paleoecologycr1

8/7/2019 paleoecologycr1

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Adam DubbinMSc Scientific Methods

Paleoecology Critical Review 1

Phytoliths and the domestication of rice (Oryza sativa L.): A critical

review

The use of phytoliths as a diagnostic tool in archaeobotanical and paleobotanical

investigations has gained much support and utility over the last decade. Phytoliths are

siliceous plant microstructures that are the result of silica deposition, from solute in

groundwater, into plant tissues (Piperno, 1988:11). The term, from the Greek phytos

(plant) and lithos (stone), literally refers to mineralized (siliceous or calcareous)

substances in plant tissues; for purpose of specificity, the term in archaeobotanical

contexts refers strictly to the silicified remains (ibid.). These structures can also be found

in the literature as ‘opal phytoliths, plant opals, opaline silica, or grass opals’ (ibid.).

Regardless of nomenclature, phytoliths possess inherent qualities as a paleoecological

indicator as well as specific evidence of plant presence in archaeological contexts. These

structures are mostly restricted to angiosperms, most importantly the monocotyledonous

grasses, although they may also be found in gymnosperm family Pinaceae and in some

pteridophytes families, such as Equisetaceae (Piperno 1988: Table 2.1).

In plants, the deposition of silica can occur in any plant organ as well as many

varieties of tissues and cells (Piperno 1988:43). On the cellular level, silica can be

deposited in the cell lumen, within the cell wall, or in intercellular spaces. When formed

intercellularly, they typically are stored in idioblasts and the individual morphology is not

related to the shape of the cell (Piperno 1988:18). In these cases, the formation of

phytoliths is poorly understood (ibid ); shapes can range from cruciform to dumbbell

shaped and also amorphous silica masses. Intracellular deposits and cell wall

incrustations, conversely, are directly dependent on the morphology of the individual cell

on/in which it formed and therefore retains the original shape and forms a ‘skeletal’

impression of the erstwhile cell. These cells include epidermal cells and ‘hair cells,’ and

are often extremely valuable for taxonomic interpretation based upon morphology.

In an article from Geoarchaeology, Zhao and Piperno (2000) endeavor to identify

domesticated rice (Oryza sativa) in the paleological record using soil cores from Lake

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Poyang, Jiangxi province, China. Because rice has been demonstrated as one of the

highest producers of phytoliths in the Gramineae (Wadham & Parry 1981), this query has

appreciable potential for success. This study also demonstrates a solution to the problem

with rice pollen: it cannot be “unequivocally identified” (Zhao & Piperno 2000:211). As

well, the authors individually and jointly have a respectable track record in phytoliths

analysis and have published recent works on rice phytoliths (Pearsall et al. 1995; Zhao et

al. 1998).

First of all, let us examine the sampling strategies employed by the researchers.

Because the authors ventured to verify the presence of rice in terms of time before

present, what they were looking for was evidence of ‘domesticated’ rice as far deep in the

soil core as possible. The specific phytoliths shape in rice that discerns ‘domesticated’

from ‘wild’ is the double-peaked glume cell phytoliths (Zhao et al. 1998). The samples

were taken from a single core out of a total of two test cores. Ordinarily, the examination

of only one core would appear insufficient, if not the drilling of only two total tests cores.

If a paleoenvironmental model is to be developed, multiple test cores should be drilled to

help assess ancient inconsistencies that may be associated with a lakebed (Piperno 1993).

However, this should neither make the presence of taxonomic-specific phytoliths

diminutive for paleoecological interpretation nor should the presence or absence of

‘domesticate’ rice phytoliths be affected. In examining the cores, two proven methods

were appropriately hybridized (Piperno 1988; Zhao & Pearsall 1998), assuring

consistency with previous works performed on similar materials.

The use of AMS14

C dating in conjunction with sedimentation patterns for the

estimation of dates is also standard procedure. The authors describe one of the dates as an

outlier, as it is not sequential with the stratigraphy; however, their explanation that “more

recent carbon percolated downward through the sands” (p. 208) is an extremely viable

cause of contamination. As well, all of that dates appear reasonable with appreciable

error calibrations and tested on the appropriate materials. In these dates, a hiatus occurs

which the authors interpret as a change in the hydrological and topographic features of

the coring area, which is a reasonable interpretation. According to sedimentation rates

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and AMS14

C dating, this hiatus probably began 10,500 B.P., contemporary with the Late

Pleistocene/Holocene shift. However, because only one core was sampled, the only

conclusions that can be derived from dating this core are dates relative to the phytoliths

found in the sediments.

The phytoliths themselves were found abundantly throughout the lake core.

Many of those found bore substantial resemblances to modern analogues of the region

and a great proportion of those belong to the grasses, in the form of short-cell phytoliths.

There were some 3 types of phytoliths that were left classified but unidentified. The

authors also discovered other plant microfossils such as sclereids, cystoliths and rough

spheres that also have significance in paleobotanical taxonomy. Those microfossils

proved useful in the development of a paleoecological model.

The conclusive literature is presented legibly and lucidly, facilitated by the

appropriate figures and illustrations. The authors take care to note factors that would

affect the contents of their core, such as the effects of changing hydrology ( i.e. inflowing

rivers and streams) and the sedentary nature of phytoliths. Because this lake lies between

two major Chinese rivers and the volatile nature of Chinese monsoons, one would

deductively conclude that this would certainly be a factor. In regards to the rice

phytoliths, the authors found the single-peaked glume phytoliths terminating at about

13,000 years B.P. and a lack of double-peaked in sediments older than this, indicating a

Late Pleistocene date for the origin of rice ‘domestication.’ This conclusion is coupled

with a paleoclimatic model based upon the quantitative phytolith data in the respective

soil layers. This model, despite the exclusion of the early Holocene from the record,

shows a much drier Late Pleistocene based on the lack of arboreal microfossils and an

abundance of grass phytoliths. While all of these conclusions appear sound within its

realm, the single fact that only one core was examined discounts a deal of credibility to

the environmental interpretations but leaves the rice domestication issue unscathed.

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References

Pearsall, D.M. 2000. Paleoethnobotany: a handbook of procedures. 2nd edition Ch. 5Academic Press, San Diego.

Pearsall, D.M., D.R. Piperno, E.H. Dinan, R. Umlauf, Z.J. Zhao and R.A. Benfer 1995.Distinguishing rice (Oryza sativa Poaceae) from wild Oryza species throughphytoliths analysis- results of preliminary research. Economic Botany 49:183-196.

Piperno, D.R. 1988. Phytolith Analysis: an archaeological and geological perspective.Academic Press: San Diego.

Piperno, D.R. 1993. Phytolith and charcoal records from deep lake cores in the Americantropics. In Pearsall, D.M. and D.R. Piperno (eds.) Current research in phytolith

analysis: applications in archaeology and paleoecology. MASCA Research

Papers in Science in Archaeology 10:58-72. Philadelphia: MASCA.

Wadham, M.D. and D.W. Parry 1981. The silicon content of Oryza sativa L. and its

effect on the grazing behavior of Agriolimax reticulates Miller. Annals of Botany

48:399-402.

Zhao, Z.J. and D.M. Pearsall 1998. Experiments for improving phytoliths extraction from

soils. Journal of Archaeological Science 25:587-598.

Zhao, Z.J., D.M. Pearsall, R.A. Benfer & D.R. Piperno 1998. Distinguishing rice (Oryza

sativa Poaceae) from wild Oryza species through phytoliths analysis, II: finalized

method. Economic Botany 52:134-145.

Zhao, Z.J. and D.R. Piperno 2000. Late Pleistocene/Holocene environments in the

Middle Yangtze River Valley, China and rice (Oryza sativa L.) domestication: thephytoliths evidence. Geoarchaeology 15:203-222.

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