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Using available shape characters we conducted an outline morphometric analysis to make family-level identifications of fossil spiders from the Florissant Formation in Colorado. In this analysis we used carapace shape because it is a character that can be observed on most fossil spiders, and we also used linear leg characters. All measurements were first made on 202 modern spiders from eight families found in localities similar to the fossil lake environment. A multiple discriminant analysis (MDA) of the eigenshape axes was used to predict family placement among the modern data set to test the accuracy of the predictions. The modern spider families that were predicted correctly most often were the Salticidae (91.2%), Linyphiidae (80%), Dictynidae (76.5%), Tetragnathidae (68.2%), Clubionidae (66.7%), and Araneidae (65.5%). Families that produced less successful results were the Agelenidae (46.7%) and the Lycosidae (39.1%). Forty-three fossil spiders from Florissant were then added to the model to determine their family placement. All fossils were placed into modern families with varying degrees of accuracy. Only 42% of our identifications agree with those made by previous authors, but it is likely that these specimens were originally misidentified. With the addition of more taxa and characters, we believe that an outline morphometric approach shows great promise for helping to identify fossil taxa that are lacking traditional taxonomic characters.

INTRODUCTION

The anatomical completeness of fossil specimens is of great concern in studies of taxonomy and morphology. Finding preserved fossils with important identifiable characters is crucial to paleontologists, yet rare. Organisms with hard parts are more likely to be preserved as fossils, because they are more resistant to the biological, physical, and chemical processes that occur after death. However, even these fossils are susceptible to boring, chemical dissolution, and breakage (Menard and Boucot, 1951; Kidwell and Bosence, 1991; Best and Kidwell, 2000). As a result, only a subset of the fossil record may be relevant to taxonomic interpretation (Kidwell, 2001). Among the organisms that are composed of soft parts, researchers studying arthropods have found an even greater potential for disintegration and destruction (Schafer, 1972; Plotnick, 1986; Allison, 1988; Martinez-Delclos and Martinell, 1993; Briggs et al., 1998; Smith, 2000; Martinez-Delclos et al., 2004; Smith et al., 2006). Spider exoskeletons tend to be less sclerotized than other terrestrial arthropods, such as insects, thus contributing to their rarity in the fossil record.

Considering the difficulties associated with fossilization, it is no surprise that taxonomic work on fossils is so difficult (Kidwell, 2001). There are rare deposits, such as the Burgess Shale, where fossilization of soft parts has occurred and preservation is exceptional. However, diagnostic characters for these organisms are not always visible. This is especially true among the spiders (Order Araneae). Superfamily- and family-level diversity among fossil spiders have been based on a wide variety of available characters, including but not limited to leg length, leg width, and carapace shape (Penney, 2000, 2002; Penney and Selden, 2002). Making generic and specific placements of modern spider taxa typically relies on characters such as genitalia, minute cheliceral characters, and leg spine orientation are typically used (Chamberlin and Ivie, 1941; Chamberlin and Gertsch, 1958; Dondale and Redner, 1982; Levi, 1981; Roth, 1993; Ubick et al., 2005). These diagnostic characters are not usually visible on spiders preserved in shale, thus illustrating the need to find an alternative method for identifying fossil spiders. In this study we present a new method that uses a combination of outline morphometric analyses and linear characters to identify spiders to the family level.

Outline-based morphometrics is a technique used to quantify shape and size (Lohmann and Schweitzer, 1990). This method captures the available shape information, while reducing the amount of user input and potential bias when choosing distinguishing characters (Lohmann and Schweitzer, 1990), as the input is the complete shape. More importantly, the analysis allows for the discovery of covariation among the morphological variables (MacLeod, 1999), ultimately providing new variance-optimized axes summarizing the major aspects of shape variation in the fewest axes. Many workers have attempted to quantify morphological differences using shape characteristics to solve a variety of taxonomic questions (Kendall, 1984; Bookstein, 1986, 1991, 1996, 1997; Rohlf and Bookstein, 1990; Rohlf, 1993; MacLeod, 1999). This method has been useful in taxonomic and environmental studies of leaves (Jensen et al., 1993, 2002; McLellan and Endler, 1998; Premoli, 1996; MacLeod, 2002; Krieger et al., 2007), ontogenetic shape variation in fossil ostracod carapaces (Baltanas et al., 2000), and sexual dimorphism in isopods (Bertin et al., 2002). This technique has also been applied successfully to the classification of both modern (Warheit, 1992; MacLeod, 1999, 2002) and fossil taxa (Kowalewski, 1993; Baltanas et al., 2000).

The goals of this study are to (1) investigate whether morphometrics can be used to distinguish modern spiders at the family level; and (2) apply these methods to Eocene fossil spiders from Florissant.

FLORISSANT GEOLOGIC HISTORY

The Florissant Fossil Beds are an example of a fossil Lagerstätte in western Colorado that formed in an ancient lakebed 34.1 million years ago. This lake was formed by a large lahar, which dammed a stream drainage running through the Florissant valley. The Tertiary lacustrine strata of the Florissant Formation are composed of tuffaceous mudstone, fossiliferous shale layers comprising couplets with alternating layers of diatoms and ashclay, and interbedded ash layers (Fig. 1). Estimates of climate at Florissant are varied, but it is generally agreed that the climate was warm temperate with mean annual temperature averaging ∼13 °C, with a distinct dry season; estimates of the mean annual precipitation range from 50 to 80 cm (Meyer, 1992, 2001, 2003; Gregory, 1992). This area has been described as a transitional temperate environment on the basis of tropical and temperate insects that have been described from the area and leaf physiognomic studies (Scudder, 1890; Meyer, 1992, 2003; Gregory, 1992; Wolfe, 1992, 1994, 1995; Gregory, 1992; Leopold and Clay-Poole, 2001; Moe and Smith, 2005). Paleoelevation estimates for Florissant range from 1900 to 4100 m (Meyer, 1992, 2001, 2003; Gregory, 1992; Wolfe, 1992, 1994, 1995).

Figure 1. Columnar stratigraphic section of the Florissant Formation, adapted from Evanoff et al., 2001.

Figure 1. Columnar stratigraphic section of the Florissant Formation, adapted from Evanoff et al., 2001.

MATERIALS AND METHODS

Sources of Data

All modern specimens were from the spider collection at the Denver Museum of Nature & Science (DMNS). Fossil specimens were loaned from several institutions including the Florissant Fossil Bed National Monument (FLFO), the Museum of Comparative Zoology at Harvard University (MCZ), the National Museum of Natural History in Washington, D.C. (USNM), and the University of Colorado Museum in Boulder, Colorado (UCM).

Modern Material

The carapace shapes of modern spiders were studied by sampling 202 modern adult spiders from the eight most abundant families found around modern lake environments (Table 1 for families, Appendix for species examined). The genera and species examined were chosen to represent the range of morphological diversity within a family as recommended by experts in that group (B. Cutler for Salticidae, C. Dondale for Lycosidae, J. Miller for Linyphiidae, H. Levi for Tetragnathidae and Araneidae, D. Ubick for Clubionidae, 2004, personal commun.).

Although Florissant has been described as a transitional temperate environment (Leopold and Clay-Poole, 2001; Meyer, 2003), the eight spider families used in this study are also commonly found in tropical lake localities (Nentwig, 1993). For example, members of the Tetragnathidae are found almost exclusively along wetland habitats in both tropical and temperate environments (Aiken and Coyle, 2000). In addition, these families include the majority of families into which the fossil spiders had been placed by previous workers.

The morphological variability of carapace shapes within the families examined is extensive. This study focused on capturing as much of the variability in carapace shapes within families as possible by choosing multiple representatives from each of the families that captured the large-scale variation in shape. In addition, covariation among traits was also considered and taxa were chosen that would best differentiate among the smaller-scale aspects of morphology. Because this study is designed to be a first test of the viability of a morphometric technique using shape and linear characters, it is limited to the study of the eight most common families, as opposed to examining fewer representatives from more families.

Fossil Material

The first work on fossil spiders from Florissant began in the late nineteenth century with the large monograph The Tertiary Insects of North America by Samuel Scudder (1890), who placed many of the spiders into new species, within 11 extant families: Anyphaenidae, Amaurobiidae, Araneidae, Clubionidae, Linyphiidae, Salticidae, Segestriidae, Tetragnathidae, Theridiidae, Thomisidae, and Titanoecidae. Petrunkevitch (1922) reevaluated this work and corrected several taxonomic problems and classification mistakes that had been previously made. He added the Gnaphosidae, Lycosidae, Parratidae, and Theraphosidae to the families found at Florissant, but determined that the Anyphaenidae, Amaurobiidae, Salticidae, Theriidae, and Titanoecidae were not present. His revisions resulted in eight extant families being recognized from Florissant, and only one extinct family, the Parratidae.

Of the fossil specimens found at Florissant, only those spiders that were complete and preserved with the dorsal surface exposed were used in this study, and specimens that showed only the ventral surface or were preserved laterally were not included. In addition, specimens that appeared to have been distorted by the process of fossilization were excluded from the study. Forty-three fossil spiders were preserved dorsally with the carapace fully exposed, and these were used in the analysis. Of these, 24 had been identified by previous workers.

Morphometric Analysis

To generate the empirical morphospace of the modern families, we began by capturing images of the modern specimens using a Nikon 995 digital camera attached to a Leica MZ-6 microscope with a standard Leica 1× optical lens. A standard-scale TIFF image was captured of the carapace of each specimen. The TIFF images of the specimens were then placed into Photoshop 7.0 (Adobe Systems Incorporated, 1990–2002) and silhouettes were carefully made by increasing the amount of contrast in the picture. The automated thresholding approach to generating silhouettes is not always effective, and in some cases we manually captured the outline of specimens in Photoshop using the lasso tool. Silhouettes still need to be converted to a set of outline coordinates. The first step in this process is to select an initial starting point that can be repeatably located, which defines homology across specimens (Lohmann and Schweitzer, 1990). The exact midpoint of the anterior area of the carapace was chosen for this purpose, as it is a point that can be relocated along the form (Fig. 2).

Figure 2. General spider body form depicting legs I–IV and femur widths II and III. The exact midpoint of the anterior area of the carapace was chosen to be landmark 1. This starting point for the outline is easy to locate on all of the specimens.

Figure 2. General spider body form depicting legs I–IV and femur widths II and III. The exact midpoint of the anterior area of the carapace was chosen to be landmark 1. This starting point for the outline is easy to locate on all of the specimens.

After selection of the outline starting point, the silhouetted images were converted to closed outlines using tpsDig 1.28 (F.J. Rohlf, http://life.bio.sunysb.edu/morph/). tpsDig represents the outlines as a series of boundary coordinate points from the starting point around the edge of the silhouetted object. Each outline consisted of 200 evenly spaced points placed around the circumference of the carapace shape. The x, y coordinate outlines were converted to phi functions using MacLeod's xy-phi program (available for download at http://life.bio.sunysb.edu/morph/). Phi functions describe the net angular change required to follow the outline around the carapace from the starting landmark (Zahn and Roskies, 1972). Conversion to phi functions removes position, rotation, and scale from the description of carapace shape; “shape” is what remains when these three quantities have been removed (Zelditch et al., 2004). The size of each carapace shape is calculated as the geometric area of the outline polygon, which is an additional independent character that is used to further place the spiders into family groups.

Once the shapes were standardized, the eigenshape analysis was performed with a covariance matrix using MacLeod's Eigenshape software (http://www.nhm.ac.uk/hosted_sites/pale-onet/ftp/ftp.html). The analysis finds the vectors with the most comparable net angular change among all specimens, which is defined as eigenshape axis 1, and then proceeds in the same manner until all the shape covariation is analyzed (MacLeod, 1999). A 5% tolerance criterion was applied to the analysis, which reduced the number of data points from 200 points around the outline to 49 points. The first ten eigenshape axes (ES1–ES10) were examined, because they explain 99.08% of the shape variation (Table 2). After ES1 is removed, ES2-ES10 represents 87.06% of the remaining variance. The first axis represents the shape similarity among all the carapace shapes of all the specimens in the data set. Axes 2–10 represent a decreasing amount of variation among all the carapace shapes in the data set. Shape variation can be visualized along any axis in the morphospace by varying the amplitude of the eigenshape for that axis (a phi function) and averaging this with the phi function for the mean shape, a process termed “modeling” (Lohmann, 1983; MacLeod, 1999). The first ten axes were modeled using MacLeod's ESMo-del program (available for download at http://life.bio.sunysb.edu/morph/), which generated three models corresponding to the maximum, mean, and minimum scores for each eigenshape axis. These shapes were then overlain and examined to look at carapace shape variation along each axis (Fig. 3).

Figure 3. Modeled eigenshape axes 1–10. Each shape represents a dorsal view of the carapace. The lines represent the variability in carapace shapes most explained by that particular eigenshape axis. The left side of each axis is the distal end and the right side is the proximal end of the carapace. ES indicates the eigenshape axis being analyzed.

Figure 3. Modeled eigenshape axes 1–10. Each shape represents a dorsal view of the carapace. The lines represent the variability in carapace shapes most explained by that particular eigenshape axis. The left side of each axis is the distal end and the right side is the proximal end of the carapace. ES indicates the eigenshape axis being analyzed.

Linear Measurements

Previous authors have determined the taxonomic identity of fossil spiders by using many different characters (Scudder, 1890; Petrunkevitch, 1922, 1958; Levi, 1980; Penney, 2000, 2002; Penney and Selden, 2002). In this study, we use linear characters in addition to the carapace shapes used in the outline analysis. In particular, we focus on linear leg measurements, or relative differences in the lengths and widths of legs I, II, III, and IV, as these characters are often diagnostic for spiders at the family level. For example, in the Araneidae and the Tetragnathidae (compared to the other six families used in the analysis), leg I is longer than leg II, which is longer than leg IV; leg III, in both families, is the shortest leg. And in the Tetragnathidae, leg I is considerably longer than leg II, whereas in the Araneidae it is only slightly longer (Levi, 2005a, 2005b). In the Salticidae, leg I or leg IV (depending on the species) is the longest leg (Richman et al., 2005). Finally, legs are often present on fossil spider specimens.

The first character that we used was the ratio of leg II length to leg III length (Fig. 2). This character should be helpful in separating the wandering spiders (Clubionidae, Lycosidae, and Salticidae) from the web-building spiders (Agelenidae, Araneidae, Dictynidae, Linyphiidae, and Tetragnathidae). Wandering spiders tend to have similar leg lengths, which aids in rapid movement on the ground. In contrast, web-building spiders tend to have legs of differing lengths to aid in web construction (Foelix, 1996). Ratios close to one would likely be a wandering spider and those ratios less than one would likely belong to a web-building spider family.

The width-to-length ratio of the femur of leg I was also used to further distinguish between spider families (Fig. 2), and several workers have argued that different families of spiders tend to have either long or short and either robust or thin first legs (Levi, 1980; Dondale and Redner, 1990; Foelix, 1996). For example, lycosids and agelenids have long robust legs I, whereas salticids and clubionids have short, robust legs I and lyniphiids have long, thin legs I. We also used the ratio of the femur width of leg I to the femur width of leg III. Leg thickness can vary across all the legs and change in thickness. For example, Araneidae tend to have much more robust femora on leg pairs I and II than on the other legs (Levi, 1980). The above characters were added because they were thought to be most useful in further defining family groups.

We recognize that there are likely to be exceptions to the general trends that we describe in the characters above. However, it is our belief that using a combination of such characters with the outline analysis will provide more useful results than using any single character on its own.

Statistical Analysis

A multivariate analysis of variance (MANOVA) test was performed to determine whether family designation was determined by carapace shape, carapace size, and the linear characters (SPSS 12.0, 2003). Because multiple comparisons are being made, a Bonferroni post hoc comparison test was used to examine the pairwise shape differences between the taxa.

A set of multiple discriminant analyses (MDA), with leave-one-out classification, were performed. We used leave-one-out classifications as means to test the quality of the modern data sets for prediction; leave-one-out classification removes one or more specimens from the initial analysis and treats the specimen(s) as “unknowns,” which are then reprojected into the MDA and classified. This provides a way to perform a relatively conservative test of the efficiency of the MDA procedure. The first set of MDAs examined only modern spiders and the initial analysis used only eigenshape axes (ES1–ES10) to determine how well the carapace shape variation could differentiate families (SPSS 12.0, 2003). Linear characters and size were then sequentially added as independent variables to further differentiate the family groupings in additional MDA analyses.

Analysis of Fossil Specimens

After the MDAs based on the modern specimens were run, spider fossils from Florissant were examined. Seven of the eight families included in this analysis had been previously described from Florissant (all but Dictynidae) (Scudder, 1890; Petrunkevitch, 1922; Eskov and Zonshtein, 1990), and all of the families are known from deposits older than the Eocene, especially amber deposits (Petrunkevitch, 1958; Wunderlich, 1988; Selden, 1989; Penney, 2001, 2002; Penney and Selden, 2002; Selden 2002; Penney et al., 2003; Penney and Langan 2006; Penney and Ortuño, 2006).

The carapace shapes of the 43 fossil specimens were projected into the modern family shape space using Eigenshape Project (http://www.nhm.ac.uk/hosted_sites/paleonet/ftp/ftp.html), and another MDA using all the shape and size information (carapace shape/size and leg ratios) was run to make predictions concerning the family placement of the fossil specimens.

RESULTS

Shape Variations among Eigenshape Axes

The first ten axes were modeled to determine the carapace shape variation represented by each axis (Fig. 3). The mean shape of eigenshape axis 1 (ES1) represents the average shape of all spider specimens in the data set (Fig. 3A). This is a shape-similarity axis that represents all of the shape similarity. Wide carapace shapes score low on this axis. Carapace shapes that are narrower score high. The variation among families is slight, with the Dictynidae and Araneidae having the highest average scores, and the Agelenidae, Salticidae, and Linyphiidae having the lowest scores.

Eigenshape axis 2 (ES2) shows variation primarily along the anterior region of the carapace (Fig. 3B). Shapes that have little curvature along the posterior region and are narrow in the anterior region score high and include the Agelenidae and the Lycosidae. Shapes that have greater curvature along both the posterior and anterior regions score low and include the Salticidae. On Eigenshape axis 3 (ES3), carapace shapes that have a greater curvature in the posterior region while slighter in the anterior region score high, and carapace shapes that have a slighter curvature throughout the carapace score low (Fig. 3C). Families that score high include members of the Araneidae, whereas families that score low include Dictynidae, Clubionidae, and Salticidae.

Eigenshape axis 4 (ES4) represents the carapace curvature variation along the posterior region and the overall length (Fig. 3D). The carapace shapes that have little curvature score high and include members of the Linyphiidae, Agelenidae, and Lycosidae. Those carapace shapes that have a greater curvature in the posterior region score low and include Dictynidae, Araneidae, and Clubionidae.

Eigenshape axes 5–10 (ES5–ES10) represent variation along the lateral edge of the carapace shape (Fig. 3E–J). There is slight family-level variation along these axes. For example, on ES5 members of the Tetragnathidae score higher than all other families, whereas Linyphiidae score lowest (Fig. 3E). On ES6, members of the Dictynidae and Agelenidae score higher than all other families on this axis, and Araneidae and Clubionidae score lowest (Fig. 3F). Along ES7, members of the Dictynidae and Araneidae score high and Clubionidae score low (Fig. 3G). Along ES8, members of the Clubionidae and Dictynidae score high, and Agelenidae score low (Fig. 3H). On ES9, members of the Clubionidae score high and Araneidae and Salticidae score low (Fig. 3I), although it should be noted that family differences along this axis were not statistically significant. Finally, along ES10, members of the Tetragnathidae score high, and Salticidae score low (Fig. 3J).

Examining the Morphometric and Discriminant Analysis Tests

We found that family designation was defined by carapace shape (all axes but ES9), carapace size, leg length ratio of leg II to leg III, and the two femur ratios. The results of the MANOVA can be found in Table 3. The results of the MDA performed on the ten eigenshape axes and eight family groups are shown in Tables 4–8. Seven functions are given, which represent the variance of carapace shapes among all family groups. Only the first three functions will be reported, because they represent 87.06% of the variance (Table 2). In the first discriminant analysis, all eigenshape axes 1–10 were significant in their ability to distinguish family groupings (Function 1: Wilks' Lambda L = 0.12, p < .0001; Function 2: L = 0.309, p < .0001; Function 3: L = 0.494, p < .0001) (Table 5). However, the eigenshape axes were only able to place 41.6% of the modern spiders into the correct family (Table 6).

The second discriminant analysis, using the eigenshape axes, the leg length ratio, leg width ratios, and carapace size, also resulted in an ability to distinguish family groupings of the modern spiders based on all characters that was statistically significant (Function 1: Wilks. Lambda L = 0.015, p < .0001; Function 2: L = 0.055, p < .0001; Function 3: L = 0.166, p < .0001) (Table 7). The eigenshape axes, linear characters, and size accurately predicted the family membership of 70.3% of the modern spiders sampled, and some groups in this analysis had very high rates of prediction when all the shape and size data sere used (Table 6). Tetragnathidae was most often confused with the Araneidae (3 of 15 spiders), Linyphiidae (2 of 15 spiders), and Dictynidae (2 of 15 spiders). The Clubionidae were also confused with the Linyphiidae (3 of 12 spiders), although when these two families are combined there is 75% accuracy between these two groups. Agelenidae were most often confused with Lycosidae (6 of 15 spiders) and Lycosidae was most often confused with Agelenidae (6 out of 23 spiders). Once combined, there is 78.2% accuracy for these two groups.

In the final discriminant analysis, fossils were placed into one of the eight families analyzed. There were no fossil spiders identified as Agelenidae. When specimens were identified as Salticidae, Linyphiidae, or Dictynidae there was a greater than 76.5% chance that these groupings were correct and that the specimens were placed into the appropriate family.

DISCUSSION

The goal of this study was to test a method that could be used with the types of characters that are typically preserved in the fossil record to identify spiders to the family level. By using a morphometric study of the carapace of extant spiders, subtleties of the shape could be analyzed and used to help identify modern spider families. Combining the morphometric analysis with linear leg measurements greatly improved the method. The results of this analysis provide a starting point for family-level identification of two-dimensional fossils. Even for families with low prediction rates, it is possible to assess whether the fossil may represent one of two or three families commonly confused in the analysis.

Character Combinations

The combination of carapace shape and the leg length ratio character helped to distinguish many of the families, especially between the major spider feeding guilds. These feeding guilds, the web-spinning spiders and the wandering spiders, have very different leg sizes depending on their feeding strategy (Chamberlin and Ivie, 1941; Levi, 1980, 1981, 1986). For example, many of the web builders like Agelenidae, Araneidae, and Tetragnathidae were separated from the wandering spiders like Lycosidae and Salticidae. Carapace shape further separated family members within each feeding guild. Subtleties in the shape that were likely to have remained unnoticed by casual observation became clear and usable through an outline morphometric approach.

Popular identification keys for spider families reflect unique character suites (Nentwig, 1993). Using a combination of outline and linear characters, represented by eigenshape axes and leg characteristics in this study, was found to be more useful than using any one of those characters alone. As a result, any character combination (carapace shape, size, and leg characters) gives a much higher prediction rate than do those characters analyzed separately. This character suite strengthens the overall prediction rate of each family, showing that the signals produced by each individual character increase the tendency for correct family placement.

Misidentified Families

Some of the spiders were consistently misplaced into a family. Among these were the Agelenidae, Clubionidae, and Lycosidae. However, when some of these families were grouped into combinations, the prediction rates increased. For example, when the Agelenidae and Lycosidae were combined, there was an 86.7% prediction rate that a fossil belonged to one of these two families. When Clubionidae was grouped with the Linyphiidae, the prediction rate jumped to 91.7%. The similarity in form of these families cannot be explained by phylogeny, because they are distantly related (Coddington and Levi, 1991).

Although the results do not attest to a complete success of the method, it is nevertheless useful in its capability to determine a specimen's identification to one of two groups, which is far superior to trying to rule out multiple groups. At the very least, for those taxa with low prediction rates, this method allows one to narrow the list of suspects and focus on specific characters.

Additions to Improve the Analysis

Although there was high overall accuracy for distinguishing family groupings, the potential accuracy was highly variable depending on the family. There are several ways the methods could be further improved to increase accuracy and make the model more useful to future researchers.

In addition to carapace shape, the carapace size was a useful character for identifying modern taxa. For example, Dictynidae and Linyphiidae tend to be composed of taxa that have relatively small bodies—most species are less than 4 mm as adults (although there are some genera with large body sizes worldwide that were not included in this analysis) (Chamberlin and Gertsch 1958). Because all the modern specimens used in the analysis were adult, this is a reliable character. However, size is not likely to be a character that is useful for the placement of fossil specimens. With fossil spiders, unless genitalia are preserved it can be difficult to determine whether a small spider is actually just a juvenile. Therefore, other morphological characters, in addition to size, must be included in an analysis to validate family placements for fossil specimens.

The inclusion of additional characters should also help better distinguish between family groups. We found that increasing the number of characters used in the analysis greatly increased the accuracy of family-level placement and we expect that improvements would continue with the addition of more characters. However, we realize that finding more characters that are consistently available on fossil specimens can be a challenge. In addition to the number of characters, expanding the number of families (and the representative genera within these families) to be included in the analysis should make the method much more applicable to a wider range of fossil deposits. The families used in this analysis were chosen because of their abundance in modern temperate lake environments. Other families common to lake environments and other climate regions should also be included, such as the Theridiidae, Thomisidae, and Philodromidae.

Fossil Family Placements

Every fossil spider was placed into a modern family, but the predicted accuracy of these placements was highly variable. Only ten of the previously identified specimens (42%) were placed in the family to which they had been assigned by the previous author. We believe that many of the identifications made by previous authors are, in fact, incorrect (Roberts 2004). However, we had greater confidence in our identification of fossil spiders when we placed them into families whose modern representatives had prediction rates of over 70%. Although nearly all of the spider families described from Florissant are extant, there is one specimen that has been attributed to the extinct family Parratidae. This specimen was not included in the present analyses because we focused on modern families. However, including this specimen in future morphometric analyses with a larger data set of extant families should yield interesting results. Given the modern composition of the Florissant spider assemblage and the large number of misidentifications made by previous workers, we would not be surprised to learn that the placement of this specimen within an extinct family is also erroneous.

Implications

Obtaining fossils that possess the characters needed for identification is often difficult. Some paleontologists believe that more fossils would be identifiable if better characters were available (Schopf et al., 1975; Schopf, 1981; Kowalewski et al., 1997). Workers interested in fossil spiders have had a difficult time understanding familial diversity and evolutionary rates because of the scarcity of characters preserved in shale (Eskov and Zonshtein, 1990; Coddington and Levi, 1991). It is, therefore, important to capture as much morphological information from a fossil as possible, as this increases the likelihood of a correct identification.

The scarcity of preservable characters has especially been a problem for identifying the fossil spiders of Florissant, and past researchers have painted a picture of spider diversity based on a much more limited data set. The use of this new methodology provides a view of the taxonomic diversity of spiders at Florissant that is dramatically different from previous interpretations. Morphometrics has allowed for the use of additional characters in making objective family placements. The results here suggest that we can make accurate predictions based on a combination of both non-traditional and traditional morphological characters that are commonly preserved in fossil shale deposits.

The methods used here are likely to be extendable to other arthropod groups. For example, insects are abundant in shale deposits and can be difficult to identify without adequate preservation of important characters (e.g., genitalia, color patterns). Diptera and many Hymenoptera, for example, can be identified to species by wing venation characters (Leach, 1815; Macquart, 1835; Newman, 1835) and wings are common in many fossil deposits (Scudder, 1890). Morphometric analysis allows shape characteristics to be quantified and has the potential to aid in identification. In short, this technique has great potential and appeal to any paleontologist in situations where character preservation among fossils is problematic.

Most importantly, approaches like the one detailed here move paleobiologists away from subjective identifications and toward more objective methods for taxonomic placement, and allow one to evaluate the uncertainty involved in such placements. Having some measure of accuracy when using morphology for the placement of modern families gives us the ability to assess the validity of our placements. This is a positive step toward recovering the baseline biodiversity information necessary for almost all paleo-biological inferences.

TABLE 1. FAMILIES USED IN THE MORPHOMETRIC AND MULTIPLE DISCRIMINANT ANALYSIS

TABLE 2. THE PERCENT VARIATION EXPLAINED BY EACH EIGENSHAPE AXIS

TABLE 3. RESULTS OF MANOVA TEST WITH BONFERRONI CORRECTION

TABLE 4. RESULTS OF MULTIPLE DISCRIMINANT ANALYSIS TEST INCLUDING ONLY THE MODERN SPECIMENS AS THE GROUPING VARIABLE AND ONLY EIGENSHAPE AXES 1–10 AS INDEPENDENT VARIABLES

TABLE 5. RESULTS OF STRENGTH TEST APPLIED TO THE MULTIPLE DISCRIMINANT ANALYSIS INCLUDING ONLY THE MODERN SPECIMENS AS THE GROUPING VARIABLES AND ONLY EIGENSHAPE AXES 1–10 AS INDEPENDENT VARIABLES

TABLE 6. BOOT-STRAPPED PREDICTED GROUP MEASUREMENTS FOR THE EIGHT FAMILIES STUDIED

TABLE 7. THE RESULTS OF THE MULTIPLE DISCRIMINANT ANALYSIS TEST WITH ALL CHARACTERS

TABLE 8. RESULTS OF STRENGTH TEST APPLIED TO THE MULTIPLE DISCRIMINANT ANALYSIS WITH ALL CHARACTERS

APPENDIX. MODERN SPIDER SPECIES USED IN THE MORPHOMETRIC ANALYSES. NUMBER IN PARENTHESES INDICATES NUMBER OF SPECIMENS, IF MORE THAN ONE WAS USED FROM A PARTICULAR SPECIES

  • Family Agelenidae

    • Agelenopsis aperta

    • Agelenopsis emertoni

    • Agelenopsis naevia

    • Agelenopsis oklahoma

    • Agelenopsis pennsylvanica

    • Agelenopsis spatula

    • Agelenopsis utahana

    • Calilena arizonica

    • Hololena dana

    • Hololena hola

    • Hololena nevada

    • Hololena oquirrhensis

    • Tegenaria agrestis

    • Tegenaria domestica

    • Tegenaria duellica

  • Family Araneidae

    • Acanthepeira stellata

    • Aculepeira carbonaria

    • Aculepeira packardi

    • Araneus marmoreus

    • Araneus normandi

    • Araneus ocellatulus

    • Araneus pegina (2)

    • Araneus pratensis

    • Araniella cucurbitina (2)

    • Araniella displicata (2)

    • Araniella sp.

    • Argiope argentata

    • Argiope aurantia (2)

    • Mangora mobilis

    • Mangora passiva

    • Mangora placida

    • Mastophora bisaccata

    • Metepeira arizonica

    • Metepeira minima

    • Neoscona arabesca (2)

    • Neoscona crucifera

    • Neoscona domiciliorum

    • Verrucosa arenata

    • Zygiella x-notata

  • Family Clubionidae

    • Cheiracanthium inclusium

    • Cheiracanthium mildei

    • Clubiona abboti

    • Clubiona canadensis

    • Clubiona johnsoni

    • Clubiona kastoni

    • Clubiona kulczynskii

    • Clubiona maritima

    • Clubiona riparia

    • Clubiona sp.

    • Clubiona spiralis

    • Elaver exceptus

  • Family Dictynidae

    • Dictyna foliacea (2)

    • Dictyna idahoana

    • Dictyna longispina

    • Dictyna nebraska

    • Dictyna sancta

    • Dictyna terrestris

    • Dictyna tridentata

    • Dictyna volucripes

    • Emblyna borealis

    • Emblyna completa

    • Emblyna completoides

    • Emblyna palomara

    • Neoantistea agilis

    • Neoantistea crandalli

    • Neoantistea magna

  • Family Linyphiidae

    • Bathyphantes latescens

    • Ceratinopsis purpurea (2)

    • Collinsia holmgreni (2)

    • Eperigone contorta

    • Eperigone maculata (2)

    • Erigone acanthagnatha

    • Erigone aletris

    • Erigone atra

    • Erigone blaesa

    • Erigone dentigera (2)

    • Erigone ephala

    • Erigone psychrophila

    • Erigone sp. (2)

    • Frontinella huachuca

    • Frontinella pyramitela

    • Grammonota gentilis

    • Grammonota inusiata (2)

    • Grammonota trivittata

    • Helophora insignis (2)

    • Hilaira proletaria

    • Hilaira vexatrix (2)

    • Hypselistes florens

    • Linyphantes pualla (2)

    • Microlinyphia mandiblulata

    • Microlinyphia pusilla

    • Neriene clathrata (2)

    • Neriene variabilis (2)

    • Neoantistea oklahoma

    • Oedothorax trilobatus

    • Pityohyphantes brachygynus

    • Pityohyphantes costatus (2)

    • Pityohyphantes phrygianus (2)

    • Scotinotylus pallidus

    • Walckenaeira communis

    • Walckenaeria clavicornis

    • Walckenaeria directa

    • Walckenaeria karpinskii

    • Walckenaeria vigilax

  • Family Lycosidae

    • Arctosa alpigena

    • Arctosa littoralis

    • Arctosa virgo (2)

    • Geolycosa missouriensis

    • Geolycosa wrighti

    • Hogna antelucana (2)

    • Hogna coloradensis

    • Hogna sp. (2)

    • Pardosa lapidicina (2)

    • Pardosa mercarialis

    • Pardosa mulaiki

    • Pardosa ourayensis

    • Pardosa sternalis

    • Pirata insularis

    • Pirata minutus

    • Pirata piraticus

    • Pirata sedentarius (2)

    • Traebacosa marxi

  • Family Salticidae

    • Ghela canadensis

    • Habrocestum pulex (2)

    • Habronattus brunneus

    • Habronattus clypeatus

    • Habronattus venatoris

    • Habronattus viridipes

    • Marpissa lineate

    • Marpissa pikei

    • Metaphidippus iviei

    • Neon nelli

    • Peckhamia scorpionia

    • Pelegrina aeneola

    • Pelegrina flavipes

    • Pelegrina peckhamorem

    • Pelegrina proterva

    • Pellenes jucundus

    • Pellenes levii

    • Phidippus arizonensis

    • Phidippus borealis

    • Phidippus pulcher

    • Phidippus rimator

    • Phidippus whitmani

    • Phlegra fasciata

    • Platycryptus californicus

    • Platycryptus undatus

    • Salticus scenicus

    • Sassacus papenhoei

    • Sassacus vitis

    • Sitticus pubescens

    • Sitticus ranieri

    • Thiodina sylvana

    • Tutelina harti

    • Tutelina similis

  • Family Tetragnathidae

    • Glenognatha emertoni

    • Glenognatha foxi

    • Leucauge argyra (2)

    • Leucauge mandibulata (2)

    • Leucauge sp.

    • Leucauge venusta (2)

    • Metellina curtisi (2)

    • Nephila clavipes (2)

    • Pachygnatha autumnalis

    • Pachygnatha brevis

    • Pachygnatha tristriata

    • Pachygnatha xanthostoma

    • Tetragnatha elongata

    • Tetragnatha extensa

    • Tetragnatha laboriosa

    • Tetragnatha straminea

    • Tetragnatha versicolor

The authors thank Heidi Schutz and Amanda Cook for statistical consultation. Florissant Fossil Beds National Monument, the National Museum of Natural History (Smithsonian Institution), the Museum of Comparative Zoology at Harvard, and the University of Colorado Museum provided the fossil specimens, and the Denver Museum of Nature & Science (DMNS) provided all modern specimens. The authors appreciate the assistance provided by Herbert Levi, Jeremy Miller, Charles Dondale, Bruce Cutler, and Darrell Ubick in choosing appropriate modern spider taxa. We greatly appreciate the comments that we received from Norman MacLeod, Paul Selden, an anonymous reviewer, and Herb Meyer on the earlier drafts of this manuscript. The Paleontological Society, the Evolving Earth Foundation, the University of Colorado Museum of Natural History's Walker Van Riper Fund, and the DMNS Department of Zoology provided financial support for A.K. Roberts while completing this project.

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Figures & Tables

Figure 1. Columnar stratigraphic section of the Florissant Formation, adapted from Evanoff et al., 2001.

Figure 1. Columnar stratigraphic section of the Florissant Formation, adapted from Evanoff et al., 2001.

Figure 2. General spider body form depicting legs I–IV and femur widths II and III. The exact midpoint of the anterior area of the carapace was chosen to be landmark 1. This starting point for the outline is easy to locate on all of the specimens.

Figure 2. General spider body form depicting legs I–IV and femur widths II and III. The exact midpoint of the anterior area of the carapace was chosen to be landmark 1. This starting point for the outline is easy to locate on all of the specimens.

Figure 3. Modeled eigenshape axes 1–10. Each shape represents a dorsal view of the carapace. The lines represent the variability in carapace shapes most explained by that particular eigenshape axis. The left side of each axis is the distal end and the right side is the proximal end of the carapace. ES indicates the eigenshape axis being analyzed.

Figure 3. Modeled eigenshape axes 1–10. Each shape represents a dorsal view of the carapace. The lines represent the variability in carapace shapes most explained by that particular eigenshape axis. The left side of each axis is the distal end and the right side is the proximal end of the carapace. ES indicates the eigenshape axis being analyzed.

Contents

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