Roy E. Plotnick

Lacunarity indices as measures of landscape texture

Lacunarity analysis is a multi-scaled method of determining the texture associated with patterns of spatial dispersion (i.e., habitat types or species locations) for one-, two-, and three-dimensional data. Lacunarity provides a parsimonious analysis of the overall fraction of a map or transect covered by the attribute of interest, the degree of contagion, the presence of self-similarity, the presence and scale of randomness, and the existence of hierarchical structure. For self-similar patterns, it can be used to determine the fractal dimension. The method is easily implemented on the computer and provides readily interpretable graphic results. Differences in pattern can be detected even among very sparsely occupied maps.

Taphonomy of a Modern Shrimp: Implications for the Arthropod Fossil Record

Every living organism represents a potentialfossil, although only a few are ever successfully fossilized. Taphonomic studies should explain episodes of both fossilization and of non-fossilization. This paper examines some of the taphonomic variables that may bias the arthropod fossil record. The short-term preservation potential of the modern carid shrimp, Pandalus danae, was studied in a variety of laboratory and field settings. Freshly killed specimens were buried at two intertidal localities on San Juan Island, Puget Sound, Washington. Carcasses were buried at depths of 5-20 cm for periods of one day to three weeks. Destruction by scavengers (crabs?) was the probable primary cause of carcass destruction. Further breakdown was caused by bacterial decomposition and disturbance by burrowing infauna. Shrimp remains were placed in a series of glass jars in the laboratory. Jars differed in the presence and kind of enclosed sediment and in the degree of aeration. Decomposition destroyed nearly all soft tissues within a period of two weeks. The cuticle became extremely soft, resulting in loss of physical integrity of the remains. Differences between oxic and anoxic decomposition were minor. These results, along with a consideration of the biological, physical, and chemical effects of bioturbation, suggest that disturbance by scavengers or burrowing infauna is a majorfactor in the destruction of buried arthropod remains. The absence or inhibition of bioturbation may be a necessary condition for arthropod preservation. The preservation potential of arthropods, and of other soft-bodied forms, may have declined since the Paleozoic.

A multiplicative multifractal model for originations and extinctions

Abstract Recent works have suggested that the fossil record exhibits a fractal structure; i.e., that processes, such as extinction, follow a power-law size distribution and their time series show a 1/f power spectrum. This structure has been used as evidence that evolutionary dynamics are an example of a self-organized critical (SOC) process. We have reexamined this claim by analyzing a detailed record of marine genus-level extinctions and originations. Our results indicate that neither extinctions nor origination metrics show the power-law size distribution or a 1/f power spectrum characteristic of SOC and related models. We also believe that the underlying assumptions of SOC are incompatible with our understanding of the processes controlling macroevolutionary patterns. Statistical analyses of the data sets are compatible, however, with the presence of multifractal self-similarity in both records, consistent with a hierarchical and multiplicative generating process. This model assumes that multiple causal mechanisms, acting over many spatial and temporal scales, interact to promote or inhibit originations and extinctions. In this view, the same event can have quite different impacts depending on the state of the biotic or physical system at the time that it occurs. This may at least partially explain such phenomena as the imperfect correlation between eustatic sea-level changes and macroevolutionary processes and the apparent nonlinear response of biotic systems to bolide impacts.

Ecosystem organization and extinction dynamics

Most recent explanations for extinctions have focused on abiotic mechanisms such as climatic change, bolide impact, and eustatic sea-level change. The structure and dynamics of the ecological systems that are responding to these disturbances have generally been overlooked. The internal structure of an ecosystem, especially interdependencies among its components, should strongly influence its response to environmental change, by controlling the degree to which loss of one component can produce additional extinctions. We used percolation theory to model this process, by examining how disturbances propagate among interconnected nodes on lattices. Our models suggest that the magnitude of ecosystem response to disturbance is strongly non-linear

Are the most durable shelly taxa also the most common in the marine fossil record

This paper tests whether the most common fossil brachiopod, gastropod, and bivalve genera also have intrinsically more durable shells. Commonness was quantified using occurrence frequency of the 450 most frequently occurring genera of these groups in the Paleobiology Database (PBDB). Durability was scored for each taxon on the basis of shell size, thickness, reinforcement (ribs, folds, spines), mineralogy, and microstructural organic content. Contrary to taphonomic expectation, common genera in the PBDB are as likely to be small, thin-shelled, and unreinforced as large, thick-shelled, ribbed, folded, or spiny. In fact, only six of the 30 tests we performed showed a statistically significant relationship between durability and occurrence frequency, and these six tests were equally divided in supporting or contradicting the taphonomic expectation. Thus, for the most commonly occurring genera in these three important groups, taphonomic effects are either neutral with respect to durability or compensated for by other factors (e.g., less durable taxa were more common in the original communities). These results suggest that biological information is retained in the occurrence frequency patterns of our target groups.

Molecular signature of chitin-protein complex in Paleozoic arthropods

The conventional geochemical view holds that the chitin and structural protein are not preserved in ancient fossils because they are readily degradable through microbial chitinolysis and proteolysis. Here we show a molecular signature of a relict chitin-protein complex preserved in a Pennsylvanian (310 Ma) scorpion cuticle and a Silurian (417 Ma) eurypterid cuticle via analysis with carbon, nitrogen, and oxygen X-ray absorption near edge structure (XANES) spectromicroscopy. High-resolution X-ray microscopy reveals the complex laminar variation in major biomolecule concentration across modern cuticle; XANES spectra highlight the presence of the characteristic functional groups of the chitin-protein complex. Modification of this complex is evident via changes in organic functional groups. Both fossil cuticles contain considerable aliphatic carbon relative to modern cuticle. However, the concentration of vestigial chitin-protein complex is high, 59% and 53% in the fossil scorpion and eurypterid, respectively. Preservation of a high-nitrogen-content chitin-protein residue in organic arthropod cuticle likely depends on condensation of cuticle-derived fatty acids onto a structurally modified chitin-protein molecular scaffold, thus preserving the remnant chitin-protein complex and cuticle from degradation by microorganisms.

A general model for simulating the effects of landscape heterogeneity and disturbance on community patterns

An individual-based, spatially explicit stochastic lattice model, CAPS, was designed to examine multiple processes responsible for spatial patterns of abundance and diversity of sessile species in heterogeneous landscapes. Species simulated by CAPS differ in habitat preferences (niche width), dispersal of propagules, and relative fecundity. The spatial distribution of habitat types are represented as heterogeneous gridded landscapes. The outcome of competition and establishment processes in successive generations is determined locally via a seed lottery. A series of 200 year-long simulations was performed to investigate the effects of variation in species characteristics and competition, landscape heterogeneity, and disturbance on patterns of species abundances. The outcome of competition was most sensitive to differences in fecundity between species, the spatial distribution of suitable habitat and the initial distribution of species. Species with a narrow niche were confined to a single habitat type and remained at or near their initialization sites. Broader niches resulted in increasing niche overlap and competition but enhanced species mobility, allowing abundance levels to approach expected values determined by map resources. Even so, initial distributions still affected the spatial patterns of species distributions at year 200. Disturbance regimes were simulated by varying the frequency, extent and spatial pattern of disturbances. Disturbance events removed species from affected sites but did not otherwise alter habitat characteristics. Results showed that disturbances may lead to a reversal in competition and establishment, dependent on species-specific differences in fecundity and dispersal. Although intermediate levels of disturbance frequency and extent increased the probability of species coexistence, the spatial pattern of disturbance played an unexpectedly important role in the tradeoff between dispersal and fecundity. The ability to simulate multiple factors affecting patterns of persistence, abundance and spatial distribution of species provided by CAPS allows new insight into the temporal and spatial patterns of community development.

Pennsylvanian paleokarst and cave fills from northern Illinois, USA: A window into late Carboniferous environments and landscapes

Abstract A new fault-associated paleokarst and cave fill has been discovered in north-central Illinois, emplaced in Ordovician limestones. The paleokarst preserves many original solution features, such as oriented grooves, pendants, and half tubes. Many of the ancient cave passages have rounded bottoms and flat roofs. Together these suggest that the original elliptical, phreatic cave passages grew upward by paragenesis, in which the floor of the cave is protected from dissolution by the presence of sediment, while the ceiling of the cave grows upward by dissolution. The fill is dated as Moscovian (Middle Pennsylvanian) based on palynological data and can be correlated with the Tradewater Formation. The fills are composed of a fining-upward sequence of relatively unindurated clastic sediments that contain well-preserved plant fossils, most notably voltzialean conifer and cordaite remains, representative of vegetation living in well-drained areas. Many of the macrofossils are fragmentary but charcoalified and, along with the megaspores, are uncompressed and preserve exceptional morphological and anatomical data. The presence of abundant charcoal in the fills, as well as diagnostic polycyclic aromatic hydrocarbons, indicates significant wildfire activity in this area during this interval.

Invention by evolution: functional analysis in paleobiology

Abstract Functional analysis of fossils is and should remain a key component of paleobiological research. Despite recently expressed doubts, conceptual and methodological developments over the past 25 years indicate that robust and testable claims about function can be produced. Functional statements can be made in at least three different hierarchical contexts, corresponding to the degree of structural information available, the position in the phylogenetic hierarchy, and the degree of anatomical specificity. The paradigm approach, which dominated thinking about function in the 1960s and 1970s, has been supplanted with a methodology based on biomechanics. Paleobiomechanics does not assume optimality in organismal design, but determines whether structures were capable of carrying out a given function. The paradigm approach can best be viewed as a way of generating, rather than testing, functional hypotheses. Hypotheses about function can also be developed and supported by well-corroborated phylogenetic arguments. Additional functional evidence can be derived from studies of trace fossils and of taphonomy. New computer techniques, including “Artificial Life” studies, have the potential for producing far more detailed ideas about function and mode of life than have been previously possible. Functional analysis remains the basis for studies of the history of adaptation. It is also an essential component of many paleoecological and paleoenvironmental studies.

Information landscapes and sensory ecology of the Cambrian Radiation

Abstract Organisms emit, detect, and respond to a huge array of environmental signals. The distribution of a given signal is dependent, first of all, upon the original spatial distribution of signal sources, the source landscape. The signal sources can be fixed or moving and their output can be stable or ephemeral. Different sources can also occupy the same general spatial location, such as insects living on a host plant. The emitted signals are modified by relevant transport processes, which are often strongly scale and environment dependent. Chemical signals, for example, are propagated by diffusion and turbulence. The resulting complex, three-dimensional, and dynamic distribution of signals in the environment is the signal landscape; it is the environment of potentially available information in which sensory systems function and have evolved. Organisms also differ widely in what signals they can actually detect; the distribution of signals that an organism can potentially respond to is its information landscape. Although increasing the kinds and specificity of signals that can be detected and processed can lead to improved decision making, it almost always comes at an increased cost. The greater the spatial and temporal complexity of the environment, the greater are the costs of incomplete information and the more advantageous is the development of improved information-gathering capabilities. Studies with simulation models suggest how variability in the spatial structure of source and signal landscapes may control patterns of animal movement that could be represented in the trace fossil record. Information landscapes and the corresponding sensory systems should have evolved in concert with major transitions in the history of life. The Ediacaran to Cambrian interval is one of the most intensively studied periods in the history of life, characterized by the profound environmental and biological changes associated with the bilaterian radiation. These include the advent of macroscopic predation, an increase in the size and energy content of organisms, and the transition in seafloors from laminated matgrounds to mixgrounds produced by the development of macroscopic infaunal bioturbation. The overall effect of these transitions was to markedly increase the spatial complexity of the marine environment. We suggest that this increased spatial complexity, in turn, drove the evolution of macroscopic sense organs in mobile bilaterians, leading to their first appearance during the Cambrian. The morphology and distribution of these sense organs should reflect the life habits of the animals that possessed them. Our overall hypothesis was that there was a “Cambrian Information Revolution,” a coevolutionary increase in the information content of the marine environment and in the ability of and necessity for organisms to obtain and process this information. A preliminary analysis of the Maotianshan Shale (Chengjiang) biota indicates that the distribution of eyes and antennae in these animals is consistent with predictions based on their life habit.