Journal of Paleontology

Published by: The Paleontological Society

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Journal of Paleontology 80(6):1047-1057. 2006
doi: 10.1666/0022-3360(2006)80[1047:PVITBL]2.0.CO;2

PHENOTYPIC VARIATION IN THE BRYOZOAN LEIOCLEMA PUNCTATUM (HALL, 1858) FROM MISSISSIPPIAN EPHEMERAL HOST MICROCOMMUNITIES

STEVEN J. HAGEMAN and JENNIFER A. SAWYER

Department of Geology, Appalachian State University, Boone, North Carolina 28608, < >

Abstract

The morphologic expression of microenvironmental variation is difficult to document in fossil ecosystems and therefore is poorly understood. However, documentation of environmental sources of variation in the phenotype is essential for meaningful studies of microevolution and speciation. A fossil assemblage from the Mississippian (Valmeyeran) Warsaw Formation near St. Louis, Missouri, provides necessary conditions to evaluate microenvironmentally induced phenotypic variation in the Paleozoic trepostome bryozoan Leioclema punctatum (Hall, 1858). Specimens of L. punctatum, found as fragments in 22 discrete piles, were collected in their entirety from a weathered surface. Each pile contained 20–200+ branch fragments of L. punctatum, which were all originally attached to large, soft-bodied hosts (sponges?). Multiple attachment bases were found in most piles, indicating that 1) multiple L. punctatum colonies (genotypes) are represented in each pile, and 2) each pile represents a near contemporaneous, relatively short-lived microcommunity. Morphological characters were measured (four per section) from two branches for each of two specimens from five separate piles. Results from completely random, nested, one-way ANOVA indicate that no highly significant differences exist among microcommunities or between colonies for any measured characters, but that significant variation exists within colonies and among colonies in the same microcommunity (pile). That is, submicroenvironmental variation, within and among colonies, can play a greater role in morphogenesis than environmental heterogeneity within a given environmental setting (undifferentiated facies). Microenvironmental factors affect the size and shape of mesopores (space-filling structures) more than other morphological characters.

Results are encouraging for the general application of the preserved fossil phenotypes as proxies for biological species. This conclusion is based on the absence of systematic variation at microenvironmental levels, measurable here, but not normally distinguishable in paleontological and sedimentological studies. Correct attribution of fossil species assumes, however, that the source and the relative importance of the low-level (submicroenvironmental) variation on development/ontogeny is recognized and attributed appropriately. Results call for a reevaluation of the application of within versus among colony variation used as a proxy for environmental stability.

Accepted: July 21, 2005



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APPENDIX


table

Appendix—Summary of the means, standard deviations (stdev), and number of observations (n) per factor/level. Measurements are in mm. Abbreviations defined in Figure 1 and Table 2

table

Appendix—Extended

Figure 1—1, Reconstruction of transverse section of Leioclema punctatum (Hall, 1858), ×20, showing position of measurements. Explanation of abbreviations: EnDi = endozone diameter, EnWlTk = endozone wall thickness, AuDpSp = autozooecium diaphragm spacing, MeDpSp = mesopore diaphragm spacing. 2, Reconstruction of tangential section of Leioclema punctatum, ×36.5, showing position of measurements. Explanation of abbreviations: AuDi = autozooecium diameter, AuAr = autozooecium area, AuDgSp = autozooecium diagonal spacing (within quincunx), AuLtSp = autozooecium lateral spacing (across quincunx), MeSp = mesopore spacing, MeAr = mesopore area, AuWlTk = autozooecium wall thickness, MeWlTk = mesopore wall thickness

Figure 2—Colonial base piece from a single colony of Leioclema punctatum, ×3. 1, Branch fragments for this study were taken from bases such as these. 2, The curvature of the bases suggests that specimens were growing on a columnar host, such as a sponge or algal frond. Scale bar = 1.0 cm

Figure 3—Stratigraphic column showing the Valmeyeran Warsaw Formation, after Snyder (1991, fig. 2, p. 13)

Figure 4—Experimental design for completely random, nested ANOVA design used in this study (one of five microcommunities shown). Colonies are nested within microcommunities and branches are nested within colonies and microcommunities. Four observations were made for each character (Fig. 1). N = Five microcommunities, a = two colonies per microcommunity, b = two branches per colony, c = four observations measured per branch (see Table 1)

Figure 5—Data plotted on the first two axes of principal components, which account for 36.7% of the total variance. Each symbol represents a suite of 11 measured characters for each zooecia. 1, Symbols represent microcommunities (Mc); circle = Mc-4; dash = Mc-5; triangle = Mc-10; diamond = Mc-21; and cross = Mc-13. 2, Same principal component space as 1, but with only microcommunities 4 and 5, indicated with black and gray symbols, respectively. Microcommunity 4 is highlighted by a large oval. Two colonies (A and B) of microcommunity 4 are outlined in smaller ovals. In each colony, four zooids measured from the same branch are highlighted with a white center. Four zooids measured from the other branch within the same colony have solid fills

Figure 6—Potential sources (causes) of morphological variation at several hierarchical levels. Each level of environmental variation may be a result of either changing conditions through space (position within environment), or changing conditions through time, or both. 1, Variation within a genetic clone (colony) may be due to very small-scale environmental variation or intrinsic controls on development of single genotype. 2, Variation among colonies (very close in space and time) may be due to small-scale environmental heterogeneity in space and/or time, or genetic variation among colonies, the effects of which are observable (Hageman et al., 2001). 3, Variation among groups of colonies (time correlative) may be due to small-scale environmental heterogeneity in space and/or time or genetic variation among colonies. 4, Variation among larger scales of environmental hierarchies (space and/or time) can be envisioned, but were not tested within this study

table

Table 1—Sum of squares table from nested Analysis of Variance for endozone diameter (EnDi in μm, Fig. 1), including degrees of freedom and F-ratio determination for all factors random and “c-branch” nested within “b-colony,” and “b-colony” nested within “a-microcommunity” from Zar (1999, appendix 7) (df = degrees of freedom, SoS = sum of squares, MS = mean SoS, P-value = probability that the Factor/level is due to random effects, Var. = variance accounted for by each Factor/level, %Var. = the percentage of the total variance accounted for by each Factor/level)

table

Table 2—Summary of characters based on P-value and percent of total variance for each character accounted for by each factor. Percent variance across a character through all levels plus residual sums to 100% (e.g., EnDi = 0.0% + 42.1% + 35.8% + 22.1% = 100.0%). P-values in bold are significant at the level of ≤ 0.05

table

Table 3—Summary loadings for the first five Principal components (varimax rotation). Bold values indicate the characters that are most correlated with each axis

 
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