Cathryn R. Newton

Selective extinction among end-Triassic European bivalves

Ongoing controversies surrounding the end-Triassic extinction highlight the need for identifying a causal mechanism leading to extinction. Bivalve data from Lombardia (Italy), Northern Calcareous Alps (Austria and Germany), and northwest Europe (England and Wales) provide the biologic signal of selective extinction to compare two competing extinction hypotheses: (1) sea-level change and associated anoxia and (2) reduced primary productivity. The end-Triassic extinction eliminated 71% of Lombardian species, 85% of northern alpine species, and 90% of northwest European species. The extinction was independent of body size and geographic distribution. With respect to living habits, species from the three regions show a significantly greater proportion of infaunal bivalve extinction. The greater survival of epifaunal bivalves is correlated to their more efficient feeding and suggests that the infaunal bivalves may not have been able to meet their nutritional requirements. This pattern of selective extinction is inconsistent with anoxia and/or sea-level change as a causal factor in which higher survival of infaunal detritus and filter feeders would be predicted. Instead, the pattern is consistent with a reduction of primary productivity. Several regional and global mechanisms, including bolide impact, would have been capable of altering primary productivity levels to affect the food sources for Late Triassic bivalves, thus leading to extinction.

Modern Deep-Water Coral Mounds North of Little Bahama Bank: Criteria for Recognition of Deep-Water Coral Bioherms in the Rock Record

ABSTRACT Deep-water, ahermatypic coral mounds are present at water depths of 1,000-1,300 m on the lower slope north of Little Bahama Bank. The mounds are patchily distributed over a minimum area of 2,500 km2 and typically display 5-40 m of relief above the surrounding sea bottom. A diverse benthic community exists on these apparently unlithified mounds, including l 1 genera and 16 species of ahermatypic coral (Bathypsammia, Caryophyllia, Deltocyathus, Desmophyllum, Enallopsammia, Javania, Madrepora, Polymyces, Solenosmilia, Stephanocyathus, and a previously undescribed genus), alcyonaceans, gorgonians, antipatharians, hydroids, ophiuroids, crinoids, barnacles, galatheid crabs, polychaetes, gastropods, bivalves, and sponges. Conspicuously absent from the coral fauna are Lop elia and Dendrophyllia, common deep-water corals in other parts of the Atlantic. Radiocarbon dates on fresh coral and gorgonian fragments of 940 ± 40 and 860 ± 50 years indicate the mounds are at least in part Recent and are probably actively forming today. Bored and stained corals date at around 22 10314C years B. P., which establishes a minimum age for these mounds. We speculate that the mounds develop on sea-floor perturbations in areas where strong bottom currents provide needed oxygen and nutrients to the fauna. The mounds may have undergone multi-stage evolution from colony to thicket to coppice to bank. This evolution may be accomplished through the in situ contribution of skeletal material along with the baffling and trapping of fine-grained carbonate sediment winnowed from adjacent areas by bottom currents. If found in the rock record, the coral mounds north of Little Bahama Bank would probably be considered bioherms. Geologists should thus be aware that scleractinian bioherms are not uniquely shallow-water in origin. The distinction between ancient deep- and shallow-water coral buildups involves using multiple recognition criteria, including: 1) presence or absence of algae; 2) diversity of corals; 3) coral morphology and microstructure; 4) abundance of planktonic/benthonic components; 5) microborings; 6) surrounding litho- and biofacies; 7) stable isotopes of carbon and oxygen; and 8) trace element geochemistry, particularly Sr and U concentrations. The distinction between deep- and shallow-water bioherms is crucial to regional paleoenvironmental and stratigraphic interpretations.

Ecological reinterpretation of the dysaerobic Leiorhynchus fauna; Upper Devonian Geneseo Black Shale, central New York

The anomalous Leiorhynchus brachiopod fauna of the Upper Devonian Geneseo Shale, formerly considered an epiplanktonic biota, is here reinterpreted as a gregarious and opportunistic epifaunal suspension-feeding assemblage that inhabited dysaerobic environments within the Appalachian Basin. This reinterpretation is based on detailed paleoecology, petrography, x-radiography, scanning electron microscopy, and organic analyses of the upper portion of the Geneseo Shale. Available evidence indicates a reclining mode of life for Leiorhynchus quadracostatus, with the brachial valve oriented up. Abundant Leiorhynchus occur in aggregates or clumps with a mean density of 14/i2. The zone of abundant Leiorhynchus also correlates with the highest measured weight percent of organics (6.0%o) within the section. The Leiorhynchus are also strongly aligned to a weak current within the basin; mean direction of Leiorhynchus beaks is 420, mean hinge direction is 1220. The presence of a weak current is also supported by small-scale cross-laminations and small starved ripples in silty layers. These suspension-feeding gregarious brachiopods may have obtained some benefit from occupying the fringes of an ancient oxycline. Water-mass stratification makes probable the development and propagation of internal waves and their interaction with sediments at sites where the waves strike the sea floor (Woodrow, 1985). Material suspended by these internal waves may have contained abundant organic matter and bacteria that accumulated on the fringes of the oxycline boundary, creating an ideal situation for gregarious, opportunistic suspension-feeding taxa capable of withstanding lower oxygen levels.

End‐Triassic bivalve extinction: Lombardian Alps, Italy

A major biotic crisis affecting virtually all major marine invertebrate clades occurred at the close of the Triassic. Species‐level data on bivalves from the Lombardian Alps of Italy record the extinction and suggest a possible causal mechanism. A significant decline in species richness is observed during the lower Rhaetian, where 51% of bivalve species, equally distributed among infaunal and epifaunal filter‐feeders, went extinct. The taxonomic loss at the middle Rhaetian was more severe, where 71% of the bivalve species were eliminated, including all infaunal and 50% of the epifaunal species. The data indicate that the extinction selectively eliminated infaunal bivalves. An initial loss of bivalve species richness during the middle and upper Rhaetian correlates with changes in sedimentary facies related to a fall in relative sea level. This sea level fall is marked by the onset of peritidal micrites and shifting ooid shoals which may have rendered substrates unsuitable for both epifaunal and infaunal bi...

Statistical testing of community patterns: Uppermost Hamilton Group, Middle Devonian (New York State: USA)

Abstract We present an extensive and rigorously controlled quantitative paleoecological study within an interval of inferred coordinated stasis. This Middle Devonian Hamilton Group study completes a 20-yr project by providing data within the unresolved upper Hamilton Group section. Together with other rigorously controlled studies, these data sets have the potential to address the larger question of coordinated stasis in the fossil record. We collected data from the Windom Member, Moscow Formation (uppermost Hamilton Group), to test different statistical approaches to define paleocommunities. We evaluate various techniques, including non-parametric multidimensional scaling and agglomerative hierarchical clustering to decipher community patterns. Additionally, we advocate regular use of cluster significance testing along with ANOSIM (i.e. analysis of similarities) when examining ecological data. Together these techniques test the significance of sample groups more rigorously than conventional testing (e.g. discriminant analysis or analysis of variance (ANOVA)). Our results indicate that communities within this upper Hamilton Group interval exhibit variable taxonomic membership within a relatively stable ecological structure.

Does coordinated stasis yield taxonomic and ecologic stability?: Middle Devonian Hamilton Group of central New York

Statistical tests of coordinated stasis within the Middle Devonian Hamilton Group demonstrate significant temporal changes in taxonomic composition and ecological structure of the macrofauna throughout a 5‐6 m.y. time span. The analysis, based upon a collection of .38,000 specimens obtained over a 20 yr period from the Hamilton Group of central New York, used highly controlled sampling techniques, applied within a single, well-defined lithofacies. Assemblages were tested for stability through time, as would be predicted by the model of coordinated stasis. Our results reveal that within at least one major Hamilton environment, taxonomic and ecological stability are not statistically significant and therefore do not support the hypothesis of coordinated stasis.

Late Triassic transition from biogenic to arc sedimentation on the Peninsular terrane: Puale Bay, Alaska Peninsula

The thick (∼700 m) section of Upper Triassic (upper Norian) carbonates, cherts, and volcaniclastics exposed at Puale Bay, Alaska Peninsula, preserves an excellent record of early Mesozoic arc-related sedimentation on the Peninsular Terrane. The Puale Bay sedimentary basin was probably part of a back-arc basin with two stages of evolution: (1) an initial upward-deepening sequence, as reflected in shallow-water carbonates succeeded by spicular chert deposits; and (2) increased magmatic-arc activity and an upward-shallowing sequence near the Triassic-Jurassic boundary, as shown by increased influx of volcaniclastics and deposition of thick, coarse-grained turbidites. This deepening-shallowing cycle of the basin is recorded in kerogen types and in authigenic carbonates that precipitated under marine pore-water influence during the deepening phase, and under meteoric or hydrothermal pore-fluid influence during the upward-shallowing phase. The marine-dominant phase is characterized by types I and II kerogen, δ 13 C (calcite) ratios near 0.0‰ PDB, and Sr values near 700 ppm. The upward-shallowing phase is characterized by type III kerogen, δ 13 C (calcite) ratios with large negative excursions (up to -7.2‰ PDB), and low Sr values down to 33 ppm. The increased volcanic activity, the transition to a terrestrial kerogen source, and the dominance of nonmarine pore fluids near the Triassic-Jurassic boundary are all suggestive of the development of a nearby volcanic arc. This early Mesozoic arc volcanism may reflect closure of ocean basins, between the Peninsular, Wrangellia, and Alexander terranes. Hence, this initiation of arc volcanism may represent an early phase of tectonic amalgamation in southern Alaska.

Biogeographic complexity in Triassic bivalves of the Wallowa terrane, northwestern United States: Oceanic islands, not continents, provide the best analogues.

High levels of endemism and complex, overlapping biogeographic patterns characterize modern molluscan faunas of the Hawaiian Islands and the Triassic bivalve fauna of the Wallowa volcanic-arc terrane in Hells Canyon, Oregon. Such biogeographic complexities and high levels of endemism in many modern and Mesozoic island settings constrain the use of faunal data as a primary basis for paleogeographic reconstruction of accreted terranes. Large, taxonomically diver samples are required to identify genuine biogeographic patterns in these insular settings. Selective use of individual species, genera, or families to reconstruct terrane paleogeography may give misleading results.

Benthic invertebrates of a modern carbonate ramp; a preliminary survey

A preliminary survey of benthic invertebrates off central west Florida provides documentation of modern epifaunal communities on a low-gradient carbonate slope. Three large-scale biofacies occur in soft-sediment carbonate environments between 200 and 2,000 m: an Echinoderm biofacies (200–550 m) dominated by a diverse assemblage of echinoderms, gastropods, and decapod crustaceans; a Penaeid shrimp–conical mound biofacies (550–1,200 m) characterized by large bioturbation structures; and a Microbial mat biofacies (1,200–2,000 m) with only rare epifaunal invertebrates. A fourth, hard-substrate biofacies reflects the presence of localized Miocene and Pleistocene hardgrounds in water depths of 200–600 m. This illustrates that hard-substrate biofacies may be laterally correlative with soft-sediment biofacies in a slope setting, thus producing a mosaic of contrasting faunal associations. All four biofacies have low population densities, presumably as a consequence of relatively low surface productivity. All four biofacies also show biogeographic affinity with other faunas at intermediate depths in the Caribbean region. Depth-related faunal transitions on the west Florida slope correlate with substrate and current velocity. Decreasing species diversity and abundance and a biofacies transition from suspension-feeding to deposit-feeding assemblages correlate with increasing depth, a decrease in mean grain size, and an increase in organic content of the sediment. This biofacies model may provide a modern analogue for faunas of ancient low-gradient slopes such as those of Cretaceous “shelf-sea” chalks of northwestern Europe.

Faunal Succession Within Deep-Water Coral Mounds North of Little Bahama Bank: ABSTRACT

Deep-water coral mounds of 5 to 40 m relief occur at depths of 1,000 to 1,300 m over a 2,500 sq km area of the lower slope north of Little Bahama Bank. These coral/gorgonian buildups, apparently unlithified, have yielded radiocarbon ages of 860 ± 50 and 940 ± 40 years for the best preserved corals and gorgonians, and preliminary dates of 22,100 years for the most intensively bored corals, the youngest deep-water coral mounds ever reported. Eight genera of deep-water coral represent the highest diversity recorded from a single locality. These ahermatypes are predominantly solitary, although branching and weakly branched forms are also present. The colonial ahermatypes from the mounds possess large-diameter corallites and relatively few corallites per specimen. Se eral of the coral general, most notably Thecopsammia, have significant stereomal deposits in the skeleton, a feature common among deep-water corals. The scleractinians are associated with a diverse fauna. The primary framework builders of the mounds, however, appear to be branching corals and gorgonians. Based on the relative amounts of boring and Mn-oxide coating on coral specimens recovered from dredge hauls, there appears to be a crude faunal succession within the mounds. Branching colonial corals and gorgonians seem to be the pioneer forms, colonizing hardgrounds. These initial coral thickets form a baffle for sediment as well as substrates for later stages of attached and free-lying ahermatypes such as Desmophyllum, Stephanocyathus, and Deltocyathus. Thus the mounds grow through a combination of sediment trapping and colonization by a greater diversity of coral and other invertebrates. The coarse nature of intermound sediments and the presence of scour and ripple marks in underwater photographs indicate that bottom currents are vital to the development of these deep-water coral s ructures. End_of_Article - Last_Page 965------------