PHYTOLITHS : INTERPRETATION
Phytolith taphonomy: understanding assemblage formation processes
Phytolith dispersal is a fundamental issue for interpreting phytolith assemblages, especially in palaeoecological studies (e.g. Osterrieth et al 2009). Phytoliths, unlikely pollen, do not form for being transported by wind or animals but it is still pertinent to ask what spatial level they represent when an assemblage is studied (Piperno 2006):
- Local (< 20 m)
- Extralocal (between 20 and several hundred meters)
- Regional (range in Km)
Unless there is reason to suspect greater distances of origin (e.g. a lake sediment could gather phytoliths from the entire drainage basin; strong winds can transport ashes from natural fires for very long distances) it is generally considered that phytoliths represents local or extralocal vegetation.
The interpretation of phytoliths origin in archaeological sites is a simpler matter because the plants that originated the phytolith input derived directly from human use and were discarded, often abundantly, in the cultural deposits. The assemblages can therefore be interpreted as of local input but this does not mean that the plant(s) was/were of local origin. This is an important point to remember because the plant(s) from which the phytoliths originated could have been transported to the site from areas much further away. The amounts of phytoliths incorporated into the cultural deposits by human actions are proportional to the frequency of these actions and the amount of plant(s) involved. Most of the time these actions result in great quantities deposited, so much so that the anthropic signal completely obliterates any inherited assemblage (see below diagram of phytolith assemblage formation).
Piperno, D. 2006. Phytoliths. A Comprehensive Guide for Archaeologists and Paleoecologists. 238 pp. Lanham, New York, Toronto, Oxford: AltaMira Press (Rowman & Littlefield).
The following is a list of some human actions that can produce substantial phytoliths deposition:
- Crop processing;
- Use of fuel in domestic or industrial settings (wood or dung);
- Constructions such as huts, roofs, etc.;
- Use of adobe;
- Use of matting;
- Storage of fodder;
- Production of pottery (chaff used as temper).
Identifying crops and understanding plant domestication
The following list of crops that can be investigated using phytoliths (being these as single, disarticulated particles or as silica skeletons) is by no means exhaustive. Indeed, phytolith studies are now regularly featured in most archaeological and natural sciences journals and it is strongly suggested to consult search engines like ScienceDirect™ or Scirus™ to look up for the most up-to-date publications.
A good review of Central and South America crops that can be identified using phytoliths (and starch) is:
- Piperno, D 2009. Identifying crop plants with phytoliths (and starch grains) in Central and South America: A review and an update of the evidence. Quaternary International 193:146-159.
For a general review see:
- Piperno, D. 2006. Phytoliths. A Comprehensive Guide for Archaeologists and Paleoecologists. 238 pp. Lanham, New York, Toronto, Oxford: AltaMira Press (Rowman & Littlefield).
Wheat (Triticum sspp.) and Barley (Hordeum sspp.)
The phytoliths produced in the inflorescence bracts from four species of wheat and two species of barley were analyzed by computer-assisted image and statistical analysis. A classification was created that was based on significant differences among the mean morphometries of the inflorescence phytoliths. Discriminant analysis was also conducted to determine the soundness of the statistical procedure. The results demonstrated that the classification key was most reliable to distinguish among the taxa at species level. Silica skeletons (phytolith in anatomical connection) of wheat and barley can also be identified at genus and species level using the junction pattern of the long cells and the morphometry of the long cells and papillae (Rosen 1992; Ball et al 1996; 2001).
- Ball, T.B., J.S. Gardner, and Brotherson, J.D. 1996. Identifying Phytoliths Produced by the Inflorescence Bracts of Three Species of Wheat (Triticum monococcum L., T. dicoccon Schrank., and T. aestivum L.) Using Computer-Assisted Image and Statistical Analyses. Journal of Archaeological Science 23:619-632.
- Ball, T.B., J.S. Gardner, and N. Anderson. 1999. Identifying inflorescence phytoliths from selected species of wheat (Triticum monoccum, T. dicoccon, T. dicoccoides, and T. aestivum) and barley (Hordeum vulgare and H. spontaneum). American Journal of Botany 86:1615-1623.
- Ball, T. B., Gardner, J. S. and Anderson, N. 2001. An approach to identifying inflorescence phjytoliths from selected species of wheat and barley. In Meunier, J. D. and Colin, F. (eds.) Phytoliths: Applications in Earth Sciences and Human History. Lisse: Balkema, 289-301.
- Rosen, A. M. (1992). Preliminary identification of silica skeletons from Near Eastern archaeological sites: An anatomical approach. Phytolith Systematics. Emerging Issues. G. J. Rapp and S. C. Mulholland (eds.) Advances in Archaeological and Museum Science 1. Plenum Press, New York. 129-147.
Oat (Avena sativa and A. strigosa)
The findings from Portillo et al (2006) indicate that while the phytoliths produced by the inflorescences of A. sativa and A. strigosa cannot be discriminated on the basis of their typologies, they can be discriminated morphometrically. This is because they produce the same types of phytoliths but those phytoliths have significantly different measurements of size and shape.
- Portillo, M., T. Ball, and J. Manwaring. 2006. Morphometric Analysis of Inflorescence Phytoliths Produced by Avena sativa L. and Avena strigosa Schreb. Economic Botany 60:121-129.
Foxtail Millet (Setaria italica) and Common Millet (Panicum miliaceum)
Foxtail millet and Common millet were the staple food in many areas of South and Southest Asia, especially in arid and semi-arid environment and they also played an important food role in Eurasia ta large, at least until the introduction of wheat and rice. The identification of the charred de-husked grains of these two crops is not always straightforward and they have been often misidentified (Fuller 2006). In a recent work by Lu et al (2009), five key diagnostic characteristics of phytolith morphology has been identified for distinguishing Foxtail millet from Common millet. These are:
- The presence in the lower lemma and glumes of cross (S. italica) vs. bilobate (P. miliaceum) types;
- The regularly arranged papillae on the surface of the upper lemma and palea are peculiar to S. italica;
- The epidermal dendritic long cells wall: Ω-undulated (S. italica) vs. the η-undulated (P. miliaceum);
- The endings structures of the epidermal dendritic long cell;
- The surface ridgy line sculpture of the upper lemma.
- Fuller, D. Q. 2006. Agricultural Origins and Frontiers in South Asia: A Working Synthesis. Journal of World Prehistory 20:1–86.
- Lu, H., Zhang, J., Wu, N., Liu, K-b, Xu, D. and Li, Q. 2009. Phytoliths Analysis for the Discrimination of Foxtail Millet (Setaria italica) and Common Millet (Panicum miliaceum). PLoS ONE 4(2):e4448.
Maize (Zea mays) and Teosinte (Zea mays ssp. parviglumis)
Maize and its wild ancestor, teosinte, are some of those domesticates that have seen the most phytoliths investigation.
Several authors (see bibliography below) have studied maize leaf and cob phytoliths and compared them against phytoliths from large collections (more than 500 species) of wild grasses.
The data accumulated so far show that a set of criteria can be used to identify maize leaves and cobs in archaeological contexts. Two are the phytoliths types routinely used for the identification of maize and to distinguish it from its wild ancestor teosinte:
- cross-shaped (occurring mostly in leaves);
- rondels (occuring mainly in the glumes and cupules of cobs).
- Iriarte, J. 2003. Assessing the feasibility of identifying previous termmaizenext term through the analysis of cross-shape size and three-dimensional morphology of phytoliths in the grasslands of southeastern South America. Journal of Archaeological Science 29:1085–1904.
- Pearsall, D. M., Chandler-Ezell, K. and Chandler-Ezell, A. 2003. Identifying maize in neotropical sediments and soilsusing cob phytoliths. Journal of Archaeological Science 30:611-627.
- Piperno, D. R. 1984. A comparison and differentiation of phytoliths from maize and wild grasses: use of morphological criteria. American Antiquity 49:361–383.
- Piperno, D. 2009. Identifying crop plants with phytoliths (and starch grains) in Central and South America: A review and an update of the evidence. Quaternary International 193:146-159.
- Piperno, D. and Pearsall, D. M. 1993. Phytoliths in the Reproductive Structures of Maize and Teosinte: Implications for the Study of Maize Evolution. Journal of Archaeological Science 20:337-362.
Rice (Oryza sativa)
Rice is a major crop in Asia. It is not possible, however, to have a complete view on the importance of rice as a crop only on the basis of the charred remains. Indeed, de-husking waste would be dominated by chaff, which burns to ash or decay (but leaves behind phytoliths), and only very few grains. Accidental charring can happen occasionally during cooking. However, it probably contributes very little to the archaeological record. Cooked grain has normally been soaked and, consequentially, distorted and therefore unlikely to survive in any recognisable form. Two types of phytoliths have been used to identify rice in archaeological sediments:
- bulliforms (also wrongly called fan-shaped);
- double-peaked cells.
Not only it is possible to evidence the presence of rice but also to distinguish wild from cultivated rice on the basis, again, of a morphometric analysis.
- Itzstein-Davey, F., Taylor, D., Dodson, J., Atahan, P. and Zheng, H 2007. Wild and domesticated forms of rice (Oryza sp.) in early agriculture at Qingpu, lower Yangtze, China: evidence from phytoliths. Journal of Archaeological Science 34:2101-2108.
- Pearsall, D.M., Piperno, D.R., Dinan, E.H., Umlauf, M., Zhao, Z.J. and Benfer, R.A. Jr. 1995. Identifying rice (Oryza sativa), Poaceae, through phytolith analysis. Economic Botany 49:183–196.
- Zhao, Z.J., Pearsall, D.M., Benfer, R.A. Jr., & Piperno D.R. 1998. Distinguishing rice (Oryza sativa Poaceae) from wild Oryza species through phytolith analysis, II: Finalized method. Economic Botany 52:134–145.
Banana (Musa sp.)
Domestication and use of fruits such as banana (Musa sp.) are difficult to trace through charred remains as edible bananas are sterile and therefore do not produce seeds. Also, these kind of fruits would rarely be exposed to fire apart in exceptional cases (sudden conflagrations, accidental burning. etc.).
Phytoliths provide a firmer basis for tracking cultivated bananas (Wilson 1985; Mbida et al 2000) by using the volcaniform morphotypes (Ball et al 2006).
- Ball, T., Vrydaghs, L., Van Den Hauwe, I., Manwaring, J. and De Langhe, E. 2006. Differentiating banana phytoliths: wild and edible Musa acuminata and Musa balbisiana. Journal of Archaeological Science 33:1228-1236.
- Fuller, D. Q. and Madella, M. in press. Banana cultivation in South Asia and East Asia: a review of the evidence from archaeology and linguistics.
- Lentfer, C. J. and Green, R. C. 2004. Phytoliths and the evidence for banana cultivation at the Lapita Reber-Rakival Site on Watom Island, Papua New Guinea. In A Pacific Odyssey: Archaeology and Anthropology in the Western Pacific. Papers in Honour of Jim Specht, ed. Val Attenbrow and Richard Fullagar, pp. 75–88. Records of the Australian Museum, Supplement 29. Sydney: Australian Museum. (free download)
- Mbida, C. M., Van Neer, W., Doutrelepont, H. and Vrydaghs, L. 2000. Evidence for Banana Cultivation and Animal Husbandry During the First Millennium bc in the Forest of Southern Cameroon. Journal of Archaeological Science 27:151-162.
- Wilson, S.M. 1985. Phytolith analysis at Kuk, an early agricultural site in Papua New Guinea. Archaeology in Oceania 230(3):90-97.
Assessing the relative economic importance of plant's parts
Phytoliths can be very useful to understand the use and economic importance of non-dietary grass plant's parts as well as of broad-leaved plants. Inflorescences, leaves and stems are all utilized for one reason or another (see list below) but they often do not appear in the archaeological record because these organs are not easily preserved and even when burned are difficult to identify. Exceptions are the waterlogged sites or the ones in extremely dry conditions.
The different tissues in the inflorescence, leaves and stems have cells that often present distinct morphologies and when these cells silicify they produce phytoliths that are characteristics of the various organs (below left). It is also important to keep in mind that sometime different phytolith input can be mixed by human action like the one illustrated in here (below right) where the phytolith from the dung are mixed with the phytolith from straw.
- Albert, R. M., Shahack-Gross, R., Cabanes, D., Gilboa, A., Lev-Yadun, S., Portillo, M., Sharon, I., Boaretto, E. and Weiner, S. 2008. Phytolith-rich layers from the Late Bronze and Iron Ages at Tel Dor (Israel): mode of formation and archaeological significance. Journal of Archaeological Science 35:57-75.
- Delhon, C., Martin, L., Argant, J. and Thiébault, S. 2008. Shepherds and plants in the Alps: multi-proxy archaeobotanical analysis of neolithic dung from “La Grande Rivoire” (Isère, France). Journal of Archaeological Science 35:2937-2952.
- Madella, M. 2007. The Silica Skeletons from the Anthropic Deposits. In A. Whittle (ed.) The Early Neolithic on the Great Hungarian Plain Investigations of the Körös culture site of Ecsegfalva 23, County Békés. Varia Archaeologica Hungarica XXI. Archaeological Institute of the Hungarian Academy of Sciences. Budapest, pp. 447-460. Download Text Download References
Understanding agricultural practices and plant processing
Water availability and management have been critical factors in the ecology of prehistoric agriculture. This was especially so in the arid regions of the world and the issue of the presence of irrigation in an early stage of agriculture or for agricultural intensification has been central to the archaeological debate of the last decades (see for example World Archaeology volume 41.1). Grasses (Poaceae) in general and cereals in particular are between the plants with the highest deposition of phytoliths and it was possible to establish a correlation between biogenic opal silica from cereals and the plant-water conditions under which it has formed (Madella et al 2009; Rosen and Weiner 1994). The experiments of Rosen and Weiner (1994) showed that emmer wheat grown with irrigation in semi-arid plots produces greater numbers of silicified cells (phytolith) than wheat that was dry-farmed. The Madella et al (2009) approach focused on investigation of wheat and barley single phytolith morphologies and X-ray analysis at the SEM. The major points resulting from this study were that:
- Phytoliths morphologies sensitive to the environmental conditions are produced in higher quantities when more water is available to the plants;
- Phytoliths trap more water molecules in the opal silica lattice when more water is available to the plants.
These studies of the morphological and chemical characteristics of phytoliths from cereal crops reveals that there are huge potentials for the use of phytoliths in investigating ancient irrigation and past water regimes in environments where water was constantly a constraining factor.
- Madella, M., Jones, M.K., Echlin, P., Powers-Jones, A. and M. Moore 2009. Plant water availability and analytical microscopy of phytoliths: Implications for ancient irrigation in arid zones. Quaternary International 193:32-40.
- Mithen, S., Jenkins, E. L., Jamjoum, K., Nuimat, S., Nortcliff, S. and Finlayson, B. L. 2008. Experimental crop growing in Jordan to develop a methodology for the identification of ancient crop irrigation. World Archaeology, 40:7-25.
- Rosen, A. M. and Weiner, S. 1994. Ancient Irrigation: a New Method Using Opaline Phytoliths from Emmer Wheat. Journal of Archaeological Science 21:125-132.
Crop processing is another area of archaeological investigation were phytoliths can dramatically increase the available information. Indeed, our current knowledge on crop processing is mainly based on data gathered from carbonised remains. Unfortunately this kind of data is not always available and for some crops (e.g. rice) charred remains are even more scantily recovered because they do not need fire for dehusking.
The different parts (e.g. leaf, culm, chaff) of domesticated cereals often produce characteristic phytolith types that can aid in identifying
Use of plants as fuel
Studies on archaeological sites dated from the Palaeolithic onward have shown that quantitative analysis of phytoliths together with mineralogical analyses can provide important information on the use of plants as fuel. This approach is especially effective if the phytolith analysis is coupled with micromorphology analysis of the same sediments to understand ash deposition and taphonomy. Phytoliths can help identifying the type of fuel (e.g. wood or dung) as well as the taxon that were burnt. Using phytolith analysis in several Near Eastern caves with deposits dated from the Middle Palaeolithic to the Natufian it has been possible to understand the use of plant material as fuel and the woody species involved even when the wood charcoal was not preserved (e.g. Albert et al 2000, Albert et al 2003, Madella et al 2002).
- Albert, R. M., Weiner, S., Bar-Yosef, O. and Meignen, L. 2000. Phytoliths in the Middle Palaeolithic Deposits of Kebara Cave, Mt Carmel, Israel: Study of the Plant Materials used for Fuel and Other Purposes. Journal of Archaeological Science 27:931-947.
- Albert, R. M., Bar-Yosef, O., Meignen, L. and Weiner, S. 2003. Quantitative Phytolith Study of Hearths from the Natufian and Middle Palaeolithic Levels of Hayonim Cave (Galilee, Israel). Journal of Archaeological Science 30:461-480.
- Madella, M., Jones, M. K., Goldberg, P., Goren, Y. and Hovers, E. 2002. The Exploitation of Plant Resources by Neanderthals in Amud Cave (Israel): The Evidence from Phytolith Studies. Journal of Archaeological Science 29:703–719.
Interpreting diet from calculus and coprolites (and other palaeofecal sources)
Recovery and identification of phytoliths has provided in cases a way to reconstruct a more comprehensive view of the diet of past human populations. The phytolith extracted from dental calculus and coprolites may record the use of plants that were eaten. The formation of dental calculus (calcium phosphate deposits) over the life of the individual traps food particles, including phytoliths. Phytoliths in the calculus are well-protected and survive well in archaeological deposits. Once recovered from the teeth, they can provide a direct record of the plants ingested or treated using the teeth (e.g. some reeds might be chewed not for nutritional reason but to mollify them before they are worked into baskets or mats). In the interpretation of phytolith from dental calculus there is the need to be cautious, however, because they are often recovered in small numbers, making it difficult to interpret their significance. Both starches and phytoliths have been recovered from human calculus dating to Neolithic and more recent periods (Lalueza Fox and Pérez-Pérez, 1994; Lalueza Fox et al., 1996; Juan-Tresserras et al., 1997; Scott Cummings and Magennis, 1997).
- Henry A. and Piperno D. R. 2007. Using plant microfossils from dental calculus to recover human diet: a case study from Tell al-Raqā'i, Syria. Journal of Archaeological Science 35(7):1943-1950.
- Juan-Tresserras, C. Lalueza, R.M. Albert and M. Calvo 1997. Identification of phytoliths from prehistoric human dental remains from the Iberian Peninsula and the Balearic Islands. In: A. Pinilla, J. Juan-Tresserras and M.J. Machado, Editors, Primer encuentro Europeo sobre el estudio de fitolitos, CSIC, Madrid, pp. 197–203.
- Lalueza Fox, C, Pérez-Pérez, A and Juan-Tresserras, JJ 1994. Dietary information through the examination of plant phytoliths on the enamel surface of human dentition. Journal of Archaeological Science 21:29–34.
- Lalueza Fox, C, Juan, J and Albert, RM 1996. Phytolith analysis on dental calculus, enamel surface, and burial soil: information about diet and paleoenvironment. American Journal of Physical Anthropology 101:101–113.
- Scott Cummings, L and Magennis, A 1997. A phytolith and starch record of food and grit in Mayan human tooth tartar. In: A. Pinilla, J. Juan-Tresserras and M.J. Machado, Editors, Primer encuentro Europeo sobre el estudio de fitolitos, CSIC, Madrid, pp. 211–218.
Ethnoarchaeology and experimental archaeology
The analysis of resource utilization and management as well as the spatial organization of human settlements are fundamental for understanding socio-political and economic dynamics in past societies. A way to further an understanding of these dynamics is through an ethnoarchaeological approach. This has been already proposed for many areas of archaeological analysis but it is still in its infancy for phytolith studies.
As illustrated in the following photographs, almost any structure or spatial characteristic can be sampled to understand the phytolith input. These input can then be compared with the ones from archaeological sediments to better understand past activities and/or use of space.
- Shahack-Gross, R, Marshall, F and Weiner, S 2004. Geo-Ethnoarchaeology of Pastoral Sites: The Identification of Livestock Enclosures in Abandoned Maasai Settlements. Journal of Archaeological Science Volume 30:439-459.
- Tsartsidou, G, Lev-Yadun, S, Efstratiou, N and Weiner, S 2007. Ethnoarchaeological study of phytolith assemblages from an agro-pastoral village in Northern Greece (Sarakini): development and application of a Phytolith Difference Index. Journal of Archaeological Science 35:600-613.
- Zurro, D, Madella, M, Briz, I and Vila, A 2009. Variability of the phytolith record in fisher–hunter–gatherer sites: An example from the Yamana society (Beagle Channel, Tierra del Fuego, Argentina). Quaternary International 193:184-191.
The role of domesticated plants in the diet of prehistoric societies is sometime difficult to unravel because habitation sites are small and ephemeral, archaeological evidence of gardening is rare, the plant parts of interest are starchy tissues (roots, tubers, etc.) or macroremains can be sometime nonexistent (e.g. see the dearth of charred grains from Natufian sites in the Levant). In this cases a multiproxy study from the same deposit can be a useful tool, especially when applied to non-conventional evidence like carbonized food residues, utensils use residues, etc.
Multiproxy studies can include two or more of the following analyses:
- plant microfossils (phytolith, starch and pollen);
- stable C and N isotopes;
- lipids analysis;
- proteins analysis;
- carbohydrates analysis;
- mineralogical analyses (e.g. FTIR).
Also, multiproxy studies can include any or all of the above together with more classical approaches like charred macroremains, animal bones, etc.
- Boyd, M, Varney, T, Surette, C and Surette J 2008. Reassessing the northern limit of maize consumption in North America: stable isotope, plant microfossil, and trace element content of carbonized food residue. Journal of Archaeological Science 35:2545-2556.