Roger A. Cooper

The Ordovician graptolite sequence of Australasia

The classical Ordovician graptolite succession of Victoria has long been taken as standard for the region and widely used for subdivision and correlation of Ordovician sequences around the world. We review the Victorian succession and incorporate other Australian and New Zealand graptolitic sequences into an Australasian set of zones and stages. Thirty zones are recognized and defined, of which one (Ca4, Isograptus victoriae maximodivergens) is new and two (La1, Da4) each comprise two subzones. Several zones are redefined, but the previous zone notation has been retained. Nine stages are recognized (as previously) and defined, six (Lancefieldian, Bendigonian, Chewtonian, Castlemainian, Yapeenian, Darriwilian) in the Early Ordovician and three (Gisbornian, Eastonian, Bolindian) in the Late Ordovician; reference sections for each are nominated. The Bolindian stage contains 5 zones, here designated Bol-5. The Australasian stages are correlated internationally. A census of Australasian graptolite taxa lists t...

Early Paleozoic terranes of New Zealand

Abstract Lower Paleozoic rocks of New Zealand comprise two major assemblages each with their own distinct sedimentary tectonic, metamorphic and igneous history; they thus represent two distinct tectono-stratigraphic terranes. In Nelson-Westland, the western, or Buller, terrane consists of the Western Sedimentary Belt, together with Ordovician paragneiss at Charleston and in Victoria Range. The sedimentary sequence, ranging in age from basal to Upper Ordovician, comprises continentderived quartz-rich turbidites with black shales inferred to have been deposited in submarine fans and slope basins. The eastern, Takaka terrane (Central and Eastern Sedimentary Belts) is much more varied in lithofacies, composition and age (Cambrian to Silurian) and itself comprises several tectonic, probably thrust, slices. Volcanics, volcaniclastics, siliceous and calcareous siltstone, conglomerate and turbidites dominate the Cambrian part of the sequence and indicate proximity to a Cambrian island arc. The oldest sediments ar...

A precise worldwide correlation of early Ordovician graptolite sequences

Fourteen early Ordovician (Tremadoc-Llandeilo) graptolite sequences from around the world are precisely (infrazonally) correlated, based on the stratigraphic ranges of 130 species and species groups. The composite standard sequence (CSS) of graptolites has been determined from the six best regional sequences by a nonparametric graphic correlation. Two data sets were selected: one comprised first appearance events of 103 taxa, the other, first and last appearance events of 45 taxa. The results of the two runs accord well and reveal respectively 66 and 73 successive bioevents in early Ordovician time. Event spacing averages 0.7-0.8 Ma and enables fine subdivision, correlation and homotaxial testing for diachroneity. The strong correlation between each of the six regional sequences and the CSS indicates the high level of accordance among graptolite successions around the world.

Second-Order Sequence Stratigraphic Controls on the Quality of the Fossil Record at an Active Margin: New Zealand Eocene to Recent Shelf Molluscs

Abstract New Zealand has the most complete Cenozoic molluscan fossil record in the Southern Hemisphere. In order to understand the true marine faunal history of the region, it is necessary first to identify apparent biodiversity changes that result simply from variations in the quality of the fossil record. The present study uses a range of methods to quantify both long-term, secular changes and short-term patterns of variation in sampling probability for New Zealand Cenozoic shelf molluscs. Overall, about one-third of all once-living Cenozoic species have been sampled, and average per-stage sampling probabilities are between 20% and 50%. Increase in per-stage sampling probability through time reflects the increase in outcrop area and ease of fossil recovery from older to younger stages. Short-term patterns of variation apparently are related to second-order sequence stratigraphic controls of preservation potential. Once the effects of stage duration are eliminated, patterns of stage-to-stage sampling probability reflect enhanced preservation in mid-cycle positions and, perhaps to a lesser extent, secondary post-depositional loss of stratigraphic record above and below sequence boundaries. Although this result mirrors patterns observed in Europe, it is possible that enhanced preservation mid-cycle is relatively more important at active margins, such as New Zealand, whereas secondary loss of record at the sequence boundary is more important at passive margins. Finally, it is worth noting that different methods and data compilations yield rather consistent estimates of short-term variation in sampling probability, lending confidence to the methods and suggesting that the patterns identified are likely to reflect true underlying features of the New Zealand marine fossil record.

Graptoloid evolutionary rates track Ordovician–Silurian global climate change

Graptoloid evolutionary dynamics show a marked contrast from the Ordovician to then Silurian. Subdued extinction and origination rates during the Ordovician give way, duringn the late Katian, to rates that were highly volatile and of higher mean value through then Silurian, reflecting the significantly shorter lifespan of Silurian species. Thesen patterns are revealed in high-resolution rate curves derived from the CONOP (constrainedn optimization) scaled and calibrated global composite sequence of 2094 graptoloid species.n The end-Ordovician mass depletion was driven primarily by an elevated extinction raten which lasted for c . 1.2 Ma with two main spikes during the Hirnantian.n The early Silurian recovery, although initiated by a peak in origination rate, wasn maintained by a complex interplay of origination and extinction rates, with both ratesn rising and falling sharply. The global δ 13 C curve echoes the graptoloidn evolutionary rates pattern; the prominent and well-known positive isotope excursionsn during the Late Ordovician and Silurian lie on or close to times of sharp decline inn graptoloid species richness, commonly associated with extinction rate spikes. Then graptoloid and isotope data point to a relatively steady marine environment in then Ordovician with mainly background extinction rates, changing during the Katian to a moren volatile climatic regime that prevailed through the Silurian, with several sharpn extinction episodes triggered by environmental crises. The correlation of graptoloidn species diversity with isotopic ratios was positive in the Ordovician and negative in then Silurian, suggesting different causal linkages. Throughout the history of the graptoloidn clade all major depletions in species richness except for one were caused by elevatedn extinction rate rather than decreased origination rate.

Quantitative biostratigraphy of the Taranaki Basin, New Zealand: A deterministic and probabilistic approach

A quantitative biostratigraphic analysis of the Paleocene to lower Miocene of the Taranaki Basin has enabled high precision in correlation, zonation, and assessment of depositional history. Biostratigraphic range-end events, based on 493 taxa in cuttings samples from eight wells, representing foraminifera, nannofossils, dinoflagellates, and miospores, were culled to 87 range-top events that were then analyzed by deterministic (constrained optimization [CONOP]) and probabilistic (ranking and scaling [RASC]) techniques. All except 16 of the events are found to have relatively good biostratigraphic reliability. The RASC probable sequence and probabilistic zonation give the best estimate of the sequence of events and zones to be encountered in any new well in the basin and a precise biostratigraphic scale for future exploration. The CONOP composite section, which matches well with that derived by conventional graphic correlation (GRAPHCOR), is readily related to previous zonations based on maximum ranges of taxa but gives an order-of-magnitude greater precision. CONOP provides a precise correlation framework and reveals marked variation in thickness of stages across the basin. When the composite section is calibrated against the time scale, basinwide changes in depositional rate are revealed. The upper Eocene and Oligocene mark an interval of slow deposition, whereas the Miocene marks a sharp increase in deposition. The time-calibrated composite section enables unconformities and changes in depositional rate found in individual wells to be precisely estimated. Many new unconformities are indicated, particularly in the Paleocene and Eocene.

Greenhouse−icehouse transition in the Late Ordovician marks a step change in extinction regime in the marine plankton

Significance In the graptoloids, a major group of early Paleozoic plankton, extinction selectively removed young species during times of background (low intensity) extinction. Age-independent extinction was confined to high extinction rate spikes of short duration that were related to environmental perturbations. During the extreme Late Ordovician Mass Extinction, old species were selectively removed. Graptoloids provide a sensitive indicator of marine environmental change and suggest that age selectivity of extinction in oceanic pelagic ecosystems switched rapidly and repeatedly from one mode to another and back again, a pattern that can be detected only when temporal resolution and species turnover rates are exceptionally high. Two distinct regimes of extinction dynamic are present in the major marine zooplankton group, the graptolites, during the Ordovician and Silurian periods (486−418 Ma). In conditions of “background” extinction, which dominated in the Ordovician, taxonomic evolutionary rates were relatively low and the probability of extinction was highest among newly evolved species (“background extinction mode”). A sharp change in extinction regime in the Late Ordovician marked the onset of repeated severe spikes in the extinction rate curve; evolutionary turnover increased greatly in the Silurian, and the extinction mode changed to include extinction that was independent of species age (“high-extinction mode”). This change coincides with a change in global climate, from greenhouse to icehouse conditions. During the most extreme episode of extinction, the Late Ordovician Mass Extinction, old species were selectively removed (“mass extinction mode”). Our analysis indicates that selective regimes in the Paleozoic ocean plankton switched rapidly (generally in <0.5 My) from one mode to another in response to environmental change, even when restoration of the full ecosystem was much slower (several million years). The patterns observed are not a simple consequence of geographic range effects or of taxonomic changes from Ordovician to Silurian. Our results suggest that the dominant primary controls on extinction throughout the lifespan of this clade were abiotic (environmental), probably mediated by the microphytoplankton.

Facies preference predicts extinction risk in Ordovician graptolites

Abstract The most abundant and diverse graptolite assemblages are found in offshore, deep-water black shales—the classical “graptolite facies” (deep-water or isograptid biofacies). The mean duration of Ordovician graptolite species confined to the deep-water facies (here referred to as “group 1” species) is 2.19 Myr, significantly shorter than the mean duration of species in the deep-water facies that are also known in sediments of the shallow-water shelf or platform (“group 2” species) −4.42 Myr, indicating a significantly higher extinction probability (p u200a=u200a <0.001). These figures are based on the precise age ranges of species derived from the time-calibrated composite sequence of 1446 Ordovician to early Devonian graptolites, built by the constrained optimization procedure (CONOP) from 256 measured sections worldwide, and exclude the effects of the Hirnantian mass extinction. The difference between groups cuts across families, morphological types, and pandemic/endemic distributions. An environmental influence is strongly suggested, and although both groups were planktonic, they were unlikely to have shared the same habitat in the water column. The new duration measurements therefore are interpreted as favoring a depth-stratification of graptolite habitats in the water column.

The phylogenetic relationships of the graptolites Tetragraptus phyllograptoides and Pseudophyllograptus cor

Abstract Re-examination of the type material of Tetragraptus phyllograptoides Strandmark and Pseudophyllograptus cor (Strandmark) confirms their unusual morphology and is consistent with the view that they represent morphologic intermediates between reclined Tetragraptus and Pseudophyllograptus. Their phylogenetic relationships are discussed and it is suggested that the ‘phyllograptoid’ rhabdosome (i.e. with four scandent stipes) has been independently derived in at least three separate lineages, those leading to Phyllograptus s.s., early Arenig Pseudophyllograptus and Pseudophyllograptus cor.

The fossil record and spatial structuring of environments and biodiversity in the Cenozoic of New Zealand

Abstract There is increasing evidence to suggest that drivers of bias in the fossil record have also affected actual biodiversity history, so that controls of artefact and true pattern are confounded. Here we examine the role of spatial structuring of the environment as one component of this common cause hypothesis. Our results are based on sampling standardized analyses of the post-Middle Eocene record of shelf molluscs from New Zealand. We find that spatial structuring of the environment directly influenced the quality of the fossil record. Contrary to our expectations, however, we find no evidence to suggest that spatial structuring of the environment was a strong or direct driver of taxic rates, net diversity, or spatial structuring in mollusc faunas at the scale of analysis. Stage-to-stage variation in sampling standardized diversity over the past 40 Ma exhibits two superficially independent dynamics: (a) changes in net diversity were associated primarily with changes in origination rate; and (b) an unknown common cause related extinction rate to the quality of the fossil record and, indirectly, to spatial structuring of the environment. We suggest that tectonic drivers, manifest as second-order sequence stratigraphic cycles, are likely to have been a key element of this common cause.