morphology. The modes of modification that will be de-

scribed in the following sections are not intended to cover

the complete range of modifications pertaining to process-

es other than human activity. They also do not claim to be

hitherto unknown, new observations. On the contrary, their

strength against arguments supporting human modes of

modification lies in the fact that they have previously been

explained by biological and geological phenomena and ex-

emplified in taphonomic research.

The idea for this research was stimulated by the ne-

cessity to distinguish naturally-modified shells from shell

artifacts in the colossal mollusk assemblages of coastal

Eastern Mediterranean sites. Most of the examples that are

displayed come from archaeological assemblages of these

sites or have been collected from the Eastern Mediterranean

coast in empty form or when still alive. Principles of nature

transcend conjunctures.

Forms of shell modification

Shell modification can take a variety of forms that are

caused by different agents. These agents can be classified

in a chronological order that covers the life time of a shell

find until its recovery.

7. Modifications that occur during ontogenic life time.

8. Post-mortem modifications that occur off-site.

9. Anthropogenic modifications.

10. Diagenetic modifications.

Modifications that occur during or after recovery can be

added to this classification, but these usually occur in char-

acteristic breaks that can be readily recognized as ‘fresh-

breaks’. They pose a problem in quantifying large amounts

of food refuse rather than in identifying shell artifacts.

1. Ontogenic modifications occur while organisms are

still alive. Two main causes for this type of modification are

predation and abrasion. Shelled mollusks, no matter how

well their shells are adapted to protect the organisms, are

subject to predation by other animals. Different modes of

predation leave different traces on the shell. The two most

important modes of predation that are of importance to

the discussion here are crushing and drilling.

For example, different species of arthropods (mainly

crabs and lobsters) predate on Cypraeidae (cowrie shells) by

crushing their shells with their claws (Vermeij 1993, 94, fig.

5.1). In this manner they break off the top of the snail shell.

Fig. 1: Beach-picked Chamelea gallina valve (Biga, Northwest

Anatolia) displaying typical carnivorous gastropod predation

hole.

Fig. 2: Inside view of the left valve of an Acanthocardium

tuberculatum specimen showing the abraded umbo surface. Live

collected in Troas.

Problems in determining the chain of production in shell “objects” 47

In appearance this break resembles a globally-distributed

artifact type: the cowrie beads. It might be rightfully argued

that the first cowrie beads were beach-collected versions of

this naturally-modified type, constituting the inspiration for

subsequent cowrie bead production.

Some carnivorous gastropods feed on other shelled

mollusks. They weaken their prey’s shell by secreting an

enzyme to soften the shell and drill the shell with their

radula (Vermeij 1993). This process, which sometimes lasts

for hours, if completed successfully, produces an orderly

hole on the prey’s shell (Fig. 1). The morphology of different

shell parts can be transformed by sponges, foraminifera,

bivalves, barnacles, worms, octopods, among others

(Claassen 1999, 55).

Shelled aquatic mollusks move as part of their life cycle

and survival method or can be transported by other agents

such as waves. The interaction between shell’s resistive

properties (thickness, strength, sculpture, periostracum

etc.) and ecological events may cause abrasion of the shell

structure. In burrowing bivalves, the umbo zone, where

two valves rub against each other as the valves periodically

open and close, might be thinned from the outer side. This

trait can be easily observed, particularly in older specimens

(Fig. 2). The process speeds up in coarser sands, and abrasion

might be more severe (Zuschin 2003, 45). Abrasion caused

by in situ jostling by wave action has been observed on

the umbo and outer surfaces of epifaunal bivalves that live

attached to rocks (Light 2005).

2. Processes that take place after the death of the

organism and before its deposition in the sediment can

cause abrasion, bioerosion, fragmentation, loss of sculpture,

pitting etc., depending on the length of this post-mortem

period before burial, on the types of ecological agents that

are present, and on the properties of the shell itself. Abrasion

by wave and sand action is most readily effective on the

thinnest shell parts. In gastropods, the most vulnerable

parts are outer lips and apices. In beach-picked bivalves,

umbones of some species are typically perforated due to

abrasion. Smooth edges are formed in both gastropods

and bivalves (Fig.  3). Shells with naturally perforated

umbones or apices resemble culturally-modified shells that

were most commonly used as pendants or beads (Figs. 4

and 5). Patellidae abraded similarly by sand blasting are also

demonstrated in Zuschin et al. (2003, 44, fig. 6, 62, fig. 15).

Shells with smooth surfaces have appearances in common

with scraping tools (Light 2005).

Fig. 3: Beach-collected Glycymeris valve, (Biga, Northwest

Anatolia). It was probably fragmented prior to abrasion. The shell

sculpture has been severely altered by post-mortem wave and

sand action.

3. Anthropogenic processes can include an enormous

variety of actions done for a similarly vast diversity of

purposes, the effects of which leave diverse signatures

on mollusk shells. It is, unfortunately, impossible to review

these in this paper. What is important to remember in this

context is that in many cases anthropogenic processes

leave no marks at all.

4. Diagenetic processes involve physical and chemical

agents. The first is most often related to sediment

compaction; the other relates to the properties of the

soil (E.g. pH level). These processes cause fragmentation

that might lead to the complete dissolution of organic

remains (Zuschin et al. 2003). Vulnerability to the effects of

post-depositional processes is specific to both the species

and the shell portion. Fragmentation usually occurs at

structurally weak points where changes in shell sculpture

and growth interruptions (or slowly formed increments)

are located (Claassen 1998). For example, the shell parts

that are most often absent in archaeological cockle shells

(Cerastoderma glaucum) are ventral-posterior ends where

Fig. 4: Two worn Conus mediterraneus specimens (Biga, Northwest

Anatolia). Wave and sand activity created holes on the apices and

abraded the body to create these bead-like shells.

48 Canan Çakırlar

the shell is thinnest. Another type of diagenetic process

that affects the shells of this species is the shedding of

different shell layers. A cockle valve consists of three layers:

Periostracum, as mentioned above, covers the shell and

is completely organic. This layer completely disintegrates

shortly after burial – except in rare anoxic conditions. Below

this are an outer and an inner calcareous layer, which are

held together with a protein matrix. The calcareous layers

have different types of crystalline microstructure, which

causes degradation at differential rates in the same shell.

This results in the occurrence of virtually whole, unbroken

valves whose outer shell layers are partially dissolved in the

matrix. Following this, the inner calcareous layer is exposed

to sedimentary agents and dissolves next. This process can

affect all parts of the shell, but is more often observed in

the zone close to the umbo (Fig.  6). It is known that this

zone was perforated out and used to make cockle beads

in the younger periods of the Franchthi Cave (Miller 1996).

Diagenic limpet (Patella spp.) fragmentation occurs in a

diagnostic pattern, usually around weaker growth lines

(Fig. 7).

Discussion and conclusions

The paths that mollusk shells go through from the initial

steps of shell formation until recovery and recording can di-

versify in many other ways than outlined here. These paths

can also overlap. Modification traces on a single shell can

represent a variety of evidence. Shells that bear holes due

to abrasion can be picked up from the beach, brought to

the human habitation area, and worn without further modi-

fication. Marine shells in non-coastal sites are remnants of

distant trade-connections. Some of these traveling shells

are specimens that were modified only by natural agents.

Examples of cone shells (Conus mediterraneus) and bitter-

sweet clams (Glycymeris glycymeris) that bear water-worn

holes have been found in association with other ornamen-

tal shell finds in inland sites (E.g. Reese 1990 and 1986). In

other cases, natural holes can be worked neatly into more

regular ones and subsequently used as ornaments. Valves

that are already abraded at the margins can be used as

scrapers or polishers. Shells that were collected for food can

later be perforated by humans or used as tools.

It is necessary to recognize and distinguish the dif-

ferent layers of natural and anthropogenic processes on

modified shells, instead of readily accepting a shell’s state

Fig. 5: Beach-picked Patella caerulea specimens (Assos, Northwest

Anatolia) with different levels of fragmentation.

Fig. 6: Cockle (C. glaucum) shell in the process of developing a

hole. Archaeological specimen from Yenibademli, Gökçeada,

North Aegean.

Fig. 7: Breakage pattern of Patella caerulea shell remains from

Yenibademli, Gökçeada, North Aegean. The specimens have

been recovered along with thousands of better preserved Patella

shells from the site that represent shellfish gathering from rocky

splash-zones of the coastal line and subsequent consumption for

food.

Problems in determining the chain of production in shell “objects” 49

during recovery as completely human-induced. Failure to

recognize the variety of factors that were responsible for

shell modification can result in pseudo-artifacts. Patella

specimens fragmented exactly as displayed in Figs. 5 and

7 were identified as artifacts in excavation reports and in

more specialized archaeological literature (E.g. Korfmann

2000, 43, fig. 39, Karali 1999, 112, fig. 24; Hood 1981, 677,

pl. 142/62).

Experimental studies can play a key role in explaining

the processes that influenced the modifications on shell

remains, but it is probably impossible to reproduce the

effects of meters-thick sediments that cover cultural lay-

ers on the shell surface or the effects of human activities

that produced these sediments. Taphonomic descriptions

of fossil or sub-fossil shell remains from purely geological

sediments should be used to support arguments that label

modified archaeological shells as evidence for human cog-

nition, symbolism and technology.

Acknowledgments

The first version of this paper was presented at the Worked

Bone Research Group in Veliko Turnovo in September 2005.

I would like to thank my Tübingen “Kamerad” Petar Zidarov

for shaping the idea for this paper with me over several in-

formal discussions in Tübingen and in Troia, and Dr. Martin

Zuschin for his readiness to answer questions about shell

taphonomy. Special thanks also go to Drs. Pamela Crabtree

and Douglas Campana for their revisions on the final ver-

sion of the text. I also would like to thank the editors for

undertaking the large task of preparing this Festschrift for

publication. And last but not the least; I want to express my

most sincere appreciation for the Uerpmanns, to whom I

am indebted for a great many deeds I cannot possibly list

here. The responsibility for the ideas and information that

are presented here are of course mine.

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Dr. Canan Çakırlar   Universität Tübingen

Institut für Ur- und Frühgeschichteund Archäologie des Mittelalters

Zentrum für Naturwissenschaftliche ArchäolgieArchäobiologie

Rümelinstr. 23 D-72070 Tübingen

undSmithsonian Institution

National Museum of Natural HistoryMuseum Support Center

Archaeobiology Laboratory MRC 534 4210 Silver Hill Road

Suitland, MD 20746-2863, USA  cakirlarc@si.edu