Society for American Archaeology
Quarter-Inch Screening: Understanding Biases in Recovery of Vertebrate Faunal Remains
Author(s): Brian S. Shaffer
Source: American Antiquity, Vol. 57, No. 1 (Jan., 1992), pp. 129-136
Published by: Society for American Archaeology
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QUARTER-INCH SCREENING: UNDERSTANDING BIASES
IN RECOVERY OF VERTEBRATE FAUNAL REMAINS
Brian S. Shaffer
Fine screeningof archaeologicalmaterialsis often too expensiveor too time-consumingforlarge assemblages.
Consequently,1?4'has becomea standardsize of mesh usedamongNorthAmericanarchaeologists.Unfortunately,
the effectsof ?/4'screeningon bone recoveryand the biases in interpretationsarepoorlydocumented.One-quarterinch screeningbiasesfaunal recoverytowards "larger"bone specimens, but previousstudiesfail to document
biasesin the recoveryof specifictaxa or elements.To betterunderstandthesebiases,screeningtests wereconducted
on 26 modern,comparativespecimens.Results of these tests indicatethat recoveryand loss of specificelements
for each taxon can be predicted.Mammals with live weightsof less than 140 g are almost completelylost by 1?4'
screening.Specimensweighingfrom 71 to 340 g are poorly represented,whilespecimensweighingfrom 340 to
3,100 g are representedby most elementsexceptfoot bones. Taxa greaterthan 4,500 g are representedby most
elements.
El tamizadofinode materialesarqueol6gicoses a menudoexcesivamentecostosoo consumedemasiadotiempo
en el caso de grandesconjuntos.Como resultado,el uso de zarandasde 1?4' se ha convertidoen prdcticacorriente
entre arque6logosnorteamericanos.Desgraciadamente,los efectos del tamizado con zarandas de 1?4' sobre la
recuperaci6nde huesosy los resultantessesgos en la interpretaci6nhan sido escasamentedocumentados.A pesar
de que el uso de este procedimientodistorsionala recuperaci6ndefauna en favor de huesos de mayor tamano,
previosestudiosno documentansesgos en la recuperaci6nde taxones o elementosespecificos.Con el prop6sito
de comprendermejorestossesgos,se realizaronpruebasde tamizadocon 26 especimenesmodernoscomparativos.
Los resultadosde estaspruebasindicanque es posiblepredecirla recuperaci6ny perdidade elementosespecificos
para cada tax6n. Mamiferoscon peso en vida menorque 140 g se pierdencasi por completoal tamizar con 1?4'.
Especimenesquepesan entre 71 y 340 g se encuentranpobrementerepresentados,en tanto que aquellosconpeso
entre340 y 3.100 g estdn representadospor la mayoria de los elementos,exceptolos huesosdel pie. Casi todos
los elementosse encuentranpresentesen taxones mayoresque 4.500 g.
Microfauna can often be recovered from archaeological sites through fine-screening procedures
(Casteel 1972; Clason and Prummel 1977; DeMarcay and Steele 1986; Dye and Moore 1978;
Meighan 1969:418; Payne 1972:53, 1975; Struever 1968:353). However, fine screening can be
expensive and time-consuming (Barker 1975:62; DeMarcay and Steele 1986:260; Kobori 1979:229;
Payne 1975:16-17). In shell middens, use of '/16' screens can result in a 500 percent increase in
screening time relative to use of 1/4? screens (Meighan 1969:418). Due to the expense in both time
and funds, ?/4'-mesh screening has become a standard recovery method on many North American
archaeological sites. Fine screening and flotation are often used only for random samples and for
specific archaeological contexts such as features (see examples in Baker et al. 1991:140-141; DeMarcay
and Steele 1986:25 1; and Shaffer 1989:172).
Grayson (1984:168-169) noted that the choice of screen size will have a significant impact on
the type of material lost or recovered. To understand taxonomic loss from 1/4"screening, many sites
have been sampled with fine-screen methods (e.g., Baker et al. 1991:140-141; Casteel 1972:383387; DeMarcay and Steele 1986:250-264; Kobori 1979:228-229; Payne 1975; Thomas 1969:392401; Yates 1987:87). These studies focused on comparing and contrasting differences in recovery
between 1/4? and fine-screen mesh (1/8'or 1/16') or on the cost efficiency of different methods. While
information about specific sites was presented, an overall framework for understanding ?/4"-mesh
biases was not presented.
Thomas's (1969:392-401) discussion of recovery of microfauna from Great Basin sites in
Nevada provided a preliminary assessment of sieving biases. Thomas examined frequencies of
Brian S. Shaffer,Departmentof Anthropology,Texas A&M University,CollegeStation, TX 77843-4352
AmericanAntiquity, 57(1), 1992, pp. 129-136.
CopyrightC) 1992 by the Society for AmericanArchaeology
129
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130
AMERICANANTIQUITY
[Vol. 57, No. 1, 1992]
recovery of five arbitrarily defined weight classes of mammals from 1/4?, 1/8", and '/16" mesh. He
documented percentage loss for the two larger sizes of screen for each of the five weight classes. For
example, 1/4"screening resulted in a 74-100 percent loss of elements among taxa with live weights
less than 100 g. Specimens weighing from 100 to 700 g exhibited losses of 39-100 percent. This
work clearly demonstrated the value of fine screening in the recovery of microvertebrates and
documented that larger-sized mesh screens bias the recovery of faunal material to larger specimens.
It did not, however, document the loss of taxon-specific elements from the 1/4"screen. To analyze
taxonomic loss stemming from use of 1/4" screens, screening tests were conducted on skeletons of
modem mammals.
METHODS
This study used 26 disarticulated mammal skeletons representing 25 species (Table 1). Most
specimens did not have a recorded live weight from the time of collection. Therefore, live weight
and head-and-body-measurement ranges for each taxon were taken from Burt and Grossenheider
(1976). Because several species exhibited a wide range of size or weight, the greatest length of the
femur of each individual was measured using dial or electronic calipers. Measurements were taken
following those described by von den Driesch (1976:84-85) and provided additional relative size
information (Table 1).
Specimens were selected based on fusion of elements, skeletal completeness, state of disarticulation, and lack of bone fragmentation. All but two specimens had fully fused long-bone epiphyses.
The two exceptions were a muskrat (Ondatra zibethicus) and a Mexican ground squirrel (Spermophilus mexicanus). The choice to use individuals with fully fused long-bone epiphyses simplified
tabulating procedures and provided a control for element completeness.
Each disarticulated animal skeleton was individually placed in a ?/4"-wiremesh screen and shaken
for 30 seconds. This process was repeated nine times with complete replacement of elements for
each taxon for each test. Exceptions to this are the canids. These skeletons were only screened five
times each because there was essentially no change between tests. After each test, elements that
passed through the screen were recorded. Repetition of tests allowed verification of -early results
and helped control for variation in the aggressiveness of the shaking of the screen from test to test.
Recovery of loose teeth was not addressed. Also, due to the necessity for preserving comparativespecimen collections, specimens were not altered or damaged in any way.
Tests conducted represent situations where every element is complete. This condition, of course,
is unlikely to occur in archaeological sites. Moreover, the skeletal elements were placed in the screen
without other material such as sediment, plant remains, or rocks. Obviously, other materials in the
screen may affect recovery potential. Plant materials may clog screens and increase recovery of
elements, while rocks may break bones during the screening process and cause additional losses.
The sediment in the screen also may affect recovery of smaller elements in several ways. By turning
small long bones on end so that they may pass through the screen, dry matrix may cause a loss of
material. Conversely, moist matrix may adhere to specimens, preventing passage through the screen.
Another consideration that cannot be controlled for is the role of the field technician operating
the screen. If attentive, the field technician may have the opportunity to remove small bones from
the screen before they are lost or may pick up bones that they see pass through the screen. In
addition, some small bones may be recovered from the matrix before it is screened, although this
is unlikely (Casteel 1972:382; Struever 1968:353; Watson 1972:221-223). Thus, tests reported here
cannot emulate all possible field circumstances. It is hoped that by providing this idealized data
set, researchers can use it as a guide for evaluation of faunal material recovered from specific sites,
taking into account the conditions of recovery at each site.
RESULTS AND DISCUSSION OF TESTS
Table 2 documents the results of the ?/4"-screentests. Table 1 provides taxon reference numbers
for Table 2, taxa names, live-weight ranges, head and body lengths, and the femur length for each
specimen tested. With few exceptions, body-weight range (Table 1) correlates well with the number
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REPORTS
131
Table 1. Weight and Measurements in Ascending Order by Lowest Weight.
Reference
Number
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
Taxon
Least shrew (Cryptotis parva)
Pygmy mouse (Baiomys taylorl)
Evening bat (Nycticeus humeralis)
House mouse (Mus musculus)
Shorttail shrew (Blarina brevicauda)
Deer mouse (Peromyscus maniculatus)
Mexican pocket mouse (Liomys irroratus)
Kangaroo rat (Dipodomys ordil)
Eastern mole (Scalopus aquaticus)
Valley pocket gopher (Thomomys bottae)
Pocket gopher (Thomomys sp.) (LA 15049
Grant County, NM)
Cotton rat (Sigmodon hispidis)
Plains pocket gopher (Geomys bursarius)
Wood rat (Neotoma albigula)
Thirteen-lined ground squirrel (Spermophilus
tridemcimlineatus)
Red squirrel (Tamiasciurus hudsonicus)
Mexican ground squirrel (Spermophilus
mexicanus)
Mink (Mustela vison-male)
Gray squirrel (Sciurus carolinensis)
Spotted skunk (Spilogale putorius)
Cottontail (Sylvilagus auduboni)
Ringtail cat (Bassariscus astutus)
Muskrat (Ondatra zibethicus)
Jackrabbit (Lepus californicus)
Red fox (Vulpesfulva)
Coyote (Canis latrans)
Weight
(g)
Length
(mm)
Femur
(mm)
4-7
7-9
7-9
11-22
11-22
18-35
34-50
42-72
67-140
71-250
56-64
51-64
36-38
81-86
76-102
71-102
102-127
102-114
114-165
122-178
7.0
9.3
13.1
12.2
9.1
15.1
22.2
24.6
13.0
19.0
?
113-198
127-354
135-283
?
127-203
140-229
190-216
22.4
31.7
29.0
33.6
140-252
198-250
114-165
178-203
29.0
40.5
198-340
198-340
340-726
363-999
600-1,200
900-1,130
908-1,816
1,300-3,100
4,500-6,700
9,000-22,000
171-190
228-266
200-250
230-340
300-380
360-410
250-360
430-530
5,600-6,300
8,100-9,400
30.4
54.7
52.2
46.4
65.2
63.8
43.6
106.2
132.6
176.0
of elements recovered (Table 2, Figure 1). For example, the only elements consistently recovered
for animals weighing < 140 g were crania, with innominates and sacrae also being recovered for
some specimens. The only long bone consistently recovered for a specimen < 140 g is the humerus
of the mole (Scalopus aquaticus). The unique, circularly shaped bone cannot pass through 1/4"mesh
from any direction.
Clearly, the use of 1/4"screening will miss most elements of mammals listed in this first weight
class. Thus, quantitative analysis of this category recovered by 1/4"mesh will not yield an accurate
assessment of taxonomic abundance or even presence vs. absence. For example, Driver (1985:5)
noted that the only small fauna recovered in this size category (from 1/4?mesh) from six sites in the
Sierra Blanca Region of New Mexico were Peromyscus (two specimens) and Microtus (two specimens). These specimens were represented only by crania and mandibles.
Unfortunately, clear distinctions between each of the size categories could not be made. Consequently, these categories overlap in size. Delineation between the first two categories is based on
the recovery of mandibles and scapulae. The largest specimen in the < 140 g weight class is a mole,
but 1/4?screening did not consistently recover the mandibles, scapulae, or most long bones. Most
elements from animals weighing 71-340 g also will be missed with 1/4?screening. However, these
taxa are often large enough for several commonly identifiable elements to be recovered (pelves,
scapulae, femora, humeri, skulls, and mandibles).
The next weight category is 340-3,100 g. Mammals in this category should be fairly well represented with ?/4"-screenrecovery methods. Even though many identifiable elements are recovered,
other elements such as caudal vertebrae, ribs, stemae, patellae, sesamoids, podials, metapodials,
and phalanges were not recovered consistently. Distinction between this size category and the
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[Vol. 57, No. 1, 1992]
ANTIQUITY
AMERICAN
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AMERICANANTIQUITY
[Vol. 57, No. 1, 1992]
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Taxa (See Table 1 for taxa description)
Legend
Elements
Figure 1.
The number of elements
Recovered
recovered per taxon with 1/4' screening.
previous size category is based on the maximum size for the Mexican ground squirrel (Spermophilus
mexicanus) and the minimum size for the gray squirrel (Sciurus carolinensis) (see Table 1). The
mink (Mustela vison) is included in this category because the individual was a domesticated animal
and therefore was probably larger in size than wild individuals. Also the recovery of elements for
this individual was greater than for others in the 71-340 g category. This level represents the first
size range at which the presacral and sacral vertebrae, and the majority of commonly identified
elements are recovered from 1/4? mesh screen.
In the > 4,500 g category, most elements except sesamoids, carpals, patellae, and middle and
distal phalanges were recovered in 70 percent or more of the tests conducted. The 4,500-g level
marks the point at which 1/4? screening does not greatly bias the sample toward "larger specimens,"
since virtually all of the specimens are "larger," and few small elements are present in animals of
this size.
Illustrated in Figure 1 are the number of elements recovered with 1/4?screen in the tests conducted.
The sum of the elements that were recovered consistently (Table 2) was plotted for each taxon.
Generally, as taxa increased in size, the number of elements recovered increased. Exceptions to this
were the small squirrels (Taxa 15-17) and the muskrat (Taxon 23). Examination of Table 1 reveals
that the small squirrels and the muskrat are smaller in length than preceding taxa and most also
had smaller femora.
Increases in the number of bones recovered for each size class are due to the recovery of new
element categories for each size class. For the 71-340-g class, additions included rami and long
bones. For the 340-3, 100-g class, additional elements recovered included vertebrae and certain long
bones. The large increases noted in the red fox and coyote (Taxa 25 and 26) are due primarily to
the recovery of podials, metapodials, and phalanges.
These results indicate that only taxa with similar recovery rates should be quantitatively compared.
That is, regardless of cultural and taphonomic factors that may have influenced assemblage composition, quantitative comparison of taxa can be conducted only when recovery methods have not
produced biased results.
CONCLUSIONS
Fine-screen techniques continue to be used on a limited scale at many archaeological sites. Even
though 1/4?screening provides the bulk of recovered faunal remains from many sites, screening biases
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REPORTS
135
largely have been overlooked. Recovery tests conducted on 25 species of mammals using ?/4" mesh
revealed that coarse screening not only biases samples towards larger specimens, but that the bias
against smaller specimens can be predicted. Additionally, the lack of recovery in archaeological
assemblages of those elements consistently found to be recovered by '/4"-screenmesh in this study
should signal the zooarchaeologist to question why the material was not recovered from the site.
These results provide zooarchaeologists with a useful tool to aid in the interpretation of faunal data.
This study also suggests that differential recovery compromises the utility of quantitative comparisons of different sizes of taxa. Based on the data presented, only taxa with similar recovery rates
should be compared unless recovery biases are taken into account. For different-sized taxa, only
elements with comparable recovery rates should be used for quantitative comparison.
Acknowledgments. BarryW. Baker,John E. Dockall, Joseph F. Powell, Julia L. Sanchez, HarryJ. Shafer,
D. Gentry Steele, and four anonymous AmericanAntiquityreviewers provided valuable comments on this
manuscript.Teresita Majewskiis thanked for her editorial assistance. Bonnie C. Yates provided additional
consultation.Specimensused in this study were borrowedfrom the followingcollections:(1) the Comparative
ZooarchaeologicalResearchCollection,Departmentof Anthropology,Texas A&M University;(2) the Department of Applied Sciences ZooarchaeologyLaboratoryfaunal collection, University of North Texas; (3) the
University of Texas, Departmentof Geology, VertebratePaleontologyCollection;and (4) from my personal
collection.Juan Jose Aguirretranslatedthe abstractinto Spanish.
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