Matthew E. Clapham

Phanerozoic trends in the global diversity of marine invertebrates.

It has previously been thought that there was a steep Cretaceous and Cenozoic radiation of marine invertebrates. This pattern can be replicated with a new data set of fossil occurrences representing 3.5 million specimens, but only when older analytical protocols are used. Moreover, analyses that employ sampling standardization and more robust counting methods show a modest rise in diversity with no clear trend after the mid-Cretaceous. Globally, locally, and at both high and low latitudes, diversity was less than twice as high in the Neogene as in the mid-Paleozoic. The ratio of global to local richness has changed little, and a latitudinal diversity gradient was present in the early Paleozoic.

Paleoecology of the oldest known animal communities: Ediacaran assemblages at Mistaken Point, Newfoundland

Abstract Ediacaran fossils at Mistaken Point, southeastern Newfoundland (terminal Neoproterozoic; 565–575 Ma) represent the oldest known animal communities. In contrast to most Phanerozoic fossil assemblages, in which postmortem transportation, bioturbation, and the accumulation of hardparts obscure community relationships, all fossils in the Mistaken Point assemblages were sessile, soft-bodied organisms that show no evidence of mobility in life or transportation after death. Mistaken Point assemblages are spectacularly preserved on large bedding planes as in situ census populations of hundreds to thousands of fossils, recording the living soft-bodied benthic community at the moment it was smothered by volcanic ash. This unique preservation style allows ecological tests routinely conducted in modern communities (e.g., species richness, abundance, “biomass,” diversity, and evenness, as well as statistical tests of nearest-neighbor interactions) to be applied to the fossil communities. Observed patterns of community variability are consistent with the theory that Mistaken Point fossil surfaces are “snapshots” recording different stages of ecological succession, progressing from communities of low-level feeders (e.g., pectinates and spindles) to frond-dominated communities with complex tiering and spatial structure. The presence of diverse slope communities at Mistaken Point suggests that the deep sea was colonized rapidly during the evolution of complex organisms. Species richness, abundance, and diversity values, as well as levels of intraspecific interaction, all fall within the typical range observed in modern slope communities. These structural similarities imply that ecological processes present in Ediacaran communities at Mistaken Point were strikingly similar to the processes that operate in modern deep-sea animal communities.

The double mass extinction revisited: reassessing the severity, selectivity, and causes of the end-Guadalupian biotic crisis (Late Permian)

Abstract The end-Guadalupian extinction, at the end of the Middle Permian, is thought to have been one of the largest biotic crises in the Phanerozoic. Previous estimates suggest that the crisis eliminated 58% of marine invertebrate genera during the Capitanian stage and that its selectivity helped the Modern evolutionary fauna become more diverse than the Paleozoic fauna before the end-Permian mass extinction. However, a new sampling-standardized analysis of Permian diversity trends, based on 53731 marine invertebrate fossil occurrences from 9790 collections, indicates that the end-Guadalupian “extinction” was actually a prolonged but gradual decrease in diversity from the Wordian to the end of the Permian. There was no peak in extinction rates; reduced genus richness exhibited by all studied invertebrate groups and ecological guilds, and in different latitudinal belts, was instead driven by a sharp decrease in origination rates during the Capitanian and Wuchiapingian. The global diversity decrease was exacerbated by changes in beta diversity, most notably a reduction in provinciality due to the loss of marine habitat area and a pronounced decrease in geographic disparity over small distances. Disparity over moderate to large distances was unchanged, suggesting that small-scale beta diversity changes may have resulted from compression of bathymetric ranges and homogenization of onshore-offshore faunal gradients stemming from the spread of deep-water anoxia around the Guadalupian/Lopingian boundary. Although tropical invertebrate genera were no more likely than extratropical ones to become extinct, the marked reduction in origination rates during the Capitanian and Wuchiapingian is consistent with the effects of global cooling (the Kamura Event), but may also be consistent with other environmental stresses such as anoxia. However, a gradual reduction in diversity, rather than a sharp end-Guadalupian extinction, precludes the need to invoke drastic extinction mechanisms and indicates that taxonomic loss at the end of the Paleozoic was concentrated in the traditional end-Permian (end-Changhsingian) extinction, which eliminated 78% of all marine invertebrate genera.

Understanding mechanisms for the end-Permian mass extinction and the protracted Early Triassic aftermath and recovery

Modern study of the end-Permian mass extinction in the marine realm has involved intensive documentation of the fossil content, sedimentology, and chemostratigraphy of individual stratigraphic sections where the mass extinction interval is well preserved. These studies, coupled with innovative modeling of environmental conditions, have produced specific hypotheses for the mechanisms that caused the mass extinction and associated environmental stress. New paleobiological studies on the environmental distribution and ecological importance of brachiopods, benthic molluscs, and bryozoans support the hypothesis that stressful ocean conditions—primarily elevated H2S levels (euxinia) but also heightened CO2 concentrations—were the prime causes of the end-Permian mass extinction. These studies also further demonstrate that both the Late Permian interval preceding this mass extinction and the Early Triassic interval that followed were times of similar elevated environmental stress. In the low-diversity Early Triassic biosphere, huge numbers of benthic molluscs, in particular four cosmopolitan genera of bivalves, typically covered the seafloor. That a few marine genera thrived during this time indicates a greater than usual tolerance to some combination of marine anoxia, as well as elevated CO2 and/or increased H2S concentrations. Research focusing on experiments with modern organisms similar to those that died, as well as those that thrived, in microcosms where levels of O2, CO2, and H2S can be experimentally manipulated will enable an even more detailed understanding of the nature of this greatest Phanerozoic biotic crisis.

Ediacaran epifaunal tiering

Epifaunal tiering, the subdivision of vertical space within a community, is a fundamental attribute of Phanerozoic suspension-feeding communities. This paper documents tiering, including the presence of meter-tall organisms, in Neoproterozoic Ediacaran communities. Ediacaran tiering was studied from three exceptionally preserved deep-water communities at Mistaken Point, Newfoundland, which contain in situ census populations of hundreds to thousands of organisms. Tiering consists of overlapping populations of dominant organisms and is characterized by gradational, rather than abrupt, tier boundaries. At least three tiers are apparent: a lower level 0–8 cm above the seafloor, an intermediate level between 8–22 cm above the seafloor, and an upper level that extends as high as 120 cm. Tier boundaries are relatively consistent between communities, but the constituent organisms in each level are variable, suggesting that some Ediacaran taxa could fill different tiers interchangeably. Development of a tiered epifaunal structure is consistent with suspension feeding or absorbing dissolved nutrients directly from seawater. Despite the common occurrence of tall organisms, all communities share a similar population structure in which biomass is concentrated in the basal 10 cm above the seafloor. Comparison with shallow-water Ediacaran localities suggests that the observed tiering structure is typical of Ediacaran communities. Ediacaran tierers also show the fundamental subdivision between organisms and/or colonies that fed along their entire length and those that developed a specialized feeding apparatus, implying that the features of Phanerozoic tiered skeletal ecosystems were first initiated in soft-bodied communities in the late Neoproterozoic.

Acidification, anoxia, and extinction: A multiple logistic regression analysis of extinction selectivity during the Middle and Late Permian

Patterns of taxonomic and ecologic selectivity are the most direct record of processes infl uencing survival during background and mass extinctions. The Guadalupian (Capitanian) and end-Permian (Changhsingian) extinctions have both been linked to environmental degradation from eruption of large fl ood basalts; however, the extent to which taxonomic selectivity conforms to the expected stresses remains incompletely understood because many of the relevant biological traits are mutually correlated. Here we use a large occurrencebased database to quantify extinction selectivity during background and mass extinction intervals from the Kungurian (latest Early Permian) to Changhsingian. Our multiple logistic regression analysis confi rms that the end-Permian extinction was a physiological crisis, selecting against genera with poorly buffered respiratory physiology and calcareous shells. Genera with unbuffered physiology also fared poorly in the Guadalupian extinction, consistent with recognition of a pronounced crisis only among protists and reef-builders and implying similar respiratory physiological stresses. Despite sharing a similar trigger, the end-Permian extinction was considerably more severe than the Guadalupian or other Phanerozoic physiological crises. Its magnitude may have resulted from a larger environmental perturbation, although the combination of warming, hypercapnia, ocean acidifi cation, and hypoxia during the end-Permian extinction likely exacerbated the crisis because of the multiplicative effects of those stresses. Although ocean carbon cycle and evolutionary changes have reduced the sensitivity of modern ecosystems to physiological stresses, extant marine invertebrates face the same synergistic effects of multiple stressors that were so severe during the end-Permian extinction.

Population structure of the oldest known macroscopic communities from Mistaken Point, Newfoundland

Abstract The presumed affinities of the Terminal Neoproterozoic Ediacara biota have been much debated. However, even in the absence of concrete evidence for phylogenetic affinity, numerical paleoecological approaches can be effectively used to make inferences about organismal biology, the nature of biotic interactions, and life history. Here, we examine the population structure of three Ediacaran rangeomorph taxa (Fractofusus, Beothukis, and Pectinifrons), and one non-rangeomorph taxon (Thectardis) across five fossil surfaces around the Avalon Peninsula, Newfoundland, through analysis of size-frequency distributions using Bayesian Information Criterion (BIC). Best-supported models resolve communities of all studied Ediacaran taxa at Mistaken Point as single cohorts with wide variance. This result is best explained in terms of a “continuous reproduction” model, whereby Ediacaran organisms reproduce aseasonally, so that multiple size modes are absent from preserved communities. Modern benthic invertebrates (both as a whole and within specific taxonomic groups) in deeper-water settings reproduce both seasonally and aseasonally; distinguishing between biological (i.e., continuous reproductive strategies) and environmental (lack of a seasonal trigger) causes for this pattern is therefore difficult. However, we hypothesize that the observed population structure could reflect the lack of a trigger for reproduction in deepwater settings (i.e., seasonal flux of organic matter), until the explosive appearance of mesozooplankton near the base of the Cambrian.

ASSESSING THE ECOLOGICAL DOMINANCE OF PHANEROZOIC MARINE INVERTEBRATES

Abstract Ecological studies have revealed that the functional roles of dominant species in modern communities are often more important than overall diversity in governing community composition and functioning. Despite this recognition that abundance and diversity data are both required for a complete understanding of ecological processes, many paleoecological studies focus on presence-absence data, possibly because of concerns regarding the taphonomic fidelity of time-averaged fossil accumulations. However, the abundance of organisms in shell beds has been shown to provide a fairly accurate record of the living community, suggesting that the benefits of relative-abundance data should be reconsidered. Recognition of ecologically dominant species in local fossil assemblages should be based on counts of relative abundance and assessment of ecological role. Ecological dominance at larger spatial or temporal scales can be quantified using the mean rank order of a clade and the proportion of assemblages where the clade is present, providing unbiased, quantitative values for measuring the ecological importance of a clade. Their utility has been tested with three case studies encompassing a range of geographic and taxonomic scales, using a database of 1221 Ordovician–Paleogene quantitative fossil collections. The dominance metrics for rhynchonelliform brachiopods, bivalves, and gastropods broadly parallel anecdotal trends, even including some more detailed patterns documented by regional studies. An examination of substrate preferences for brachiopod and bivalve orders confirms the abundance of infaunal bivalves in siliciclastics and epifaunal bivalves in carbonates, but it also reveals intriguing patterns regarding substrate preferences among rhynchonelliform brachiopod orders. The final case study analyzed changes in dominance between early Mesozoic fossil assemblages from Tethys and Panthalassa, documenting significant geographic differences in the ecological importance of rhynchonelliform brachiopods and bivalves. These large-scale dominance patterns often approximately matched those inferred from diversity trends; however, there are also times when dominance was decoupled from diversity, indicating that further investigation of ecological dominance will provide additional insights into ecological influences on the Phanerozoic history of life. “Are most species simply passengers in ecosystems that are run basically by a few dominants?” (Worm and Duffy, 2003, p. 631)

Environmental and biotic controls on the evolutionary history of insect body size

Giant insects, with wingspans as large as 70 cm, ruled the Carboniferous and Permian skies. Gigantism has been linked to hyperoxic conditions because oxygen concentration is a key physiological control on body size, particularly in groups like flying insects that have high metabolic oxygen demands. Here we show, using a dataset of more than 10,500 fossil insect wing lengths, that size tracked atmospheric oxygen concentrations only for the first 150 Myr of insect evolution. The data are best explained by a model relating maximum size to atmospheric environmental oxygen concentration (pO2) until the end of the Jurassic, and then at constant sizes, independent of oxygen fluctuations, during the Cretaceous and, at a smaller size, the Cenozoic. Maximum insect size decreased even as atmospheric pO2 rose in the Early Cretaceous following the evolution and radiation of early birds, particularly as birds acquired adaptations that allowed more agile flight. A further decrease in maximum size during the Cenozoic may relate to the evolution of bats, the Cretaceous mass extinction, or further specialization of flying birds. The decoupling of insect size and atmospheric pO2 coincident with the radiation of birds suggests that biotic interactions, such as predation and competition, superseded oxygen as the most important constraint on maximum body size of the largest insects.

THECTARDIS AVALONENSIS: A NEW EDIACARAN FOSSIL FROM THE MISTAKEN POINT BIOTA, NEWFOUNDLAND

Abstract The Neoproterozoic Ediacara biota at Mistaken Point contains the oldest diverse Ediacaran assemblages and is one of the few known deepwater localities, yet the biota is dominated by endemic forms, nearly all of which remain undescribed. Thectardis avalonensis new genus and species, one of these endemic forms, is a cm-scale triangular fossil with a raised rim and a featureless-to-faintly-segmented central depression. More than 200 specimens occur on two bedding plane surfaces: the 565 Ma E surface and the 575 Ma Pigeon Cove surface, nearly 2,000 m lower in the succession. Morphological and taphonomic data suggest that the organism was an elongate cone that may have lived as a suspension-feeding “mat sticker” with its pointed base inserted into the microbially bound sediment. If true, Thectardis n. gen. would be the tallest-known mat sticker, reaching a maximum height of over 15 cm. Specimens display little ontogenetic change in length:width ratio, suggesting that Thectardis grew uniformly by incremental addition of material to its distal end. Morphological differences between specimens at two well-separated stratigraphic levels may have resulted from evolutionary or ecophenotypic variation.