Mary L. Droser

The Great Ordovician Biodiversification Event

The Global Ordovician Biodiversification Event (GOBE) was undoubtedly one of the most significant evolutionary events in the history of the marine biosphere. A continuous increase in ichnodiversity occurs through the Ordovician in both shallow- and deep-marine environments. The earlier view that early Paleozoic deep-marine ichnofaunas are of low alpha diversity has been challenged by discoveries of moderately diverse associations. Interestingly, however, the increase in global ichnodiversity through the Ordovician is not paralleled by an increase in ichnodisparity of bioturbation structures. In fact, whereas global ichnodiversity in the Ordovician almost doubled Cambrian levels, Ordovician ichnodisparity of bioturbation structures is roughly similar to that resulting from the Cambrian explosion. Macroboring organisms also display significant evolutionary innovation and diversification in shallow-water hardgrounds and other carbonate substrates, resulting in the Ordovician Bioerosion Revolution. Along with this macroboring ichnodiversity and ichnodisparity increase is a significant rise in the rate of bioerosion in carbonate substrates. Ichnofaunal changes in lower-shoreface and offshore siliciclastic deposits through the Ordovician reveal faunal turnovers resulting from the evolutionary radiation. Lower Ordovician deposits tend to be dominated by abundant trilobite-produced trace fossils. Middle to Upper Ordovician shallow-marine ichnofaunas tend to show more varied behavioral patterns and trilobite trace fossils are rarely the dominant components. During the early Paleozoic, the tiering structure of ichnofaunas became more complex, as a result of both the addition of deeper tiers and of a wider variety of biogenic structures in previously occupied tiers. Infaunalization by deposit feeders in offshore siliciclastic environments was most likely diachronous, with the establishment of a mid-tier infauna first in Laurentia and Baltica, and only subsequently in Gondwana.

Late Precambrian Oxygenation; Inception of the Clay Mineral Factory

An enigmatic stepwise increase in oxygen in the late Precambrian is widely considered a prerequisite for the expansion of animal life. Accumulation of oxygen requires organic matter burial in sediments, which is largely controlled by the sheltering or preservational effects of detrital clay minerals in modern marine continental margin depocenters. Here, we show mineralogical and geochemical evidence for an increase in clay mineral deposition in the Neoproterozoic that immediately predated the first metazoans. Today most clay minerals originate in biologically active soils, so initial expansion of a primitive land biota would greatly enhance production of pedogenic clay minerals (the “clay mineral factory”), leading to increased marine burial of organic carbon via mineral surface preservation.

Trends in depth and extent of bioturbation in Cambrian carbonate marine environments, western United States

Cambrian carbonate strata of the Great Basin deposited below fairweather wave base but above mean storm wave base (inner shelf), and below mean storm wave base but above maximum storm wave base (middle shelf) were examined for extent and depth of bioturbation. Data indicate that initial extensive colonization of marine infaunal habitats occurred between Tommotian and Atdabanian time. Throughout the remaining 65 m.y. of Cambrian time bioturbation increased, although depth of bioturbation remained relatively constant at less than 6 cm. During this time, the amount of bioturbation in the inner shelf was greater than that in the middle shelf.

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.

Ediacara-type fossils in Cambrian sediments

Fossil assemblages that preserve soft-bodied organisms are essential for our understanding of the composition and diversity of past life. The worldwide terminal Proterozoic Ediacara-type fossils (from ∼600–544 Myr BP) are unique in consisting of soft-bodied animals, which are typically preserved as impressions in coarse-grained sediments. These Lagerstätten are also special because they pre-date the major burst of skeletonization, which occurred near the start of the Cambrian period. Most Ediacara-type fossils are interpreted to be cnidarians, but higher metazoans such as annelids and molluscs may also be represented. However, the unique style of preservation and difficulties in finding convincing morphological homologies with definite animals have led some specialists to prefer non-metazoan interpretations, such as Vendobionta. In addition, the rarity of Ediacara-type fossils in younger sediments has led to suggestions of a terminal Proterozoic mass extinction. Here we report typical Ediarcara-type frond-shaped fossils that occur together with an assemblage of Cambrian-type trace fossils in unequivocally Cambrian-aged sediments of the Uratanna Formation, South Australia. This occurrence bridges the apparent divide between the terminal Proterozoic and Cambrian fossil assemblages, and also suggests that closure of a taphonomic window (an interval of time with unique preservational conditions) was as important as extinction in the disappearance of Ediacara-type organisms.

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.

Trace fossils and substrates of the terminal Proterozoic–Cambrian transition: Implications for the record of early bilaterians and sediment mixing

The trace fossil record is important in determining the timing of the appearance of bilaterian animals. A conservative estimate puts this time at ≈555 million years ago. The preservational potential of traces made close to the sediment–water interface is crucial to detecting early benthic activity. Our studies on earliest Cambrian sediments suggest that shallow tiers were preserved to a greater extent than typical for most of the Phanerozoic, which can be attributed both directly and indirectly to the low levels of sediment mixing. The low levels of sediment mixing meant that thin event beds were preserved. The shallow depth of sediment mixing also meant that muddy sediments were firm close to the sediment–water interface, increasing the likelihood of recording shallow-tier trace fossils in muddy sediments. Overall, trace fossils can provide a sound record of the onset of bilaterian benthic activity.

Ordovician increase in extent and depth of bioturbation: Implications for understanding early Paleozoic ecospace utilization

The infaunal habitat was colonized in the early Phanerozoic primarily by soft-bodied organisms. To understand the early biotic history of this environment, it is necessary to examine the overall record of bioturbation, or ichnofabric. We present here results documenting trends in ichnofabric from Ordovician strata of the Great Basin in California, Nevada, and Utah, collected by a method devised to semiquantitatively measure the extent of early Paleozoic utilization of infaunal ecospace. When combined with similar Cambrian data, a two-phase stepwise pattern of increasing bioturbation contemporaneous with the Cambrian and Ordovician radiations of skeletonized metazoans is apparent. These data represent evidence independent of the body fossil record of increasing ecospace utilization in the early Paleozoic.

Burrowing below the basal Cambrian GSSP, Fortune Head, Newfoundland

The range of Treptichnus pedum , the index trace fossil for the Treptichnus pedum Zone, extends some 4 m below the Global Standard Stratotype-section and Point for the base of the Cambrian Period at Fortune Head on the Burin Peninsula in southeastern Newfoundland. The identification of zigzag traces of Treptichnus isp., even further below the GSSP than T. pedum in the Fortune Head section, and in other terminal Proterozoic successions around the globe, supports the concept of a gradational onset of three-dimensional burrowing across the Proterozoic–Cambrian boundary. Although T. pedum remains a reasonable indicator for the base of the Cambrian Period, greater precision in the stratotype section can be achieved by a detailed re-evaluation of the stratigraphic ranges and the morphological variation of ichnotaxa included in the T. pedum Zone.

Eight-armed Ediacara fossil preserved in contrasting taphonomic windows from China and Australia

We report the preservation of the eight-armed Ediacara fossil Eoandromeda octobrachiata as carbonaceous compressions in the Doushantuo black shale of south China and as casts and molds in the Rawnsley Quartzite in South Australia. The contrasting preservational styles in two taphonomic windows indicate that E. octobrachiata may have had a relatively recalcitrant organic integument, which rules out its close comparison with giant agglutinated foraminifers such as xenophyophores. Its octaradial symmetry and dextrally spiraling arms suggest that it may be a diploblastic-grade animal sharing some features with cnidarians and ctenophores, although its phylogenetic affinity remains open. It is the first and only unambiguously identified Ediacaran macrofossil that occurs in two drastically different taphonomic windows, thus bridging the conventional biological and taxonomic gaps between the Ediacara and Miaohe biotas, which collectively record the earliest known macroscopic and complex life.