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Issues and directions in phytolith analysis
Journal of Archaeological Science xxx (2016) 1e8
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Issues and directions in phytolith analysis Thomas C. Hart Anthropology Department, University of Texas at Austin, Austin, TX 78712, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 February 2016 Received in revised form 26 February 2016 Accepted 1 March 2016 Available online xxx
This special issue examines new trends in phytolith scholarship and assesses the future direction of this field of research. The papers presented represent a broader shift in phytolith research into a new phase called the “Period of Expanding Applications”. It is characterized by 1) a rapid increase in the number of phytolith publications; 2) a diversification of research topics; 3) a reassessment of the use of radiocarbon and other isotopes in phytoliths; 4) the development of digital technologies for refining and sharing phytolith identifications; 5) renewed efforts for standardization of phytolith nomenclature and laboratory protocol; and 6) the development of the field of applied phytolith research. This paper argues that interdisciplinary collaborations and a continued effort to understand the basics of phytolith production patterns are essential for the growth of the discipline and its application in archaeological studies. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Phytoliths Archaeology Paleoenvironment Archaeobotany Paleoethnobotany
1. Introduction Exploring how phytoliths are produced in plants, and what they can tell us about human activities in the past, has been the main focus of phytolith scholarship for more than 180 years. With the explosion of phytolith research across the globe starting around the year 2000, phytolith scholarship has entered a phase titled the “Period of Expanding Applications”. In this paper I review the historical trends in phytolith analysis leading up to and including this new era, discuss how the papers presented in this special edition fit into these trends, and offer some suggestions and cautionary notes about the direction of phytolith research in archaeology going forward. 2. History of phytolith research The history of phytolith research is a tale of dueling interests. Since the discovery of phytoliths, researchers have alternated between the need to do botanical research and the application of phytoliths to environmental and anthropological research questions. The first phytolith publication, during what Piperno (1988, 2006, p. 2) terms the “Discovery and Exploratory Phase1835e1895”, examined phytolith production in living plant tissues (Piperno, 1988, 2006; Struve, 1835). Scholars in Germany were quick to realize that phytoliths could be used in environmental
reconstructions (Ehrenberg, 1841, 1854; Piperno, 1988, 2006). From 1895 to 1936, during the “Botanical Phase of Research” (Piperno, 1988, 2006, p. 2), phytolith scholarship was centered on understanding comparative plant physiology and phytolith formation. The first archaeological applications occurred in the early twentieth century of this period (Netolitzky, 1900, 1914; Schellenberg, 1908). In the “Period of Ecological Research” from 1955 to 1975 soil scientists, ecologists, agronomists, and botanists continued to conduct botanical research but also conducted some of the first paleobotanical and paleoecological studies of ancient sediments (Piperno, 1988, 2006). The paper by Rovner (1971) is widely credited with bringing wider attention to the use of phytoliths for palaeoethnobotanical and archaeological research (Piperno, 1988, 2006). In the “Modern Period of Archaeological and Paleoenvironmental Research” (1978e2000) phytolith scholarship really established itself as an independent, important area of archaeology (Piperno, 1988, 2006). This period saw the first major expansion of archaeological phytolith research across the globe with a large number of projects focusing on reconstructing past environments and uncovering the origins and intensification of agriculture (Pearsall, in press). The International Society of Phytolith Research, now known as the International Phytolith Society, was founded in 1996. At the biennial conferences phytolith experts focus on the finer points of phytolith method and theory (e.g., Madella et al., 2005). Phytolith papers now regularly appear in a wide variety of journals ranging from publications with broad audiences like Science to the specialist reports in The Phytolitherian.
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3. The period of expanding applications, 2001 e present Since the turn of the century, the coalition of a number of factors has been leading us into a new phase of phytolith scholarship which I have termed the “Period of Expanding Applications.” It is characterized by 1) a rapid increase in the number of phytolith publications; 2) a diversification of research topics; 3) a reassessment of the use of carbon 14 and other isotopes in phytoliths; 4) the development of digital technologies for refining and sharing phytolith identifications; 5) renewed efforts for standardization of phytolith nomenclature and laboratory protocol; and 6) the development of the field of applied phytolith research. The papers in this special edition represent current topics of interest in phytolith research and point the way to the future.
3.1. Increase in the number of phytolith publications The first indication that we were entering a new period of phytolith research is the overall increase in number of phytolith publications since 1996. A basic English language search of the databases of the five major international publishing companies, Reed-Elsevier, Springer, Wiley-Blackwell, Taylor & Francis and Sage using the term “phytolith” in the publication title reveals an increase in the number of phytolith publications from around 1997 (Fig. 1). The average number of phytolith publications published per year from 1971 to 1996 was 1.03 (std ± 1.61). This increased dramatically between 1997 and 2015 (Fig. 1) to 13.58 (std ± 8.71) publications per year. In addition, the average number of journals that published articles with the word “phytolith” in the title also increased from 0.88 (std ± 1.34) per year before 1997 to 8.37 (std ± 5.09) journals per year (Fig. 2). These are likely to be conservative estimates because this search was limited to papers that only include the word “phytoliths” in the title of English language journals and does not reflect the diversity and growth of phytolith scholarship in other languages. This search
is also a conservative estimate because many papers involve phytolith research without explicitly stating so up front. While much of the increase in publications and journals can be attributed to the overall growth of the global economy and the development of new research facilities, it still represents a sizable change in the quantity of phytolith scholarship.
3.2. Diversification of research topics The second indication that we are entering a new period of research is the expansion and diversification of research topics. New areas in which phytoliths have contributed to archaeological research include plant use by early Homo sapiens and other hominins; ritual and burial practices; study of agricultural fields and paleosols; measures of anthropogenic burning; identification of beverages and spices; hunter-gatherer food ways; and agricultural and herding food traditions; (Pearsall, 2015, pp. 266e267). While artifact and dental calculus analyses are not entirely new lines of research, there has been a substantial increase in the number of studies as well as a concentrated effort to refine methodological and taphonomic issues (García-Granero et al., 2016; Hart, 2011; Henry et al., 2011; Henry and Piperno, 2008; Raviele, 2010, 2011). Scholars have expanded the pool of research topics that can be studied through phytolith analysis by examining phytolith production in previously untested, yet potentially important plant taxa. Prior to the Period of Expanding Research Applications, some of the most important comparative phytolith work focused on the origins and intensification of agriculture. This research was largely limited to the Lowland Neotropics (Piperno and Pearsall, 1998), Southwest Asia (Rosen, 1992) and, to a lesser extent, East Asia (Fujiwara, 1976, 1993), Southeast Asia (Wilson, 1985), and Africa (Alexandre et al., 1997). Research in the new millennium has refined some of these identifications such as bananas (Ball et al., 2006; Lentfer, 2009) and expanded to new crops such as Setaria and Panicum millets (Lu et al., 2009). For this special issue many of the leading experts in the field
Fig. 1. Number of papers with the word “phytolith” in the title when searching Reed-Elsevier, Springer, Wiley-Blackwell, Taylor & Francis and Sage publication companies. Note the dramatic rise in the number of publications after 1997.
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Fig. 2. Number of journals that include papers with the word “phytolith” in the title when searching Reed-Elsevier, Springer, Wiley-Blackwell, Taylor & Francis and Sage publication companies. Note the dramatic rise in the number of journals after 1997.
collaborated on a detailed and useful summary on how to identify crop phytoliths from around the world (Ball et al., in press a). This article centralizes what is known about these crops and provides the readers with the basics of how to identify crop phytoliths and where they can find additional bibliographic resources. Their study discusses many of the classic crops identified during the Modern Period of Archaeological and Paleoecological Research, such as maize and wheat, alongside some of the crops identified during the new Period of Expanding Research Applications, such as Setaria and Panicum millets. Overall, the majority of crops that can be identified through phytolith analysis belong to the grass family, partly because grasses produce large amounts of silica and because domesticated grasses play such an important role in crop production worldwide (Pearsall, 2015; Piperno, 2006). Comparative studies should continue during this new period by expanding to food taxa that are not as well known or understood archaeologically. The paper by Gu et al. (in press) represents renewed scholarship into economically important non-food crops such as bamboo. Previous research into non-food crops began in earnest during what Piperno (1988, 2006, p. 3) refers to as the “Botanical Phase of Research” (1895e1936), was halted for World War II, and then continued in the subsequent “Period of Ecological Research” (1955e1975). Gu et al. (in press) studies 26 woody bamboo species from the Dendrocalamopsis, Bambusa, and Dendrocalamus genera from southwest China. This study was designed to determine if these taxa can be identified through phytolith analysis and what this can tell us about their genetic relationships. Hierarchical cluster analysis of bulliform and saddle phytoliths reveals that Dendocalamopsis can be differentiated from Bambusa and Dendrocalamus bamboo taxa through their phytolith shapes and that Dendrocalamopsos should be an independent genus within Bambusoideae. This study reaffirms the interdisciplinary nature of phytolith research and the study of non-food crops can broaden the range of taxa that can be identified archaeologically. Lastly, the diversification of research topics starting in the new
millennium has extended beyond the field of archaeology to paleontology, paleobotany, and primatology. The application of phytolith studies to paleontological and paleobotanical research after the year 2000 has dispelled the notion that research into plants, diet, and the environment are limited to the recent past. For instance, phytoliths recovered from paleosols were used to argue that C3 grasses dominated the grass populations of North America € mberg, 2002). Phytoliths as until about 16 million years ago (Stro indicators of diet are also preserved in ancient Cretaceous period dinosaur corpolites (Piperno and Sues, 2005; Prasad, 2005). The recovery of phytoliths contained in modern primate feces (Phillips and Lancelotti, 2014) and dental calculus (Henry, 2012; Power et al., 2015) broadens the research possibilities for archaeologists by providing for potential primate analogs for studying diet and behavior among early hominins.
3.3. A reassessment of the use of carbon 14 and other isotopes in phytolith analysis Radiocarbon dating and isotope analyses are two very important components of contemporary archaeological and paleoecological research Their role in archaeology, and the challenges they face, has been the subject of much recent consideration (Canti and Huisman, 2015; Hellstrom and Pickering, 2015; Makarewicz and Sealy, 2015; Wood, 2015). Radiocarbon dating of phytoliths is a long established technique that can be traced back to the Period of Ecological Research (1955e1975) (Piperno, 1988, 2006; Wilding, 1967). Recently, several studies (Santos et al., 2010, 2012; Yin et al., 2014) have begun to question whether phytoliths are a reliable source of carbon for radiocarbon dating due to abnormal radiocarbon readings. Piperno (in press) reviews these latest discussions and provides new evidence in support of phytolith radiocarbon dating. Traditionally, phytoliths have been thought to be a reliable source of C14 because they encapsulate carbon taken in from the atmosphere during photosynthesis. However, modern grass samples from North America, France, and Australia were tested for
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radiocarbon dates and gave abnormal age ranges from 2000 to 8000 BP (Santos et al., 2010; Sullivan and Parr, 2008). In addition, modern Chinese rice and millet samples yielded dates of several hundred to 1000 years old (Yin et al., 2014). Santos et al. (2012) propose that there are actually two pools of carbon contained in phytoliths, older carbon from dissolved soil organic matter (SOM) and younger carbon taken in from the atmosphere. The presence of older SOM in the silica provides for the older ages of the plant phytoliths. In contrast, Yin et al. (2014) suggest that differences in combustion resistance may account for the age differences they encountered. Piperno (in press) obtained radiocarbon dates on modern plant samples of Zea mays, Cucurbita ecuadorensis, Cucurbita ficifolia, Hirtella americana, and Socratea durissima. Almost all of the samples returned expected postbomb dates associated with when they were collected in the field. The one exception was a sample of Hirtella americana that had a date of 1570e1420 cal BP. An investigation of the herbarium practices used to preserve the reference sample revealed that the voucher specimen was originally treated with lauryl pentachlorophenate and paradichlorobenzene, chemicals that are rich in radiocarbon-dead carbon. These chemicals, and other similar factors, can give false radiocarbon dates if not properly removed during pretreatment. This study demonstrates the importance of understanding the effectiveness of the pretreatment of radiocarbon-dated materials given the variety of processing procedures available. Studying how carbon becomes trapped in phytoliths will undoubtedly continue given its importance as a tool for radiocarbon dating and as a potential means of removing carbon dioxide from the atmosphere. Another important area of research for the future is understanding how other isotopes, such as strontium and calcium, are deposited in phytoliths. Hodson's (in press) paper, brings together the available information in the literature and helps clarify some misconceptions. Soluble silica, which is usually present as monosilic acid (H4SiO4) in soils with a pH at or below 9, is usually transported through the xylem and is deposited in three possible locations: the cell lumen; the cell wall; and/or the intercellular spaces or in an extracellular layer. Since phytoliths are formed differently in these locations, Hodson (in press) argues that we should expect they would have different isotopic signatures. Phytoliths contain at least 20 different elements in their silica and trapped organic matter. Hodson (in press) discusses the presence of calcium, aluminum, nitrogen, silicon, oxygen, and carbon isotopes. In each one of the isotopes, he discusses what is known about them and how they potentially could be used in archaeological research. For example, Hodson (in press) discusses how silicon isotopes fractionate within plants depending upon where they are located in the plant structure. The differential concentration of heavy and light silicon isotopes within a plant could provide another avenue for identifying plant parts archaeologically. Hodson (in press) proposes that a poor understanding of the fractionation of isotopes within the plant remains the main obstacle for using phytolith isotopes for paleoenvironmental reconstruction. This problem can only be remedied through further analysis of how phytoliths are formed in plant tissues.
3.4. The development of digital technologies for refining and sharing phytolith identifications The combination of new digital technologies and the development of online databases is another key indicator of a new period of phytolith research. Several of the papers in this special issue address these topics and provide examples of how new digital technology can improve phytolith research.
3.4.1. Advances in morphometric analysis As new computer technologies develop, so does the ability to describe, measure, and assign phytoliths to specific plant taxa through morphometric analysis. In morphotypic analysis, as illustrated by Ball et al. (in press a) and Gu et al. (in press), a hierarchical system of simple to complex phytolith features is useful for describing and assigning taxonomic affiliation to individual phytolith types. The key element of this kind of analysis is that morphotypes are easily distinguished from each other due to some unique feature such as size and/or shape. Morphometric analysis, in contrast, is an approach that uses an increasingly complex set of criteria to differentiate between phytoliths that have similar morphotypes through the use of computer imaging technology and statistical software programs (Ball et al., 1996, 1999). Building upon the morphometric guidelines presented in Ball et al. (in press b), Evett and Cuthrell (in press) lay out the conceptual framework and steps needed for the development of a computer-assisted phytolith identification system. The benefits of developing a computer assisted analysis program include minimizing observer bias, establishing consistency in identification between labs, and developing finer grained identifications. The ultimate goal is the development of a software program that automatically identifies and counts phytoliths in sediment samples, thereby standardizing identifications and reducing overall research time. Another advancement in morphometric analysis is the integration of phytolith analysis with micromorphology, and morphometric approaches used in soil analysis. There are two issues that have continued to pose problems in phytolith identifications: multiplicity and redundancy. Multiplicity arises when a single taxon produces a range of phytolith shapes and sizes. Redundancy is when these same shapes are produced by many different taxa making taxonomic identification difficult. This issue is compounded further when the phytoliths become disarticulated either during deposition in the sediments or as a consequence of routine phytolith extraction protocols. Disarticulation makes morphometric analyses, such as those used by Ball et al. (in press b) and Evett and Cuthrell (in press), more challenging because articulated form are necessary for this type of analysis. Vrydaghs et al. (in press) attempt to tackle redundancy and multiplicity by comparing the identification of phytoliths using micromorphological techniques with a standardized phytolith extraction protocol. In their study, thin sections were analyzed from five historic period sites in downtown Brussels and compared with two typical phytolith extractions from the same contexts. The phytoliths recovered from the traditional approach were more disarticulated than the ones analyzed in the thin section making it difficult to conduct morphometric analysis. In contrast, the articulated phytoliths preserved in the thin sections provided better opportunities for morphometric analyses because the researchers were confident that the phytoliths were all deposited together and came from the same plant, a critical step forward in the application of morphometric analysis to phytolith identification. 3.4.2. Advances in online databases Phytolith scholars have been quick to recognize the potential of the Internet for sharing and discussing phytoliths through websites and online databases. Some scholars have created websites composed of a collection of phytolith images organized by plant taxon and by region (Table 1). Other scholars established searchable databases online that were the result of specific comparative collection studies or comparative collections associated with archaeological research. At the time of this publication, two searchable databases were exclusively devoted to phytoliths, the Flora of Ecuador database and
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Table 1 Digital collections of phytolith images. Website
Web address
Dorian Fuller (Old World Phytoliths) Mikhail Blinnikov- (Pacific Northwest) Dolores Piperno- (Lowland Neotropics)
http://www.homepages.ucl.ac.uk/~tcrndfu/phytoliths.html http://web.stcloudstate.edu/msblinnikov/phd/phyt.html http://www.mnh.si.edu/highlight/phytoliths/
the PhytoCore database. A third database, also provides a searchable database of phytoliths that also includes pollen, starch, and macrobotanical images as well (Table 2). Unfortunately, as quickly as some of these online tools became available, many of them have disappeared. Staples of online phytolith research that are no longer active include Terry Ball's New World and Old World phytolith image collection, the Colonial Williamsburg Database and the Wits Online Phytolith Database (Table 2). The absence of these databases are felt throughout the phytolith community and provide good examples as to why phytolith researchers should not rely upon online databases as their sole source of reference material information. Despite the potential loss of information when a website disappears, the online sharing of archaeological and comparative plant data is critical for the advancement of phytolith research and is continuing to grow during the New Era of Phytolith Research. The Phytoliths in the Flora of Ecuador (Pearsall, in press) and the PhytCore 2.0 (Albert et al., in press) represent two different approaches to online phytolith database systems. The first is a database that is an outgrowth from an individualized project focused on defining phytolith types and vegetation signatures for a specific region. In contrast, PhytCore 2.0 represents the next step in online database management because it can incorporate the data from a project such as the Phytoliths in the Flora of Ecuador into a larger database that can be used to compare phytolith types around the globe thereby facilitating advances in phytolith morphologies and identification.
research, definitions and mathematical formulas for the morphometric parameters to be measured, and protocol for making these data accessible to interested scholars. These recommendations fall in line with similar efforts by the International Phytolith Society to standardize methodologies and terminology such as the International Code for Phytolith Nomenclature (Madella et al., 2005). Following on from these recommendations Zurro et al. (in press) set out to provide some standardized guidelines and minimum standards for phytolith research around the globe. The authors cover a broad range of topics stretching from the formation of research questions, which must be made explicit, to the kinds of new research that would advance the discipline. They note that phytolith deposition and preservation are issues that any researcher should take into consideration when interpreting the phytolith record. They acknowledge that there are many different processing procedures but that they mostly share similar recovery rates. The phrase “phytolith taxonomy” should be used for the description, identification, and classification of phytoliths rather than “systematics” because very few phytolith studies deal with phylogenetic or evolutionary aspects of plants (Zurro et al., in press). One of the more interesting suggestions they make is that measures of ubiquity and frequency should become incorporated into more phytolith studies. Ubiquity and frequency are common measures of assessing the overrepresentation and underrepresentation of taxa in macrobotanical studies (Pearsall, 2015). The incorporation of these types of analyses would greatly aid in our understanding of phytolith depositional patterns.
3.5. Renewed efforts at standardization
3.6. Applied phytolith research
With the expansion of phytolith scholarship at the turn of the millennium comes renewed efforts at standardizing phytolith terminology and methodologies. An International Committee for Phytolith Morphometrics was created at the 8th International Meetings for Phytoliths in 2011 in order to standardize the terminology used in morphometric studies and to recommend methods, measurements, and protocols for morphometric analysis in response to these advances in technology and the proliferation of morphometric studies (Ball et al., in press a, b). Previously, the techniques used in morphometric analysis were scattered throughout the literature making it very difficult to 1) reproduce the results of many of the studies and 2) compare the results in the literature due to a lack of standardized terminology and protocol. This paper provides a flow chart of how to conduct morphometric
The last and final set of evidence that we have entered a new period of archaeological phytolith research is the development of the field of applied phytolith research. Adapting the definition of applied anthropology originally developed by Chambers (1987, p. 309), applied phytolith research can be characterized as the field of inquiry concerned with the relationships between phytolith-based inquiry and the uses of that knowledge in other fields. One important area of research is the use of phytoliths for carbon sequestration as a tool for removing carbon dioxide from the atmosphere to combat climate change (Jansson et al., 2010; Li et al., 2013; Parr et al., 2009, 2010; Parr and Sullivan, 2005, 2011, 2014; Song et al., 2012, 2013a, 2013b; Yang et al., 2015a, b; Zuo et al., 2014; Zuo and Lü, 2011). Phytoliths are also being used in the new field of forensic botany (Blackledge, 2007; Charlier et al., 2010;
Table 2 Active and inactive online phytolith databases. Website
Web address
Active Flora of Ecuador database http://phytolith.missouri.edu/ PhytoCore database http://www.phytcore.org/phytolith/index.php?rdm¼JwJIyWMKbZ&action¼home Paleobot.org http://www.paleobot.org/ Inactive Terry Ball's New World and Old World phytolith image collection http://webpub.byu.net/tbb/ The Colonial Williamsburg database http://research.history.org/archaeological_research/collections/collarchchaeobot/phytolithSearch.cfm Wits Online Phytolith database http://www.wits.ac.za/Academic/Science/GeoSciences/BPI/Research/WOPD/WOPDHome.htm
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Houk, 2004) and hold potential for use in the nanotechnology industry (Neethirajan et al., 2009). Finally, phytoliths are playing an important role in vegetation restoration efforts in the Great Basin Desert, USA (Morris et al., 2009, 2010; Pearsall, 2015), California grasslands, USA, (Evett and Bartolome, 2013), James Madison's Montpelier, Virginia, USA (Hart, 2015), and on the Chinese Loess Plateau (Jiao et al., 2012; Lu et al., 1999, 2006, 2007). While not directly related to archaeology, many of the above projects have their roots in archaeological phytolith research.
our knowledge of the relationship between phytoliths and plant anatomy and physiology and is an interdisciplinary endeavor by its very nature (Fig. 3). Because of this, archaeologists are able to use phytoliths to make significant contributions to our understanding of the human past. However, as the number and diversity of archaeological applications continues to grow, so too should the number and diversity of comparative phytolith projects. Going forward, continued comparative research is essential for understanding isotopic deposition in phytoliths as well as finding and publishing the limitations of using phytoliths to identify ancient plant taxa.
4. Concluding thoughts and cautionary note It has been 181 years since phytolith research began as a dissertation project in Germany that sought to understand the shapes of silica bodies produced in living plants (Piperno, 1988, 2006; Struve, 1835). Since that time, research interests have alternated between comparative plant studies and archaeological and paleoecological studies. Starting around the year 2000, archaeological applications of phytolith scholarship began to grow and diversify ushering in the “Period of Expanding Applications”. This era is characterized by a rapid increase in the number of articles published per year; a diversification of research topics; a reassessment of the use of carbon 14 and isotopes in phytoliths; the development of digital technologies for refining and sharing phytolith identifications; renewed efforts for standardization of phytolith nomenclature and laboratory protocol; and the development of the field of applied phytolith research. The foundation of archaeological phytolith research stems from
Role of the funding source This project was funded through personal finances. No outside source of funding was involved in any stage of this project. Acknowledgments This special edition was truly an international effort and was not possible without the excitement, drive, and steadfastness of all those involved, so thank you to all who participated in this special edition. This special edition was an outgrowth of the 2014 session titled “Issues and Directions in Phytolith Research” held at the Society for American Archaeology annual meetings in Austin, Texas. Thank you to all of the session participants for their presentations and the ensuing discussions over food and drinks. It was a pleasure to work with such fine scholars. I want to thank all the authors for
Fig. 3. A phytolith “Tree of Life”. This illustrates the interconnectedness of phytolith research and how all research can be traced back to a basic understanding of plant anatomy.
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their hard work in making this volume dedicated to phytolith research a success. Thank you to all the reviewers who reviewed the special edition manuscripts; your participation and intellectual input were critical for improving these papers. A special thank you goes out to Robin Torrence and the editorial staff at the Journal of Archaeological Science for their willingness to take on such a project and for their patience and assistance in seeing this special edition through to completion. I would also like to extend a special thank you to Deborah Pearsall, Dolores Piperno, Alexia Smith, Natalie Munro, and Arlene Rosen who helped with various stages of this project and politely replied to e-mails, phone calls, and office hour visits too numerous to count. And lastly, I'd like to thank the anonymous reviewers for their comments and suggestions in helping to improve this article. References Albert R.M., Ruíz J.A. and Sans A. 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Journal of Archaeological Science journal homepage: http://www.elsevier.com/locate/jas
Issues and directions in phytolith analysis Thomas C. Hart Anthropology Department, University of Texas at Austin, Austin, TX 78712, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 February 2016 Received in revised form 26 February 2016 Accepted 1 March 2016 Available online xxx
This special issue examines new trends in phytolith scholarship and assesses the future direction of this field of research. The papers presented represent a broader shift in phytolith research into a new phase called the “Period of Expanding Applications”. It is characterized by 1) a rapid increase in the number of phytolith publications; 2) a diversification of research topics; 3) a reassessment of the use of radiocarbon and other isotopes in phytoliths; 4) the development of digital technologies for refining and sharing phytolith identifications; 5) renewed efforts for standardization of phytolith nomenclature and laboratory protocol; and 6) the development of the field of applied phytolith research. This paper argues that interdisciplinary collaborations and a continued effort to understand the basics of phytolith production patterns are essential for the growth of the discipline and its application in archaeological studies. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Phytoliths Archaeology Paleoenvironment Archaeobotany Paleoethnobotany
1. Introduction Exploring how phytoliths are produced in plants, and what they can tell us about human activities in the past, has been the main focus of phytolith scholarship for more than 180 years. With the explosion of phytolith research across the globe starting around the year 2000, phytolith scholarship has entered a phase titled the “Period of Expanding Applications”. In this paper I review the historical trends in phytolith analysis leading up to and including this new era, discuss how the papers presented in this special edition fit into these trends, and offer some suggestions and cautionary notes about the direction of phytolith research in archaeology going forward. 2. History of phytolith research The history of phytolith research is a tale of dueling interests. Since the discovery of phytoliths, researchers have alternated between the need to do botanical research and the application of phytoliths to environmental and anthropological research questions. The first phytolith publication, during what Piperno (1988, 2006, p. 2) terms the “Discovery and Exploratory Phase1835e1895”, examined phytolith production in living plant tissues (Piperno, 1988, 2006; Struve, 1835). Scholars in Germany were quick to realize that phytoliths could be used in environmental
reconstructions (Ehrenberg, 1841, 1854; Piperno, 1988, 2006). From 1895 to 1936, during the “Botanical Phase of Research” (Piperno, 1988, 2006, p. 2), phytolith scholarship was centered on understanding comparative plant physiology and phytolith formation. The first archaeological applications occurred in the early twentieth century of this period (Netolitzky, 1900, 1914; Schellenberg, 1908). In the “Period of Ecological Research” from 1955 to 1975 soil scientists, ecologists, agronomists, and botanists continued to conduct botanical research but also conducted some of the first paleobotanical and paleoecological studies of ancient sediments (Piperno, 1988, 2006). The paper by Rovner (1971) is widely credited with bringing wider attention to the use of phytoliths for palaeoethnobotanical and archaeological research (Piperno, 1988, 2006). In the “Modern Period of Archaeological and Paleoenvironmental Research” (1978e2000) phytolith scholarship really established itself as an independent, important area of archaeology (Piperno, 1988, 2006). This period saw the first major expansion of archaeological phytolith research across the globe with a large number of projects focusing on reconstructing past environments and uncovering the origins and intensification of agriculture (Pearsall, in press). The International Society of Phytolith Research, now known as the International Phytolith Society, was founded in 1996. At the biennial conferences phytolith experts focus on the finer points of phytolith method and theory (e.g., Madella et al., 2005). Phytolith papers now regularly appear in a wide variety of journals ranging from publications with broad audiences like Science to the specialist reports in The Phytolitherian.
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.jas.2016.03.001 0305-4403/© 2016 Elsevier Ltd. All rights reserved.
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3. The period of expanding applications, 2001 e present Since the turn of the century, the coalition of a number of factors has been leading us into a new phase of phytolith scholarship which I have termed the “Period of Expanding Applications.” It is characterized by 1) a rapid increase in the number of phytolith publications; 2) a diversification of research topics; 3) a reassessment of the use of carbon 14 and other isotopes in phytoliths; 4) the development of digital technologies for refining and sharing phytolith identifications; 5) renewed efforts for standardization of phytolith nomenclature and laboratory protocol; and 6) the development of the field of applied phytolith research. The papers in this special edition represent current topics of interest in phytolith research and point the way to the future.
3.1. Increase in the number of phytolith publications The first indication that we were entering a new period of phytolith research is the overall increase in number of phytolith publications since 1996. A basic English language search of the databases of the five major international publishing companies, Reed-Elsevier, Springer, Wiley-Blackwell, Taylor & Francis and Sage using the term “phytolith” in the publication title reveals an increase in the number of phytolith publications from around 1997 (Fig. 1). The average number of phytolith publications published per year from 1971 to 1996 was 1.03 (std ± 1.61). This increased dramatically between 1997 and 2015 (Fig. 1) to 13.58 (std ± 8.71) publications per year. In addition, the average number of journals that published articles with the word “phytolith” in the title also increased from 0.88 (std ± 1.34) per year before 1997 to 8.37 (std ± 5.09) journals per year (Fig. 2). These are likely to be conservative estimates because this search was limited to papers that only include the word “phytoliths” in the title of English language journals and does not reflect the diversity and growth of phytolith scholarship in other languages. This search
is also a conservative estimate because many papers involve phytolith research without explicitly stating so up front. While much of the increase in publications and journals can be attributed to the overall growth of the global economy and the development of new research facilities, it still represents a sizable change in the quantity of phytolith scholarship.
3.2. Diversification of research topics The second indication that we are entering a new period of research is the expansion and diversification of research topics. New areas in which phytoliths have contributed to archaeological research include plant use by early Homo sapiens and other hominins; ritual and burial practices; study of agricultural fields and paleosols; measures of anthropogenic burning; identification of beverages and spices; hunter-gatherer food ways; and agricultural and herding food traditions; (Pearsall, 2015, pp. 266e267). While artifact and dental calculus analyses are not entirely new lines of research, there has been a substantial increase in the number of studies as well as a concentrated effort to refine methodological and taphonomic issues (García-Granero et al., 2016; Hart, 2011; Henry et al., 2011; Henry and Piperno, 2008; Raviele, 2010, 2011). Scholars have expanded the pool of research topics that can be studied through phytolith analysis by examining phytolith production in previously untested, yet potentially important plant taxa. Prior to the Period of Expanding Research Applications, some of the most important comparative phytolith work focused on the origins and intensification of agriculture. This research was largely limited to the Lowland Neotropics (Piperno and Pearsall, 1998), Southwest Asia (Rosen, 1992) and, to a lesser extent, East Asia (Fujiwara, 1976, 1993), Southeast Asia (Wilson, 1985), and Africa (Alexandre et al., 1997). Research in the new millennium has refined some of these identifications such as bananas (Ball et al., 2006; Lentfer, 2009) and expanded to new crops such as Setaria and Panicum millets (Lu et al., 2009). For this special issue many of the leading experts in the field
Fig. 1. Number of papers with the word “phytolith” in the title when searching Reed-Elsevier, Springer, Wiley-Blackwell, Taylor & Francis and Sage publication companies. Note the dramatic rise in the number of publications after 1997.
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Fig. 2. Number of journals that include papers with the word “phytolith” in the title when searching Reed-Elsevier, Springer, Wiley-Blackwell, Taylor & Francis and Sage publication companies. Note the dramatic rise in the number of journals after 1997.
collaborated on a detailed and useful summary on how to identify crop phytoliths from around the world (Ball et al., in press a). This article centralizes what is known about these crops and provides the readers with the basics of how to identify crop phytoliths and where they can find additional bibliographic resources. Their study discusses many of the classic crops identified during the Modern Period of Archaeological and Paleoecological Research, such as maize and wheat, alongside some of the crops identified during the new Period of Expanding Research Applications, such as Setaria and Panicum millets. Overall, the majority of crops that can be identified through phytolith analysis belong to the grass family, partly because grasses produce large amounts of silica and because domesticated grasses play such an important role in crop production worldwide (Pearsall, 2015; Piperno, 2006). Comparative studies should continue during this new period by expanding to food taxa that are not as well known or understood archaeologically. The paper by Gu et al. (in press) represents renewed scholarship into economically important non-food crops such as bamboo. Previous research into non-food crops began in earnest during what Piperno (1988, 2006, p. 3) refers to as the “Botanical Phase of Research” (1895e1936), was halted for World War II, and then continued in the subsequent “Period of Ecological Research” (1955e1975). Gu et al. (in press) studies 26 woody bamboo species from the Dendrocalamopsis, Bambusa, and Dendrocalamus genera from southwest China. This study was designed to determine if these taxa can be identified through phytolith analysis and what this can tell us about their genetic relationships. Hierarchical cluster analysis of bulliform and saddle phytoliths reveals that Dendocalamopsis can be differentiated from Bambusa and Dendrocalamus bamboo taxa through their phytolith shapes and that Dendrocalamopsos should be an independent genus within Bambusoideae. This study reaffirms the interdisciplinary nature of phytolith research and the study of non-food crops can broaden the range of taxa that can be identified archaeologically. Lastly, the diversification of research topics starting in the new
millennium has extended beyond the field of archaeology to paleontology, paleobotany, and primatology. The application of phytolith studies to paleontological and paleobotanical research after the year 2000 has dispelled the notion that research into plants, diet, and the environment are limited to the recent past. For instance, phytoliths recovered from paleosols were used to argue that C3 grasses dominated the grass populations of North America € mberg, 2002). Phytoliths as until about 16 million years ago (Stro indicators of diet are also preserved in ancient Cretaceous period dinosaur corpolites (Piperno and Sues, 2005; Prasad, 2005). The recovery of phytoliths contained in modern primate feces (Phillips and Lancelotti, 2014) and dental calculus (Henry, 2012; Power et al., 2015) broadens the research possibilities for archaeologists by providing for potential primate analogs for studying diet and behavior among early hominins.
3.3. A reassessment of the use of carbon 14 and other isotopes in phytolith analysis Radiocarbon dating and isotope analyses are two very important components of contemporary archaeological and paleoecological research Their role in archaeology, and the challenges they face, has been the subject of much recent consideration (Canti and Huisman, 2015; Hellstrom and Pickering, 2015; Makarewicz and Sealy, 2015; Wood, 2015). Radiocarbon dating of phytoliths is a long established technique that can be traced back to the Period of Ecological Research (1955e1975) (Piperno, 1988, 2006; Wilding, 1967). Recently, several studies (Santos et al., 2010, 2012; Yin et al., 2014) have begun to question whether phytoliths are a reliable source of carbon for radiocarbon dating due to abnormal radiocarbon readings. Piperno (in press) reviews these latest discussions and provides new evidence in support of phytolith radiocarbon dating. Traditionally, phytoliths have been thought to be a reliable source of C14 because they encapsulate carbon taken in from the atmosphere during photosynthesis. However, modern grass samples from North America, France, and Australia were tested for
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radiocarbon dates and gave abnormal age ranges from 2000 to 8000 BP (Santos et al., 2010; Sullivan and Parr, 2008). In addition, modern Chinese rice and millet samples yielded dates of several hundred to 1000 years old (Yin et al., 2014). Santos et al. (2012) propose that there are actually two pools of carbon contained in phytoliths, older carbon from dissolved soil organic matter (SOM) and younger carbon taken in from the atmosphere. The presence of older SOM in the silica provides for the older ages of the plant phytoliths. In contrast, Yin et al. (2014) suggest that differences in combustion resistance may account for the age differences they encountered. Piperno (in press) obtained radiocarbon dates on modern plant samples of Zea mays, Cucurbita ecuadorensis, Cucurbita ficifolia, Hirtella americana, and Socratea durissima. Almost all of the samples returned expected postbomb dates associated with when they were collected in the field. The one exception was a sample of Hirtella americana that had a date of 1570e1420 cal BP. An investigation of the herbarium practices used to preserve the reference sample revealed that the voucher specimen was originally treated with lauryl pentachlorophenate and paradichlorobenzene, chemicals that are rich in radiocarbon-dead carbon. These chemicals, and other similar factors, can give false radiocarbon dates if not properly removed during pretreatment. This study demonstrates the importance of understanding the effectiveness of the pretreatment of radiocarbon-dated materials given the variety of processing procedures available. Studying how carbon becomes trapped in phytoliths will undoubtedly continue given its importance as a tool for radiocarbon dating and as a potential means of removing carbon dioxide from the atmosphere. Another important area of research for the future is understanding how other isotopes, such as strontium and calcium, are deposited in phytoliths. Hodson's (in press) paper, brings together the available information in the literature and helps clarify some misconceptions. Soluble silica, which is usually present as monosilic acid (H4SiO4) in soils with a pH at or below 9, is usually transported through the xylem and is deposited in three possible locations: the cell lumen; the cell wall; and/or the intercellular spaces or in an extracellular layer. Since phytoliths are formed differently in these locations, Hodson (in press) argues that we should expect they would have different isotopic signatures. Phytoliths contain at least 20 different elements in their silica and trapped organic matter. Hodson (in press) discusses the presence of calcium, aluminum, nitrogen, silicon, oxygen, and carbon isotopes. In each one of the isotopes, he discusses what is known about them and how they potentially could be used in archaeological research. For example, Hodson (in press) discusses how silicon isotopes fractionate within plants depending upon where they are located in the plant structure. The differential concentration of heavy and light silicon isotopes within a plant could provide another avenue for identifying plant parts archaeologically. Hodson (in press) proposes that a poor understanding of the fractionation of isotopes within the plant remains the main obstacle for using phytolith isotopes for paleoenvironmental reconstruction. This problem can only be remedied through further analysis of how phytoliths are formed in plant tissues.
3.4. The development of digital technologies for refining and sharing phytolith identifications The combination of new digital technologies and the development of online databases is another key indicator of a new period of phytolith research. Several of the papers in this special issue address these topics and provide examples of how new digital technology can improve phytolith research.
3.4.1. Advances in morphometric analysis As new computer technologies develop, so does the ability to describe, measure, and assign phytoliths to specific plant taxa through morphometric analysis. In morphotypic analysis, as illustrated by Ball et al. (in press a) and Gu et al. (in press), a hierarchical system of simple to complex phytolith features is useful for describing and assigning taxonomic affiliation to individual phytolith types. The key element of this kind of analysis is that morphotypes are easily distinguished from each other due to some unique feature such as size and/or shape. Morphometric analysis, in contrast, is an approach that uses an increasingly complex set of criteria to differentiate between phytoliths that have similar morphotypes through the use of computer imaging technology and statistical software programs (Ball et al., 1996, 1999). Building upon the morphometric guidelines presented in Ball et al. (in press b), Evett and Cuthrell (in press) lay out the conceptual framework and steps needed for the development of a computer-assisted phytolith identification system. The benefits of developing a computer assisted analysis program include minimizing observer bias, establishing consistency in identification between labs, and developing finer grained identifications. The ultimate goal is the development of a software program that automatically identifies and counts phytoliths in sediment samples, thereby standardizing identifications and reducing overall research time. Another advancement in morphometric analysis is the integration of phytolith analysis with micromorphology, and morphometric approaches used in soil analysis. There are two issues that have continued to pose problems in phytolith identifications: multiplicity and redundancy. Multiplicity arises when a single taxon produces a range of phytolith shapes and sizes. Redundancy is when these same shapes are produced by many different taxa making taxonomic identification difficult. This issue is compounded further when the phytoliths become disarticulated either during deposition in the sediments or as a consequence of routine phytolith extraction protocols. Disarticulation makes morphometric analyses, such as those used by Ball et al. (in press b) and Evett and Cuthrell (in press), more challenging because articulated form are necessary for this type of analysis. Vrydaghs et al. (in press) attempt to tackle redundancy and multiplicity by comparing the identification of phytoliths using micromorphological techniques with a standardized phytolith extraction protocol. In their study, thin sections were analyzed from five historic period sites in downtown Brussels and compared with two typical phytolith extractions from the same contexts. The phytoliths recovered from the traditional approach were more disarticulated than the ones analyzed in the thin section making it difficult to conduct morphometric analysis. In contrast, the articulated phytoliths preserved in the thin sections provided better opportunities for morphometric analyses because the researchers were confident that the phytoliths were all deposited together and came from the same plant, a critical step forward in the application of morphometric analysis to phytolith identification. 3.4.2. Advances in online databases Phytolith scholars have been quick to recognize the potential of the Internet for sharing and discussing phytoliths through websites and online databases. Some scholars have created websites composed of a collection of phytolith images organized by plant taxon and by region (Table 1). Other scholars established searchable databases online that were the result of specific comparative collection studies or comparative collections associated with archaeological research. At the time of this publication, two searchable databases were exclusively devoted to phytoliths, the Flora of Ecuador database and
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Table 1 Digital collections of phytolith images. Website
Web address
Dorian Fuller (Old World Phytoliths) Mikhail Blinnikov- (Pacific Northwest) Dolores Piperno- (Lowland Neotropics)
http://www.homepages.ucl.ac.uk/~tcrndfu/phytoliths.html http://web.stcloudstate.edu/msblinnikov/phd/phyt.html http://www.mnh.si.edu/highlight/phytoliths/
the PhytoCore database. A third database, also provides a searchable database of phytoliths that also includes pollen, starch, and macrobotanical images as well (Table 2). Unfortunately, as quickly as some of these online tools became available, many of them have disappeared. Staples of online phytolith research that are no longer active include Terry Ball's New World and Old World phytolith image collection, the Colonial Williamsburg Database and the Wits Online Phytolith Database (Table 2). The absence of these databases are felt throughout the phytolith community and provide good examples as to why phytolith researchers should not rely upon online databases as their sole source of reference material information. Despite the potential loss of information when a website disappears, the online sharing of archaeological and comparative plant data is critical for the advancement of phytolith research and is continuing to grow during the New Era of Phytolith Research. The Phytoliths in the Flora of Ecuador (Pearsall, in press) and the PhytCore 2.0 (Albert et al., in press) represent two different approaches to online phytolith database systems. The first is a database that is an outgrowth from an individualized project focused on defining phytolith types and vegetation signatures for a specific region. In contrast, PhytCore 2.0 represents the next step in online database management because it can incorporate the data from a project such as the Phytoliths in the Flora of Ecuador into a larger database that can be used to compare phytolith types around the globe thereby facilitating advances in phytolith morphologies and identification.
research, definitions and mathematical formulas for the morphometric parameters to be measured, and protocol for making these data accessible to interested scholars. These recommendations fall in line with similar efforts by the International Phytolith Society to standardize methodologies and terminology such as the International Code for Phytolith Nomenclature (Madella et al., 2005). Following on from these recommendations Zurro et al. (in press) set out to provide some standardized guidelines and minimum standards for phytolith research around the globe. The authors cover a broad range of topics stretching from the formation of research questions, which must be made explicit, to the kinds of new research that would advance the discipline. They note that phytolith deposition and preservation are issues that any researcher should take into consideration when interpreting the phytolith record. They acknowledge that there are many different processing procedures but that they mostly share similar recovery rates. The phrase “phytolith taxonomy” should be used for the description, identification, and classification of phytoliths rather than “systematics” because very few phytolith studies deal with phylogenetic or evolutionary aspects of plants (Zurro et al., in press). One of the more interesting suggestions they make is that measures of ubiquity and frequency should become incorporated into more phytolith studies. Ubiquity and frequency are common measures of assessing the overrepresentation and underrepresentation of taxa in macrobotanical studies (Pearsall, 2015). The incorporation of these types of analyses would greatly aid in our understanding of phytolith depositional patterns.
3.5. Renewed efforts at standardization
3.6. Applied phytolith research
With the expansion of phytolith scholarship at the turn of the millennium comes renewed efforts at standardizing phytolith terminology and methodologies. An International Committee for Phytolith Morphometrics was created at the 8th International Meetings for Phytoliths in 2011 in order to standardize the terminology used in morphometric studies and to recommend methods, measurements, and protocols for morphometric analysis in response to these advances in technology and the proliferation of morphometric studies (Ball et al., in press a, b). Previously, the techniques used in morphometric analysis were scattered throughout the literature making it very difficult to 1) reproduce the results of many of the studies and 2) compare the results in the literature due to a lack of standardized terminology and protocol. This paper provides a flow chart of how to conduct morphometric
The last and final set of evidence that we have entered a new period of archaeological phytolith research is the development of the field of applied phytolith research. Adapting the definition of applied anthropology originally developed by Chambers (1987, p. 309), applied phytolith research can be characterized as the field of inquiry concerned with the relationships between phytolith-based inquiry and the uses of that knowledge in other fields. One important area of research is the use of phytoliths for carbon sequestration as a tool for removing carbon dioxide from the atmosphere to combat climate change (Jansson et al., 2010; Li et al., 2013; Parr et al., 2009, 2010; Parr and Sullivan, 2005, 2011, 2014; Song et al., 2012, 2013a, 2013b; Yang et al., 2015a, b; Zuo et al., 2014; Zuo and Lü, 2011). Phytoliths are also being used in the new field of forensic botany (Blackledge, 2007; Charlier et al., 2010;
Table 2 Active and inactive online phytolith databases. Website
Web address
Active Flora of Ecuador database http://phytolith.missouri.edu/ PhytoCore database http://www.phytcore.org/phytolith/index.php?rdm¼JwJIyWMKbZ&action¼home Paleobot.org http://www.paleobot.org/ Inactive Terry Ball's New World and Old World phytolith image collection http://webpub.byu.net/tbb/ The Colonial Williamsburg database http://research.history.org/archaeological_research/collections/collarchchaeobot/phytolithSearch.cfm Wits Online Phytolith database http://www.wits.ac.za/Academic/Science/GeoSciences/BPI/Research/WOPD/WOPDHome.htm
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Houk, 2004) and hold potential for use in the nanotechnology industry (Neethirajan et al., 2009). Finally, phytoliths are playing an important role in vegetation restoration efforts in the Great Basin Desert, USA (Morris et al., 2009, 2010; Pearsall, 2015), California grasslands, USA, (Evett and Bartolome, 2013), James Madison's Montpelier, Virginia, USA (Hart, 2015), and on the Chinese Loess Plateau (Jiao et al., 2012; Lu et al., 1999, 2006, 2007). While not directly related to archaeology, many of the above projects have their roots in archaeological phytolith research.
our knowledge of the relationship between phytoliths and plant anatomy and physiology and is an interdisciplinary endeavor by its very nature (Fig. 3). Because of this, archaeologists are able to use phytoliths to make significant contributions to our understanding of the human past. However, as the number and diversity of archaeological applications continues to grow, so too should the number and diversity of comparative phytolith projects. Going forward, continued comparative research is essential for understanding isotopic deposition in phytoliths as well as finding and publishing the limitations of using phytoliths to identify ancient plant taxa.
4. Concluding thoughts and cautionary note It has been 181 years since phytolith research began as a dissertation project in Germany that sought to understand the shapes of silica bodies produced in living plants (Piperno, 1988, 2006; Struve, 1835). Since that time, research interests have alternated between comparative plant studies and archaeological and paleoecological studies. Starting around the year 2000, archaeological applications of phytolith scholarship began to grow and diversify ushering in the “Period of Expanding Applications”. This era is characterized by a rapid increase in the number of articles published per year; a diversification of research topics; a reassessment of the use of carbon 14 and isotopes in phytoliths; the development of digital technologies for refining and sharing phytolith identifications; renewed efforts for standardization of phytolith nomenclature and laboratory protocol; and the development of the field of applied phytolith research. The foundation of archaeological phytolith research stems from
Role of the funding source This project was funded through personal finances. No outside source of funding was involved in any stage of this project. Acknowledgments This special edition was truly an international effort and was not possible without the excitement, drive, and steadfastness of all those involved, so thank you to all who participated in this special edition. This special edition was an outgrowth of the 2014 session titled “Issues and Directions in Phytolith Research” held at the Society for American Archaeology annual meetings in Austin, Texas. Thank you to all of the session participants for their presentations and the ensuing discussions over food and drinks. It was a pleasure to work with such fine scholars. I want to thank all the authors for
Fig. 3. A phytolith “Tree of Life”. This illustrates the interconnectedness of phytolith research and how all research can be traced back to a basic understanding of plant anatomy.
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their hard work in making this volume dedicated to phytolith research a success. Thank you to all the reviewers who reviewed the special edition manuscripts; your participation and intellectual input were critical for improving these papers. A special thank you goes out to Robin Torrence and the editorial staff at the Journal of Archaeological Science for their willingness to take on such a project and for their patience and assistance in seeing this special edition through to completion. I would also like to extend a special thank you to Deborah Pearsall, Dolores Piperno, Alexia Smith, Natalie Munro, and Arlene Rosen who helped with various stages of this project and politely replied to e-mails, phone calls, and office hour visits too numerous to count. And lastly, I'd like to thank the anonymous reviewers for their comments and suggestions in helping to improve this article. References Albert R.M., Ruíz J.A. and Sans A. 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Please cite this article in press as: Hart, T.C., Issues and directions in phytolith analysis, Journal of Archaeological Science (2016), http:// dx.doi.org/10.1016/j.jas.2016.03.001