Ten things you might not know about Phytoliths

Christian Gottfried Ehrenberg Image © Wikimedia Commons File PSM V14 D570

Christian Gottfried Ehrenberg
Image © Wikimedia Commons File PSM V14 D570

1. Phytoliths were first discovered in Germany in the 19th Century

They were named by Christian Ehrenberg in 1835, who called them Phytolitharia, or ‘plant stones’ (Piperno 2006, 5).

2. If you’ve ever taken a Phytolith sample, you have something in common with Charles Darwin

Phytoliths were identified in dust samples taken by Charles Darwin from the deck of the HMS Beagle off the coast of West Africa in 1833. Darwin recorded sending these samples to Christian Ehrenberg who identified 34 types of phytolith (Darwin 1846, 28-29). Darwin was surprised that ‘a sample smaller than half a teaspoon’ contained seventeen different types of phytolith (Darwin 1846, 28-29).

3. Phytoliths are basically small fossil plant skeletons!

They form within some plants when they absorb Monosilicic acid (Si(OH)₄) from the ground water. The Monosilicic acid can become deposited as solid or opaline silica (SiO₂·nH₂O) within or between plant cells during transpiration, essentially producing a ‘negative’ of the cell within the plant (Piperno & Pearsall 1993; Piperno 2006, 6-8). Not all plants make phytoliths – some plants have cells with a genetic susceptibility to silicification, whereas other plants may make phytoliths in response to an environmental stress or effect, such as the over absorption of Monosilicic acid (Pearsall & Piperno 1993, 9; Piperno 2006, 8).

Articulated Pearl Millet (Pennisetum glaucum) Leaf Bilobate Phytoliths Image © H. McParland-Clarke 2015

Articulated Pearl Millet (Pennisetum glaucum) Leaf Bilobate Phytoliths
Image © H. McParland-Clarke 2015

4. No one really knows for sure why plants make phytoliths

It used to be thought that silica was deposited as a waste product within the plant, but it has been suggested that phytoliths provide support for plant structures, that they help plants to resist attack by insects or make them unpalatable for consumers (Piperno & Pearsall 1993; Piperno 1991).

5. Phytoliths are preserved in a wide range of environments

Due to their high silica content, Phytoliths are not dependent on charring, waterlogging or mineralisation like plant macrofossils, and their preservation isn’t dependent upon an anoxic environment, like pollen. Pretty simply, this means that if a phytolith producing plant has degraded somewhere, the chances of finding the phytoliths in any soil type are quite high. Phytoliths are particularly useful in tropical regions of the world where the preservation of pollen or plant macrofossils might be severely limited, leaving almost no information about plant use or environment (Piperno 2006, 151). But, like everything, there are more complex mechanisms which determine phytolith survival in the archaeological record, aside from pre- and post-depositional taphonomic factors, the degree of silicification of the phytolith – some will contain other elements which may make them more susceptible to dissolution (dissolving into the soil) – has a potential impact on its survivability and subsequent archaeological visibility (Piperno 2006, 21). Soil pH will also have an impact to some degree – higher pH values above pH 9 have the potential to increase phytolith dissolution. These conditions are relatively rare, occurring in high pH coral sediments, lime plastered floor surfaces, shell middens and within confined ashy layers (Karkanas et al. 2002; Piperno 2006, 22). Essentially, the take home message is that phytoliths will survive in most archaeological contexts, most of the time.

6. Phytoliths aren’t only produced in the reproductive structure of the plant.

Most of the evidence we see archaeobotanically is in the form of seeds preserved by charring or in some cases waterlogging or mineralisation. Whilst this has the potential to tell us about food preparation, consumption patterns and, to some extent, the local environment, the mode of preservation doesn’t provide us with the full picture. Because phytoliths can be produced in the leaves, stems, flowers and fruit of some species, it enables a wider range of applications. For example, at Çatalhöyük, the preservation is so pristine that baskets are outlined within the soil, formed of chains of white phytoliths visible to the naked eye and which can be identified to plant family. Baskets or matting made of reeds (Phragmites sp.) and rushes (Scirpus sp.) have been identified at Çatalhöyük, and in the case of the basket from within Building 5, the grass or reed basket identified by the presence of bilobates, was associated with Wheat husk phytoliths, strongly suggesting that these baskets were used for the storage of Wheat grain (Rosen 2005 206-207; Jenkins et al. 2012). In addition, the presence of Date Palm (Phoenix dactylifera) globular echinates, is doubly important – demonstrating the potential use of Date Palm basketry or cordage and the import or potential trade of an organic object during the Neolithic, as Date Palms were not native to the region (Rosen 2005, 207).

Phytoliths preserving the outline of a Basket in Building 5 Image ©  http://cnx.org/contents/335b3eeb-1ca4-4553-b93c-42ab830d4df4@1.1:4/Remixing_Çatalhöyük

Phytoliths preserving the outline of a Basket in Building 5
Image © http://cnx.org/contents/335b3eeb-1ca4-4553-b93c-42ab830d4df4@1.1:4/Remixing_Çatalhöyuk

7. Phytoliths are what is known as an in situ indicator

They are released during the decay of plant material and incorporated into soils and sediments during pedogenesis (soil formation). Unlike pollen, which can be designed to be wind or water dispersed to aid plant reproduction, phytoliths are relatively unlikely to travel long distances or become subject to wind dispersal. Therefore, it is often assumed that phytoliths are preserved in situ. Given that the dust from the HMS Beagle contained such a diversity of phytoliths, I’ll leave it up to you to decide whether this is strictly true. Though wind transport is possible, there are other things to worry about when interpreting a phytolith assemblage, including pre-depositional and post-depositional taphonomy. In other words, assuming that the phytoliths were not transported by the elements, there are a range of chemical, physical and anthropogenic actions which can impact their inclusion into the archaeological record. For example, post-depositional activities such as sweeping may remove the phytolith assemblage from domestic contexts (Malinsky-Buller et al. 2011; Rosen 2005). It’s important to remember that no archaeobotanical material is without these issues, and in secure contexts and discrete features where the archaeology is well preserved, the phytolith assemblage can be representative of plant use and activity areas.

8. Phytoliths can get stuck in your teeth!

Phytoliths have been found in the dental calculus of humans (Henry et al. 2010; Henry & Piperno 2008) and grazing animals (Gobetz & Bozarth 2001; Middleton & Rovner 1994). Phytoliths from the dental calculus of herbivores have provided an insight into the way in which the herbivores were managed during the American Colonial period in Virginia (Middleton & Rovner 1994). This approach combined with new approaches to the identification of livestock dung through soil geochemistry, phytolith and spherulite analysis has the potential to reveal important information about livestock management and the use of dung in domestic contexts (Lancelotti & Madella 2012). Phytoliths have also been found within dental calculus on the teeth of American Mastodons, providing an insight into their grazing habits during the Pleistocene (Gobetz & Bozarth 2001). The phytolith assemblage was dominated by grasses, with limited dicotyledon phytoliths from deciduous trees and Celtis sp. seeds (Hackberry) (Gobetz & Bozarth 2001). Did Mastodons have a sweet tooth?

The analysis of phytoliths from human dental calculus has revealed the probable consumption of Phoenix sp. (Date Palm) fruits among Near Eastern Neanderthal populations in Shanidar Cave, Iraq (Henry et al. 2010). However, starch is more likely to become occluded within dental calculus, the recovery of starch grains from dental calculus samples taken from human populations at Tell al-Raqā’i, Syria, outnumbered phytoliths almost 30:1 (Henry & Piperno 2008). Phytolith analysis of dental calculus can be a valuable tool when undertaken with starch analysis, but it seems that phytoliths are less likely to become occluded within calculus than phytoliths, which can lead to the necessary interpretation of an extremely small sample.

Articulated Sorghum (Sorghum bicolor) Husk Phytoliths Image © H. McParland-Clarke 2015

Articulated Sorghum (Sorghum bicolor) Husk Phytoliths
Image © H. McParland-Clarke 2015

9. Phytoliths have been found in Dinosaur poo!

Samples of Dinosaur coprolites produced by Late Cretaceous Titanosaur sauropods found in India revealed a surprise component of their diet, they ate grass! The teeth of Titanosaur sauropods or ‘the most prominent plant-eaters in Gondwana’ are not specifically adapted for grass grazing, yet phytoliths within their coprolites suggest that they consumed five varieties of grass (Prasad et al. 2005, 1178). The phytolith evidence revealed that they consumed palms, conifers and dicots in addition to grasses, suggesting that they had general grazing preferences, exploiting a wide range of taxa (Prasad et al. 2005, 1179).

10. Phytoliths are Spicy Stuff!

The, let’s say ‘universal’, preservation of phytoliths enables a greater understanding of aspects of food production and consumption which are often invisible. As their preservation doesn’t rely on charring and diagnostic phytoliths are not only produced in the seed part of the plant, phytoliths have the potential to be preserved within pottery residues, providing direct evidence of the use of plants, and to some extent consumption.

Phytoliths have the potential to illuminate aspects of a complex culinary process, which are largely archaeologically invisible. A recent analysis of late Mesolithic and early Neolithic pottery residues employed lipid, stable isotope, and starch and phytolith analysis revealing evidence of the use of Alliaria petiolata (Garlic Mustard) as a spice (Saul et al. 2013). The preservation of phytoliths within the pottery residues was good, with just over a third of carbonised pottery residues analysed containing phytoliths (Saul et al. 2013). The identification of Alliaria petiolata phytoliths demonstrates the use of a nutritionally poor food, selected for its taste, suggesting that prehistoric populations of the western Baltic consumed food with flavour, not only substance (Saul et al. 2013). This discovery was widely reported by the media including BBC News,  The Telegraph, The Independent, National Geographic and The Daily Mail. Addendum: However, the relatively small sample size and a focus on a single phytolith identification have been discussed in an interesting review of the paper in a blog post by Dr Lisa-Marie Shillito, who maintains Castles and Coprolites, a successful geoarchaeology blog.

References:

Gobetz, K.E. and Bozarth, S.R. 2001. Implications for Late Pleistocene Mastodon Diet from Opal Phytoliths in Tooth Calculus in Quaternary Research, Vol. 55, Issue 2. pp. 115-122.

Henry, A.G., Brooks, A.S. and Piperno, D.R. 2010. Microfossils in calculus demonstrate consumption of plants and cooked foods in Neanderthal diets (Shanidar III, Iraq; Spy I and II, Belgium) in PNAS: proceedings of the National Academy of Sciences of the United States of America, vol. 108, no. 2.

Henry, A.G. and Piperno, D.R. 2008. Using plant microfossils from dental calculus to recover human diet: a case study from Tell al-Raqā’i, Syria in Journal of Archaeological Science, vol. 35, issue 7. pp. 1943-1950.

Jenkins, E. L., Rosen, A. M. and Otsaku, M., 2012. The Phytoliths of the BACH Area. In: Tringham, R. and Stevanović, M., eds. Last House on the Hill: BACH Area Reports from Çatalhöyük, Turkey. Los Angeles, CA, USA: Cotsen Institute of Archaeology Press, 261 – 267.

Karkanas, P., Rigaud, J-P., Simek, J.F., Albert, R.M. and Weiner, S. 2002. Ash, bones and guano: a study of the minerals and phytoliths in the sediments of Grotte XVI, Dordogne, France in Journal of Archaeological Science, vol. 29. pp. 721-732.

Lancelotti, C. and Madella, M. 2012. The ‘invisible’ product: Developing markers for identifying dung in archaeological contexts in Journal of Archaeological Science, Vol. 39, Issue 4. pp. 953-963.

Malinsky-Buller, A., Hovers, E. and Marder, O. 2011. Making Time: ‘Living floors’, ‘palimpsests’ and site formation processes – A perspective from the open-air Lower Palaeolithic site of Revadim Quarry, Israel in Journal of Anthropological Archaeology, vol. 30. pp. 89-101.

Middleton, W.D. and Rovner, I. 1994. Extraction of Opal Phytoliths from Herbivore Dental Calculus in Journal of Archaeological Science, Vol. 21, Issue 4. pp. 469-473.

Piperno, D. R. 2006. Phytoliths: A Comprehensive Guide for Archaeologists and Paleoecologists, AltaMira Press.

Piperno, D.R. and Pearsall, D. 1993. The Nature and Status of Phytolith Analysis in Pearsall, D. and Piperno, D.R. (eds.). 1993. Current Research in Phytolith Analysis: Applications in Archaeology and Paleoecology, University of Pennsylvania Press.

Prasad, V., Stromberg, C.A.E., Alimohammadian, H. and Sahn. A. 2005. Dinosaur Coprolites and the Early Evolution of Grasses and Grazers in Science, vol. 310. pp. 1177-1180.

Saul, H., Madella, M., Fischer, A., Glykou, A., Hartz, S. and Craig, O. 2013. Phytoliths in Pottery Reveal the Use of Spice in European Prehistoric Cuisine in PLoS One, vol. 8, issue 8.

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Guest Blog Post: Archaeobotany & Plant Use in Late Iron Age Silchester, Lisa Lodwick, University of Reading

Aerial view of the Late Iron Age archaeology in Insula IX Image © University of Reading Silchester Insula IX ‘Town Life’ Project

Aerial view of the Late Iron Age archaeology in Insula IX
Image © University of Reading Silchester Insula IX ‘Town Life’ Project

Archaeobotany has been a key part of archaeological science since the 1970s, and although the tried and tested techniques that form the basis of this study area (basically playing with buckets, water and sieves) have remained unchanged for decades, the research field continues to give new insights into the past as new sites and periods are studied. Archaeobotany (or palaeoethnbotany in America) usually refers to the study of plant macrofossils – seeds, fruits, leaves, twigs, nuts etc. Basically any plant remains which can be seen (just about) with the naked eye. Plant microfossils – starch and phytoliths, also come under the umbrella of archaeobotany, but I’m going to stick to the big bits here.

Plant macrofossils are all retrieved by either flotation – breaking up anywhere between 1-40L of sediment in a tank of water, and collecting the plant remains which float on the surface, or sieving – passing the sediment through a stack of sieves. The flot or sieve residues are then studied under a low power microscope, usually only up to x40 magnification. Archaeobotanists study plant remains preserved by four main types of preservation. These are very important to keep in mind, as they effect which seeds are preserved in the first place, how the seeds are separated from sediment, how hard it is to identify the seeds, and what type of information they are going to give us about past societies.

Charred Hordeum sp. (Barley) Grain  Image © Lisa Lodwick 2015

Charred Hordeum sp. (Barley) Grain
Image © Lisa Lodwick 2015

Second – mineralised plant remains. This is a pretty rare type of preservation. It normally occurs in cesspits, when calcium (from animal bones, marine shells, egg shells etc.) and phosphate (from human waste) enter into seeds/fruit stones, and form calcium-phosphate. This material will survive for millennia, but the range of casts and impressions formed (individual cell replacement, interior casts, exterior casts) can be tricky to identify. Nevertheless, mineralised plant remains provide us with fantastic evidence for diet and trade in plant foods, mainly for the Roman and Medieval periods.

Third – desiccated plant remains. In hyper-arid places (think Egypt) the bacteria which would usually decay plant remains cannot do so without any water. So, huge quantities of straw, cereal chaff, fruit stones and seeds survive in middens, mud bricks and in occupation deposits within buildings. Studies at trading settlements on the Red Sea have provided fascinating evidence for spices which were traded from India to the Mediterranean, and also more everyday activities like animal foddering and crop-processing (Van der Veen 2007).

Finally, waterlogged plant remains are in a way most similar to desiccated plant remains, but decay is stopped in this case by the lack of oxygen rather than water. Plant remains have to have been dumped into the standing water at the bottom of deep pits or wells, or discarded in towns with damp climates (like York!) where the rate of organic rubbish dumping was faster than decay. Waterlogged plant macrofossils are also preserved in off-site sequences, such as peat bogs and palaeo channels. The study of these usually falls under the realm of palaeoenvironmental studies.

The great thing about waterlogged plant remains is that they include plant foods which do not need to be exposed to fires during food preparation – like fruits, flavourings and vegetables. So, waterlogged samples typically produce a much more diverse range of plant foods than charred samples from the same site. Plus, waterlogged preservation is much more common than mineralisation in north-west Europe.

My research focuses on the development of “oppida” in the Late Iron Age and into the Roman period, and what the societies were like which inhabited these settlements. Whilst sampling for plant macrofossils has been widely undertaken for around 40 years, no oppida has been excavated during this period in a large enough scale, or with enough attention to systematic sampling (large well-recorded sediment samples from a wide range of deposits and site phases). So, we don’t know what foods the inhabitants were eating, and whether what crops they grew, and how they grew them, had to change as people decided to live in the first proto-urban settlements in Britain.

Luckily, the University of Reading Silchester Insula IX ‘Town Life’ Project has been excavating a large area of the oppidum Calleva Atrebatum over the last two decades. The reliance on wells for water supply to the settlement in the past, combined with the bulk sampling for plant remains throughout the excavation, meant a large dataset was available for study. Two Late Iron Age wells contained waterlogged plant remains. Whilst most of the sample consisted of seeds of weedy plants, which would have been growing around the mouth of the well, around 5% of the sample contents were seeds of cultivated plant foods. Several seeds of celery and coriander were present, alongside a few pieces of olive stone. These foods were previously thought to have been introduced to Britain by the Roman military after the Roman Invasion in AD 43, but we now know that some of the inhabitants of Late Iron Age Silchester were selectively adopting some of the plant foods being consumed elsewhere in the Roman world.

Mineralised Apium graveolens (Celery) seeds from a Roman cesspit at Silchester and Waterlogged Coriandrum sativum (Corainder) seed from an Early Roman well at Silchester; Excavation of the Late Iron Age Well at Silchester

Mineralised Apium graveolens (Celery) seeds from a Roman cesspit at Silchester
and Waterlogged Coriandrum sativum (Corainder) seed from an Early Roman well at Silchester; Excavation of the Late Iron Age Well at Silchester

The archaeobotanical dataset from Insula IX also includes substantial charred and mineralised datasets, showing  both continuation and change in farming and food consumption practices across centuries of occupation there. Whilst my research so far has focussed on the systematic application of long-standing archaeobotanical techniques, the development of new methods of statistical analysis, combined with exciting developments in the areas of crop stable isotope and aDNA analysis, means archaeobotany continues to provide new insights into the past.

A Late Iron Age well after excavation at Silchester  Image © University of Reading Silchester Insula IX ‘Town Life’ Project

A Late Iron Age well after excavation at Silchester
Image © University of Reading Silchester Insula IX ‘Town Life’ Project

References

Lodwick, L. (2014). Condiments before Claudius: new plant foods at the Late Iron Age oppidum at Silchester, UK. Vegetation History and Archaeobotany 23: 543–549.

Van der Veen, M. (2007). Formation processes of desiccated and carbonized plant remains – the identification of routine practice. Journal of Archaeological Science 34 (6), 968–990.

University of Sheffeld Integrated Archaeobotanical Research

About the Author

Lisa Lodwick is a Post Doctoral Researcher at the University of Reading, her current research focuses on analysis of plant macrofossils from Silchester. Lisa completed a BA in Archaeology and Anthropology and an MSt. in European Archaeology at the University of Oxford, before pursuing a PhD in Archaeology at the same institution. Lisa’s PhD research focussed on agricultural and social changes during the Late Iron Age to Roman transition period in Britain, identifying changes and continuities in consumption and production patterns. She occasionally blogs about her own research at https://theplantremains.wordpress.com/  and tweets much more frequently about archaeobotany @LisaLodwick . Lisa is also one of the team behind the successful Not Just Doormice – Food for Thought blog https://notjustdormice.wordpress.com/.