Peter M. Sheehan

The Evolution of Reef Communities

THE REEF PHENOMENON: What is a Reef? Cenozoic Reef Models: Occurrence Cenozoic Reef Models II: Cenozoic Reef Models III Cenozoic Reef Models IV: Communities Ancient Reefs Autecology and History of Major Reef Algal Autecology and History Poriferan Autecology and History Coelenterate Autecology and History of Bryozoa SYNECOLOGY AND HISTORY OF PRE- CENOZOIC REEF COMMUNITIES: Early Paleozoic Reef Communities Cambrian and Early Orodovician Middle Paleozoic Reef Communities Middle Ordovician - Late Devonian Late Paleozoic Reef Communities Mesozoic Reef Communities Overview and Conclusions Glossary Index.

A weathering hypothesis for glaciation at high atmospheric pCO2 during the Late Ordovician

New paired carbonate and organic-carbon isotope analyses from Nevada, USA, together with a consideration of the effects of mountain-building and ice-sheet coverage of the continents on atmospheric pCO2, lead to a new hypothesis for the cause of the Late Ordovician glaciation. We suggest that the Taconic orogeny, which commenced in the late-middle Ordovician, caused a long-term decline in atmospheric pCO2 through increased weatherability of silicate rocks. Ice-sheet growth was triggered when pCO2 decreased to a threshold of ∼10× present atmospheric level and proceeded by positive ice-albedo feedback. In the midst of glaciation, atmospheric pCO2 began to rise as continental silicate weathering rates declined in response to coverage of weathering terrains by ice sheets. At first, this enhanced greenhouse effect was overcompensated for by ice-albedo effects. Ultimately, however, atmospheric pCO2 reached a level which overwhelmed the cooling effects of ice albedo, and the glaciation ended. The isotope results can be interpreted to indicate that atmospheric pCO2 rose during the glaciation, consistent with other proxy information, although alternative interpretations are possible. The large, positive carbonate isotope excursion observed in Late Ordovician rocks around the world is explained as the expected response to increased carbonate-platform weathering during glacioeustatic sea-level lowstand, rather than as a response to increased organic-carbon burial.

Onshore-offshore patterns in the evolution of phanerozoic shelf communities.

Cluster analysis of Cambrian-Ordovician marine benthic communities and community-trophic analysis of Late Cretaceous shelf faunas indicate that major ecological innovations appeared in nearshore environments and then expanded outward across the shelf at the expense of older community types. This onshoreinnovation, offshore-archaic evolutionary pattern is surprising in light of the generally, higher species turnover rates of offshore clades. This pattern probably results from differential extinction rates of onshore as compared to offshore clades, or from differential origination rates of new ecological associations or evolutionary novelties in nearshore environments.

Diversification, Faunal Change, and Community Replacement during the Ordovician Radiations

The Ordovician evolutionary radiations represent a major pivotal point in the history of life on earth. During the few tens of million years between the ends of the Cambrian and Ordovician Periods, the nature of marine faunas was almost completely changed. The trilobite-dominated communities of the Cambrian were replaced by complex suspension-feeding communities dominated by brachiopods, bryozoans, and pelmatozoans, and taxonomic diversity, as seen at both local (i.e., community-wide) and global (i.e., worldwide) levels, was increased two- to threefold. These new faunal patterns then persisted with only minor change for the next 200 m.y. of the Paleozoic. Only two other events in the history of marine faunas had comparable importance: the Vendian to Early Cambrian radiations, which emplaced the first marine fauna, and the Late Permian extinctions, which destroyed the Paleozoic fauna established during the Ordovician and led to the subsequent dominance of the modern marine fauna.

Decoupling of taxonomic and ecologic severity of Phanerozoic marine mass extinctions

There have been five major mass extinctions among the marine biota during the ∼0.6 b.y. history of metazoan life on Earth. These mass extinctions have been ranked from the largest to the smallest by the severity of taxonomic diversity losses, but they have not been ranked by the severity of the ecologic changes that they produced. Here we utilize a system of paleoecological levels that allows for the ranking of ecological degradation or shifts associated with significant taxonomic events, along with an analysis of large-scale paleoenvironmental patterns of two of the great evolutionary faunas, to compare the relative ecologic degradation caused by two major mass extinctions. The Late Ordovician and Late Devonian mass extinctions produced similar taxonomic losses (marine families declined ∼22% and 21%, respectively). However, our analyses show that whereas the Late Ordovician extinction resulted in only minimal permanent ecological change, the Late Devonian extinction resulted in the complete restructuring of many components of the marine ecosystem. Thus, the large-scale taxonomic and ecological significance of these extinction events are decoupled, implying that some taxa are ecologically more critical than others.

Evaluating the ecological architecture of major events in the Phanerozoic history of marine invertebrate life

Paleoecological changes associated with Phanerozoic mass extinctions and radiations can be categorized into four nonhierarchical, nonadditive levels. First-level changes include colonization of a new ecosystem. Structural changes within an established ecosystem represent the second level, changes within an already established ecological structure are the third-level, and taxonomic changes within a community represent the fourth-level. Applying these levels to the Ordovician radiation, end-Ordovician extinction and Silurian recovery, as well as the end-Permian extinction and Triassic recovery demonstrate that paleoecological changes associated with these major events can be evaluated and compared in a more rigorous manner than previously done. Results of this analysis demonstrate that use of these levels indicates that the relative magnitude of an event as measured by taxonomic criteria may be decoupled from its paleoecological significance.

Understanding the Great Ordovician Biodiversification Event (GOBE): Influences of paleogeography, paleoclimate, or paleoecology?

“The Great Ordovician Biodiversification Event” (GOBE) was arguably the most important and sustained increase of marine biodiversity in Earth’s history. During a short time span of 25 Ma, an “explosion” of diversity at the order, family, genus, and species level occurred. The combined effects of several geological and biological processes helped generate the GOBE. The peak of the GOBE correlates with unique paleogeography, featuring the greatest continental dispersal of the Paleozoic. Rapid sea-floor spreading during this time coincided with warm climates, high sea levels, and the largest tropical shelf area of the Phanerozoic. In addition, important ecological evolutionary changes took place, with the “explosion” of both zooplankton and suspension feeding organisms, possibly based on increased phytoplankton availability and high nutrient input to the oceans driven by intense volcanic activity. Extraterrestrial causes, in the form of asteroid impacts, have also been invoked to explain this remarkable event.

Detritus feeding as a buffer to extinction at the end of the Cretaceous

At the end of the Cretaceous the principal animals that became extinct, such as dinosaurs, marine animals that lived in the water column, and benthic filter feeders, were in food chains tied directly to living plant matter. Animal groups less affected by extinction, including marine benthic scavengers and deposit feeders, small insectivorous mammals, and members of stream communities, were in food chains dependent on dead plant material. The proposal that an asteroid or comet impact at the end of the Cretaceous produced a dust cloud that cut off photosynthesis for several months is consistent with this pattern of extinction. Food chains dependent on living plant matter crashed, while food chains based on detritus were buffered from extinction because there was a food supply adequate for the interval when photosynthesis was halted.

The Extinction of the Dinosaurs in North America

Rightly or wrongly, dinosaurs are poster children for the CretaceousTertiary (K-T) extinction. The rate and cause of their extinction, however, has been contentious, at least in part because of their rarity. Nonetheless, significant data have accumulated to indicate that the dinosaur extinction, in North America at least, was geologically instantaneous. The evidence comes from field studies in geologically disparate settings involving the reconstruction of dinosaur stratigraphic ranges as well as community structure in the Late Cretaceous, and from quantitative studies of the post-Cretaceous evolution of mammals. The hypothesis of extinction by asteroid impact is concordant with what is known of the rate of the dinosaur extinction, as well as the patterns of selective vertebrate survivorship across the K-T boundary. The precise nature of the kill mechanism(s), however, remains under discussion.

Major extinctions of land-dwelling vertebrates at the Cretaceous-Tertiary boundary, eastern Montana

A large database recording species of terrestrial vertebrates present in formations above and below the Cretaceous-Tertiary (K-T) boundary in eastern Montana was assembled by J. D. Archibald and L. J. Bryant. Division of the species in this database into freshwater and land-dwelling vertebrate assemblages reveals that the K-T vertebrate extinction was concentrated in land- dwelling forms. In data corrected for the effects of rare taxa, 90% of the species in the freshwater assemblage survived into the Tertiary, but only 12% of the land-dwelling forms survived. The pattern of differential extinction of terrestrial vertebrates in eastern Montana may be in large part the result of the dependence of land-based communities on primary productivity. This is in contrast to the riverine communities, which may derive much of their organic carbon from detritus. The pattern of extinction and survival is compatible with the hypothesis of an asteroid impact after which there was a temporary cessation of primary, photosynthetic productivity.