Lindsey R. Leighton

EFFECTS OF DATA CATEGORIZATION ON PALEOCOMMUNITY ANALYSIS: A CASE STUDY FROM THE PENNSYLVANIAN FINIS SHALE OF TEXAS

Abstract Paleocommunity research efforts have explored a multitude of faunal assemblages using a wide range of sampling and analytical methods to infer a paleoecological signal. Here, we derive six secondary datasets from a single stratigraphic series of faunal assemblages in the Finis Shale (Pennsylvanian) of Jacksboro, Texas, USA, using a variety of data categorization decisions (i.e., abundance versus calcified biomass, all taxa versus selected indicator taxa, and generic versus higher clade resolution). Biomass- and abundance-derived datasets were not significantly different in terms of evenness, Shannons information index, or Simpsons diversity index. Using Bray-Curtis and nonmetric multidimensional scaling ordinations, with Sorenson and relative Sorenson distance measures, ordination axis scores of the six derived datasets were all significantly correlated with one another, suggesting little difference in their respective paleoecological signals. Three potential explanations for this consistent paleoecological signal, regardless of which data categorizations are employed, include: (1) the dominance of a few brachiopod taxa overwhelmingly influenced the community structure, (2) relatively constrained environmental conditions limited community variation, and (3) low variation in specimen size minimized potential differences among abundance and calcified biomass categorizations. We suggest that other datasets with greater diversities, greater evenness, or from a wider range of paleoenvironments might not show this consistency. Thus, to the degree possible and appropriate, paleoecological investigators should test the effects of these data categorization decisions on a paleoecological signal, regardless of the analytical method employed.

The Microstructural Record of Predation: A New Approach for Identifying Predatory Drill Holes

Abstract Drill holes in prey skeletons are the most common source of data for quantifying predator-prey interactions in the fossil record. To be useful, however, such drill holes need to be identified correctly. Field emission scanning electron microscopy (FE-SEM) and environmental scanning electron microscopy (ESEM) were applied to describe and quantify microstructural characteristics of drill holes. Various specimens, including modern limpets and mussels drilled by muricid snails in laboratory experiments, subfossil limpets collected from a tidal flat (San Juan Island, Washington state, USA), and various Miocene bivalves collected from multiple European sites, were examined for microstructural features. The microstructures observed are interpreted here as Radulichnus-like micro-rasping marks, or predatory microtraces, made by the radula of drilling gastropod predators. The mean adjacent spacing of these microtraces is notably denser than the spacing of muricid radular teeth determined by measurements taken from the literature. Because the radular marks typically overlie or crosscut each other, the denser spacing of predatory microtraces likely reflects superimposition of scratches from repeated passes of the radula. One incomplete drill hole showed a clear, chemically aided drilling dissolution signature around its outer margin, while a number of other specimens showed similar, but ambiguous, traces of dissolution. The range of organisms examined illustrates the utility of scanning electron microscopy (SEM) imaging for identifying micro-rasping marks associated with predatory drill holes in both modern and fossil specimens. These distinct microtraces offer promise for augmenting our ability to identify drill holes in the fossil record and to distinguish them from holes produced by non-predatory means.

Ecological effects of the Paleozoic-Modern faunal transition: Comparing predation on Paleozoic brachiopods and molluscs

The shift from the Paleozoic to the Modern (post-Paleozoic) Fauna involved a major influx of benthic molluscs (gastropods and bivalves) into offshore marine environments, resulting in mixed brachiopod–benthic mollusc paleocommunities as early as the late Paleozoic Era; this change might be expected to have affected contemporaneous predators. Gastropods and productidine brachiopods from five upper Pennsylvanian shales in Texas (United States) show greater repair frequencies than do other taxa, suggesting that crushing predators readily consumed gastropods early in the faunal transition but that some brachiopod taxa still composed an important component of predator diets. In contrast, drilling predators continued to prefer brachiopods almost exclusively over benthic molluscs; Paleozoic drillers may have been incapable of taking mobile prey. Contrary to previous hypotheses, brachiopods were probably not “mistaken” prey, at least during the Paleozoic. The addition of gastropods to the diet of crushing predators suggests a major change in ecological processes in response to the new fauna.

Multivariate faunal analyses of the Turonian Bissekty Formation: Variation in the degree of marine influence in temporally and spatially averaged fossil assemblages

Abstract The Bissekty Formation, exposed on the Dzharakuduk escarpment (Kyzylkum Desert) in Uzbekistan, contains the richest and most abundant Turonian terrestrial fauna known from Eurasia. This study utilizes ordination analyses to identify spatial or temporal patterns (i.e., biofacies) in the distribution of spatially averaged skeletal elements (i.e., mix of unequivocally marine faunal elements with terrestrial ones) within the laterally extensive intraformational conglomerates (IFCs) of the Bissekty Formation. Ordination analyses were used to determine similarities among the IFCs based on their absolute taxonomic abundance and presence or absence. To determine the primary factor(s) driving the ordination pattern, taxonomic abundance, richness, environmental restrictions (aquatic, semi-aquatic, and terrestrial), and skeletal element size were examined. Relative Sørensen and Sørensen in combination with Euclidean and city block metrics were used in the analysis and results were consistent across methodologies. Ordination patterns were driven by aquatic taxa, which were dominated in abundance and richness by marine and brackish-tolerant taxa. By mapping the abundance of terrestrial, semi-aquatic, and aquatic taxa of individual IFCs onto ordination space, the relative position of the coastline with reference to the Dzharakuduk escarpment during the deposition of the Bissekty Formation can be inferred. These results indicate that ordination analyses are useful tools for examining taphonomically biased samples and should be utilized more frequently in vertebrate studies.

Predation on Modern and Fossil Brachiopods: Assessing Chemical Defenses and Palatability

ABSTRACT The post-Paleozoic decline of the diversity and abundance of rhynchonelliform brachiopods has been attributed to a variety of factors. Of the possible mechanisms invoked to explain the evolutionary decline and cryptic or antitropical distribution of brachiopods, predation has frequently been dismissed due to the potentially low energetic value and suspected nonpalatability or toxicity of brachiopod tissues. Herein we demonstrate that multiple invertebrate marine predators (crustaceans, echinoderms, and gastropods) are willing and able to consume brachiopods in laboratory settings without observable negative effects after ingestion. In addition, field samples indicate predation pressure on the living brachiopod population may be substantial. Although feeding trials are consistent with previous reports that bivalves are preferred prey relative to brachiopods, predation should not be dismissed as a potentially important factor in brachiopod ecology and evolution. The results presented herein reveal that in some cases brachiopods may be the intended target of predatory attacks, especially in habitats where mollusks are rare or absent. Examination of the fossil record of predation on rhynchonelliform brachiopods is consistent with this interpretation: evidence for drilling and repair of brachiopod shells is found throughout the fossil record in multiple lineages. While it is likely that predation traces on post-Paleozoic brachiopods are generally rare, there are multiple reports of fossil localities with anomalously high drill-hole or repair-scar frequencies. This suggests that although brachiopods may be unwanted prey in the presence of energetically more desirable targets, they do appear to be edible and subject to intense predator-prey interactions under certain conditions.

Analyzing Predation from the Dawn of the Phanerozoic

Although there have been numerous reports of predation on Cambrian and older fossils, there have been relatively few quantitative studies conducted on predation during this important interval. Such studies may prove extremely important as predation has been invoked as a primary influence on the Cambrian Explosion. Traces of predatory behavior, such as pursuit traces, crushing and repair scars, and drill-holes, are recommended as the best proxy for predation intensity. This chapter reviews the evidence for predation in the Neoproterozoic and Cambrian and then suggests and explains some techniques for analyzing predation data, with special consideration and examples of analysis of data from this early phase in the history of metazoans.

Caedichnus, a New Ichnogenus Representing Predatory Attack on the Gastropod Shell Aperture

Predatory traces, in which the tracemaker has damaged the prey animals skeleton to kill and consume it, have a deep fossil history and have received much scientific attention. Several types of predatory traces have been assigned to ichnotaxa, but one of the most studied predatory traces, the wedge-shaped excision produced as a result of attacks mainly by crustaceans on the apertures of gastropod shells, has yet to be described as an ichnotaxon. We propose the ichnogenus Caedichnus to describe the shell damage produced by aperture peeling behavior. Caedichnus is produced by predators that are unable to crush their preys shells outright. Depending on the predators peeling ability and the preys withdrawal depth within the shell, the trace can extend through several whorls of the shell. Aperture peel attacks may fail, allowing such damage to be repaired by surviving gastropods. Thus, the types of attacks that produce Caedichnus may exert selective pressure on prey to evolve better-defended shells (in the case of gastropods) or to inhabit better-defended shells (in the case of hermit crabs). The identification of these trace fossils will enhance our understanding of how predation influences the morphological, and even behavioral, evolution of prey organisms.

Reexamining Sample Size Requirements for Multivariate, Abundance-Based Community Research: When Resources are Limited, the Research Does Not Have to Be

Community ecologists commonly perform multivariate techniques (e.g., ordination, cluster analysis) to assess patterns and gradients of taxonomic variation. A critical requirement for a meaningful statistical analysis is accurate information on the taxa found within an ecological sample. However, oversampling (too many individuals counted per sample) also comes at a cost, particularly for ecological systems in which identification and quantification is substantially more resource consuming than the field expedition itself. In such systems, an increasingly larger sample size will eventually result in diminishing returns in improving any pattern or gradient revealed by the data, but will also lead to continually increasing costs. Here, we examine 396 datasets: 44 previously published and 352 created datasets. Using meta-analytic and simulation-based approaches, the research within the present paper seeks (1) to determine minimal sample sizes required to produce robust multivariate statistical results when conducting abundance-based, community ecology research. Furthermore, we seek (2) to determine the dataset parameters (i.e., evenness, number of taxa, number of samples) that require larger sample sizes, regardless of resource availability. We found that in the 44 previously published and the 220 created datasets with randomly chosen abundances, a conservative estimate of a sample size of 58 produced the same multivariate results as all larger sample sizes. However, this minimal number varies as a function of evenness, where increased evenness resulted in increased minimal sample sizes. Sample sizes as small as 58 individuals are sufficient for a broad range of multivariate abundance-based research. In cases when resource availability is the limiting factor for conducting a project (e.g., small university, time to conduct the research project), statistically viable results can still be obtained with less of an investment.

Categorization of shell fragments provides a proxy for environmental energy and predation intensity

Shell fragments are extremely abundant in many marine environments; the origins of these fragments can largely be attributed to either crushing by predators or post-mortem processes such as transport. We present and test a new approach to identifying the origin of shell fragments in marine environments by examining modern mollusc assemblages from three wave-exposed, low-predation and three wave-sheltered, high-predation intertidal localities in Bamfield, British Columbia. We hypothesized that fragments with all-rounded edges (AR) or a combination of rounded and sharp edges (R&S) are indicative of taphonomic processes such as transport and so should occur more often in wave-exposed, low-predation localities whereas fragments with all sharp edges (AS) or a combination of intact and sharp edges (I&S), indicative of predation, should be more common in wave-sheltered, high-predation settings. In keeping with the prediction, All-Rounded (AR) and Rounded and Sharp (R&S) fragments are more abundant at the wave-exposed localities than at wave-sheltered localities whereas Intact and Sharp (I&S) and All-Sharp (AS) fragments are more abundant at high-predation localities than at low-predation localities. The two types of localities were statistically distinguishable for either gastropod or bivalve fragments. This supports the hypothesis that I&S and AS result from predation, whereas AR and R&S have a taphonomic genesis.

Comparing predatory drillholes to taphonomic damage from simulated wave action on a modern gastropod

Marine drilling predation, in which the predator bores a hole through shelled invertebrate prey, plays a role in the structure of benthic communities. As drilling often leaves the prey shell otherwise undamaged, the resulting holes are also an excellent proxy for drilling predation pressure in the fossil record. Considering that a large number of predation studies focus on drilling predation in the fossil record, it is crucial that we are able to distinguish true drilling predation from taphonomy. The purpose of this study is to determine damage on Olivella biplicata shells, drilled by naticid gastropods, is distinguishable from taphonomically produced damage to these shells. In addition, the potential for preferential breakage due to either the presence or whether absence of a drillhole was investigated. Drilled and non-drilled O. biplicata shells were tumbled to simulate wave action and were checked at intervals to record accumulated damage. Drilled and non-drilled shells do not show a significant difference in damage accumulated while undergoing simulated wave action. Taphonomic damage is unlikely to be mistaken for drilling damage, due to the jagged, irregular appearance of taphonomically produced holes.