Ecospace Utilization Research Articles

BIOTURBATORS AS ECOSYSTEM ENGINEERS: ASSESSING CURRENT MODELS

ABSTRACT Bioturbating organisms can dramatically alter the physical, chemical, and hydrological properties of the sediment and promote or hinder microbial growth. They are a classic example of “ecosystem engineers” as they alter the availability of resources to other species. Multiple evolutionary hypotheses evoke bioturbation as a possible driver for historical ecological change. To test these hypotheses, researchers need reliable and reproducible methods for estimating the impact of bioturbation in ancient environments. Early efforts to record and compare this impact through geologic time focused on the degree of bioturbation (e.g., bioturbation indices), the depth of bioturbation (e.g., bioturbation depth), or the structure of the infaunal community (e.g., tiering, ecospace utilization). Models which combine several parameters (e.g., functional groups, tier, motility, sediment interaction style) have been proposed and applied across the geological timescale in recent years. Here, we review all models that characterize the impact of bioturbators on the sedimentary environment (i.e., ‘ecosystem engineering'), in both modern and fossil sediments, and propose several questions. What are the assumptions of each approach? Are the current models appropriate for the metrics they wish to measure? Are they robust and reproducible? Our review highlights the nature of the sedimentary environment as an important parameter when characterizing ecosystem engineering intensity and outlines considerations for a best-practice model to measure the impact of bioturbation in geological datasets.

Resilience of infaunal ecosystems during the Early Triassic greenhouse Earth.

The Permian-Triassic mass extinction severely depleted biodiversity, primarily observed in the body fossil of well-skeletonized animals. Understanding how whole ecosystems were affected and rebuilt following the crisis requires evidence from both skeletonized and soft-bodied animals; the best comprehensive information on soft-bodied animals comes from ichnofossils. We analyzed abundant trace fossils from 26 sections across the Permian-Triassic boundary in China and report key metrics of ichnodiversity, ichnodisparity, ecospace utilization, and ecosystem engineering. We find that infaunal ecologic structure was well established in the early Smithian. Decoupling of diversity between deposit feeders and suspension feeders in carbonate ramp-platform settings implies that an effect of trophic group amensalism could have delayed the recovery of nonmotile, suspension-feeding epifauna in the Early Triassic. This differential reaction of infaunal ecosystems to variable environmental controls thus played a substantial but heretofore little appreciated evolutionary and ecologic role in the overall recovery in the hot Early Triassic ocean.

Infaunalization and resource partitioning during the Mesozoic marine revolution

Infaunalization has been regarded as representing a response to increased predation pressures and is therefore central to the Mesozoic marine revolution, which gives pre-eminence to the role that enemy-directed evolution has played as a driving force of biotic change. Our ichnologic compilation from 39 Middle Triassic to Late Jurassic shallow-marine siliciclastic units allows us to evaluate the vertical partitioning of the infaunal ecospace through the application of the ichnoguild concept. This study shows that infaunal communities experienced a marked increase in ecospace utilization during the Early Jurassic, reaching a plateau during the rest of the Jurassic. This trend is expressed by an increase in the maximum number of ichnoguilds per community and per tier and in the number of ichnotaxa per ichnoguild. This pattern shows important partitioning of the infaunal ecospace into a series of tiers and that several organisms were able to exploit the same resources available at discrete sediment zones below the sea bottom. The increase in the maximum number of ichnoguilds per community and per tier suggests that niche partitioning was a key factor in a more efficient use of the infaunal ecospace and in driving alpha diversity. However, the increase in the number of ichnotaxa per ichnoguild indicates that ichnoguilds were packed with organisms exploiting similar resources, arguing against the role of competitive exclusion in structuring communities. Because several episodes of predation increase took place during the Mesozoic, an unequivocal link between predation pressures and infaunalization cannot be demonstrated empirically.

Ordovician matground and mixground ecosystems in shoreface–offshore and barrier-island environments from Central Iran, northern Gondwana

AbstractThe shift of matground ecosystems to bioturbator-free settings was investigated in the Ordovician wave-dominated marine strata of the Shirgesht Formation in Central Iran. Ten ichnofabrics are recognized in shoreface–offshore and barrier-island sedimentary facies, representing a proximal-to-distal depositional trend along the studied profile. In the offshore settings, intensive burrowing on several tiers and bioirrigation, referred to theThalassinoides(Th),Cruziana–Skolithos–Rosselia(CrSkRo), crowdedTrichophycus(CT) andCruziana–Helminthopsis(CrHe) ichnofabrics, prevented the development of matgrounds. As a result of the Ordovician radiation, progressive ecospace utilization by suspension-, deposit- and detritus-feeders, as represented by theSkolithos(Sk),Rosselia–Skolithos(RoSk), crowdedRosselia(CR) andConichnus–Skolithos(CoSk) ichnofabrics, is regarded as a key factor for sediment mixing and reworking and thus disruption of the matgrounds in shoreface settings. However, the restriction on microbial growth was reduced in lower-offshore / shelf or tidal-flat and back-barrier settings, whereChondrites–Planolites(ChPl) andPlanolites(Pl) ichnofabrics dominate. This restricted microbial growth resulted from physico-chemical stresses on infaunal communities, as shown by the low ichnodiversity, scattered burrowing, absence of complex tiering, and prevalence of simple feeding strategies. This study suggests that following the early Palaeozoic evolutionary radiations, a considerable increase in abundance of detritus-feeders, deposit-feeders, suspension-feeders, predators, vagile bilaterian metazoans and grazers in shallow-water benthic communities caused mat-building microorganisms to migrate into lower-offshore / shelf, tidal-flat and back-barrier settings, where colonization by burrowing organisms was delayed.

Exotic facies episodes of a carbonate platform: implications for middle and late Cambrian ecosystems and impact of bioturbation in the Alborz Basin, Iran

Abstract The sedimentological and ichnological data of the Mila Group in the Alborz Basin, northern Iran indicate that the appearance of exotic facies and resurgence of microbialites resulted from the development of extraordinary palaeoceanographic conditions with widespread environmental stress and enhanced precipitation of CaCO 3 . Inhospitable environmental conditions in the Mila Group platform led to a considerable reduction in ichnodiversity, bioturbation intensity and depth and size of burrows, which led to periods of dominating calcimicrobe ecology. The development of exotic or anachronistic facies started around the Cambrian Stage 4–Wuliuan boundary, probably simultaneous with the SPICE event, whereas the Furongian exotic facies period probably took place simultaneous with the HERB event. Biotic and physico-chemical changes in the ocean during these events may have led as well to the virtual absence of the mixed layer and infaunal ecospace utilization in the deposits of the Mila Group. Nevertheless, the local presence of metazoan-microbial build-ups that established between the two suggested exotic facies developments indicates that these periods started at the Cambrian Stage 4–Wuliuan boundary interval, but a fluctuation of abnormal palaeoceanographic conditions and ambient normal marine conditions took place during the Miaolingian and Furongian in the Alborz Basin as a representative of the northern Gondwanan margin.

Quantifying deep-water marine ecospace utilization andecosystem engineering from late Permian to Middle Triassic

<p indent=0mm>With the loss of around three-fourths of marine and continental genera, the end-Permian mass extinction (EPME) event was the biggest biotic crisis in the Phanerozoic era. The shallow marine and continental ecosystem were almost collapsed due to continuous harsh and turbulent environment, e.g., global warming and anoxic event. However, the overall response strategies of biotic communities and function of marine ecosystem engineering are still uncertain. As a promising window for understanding organism-environmental interactions, trace fossils not only record the response of soft-bodied animals that are not easily preserved as fossils to the mass extinction event, but also record a large number of complex behaviors involving the construction of ecosystems, and document the entire process of the collapse and recovery of marine ecosystems during the Permian-Triassic transition. A global Permian-Triassic deep-water marine database was compiled on the basis of literatures and personal data. The total number of invertebrate ichnogenera identified as valid is 62 from a survey of 146 Permian-Triassic stratigraphic unites. Ichnodiversity simply refers to the number of ichnotaxa present at ichnogenus level. Ichnodisparity is a measure of the variability of morphological plans in trace fossils, which reveals major innovations in body plan, locomotive system, and behavioral program. The multidimensional ecospace utilization and ecosystem engineering schemes based on body fossils have been adapted for trace fossils. Ecospace utilization is quantified on the basis of the number of modes of life, following three parameters: Tiering (subdivided into surficial, semi-infaunal, shallow, intermediate and deep tiers), motility (subdivided into motile, facultatively motile and non-motile types), and feeding mode (subdivided into suspension feeding, non-specialized deposit feeding, specialized deposit feeding, predation and others). Ecosystem engineering is categorized on the basis of three parameters: Tiering (as described above), mode of sediment interaction (intrusion, compression, backfilling and excavation), and mode of sediment modification (biodiffusion, conveyors, regenerators, and gallery biodiffusion). Ecosystem engineering impact (EEI) values and ranges on the basis of three parameters: (1) Tiering (surficial tier, semi-infaunal tier, shallow tier, intermediate tier, and deep tier); (2) bioirrigation potential (improbable, probable, and possible); and (3) potential functional group, describing how substrate is modified by burrowing organisms (biodiffusion, conveyors, regenerators, and gallery biodiffusion). This paper aims to understand the patterns of EPME on the marine deep-water ecospace utilization and ecosystem engineering, and the adaptation and modification mechanism of organisms in the Early Triassic deep-water ecosystem recovery from an ecological perspective. Our results show that in the Permian-Triassic transition, ichnodiversity, ichnodisparity, bioturbation index, the ecospace utilization and ecosystem engineering cubes of deep-water marine were not significantly declined, indicating that the marine deep-water ecosystem was under relatively stable conditions during the EPME. Non-specialized deposit feeding, motile, gallery biodiffusion and compression were the most common ecosystem engineers. Notably, gallery biodiffusion, as the highest impact sediment modification functional group, is present through the transition of Permian-Triassic. Modes of life in the Early and Middle Triassic tend to be diversified and dominated by the backfilling-conveyors in the shallow tiering. Increased bioirrigation further affected the geochemical cycle of the substrate in the Early Triassic and laid the foundation the further construction and development of deep tiering ecospace utilization. In the Middle Triassic, the nutrient deep-tiering ecospace was inhabited by organisms with advanced bioirrigation capacities.

Environmental constraints on the distribution of matground and mixground ecosystems across the Cambrian Series 2–Miaolingian boundary interval in Iran: A case study for the central sector of northern Gondwana

The stratigraphic architecture, changes of depositional systems in lateral and vertical extent, and 3D-stacking pattern of second- and third-order sequences of the Cambrian successions on the northern Gondwanan margin indicates a distinct sea-level fall in the Alborz Zone and Central Iran corresponding with global sea-level fluctuations. The peak of the sea-level fluctuations is located at the boundary between the Lalun Formation and the base of the overlying Mila Group. A vertical trend from gradual sea-level rise during the Cambrian Series 2 (recorded as thick transgressive deposits of the Lalun Formation) to transgressive valley-fill system at the base of the Fasham Formation and overlying highstand subtidal carbonates (carbonate platform of the Deh-Sufiyan Formation) can be observed. This trend is paralleled by a change from mixgrounds with metazoan body fossils to microbial matgrounds and microbial boundstones (thrombolites and stromatolites). The general increase in vagile bilaterian metazoans led to a distinct proliferation in ichnofossils in the Lalun Formation indicating a considerable increase in progressive ecospace utilization and tiering complexity. These are associated with biodiffusion, sediment reworking and bioirrigation so that matgrounds were disrupted and mixed layer substrates developed. A proliferation of microbes took place at the base of the Mila Group, which is regarded as a reaction to unfavourable environmental conditions and considerable challenges for metazoans. Consequently, a dramatic degradation of marine bottom communities followed the major eustatic sea-level fall at the Cambrian Series 2–Miaolingian boundary interval and led to the decrease in ichnodiversity, bioturbation intensity, depth of bioturbation and burrow size at the base of the Mila Group. The amount of biogenic mixing significantly decreased, and eventually sedimentary mixed layers disappeared.

Quantifying ecospace utilization and ecosystem engineering during the early Phanerozoic-The role of bioturbation and bioerosion.

The Cambrian explosion (CE) and the great Ordovician biodiversification event (GOBE) are the two most important radiations in Paleozoic oceans. We quantify the role of bioturbation and bioerosion in ecospace utilization and ecosystem engineering using information from 1367 stratigraphic units. An increase in all diversity metrics is demonstrated for the Ediacaran-Cambrian transition, followed by a decrease in most values during the middle to late Cambrian, and by a more modest increase during the Ordovician. A marked increase in ichnodiversity and ichnodisparity of bioturbation is shown during the CE and of bioerosion during the GOBE. Innovations took place first in offshore settings and later expanded into marginal-marine, nearshore, deep-water, and carbonate environments. This study highlights the importance of the CE, despite its Ediacaran roots. Differences in infaunalization in offshore and shelf paleoenvironments favor the hypothesis of early Cambrian wedge-shaped oxygen minimum zones instead of a horizontally stratified ocean.

Firm evidence for a post-extinction ichnofauna: earliest CarboniferousCruziana reticulataassemblage from the Anti-Atlas of Morocco

Cruziana reticulata is an arthropod-related ichnospecies that is characterized by a conspicuous net-like scratch pattern whose initial formation and later preservation require the presence of consolidated substrates in shallow marine fine-grained bottoms. There are two scenarios in which epifaunal to shallow infaunal benthic organisms may access firm siliciclastic substrates: first, by exposure of the compacted bottom after erosion of the upper, water-saturated and usually soft portion of sediment column; and, second, by primary fast substrate stabilization in the absence of biogenic sediment mixing. Whereas the first mode occurs throughout the Phanerozoic, the latter is only prevalent in marine bottoms during the Early Palaeozoic mainly as a consequence of poorly developed infaunal bioturbation. However, by eradicating burrowing organisms, mass extinctions are known (i.e. the end-Permian extinction) to ‘reset’ intensity of ecospace utilization, which entails the return to this anachronistic style of trace fossil preservation in younger times. An earliest Carboniferous ichno-assemblage dominated by Cruziana reticulata from Morocco attests the spread of firm substrates in the aftermath of the Hangenberg Event – a major extinction at the end of the Devonian period. Since evidence for erosion is lacking in accompanying sedimentary rocks and the ichno-assemblage shows characteristics of opportunistic exploitation of the nutritious muddy seabed, we favour the interpretation of this assemblage to represent a post-extinction ichnofauna. It shows that other taxonomically less severe mass extinctions may also exhibit transient but severe ecological effects in open marine ecosystems such as the collapse of vital sediment mixing. Such post-extinction effects may distort the perception of the stratigraphical record as a firmground ichno-assemblage often taken as evidence for submarine erosion and are crucial in highlighting sequence boundaries. We, thus, encourage appreciating macroevolutionary framework of respective ichnofaunas.

Late Devonian and Early Carboniferous alpha diversity, ecospace occupation, vertebrate assemblages and bio-events of southeastern Morocco

The Late Devonian was a time of dramatic environmental perturbations affecting marine ecosystems. Both the Kellwasser (latest Frasnian) and the Hangenberg crises (latest Famennian) are primarily reported as phases of drastic decreases in marine diversity while the Hangenberg Crisis is also described as a bottleneck in vertebrate evolution. Fossil-bearing localities with Upper Devonian strata are of great interest to assess variations in the effects of environmental perturbations on biodiversity. For this purpose, we examined changes in alpha diversity and ecospace utilization of 21 Famennian (Late Devonian) and early Tournaisian (Early Carboniferous) invertebrate associations containing 9828 specimens from Madène el Mrakib and Aguelmous (southern Maïder Basin, northeastern Anti-Atlas, Morocco), where some layers yield exceptionally preserved gnathostome remains. Both the invertebrate and vertebrate associations contain predominantly opportunistic and pelagic taxa indicating oxygen depletion near the seafloor in this region. Nevertheless, the ecospace extension was fluctuating and correlated with regional and/or global sea-level changes and oxygenation of bottom waters. In the Maïder Basin, the ecospace was depleted after and during several bio-events such as the Kellwasser and Hangenberg crises, the Annulata event (middle Famennian) as well as during the early Tournaisian. Abiotic as well as biotic changes (instability of the invertebrate ecosystem) are considered to have influenced Famennian vertebrate diversity because they were more or less directly dependent on invertebrates as a food source.

Ecological disparity is more susceptible to environmental changes than familial taxonomic richness during the Cretaceous in the Alpstein region (northeastern Switzerland)

Studies of global palaeoecology through time usually ignore regional details. Such regional studies on palaeoecology are required to better understand both regional- and global-scale palaeoecolgical changes. We analyzed the palaeoecolgy of a Cretaceous sedimentary sequence in the Alpstein (cantons of Appenzell Ausserrhoden, Appenzell Innerrhoden and St. Gallen, northeastern Switzerland), which covers from the Barremian to Cenomanian stage. Two diversity indices of familial taxonomic richness and ecological disparity (ecospace occupation) with the trophic nucleus concept were employed in order to document changes in palaeocommunities through time. Our results illustrate that taxonomic richness did not change dramatically, while distinct changes occurred in ecospace occupation through time. The changes in ecospace utilization likely root in fluctuations in water depth and ocean temperature. In addition to these changes in sea level and temperature, our results suggest that water depth was higher in the eastern part of the Alpstein region. Ecospace occupation was moderately diverse through time, which is likely linked with the favorable conditions such as moderate water depth, which made the region habitable for a range of organisms. Only during the late Barremian, the sea was maybe too shallow, thus preventing highly diverse associations. Statistical tests indicate that the chronological changes of familial taxonomic richness and ecological disparity are decoupled and that the ecological disparity is more highly variable in response to environmental changes than familial taxonomic richness.

The Rise of Animals in a Changing Environment: Global Ecological Innovation in the Late Ediacaran

The evolutionary trajectory of early complex life on Earth is interpreted largely from the fossils of the Precambrian soft-bodied Ediacara Biota, which appeared and evolved during a time of dynamic biogeochemical and environmental fluctuation in the global ocean. The Ediacara Biota is historically divided into three successive Assemblages—the Avalon, the White Sea, and the Nama—which are marked by the appearance of novel biological traits and ecological strategies. In particular, the younger White Sea and Nama Assemblages record a “second wave” of ecological innovations, which included not only the development of uniquely Ediacaran body plans and ecologies, such as matground adaptations, but also the dual emergence of bilaterian-grade animals and Phanerozoic-style ecological innovations, including spatial heterogeneity, complex reproductive strategies, ecospace utilization, motility, and substrate competition. The late Ediacaran was an evolutionarily dynamic time characterized by strong environmental control over the distribution of taxa in time and space.

General models of ecological diversification. I. Conceptual synthesis

AbstractEvolutionary paleoecologists have proposed many explanations for Phanerozoic trends in ecospace utilization, including escalation, seafood through time, filling of an empty ecospace, and tiering, among others. These hypotheses can be generalized into four models of functional diversification within a life-habit ecospace framework (functional-trait space). The models also incorporate concepts in community assembly, functional diversity, evolutionary diversification, and morphological disparity. The redundancy model produces an ecospace composed of clusters of functionally similar taxa. The partitioning model produces an ecospace that is progressively subdivided by taxa along life-habit gradients. The expansion model produces an ecospace that becomes progressively enlarged by the accumulation of taxa with novel life habits. These models can be caused by a wide range of ecological and evolutionary processes, but they are all caused by particular “driven” mechanisms. A fourth, neutral model also exists, in which ecospace is filled at random by life habits: this model can serve as a passive null model. Each model produces distinct dynamics for functional diversity/disparity statistics when simulated by stochastic simulations of ecospace diversification. In this first of two companion articles, I summarize the theoretical bases of these models, describe their expected statistical dynamics, and discuss their relevance to important paleoecological trends and theories. Although most synoptic interpretations of Phanerozoic ecological history invoke one or more of the driven models, I argue that this conclusion is premature until tests are conducted that provide better statistical support for them over simpler passive models.

Persistent ecological shifts in marine molluscan assemblages across the end-Cretaceous mass extinction

Contemporary biodiversity loss and population declines threaten to push the biosphere toward a tipping point with irreversible effects on ecosystem composition and function. As a potential example of a global-scale regime shift in the geological past, we assessed ecological changes across the end-Cretaceous mass extinction based on molluscan assemblages at four well-studied sites. By contrasting preextinction and postextinction rank abundance and numerical abundance in 19 molluscan modes of life--each defined as a unique combination of mobility level, feeding mode, and position relative to the substrate--we find distinct shifts in ecospace utilization, which significantly exceed predictions from null models. The magnitude of change in functional traits relative to normal temporal fluctuations at far-flung sites indicates that molluscan assemblages shifted to differently structured systems and faunal response was global. The strengths of temporal ecological shifts, however, are mostly within the range of preextinction site-to-site variability, demonstrating that local ecological turnover was similar to geographic variation over a broad latitudinal range. In conjunction with varied site-specific temporal patterns of individual modes of life, these spatial and temporal heterogeneities argue against a concerted phase shift of molluscan assemblages from one well-defined regime to another. At a broader ecological level, by contrast, congruent tendencies emerge and suggest deterministic processes. These patterns comprise the well-known increase of deposit-feeding mollusks in postextinction assemblages and increases in predators and predator-resistant modes of life, i.e., those characterized by elevated mobility and infaunal life habits.

Late Ordovician brachiopod distribution and ecospace partitioning in the Tvären crater system, Sweden

Patterns of distribution and ecospace utilization of Late Ordovician brachiopods in a recently formed, contemporary meteorite crater are described and analyzed. Rhynchonelliformean brachiopod communities, dominated by a wide range of orthides and strophomenides, colonized the newly formed crater. At the crater rim communities were established early on, although the crater depression was not inhabited until deposition of the upper third of the remaining crater fill. The crater formed a protected but restricted microenvironment where sediments four times the thickness of the nearby basinal succession accumulated. Within this narrow space environments varied from shallow-water to deeper-waters, about 200m in depth, and from well oxygenated to hypoxic. Such varied environments generated a rough ecological landscape, facilitating niche partitioning across a relatively small geographic area. Analysis of the guild structure of the fauna permits explanation of a local biodiversity hotspot in otherwise low-diversity strata elsewhere in the Scandinavian region. The Tvären impact event had an important palaeobiologic effect upon the fossil record as it served as a local pump and reservoir for biodiversity. Moreover the development of new community types and narrowly-defined niches helped further drive both α and β biodiversity during a critical phase of the Great Ordovician Biodiversification Event.

Long-term origination rates are reset only at mass extinctions

Research Article| August 01, 2012 Long-term origination rates are reset only at mass extinctions Andrew Z. Krug; Andrew Z. Krug Department of Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, USA Search for other works by this author on: GSW Google Scholar David Jablonski David Jablonski Department of Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, USA Search for other works by this author on: GSW Google Scholar Geology (2012) 40 (8): 731–734. https://doi.org/10.1130/G33091.1 Article history received: 05 Dec 2011 rev-recd: 28 Feb 2012 accepted: 05 Mar 2012 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Andrew Z. Krug, David Jablonski; Long-term origination rates are reset only at mass extinctions. Geology 2012;; 40 (8): 731–734. doi: https://doi.org/10.1130/G33091.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Diversification during recovery intervals is rapid relative to background rates, but the impact of recovery dynamics on long-term evolutionary patterns is poorly understood. The age distributions for cohorts of marine bivalves show that intrinsic origination rates tend to remain constant, shifting only during intervals of high biotic turnover, particularly mass extinction events. Genera originating in high-turnover intervals have longer stratigraphic durations than genera arising at other intervals, and drive the magnitude of the shift following the Cretaceous–Paleogene (K-Pg) extinction. Species richness and geographic range promote survivorship and potentially control rates through ecospace utilization, and both richness and range have been observed to expand more rapidly in recovery versus background states. Post-Paleozoic origination rates, then, are directly tied to recovery dynamics following each mass extinction event. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Mass Occurrence of Penetrative Trace Fossils in Triassic Lake Deposits (Kyrgyzstan, Central Asia)

Bioturbation and the breakdown of organic detritus by burrowing macro-invertebrates are key factors for the energy flow in recent freshwater ecosystems due to the acceleration of nutrient cycling. According to the current state of knowledge, food webs similar to those of modern lakes were not operating until the Late Mesozoic, when a well-established freshwater infauna evolved. Here we describe laterally extended networks of irregularly branched burrows that constitute the most common ichnofossils in lacustrine deposits of the Middle to Late Triassic Madygen Formation, SW Kyrgyzstan. The shallow penetrative trace fossils give evidence that exploitation of lake-bottom sediments by benthic invertebrates was already in place in the Early Mesozoic. Architecture and size of the fossil burrows indicate deposit-feeding, worm-like trace makers of similar morphology and behavior to extant oligochaetes or aquatic insect larvae. Maximum intensity of bioturbation is recorded in mudstones of the transitional sublittoral to profundal lake zone, which usually represent the thermocline/chemocline level in modern stratified lakes. Taking into account the low-oxygen tolerance of many recent oligochaetes and insect larvae, we propose that ecospace utilization of deep-water lacustrine settings was originally impelled by two factors: the exploitation of additional food resources and the avoidance of predation by carnivorous animals from well-aerated lake zones. Spatial restriction of the described fossil traces could offer a basic approach to subdivide the Mermia ichnofacies.

Early Cretaceous bivalves of the Neuquén Basin, west‐central Argentina: notes on taxonomy, palaeobiogeography and palaeoecology

AbstractThis paper provides an updated taxonomic inventory of the bivalve fauna collected in the Pilmatué Member of the Agrio Formation of the Neuquén Basin, west‐central Argentina, places the fauna in its palaeobiogeographic setting, and addresses its palaeoecological significance. Thirty‐one Late Valanginian to Early Hauterivian bivalve species within 24 genera were identified. A large part (32%) of the identified bivalve species occur over a wide geographical area: from the Pacific coast of South America to Europe, North Africa, Central Asia and East and South Africa; some are also recorded in Japan. A relatively high degree of endemism (26%) is shown, suggesting that some of the bivalve species had barriers to their dispersal; larval strategy and length of larval development were probably important. A significant number (42%) of the bivalve taxa are left in open nomenclature as they are probably new species. Bivalve guilds are described to interpret palaeoecology, in particular the ecospace utilization. Guilds are based on tiering, life habit, and feeding category. Eight guilds are recognized: free‐lying epifaunal, cemented epifaunal, epibyssate, boring, endobyssate, shallow burrowing, deep burrowing and deep burrowing with symbiotic bacteria. The fauna is composed only of suspension‐feeders indicating that food resources were dominantly in suspension, in agreement with the predominantly shallow‐water aspect of the study deposits. The ecospace utilization in the shoreface is broader than in the offshore shelf, suggesting more favourable living conditions and/or a wider range of different habitat types represented. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Changes in theoretical ecospace utilization in marine fossil assemblages between the mid-Paleozoic and late Cenozoic

We present a new three-dimensional theoretical ecospace for the ecological classification of marine animals based on vertical tiering, motility level, and feeding mechanism. In this context, analyses of a database of level-bottom fossil assemblages with abundance counts demonstrate fundamental changes in marine animal ecosystems between the mid-Paleozoic (461–359 Ma) and late Cenozoic (23–0.01 Ma). The average local relative abundance of infaunal burrowers, facultatively motile animals, and predators increased, whereas surface dwellers and completely non-motile animals decreased in abundance. Considering tiering, motility, and feeding together, more modes of life had high to moderate average relative abundance in the Cenozoic than in the Paleozoic. These results are robust to the biasing effects of aragonite dissolution in Paleozoic sediments and to heterogeneities in the latitudinal and environmental distributions of collections. Theoretical ecospace provides a unified system for future analyses of the utilization of ecologic opportunities by marine metazoa.

AUTECOLOGY AND THE FILLING OF ECOSPACE: KEY METAZOAN RADIATIONS

Abstract: All possible combinations of six tiering positions in relation to the substratum/water interface, six motility levels and six feeding strategies define a complete theoretical ecospace of 216 potential modes of life for marine animals. The number of modes of life actually utilized specifies realized ecospace. Owing to constraints of effectiveness and efficiency the modern marine fauna utilizes only about half the potential number of modes of life, two‐thirds of which (62 of 92) are utilized by animals with readily preserved, mineralized hard parts. Realized ecospace has increased markedly since the early evolution of animal ecosystems. The Ediacaran fauna utilized at most 12 modes of life, with just two practised by skeletal organisms. A total of 30 modes of life are recorded in the Early and Middle Cambrian, 19 of which were utilized by skeletal organisms. The other 11 are documented from soft‐bodied animals preserved in the Chengjiang and Burgess Shale Konservat‐Lagerstätten. The number of modes of life utilized by skeletal organisms increased by more than 50 per cent during the Ordovician radiation to a Late Ordovician total of 30. Between the Late Ordovician and the Recent the number of utilized modes of life has doubled again. The autecological and taxonomic diversity histories of the marine metazoa appear to be broadly parallel, and future studies of theoretical ecospace utilization should provide more detailed tests of pattern and process in the ecological history of the metazoa.