No abstract

An abundant calcareous fauna has been discovered in the oxygen minimum zone (OMZ; < 0.5 ml/l O2) off central California at oxygen concentrations considerably less than those predicted by previous ecological models. Analysis of box core samples and bottom photographs has revealed a distinct depth zonation of echinoderms. Asteroids and ophiuroids are most abundant along the upper and lower edges of the OMZ, respectively, whereas echinoids are found near the core of this zone where oxygen levels are as low as 0.3 ml/l O2. As these heart urchins are very abundant (14/m2), they represent a potentially significant component of the fossil record of OMZ's. Locally these urchins are capable of disturbing over 90% of near surface sediment. The core of the OMZ is inhabited by hermit crabs that actively transport and recycle gastropod shells. This ‘biotransport’ results in an accumulation of potentially preservable, non‐endemic. hard‐bodied organisms which may lead to misinterpretation of paleo‐oxygenation conditions. We propose an alternative to the Rhoads & Morse (1971, Lethaia 4) biofacies model for open‐ocean, dysaerobic environments which consists of: (1) a zone devoid of maeroinvertebrates. characterized by laminated sediments (<0.1 ml/l O2); (2) a zone dominated by small (1–2 mm) soft‐bodied infauca which exhibits moderate disturbance of laminae due to bioturbation (0.1–0.3 ml/l O2); and (3) a zone inhabited by an abundant calcareous fauna characterized by highly bioturbated sediments (>0.3 ml/l O2).

Quartz grains that appear to have been shock-metamorphosed occur within three closely spaced shale beds from the uppermost Triassic ("Rhaetian") Calcare a Rhaetavicula in the Northern Apennines of Italy. The upper shale coincides with the abrupt termination of the distinctive, uppermost Triassic Rhaetavicula fauna and is overlain by the Hettangian (Lower Jurassic) Calcare Massiccio; no extinctions appear to be associated with the two lower layers, which occur 1.2 and 2.4 meters below the boundary shale. Approximately 5 to 10% of the quartz grains within these layers exhibit one or more sets of planar deformational features whose orientations cluster around the rational crystallographic planes (basal, omega, and pi) most commonly observed in shocked quartz. Textural and stratigraphic observations support an interpretation of at least three closely spaced impacts at the end of the Triassic.

Equatorial faunas of the ancient Tethyan seaway, which extended from western Europe to southeastern Asia, comprise some of the most diverse marine taxa in the fossil record. Comparable or identical "Tethyan" species that occur far from the Tethyan seaway in Paleozoic and Mesozoic rocks of the North and South American Cordillera have long been considered as a major biogeographic anomaly. Two leading theories to account for the occurrence of these anomalous "Tethyan" faunas in the Cordillera are that they were transported long distances to the east on tectonic blocks(suspect terranes that originated near the Tethys) or that they migrated westward via undiscovered marine corridors through continental areas of Pangea. An alternative model is that these "Tethyan" fauna were pantropic species that extended with attenuated diversities into the eastern proto-Pacific Ocean. This pantropic model can better account for the distribution patterns of many Paleozoic and early Mesozoic "Tethyan" species in the American Cordillera and provides a steady state hypothesis against which the other models can be tested. The distribution of pre-Cretaceous "Tethyan" faunas is similar to the known pantropic distribution of many Cretaceous and Cenozoic tropical biotas. During the Cenozoic, taxa were most diverse in the Tethys and Indo-West Pacific regions but extended with attenuated diversity to many parts of the equatorial Pacific Ocean, including the west coasts of North and South America. The eastern Pacific occurrence of many Indo-West Pacific species provides a modern analog for the occurrence of many anomalous "Tethyan" fossils in the American Cordillera.

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 sealevel 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.