Christopher A. McRoberts

A synoptical classification of the Bivalvia (Mollusca)

Preface This classification summarizes the suprageneric taxonomy of the Bivalvia for the upcoming revision of the Bivalvia volumes of the Treatise on Invertebrate Paleontology, Part N.

Palaeoenvironmental interpretation of a Triassic-Jurassic boundary section from Western Austria based on palaeoecological and geochemical data

A section spanning the Triassic-Jurassic boundary is described from near the village of Loruns in the Vorarlberg region of western Austria. At Loruns, the uppermost Triassic is characterised by bedded carbonates of the Kossen Formation supporting a stenotopic fauna indicative of a shallow sub-tidal environment of normal marine salinity. The Triassic-Jurassic boundary may be represented as a sequence boundary developed on top of a 1.1 m thick red mudstone of the lower Schattwald Shale, which is interpreted to have been deposited in a marginal marine environment, possibly a mud flat. Above the boundary beds, the upper Schattwald Shale is characterised by thin-bedded marl and dark limestone beds with an earliest Hettangian macrofauna dominated by epifaunal filter-feeding bivalves, including ostreids, mytilids and oxytomids, which suggest a shallow, subtidal, salinity-controlled environment typical of an interplatform lagoon. Carbonate production rejuvenated in the later Early Hettangian with development of the Loruns oolite, a shallow subtidal oolitic and oncolitic unit bearing echinoderms indicative of normal marine conditions. Low ThU ratios from the remainder of the section are a result of reduced thorium in carbonate-rich sediments and not from authigenic uranium in anoxic sediments. In the boundary beds evidence for marine anoxia (or dysoxia) is absent where ThU values, determined by gamma-ray spectrometry, are above 5. The negative excursion in δ13C and positive excursion in δ18O in the boundary beds may be due to secondary geochemical effects, due to organic diagenesis or the precipitation of caliche during paleosol development. Alternatively, the excursions may reflect a primary geochemical signal recording short-term decline in primary productivity. Comparison in δ18O and δ13C values between the Kossen Formation and Lorfus oolite indicate no significant long-term geochemical changes are evident in the section and suggest that any environmental perturbations were restricted to the boundary beds and possible sequence boundary.

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.

Towards accurate numerical calibration of the Late Triassic: High- precision U-Pb geochronology constraints on the duration of the Rhaetian

Numerical calibration of the Late Triassic stages is arguably the most controversial issue in Mesozoic stratigraphy, despite its importance for assessing mechanisms of environmental perturbations and associated biologic consequences preceding the end-Triassic mass extinction. Here we report new chemical abrasion–isotope dilution– thermal ionization mass spectrometry zircon U-Pb dates for volcanic ash beds within the Aramachay Formation of the Pucara Group in northern Peru that place precise constraints on the age of the Norian- Rhaetian boundary (NRB) and the duration of the Rhaetian. The sampled ash bed–bearing interval is located just above the last occurrence of the bivalve Monotis subcircularis, placing this stratigraphic sequence in the uppermost Norian, perhaps ranging into the earliest Rhaetian. Zircon U-Pb dates of ash beds constrain the deposition age of this interval to be between 205.70 ± 0.15 Ma and 205.30 ± 0.14 Ma, providing precise constraints on the age of the NRB. Combined with previously published zircon U-Pb dates for ash beds bracketing the Triassic-Jurassic boundary, we estimate a duration of 4.14 ± 0.39 m.y. for the Rhaetian. This ends a prolonged controversy about the duration of this stage and has fundamental implications for the rates of paleoenvironmental deterioration that culminated in the end-Triassic mass extinction.

Triassic bivalves and the initial marine Mesozoic revolution: A role for predators?

Marine bivalves document the long-term increase in generic richness through the early Mesozoic. Following the end-Permian crisis, the Early Triassic was marked by a gradual recovery in generic richness (57 Induan and 66 Olenekian genera). Diversity slowly increased in the Middle Triassic (98 Anisian and 121 Ladinian genera) and peaked in the Late Triassic (171 Carnian, 165 Norian, and 143 Rhaetian genera). These data support earlier hypotheses that the recovery following the end-Permian extinction was very gradual and was not completed (in terms of both richness and ecologic complexity) until the Ladinian. Although a Carnian-Norian extinction is not evident in the data and may be a regional event limited to the Tethyan realm, the end-Triassic extinction is profound—fewer than 30 genera ( ,35%) survived into the Jurassic. Diversity metrics are not equally distributed among bivalve living habits. The generally epifaunal Pteriomorphia and Isofilibranchia exhibit higher extinction rates compared to the ordinarily infaunal Heteroconchia (especially the Veneroida and Trigonoida). This pattern of selective extinction led to a gradual increase in generic richness of infaunal suspension feeders through most of the Triassic. Contrary to previous hypotheses, this increase in infaunalization may not have been related to the evolutionary expansion of major predatory groups (e.g., shell-crushing cephalopods, crustaceans, sharks, fish, and reptiles), which had typically low abundances and limited distribution during the Triassic. Drilling predators, although present during the Triassic, are not considered to be prominent causes of mortality among bivalves. Instead, the infaunalization of bivalves during the Triassic may have been due to several interconnected abiotic and biotic causes associated with the recovery after the end-Permian mass extinction.

Biochronology of Triassic bivalves

Abstract Substantial advances by numerous researchers over the past 20 years have made it possible to develop a composite biochronological scheme for the Triassic based on the bivalves Claraia, Peribositria, Enteropleura, Daonella, Halobia, Eomonotis and Monotis. These bivalves exhibit temporal durations nearly equal to ammonoids and conodonts. Widely distributed across the Tethys, Panthalassa and Boreal regions, these bivalves occur in a wide variety of marine facies and water depths, but are most notable for their thick shell accumulation in deeper-water oxygen deficient environments. They were most likely resting or reclining benthos, may have housed chemosymbionts, and were part of episodic opportunistic palaeocommunities in or near oxygen deficient settings. A new biochronological zonation for bivalves is presented that encompasses the entire Triassic and is integrated with standard ammonoid schemes. The Lower Triassic is characterized by 2–3 zones of Claraia, most notably from the eastern Tethys representing the entire Induan and lower portion of the Olenekian. Later in the Olenekian, and most notably from the Boreal realm, species of Peribositria (included by some workers within Bositra) provide useful zonal indexes. The Middle Triassic is well represented by Enteropleura (Middle Anisian) and Daonella (Upper Anisian through Ladinian) in the Tethys and North America with significant occurrences throughout the circum-Pacific and Boreal realms. The Upper Triassic can be subdivided into 8–13 bivalve zones based on the succession of Halobia, Eomonotis and Monotis sensu lato species with best representation in the Tethys, Boreal and eastern Panthalassa regions.

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...

The Triassic-Jurassic ecostratigraphic transition in the Lombardian Alps, Italy

Abstract Three paleoenvironmental phases and two declines in diversity characterize the Late Triassic to Early Jurassic history of the Lombardian Platforms. The first phase, of Late Triassic time (?Choristoceras Zone), consists of 1–5 m thick shallowing-upward subtidal cycles of molluscan, coralline, and echinoderm wackestone and packstone of the Zu Limestone. Biotic and ecostratigraphic characteristics such as typical Rhaetavicula contorta fauna and facies allow correlation to the Kossen Formation of the Northern Calcareous Alps. The second phase, of latest Triassic time (?upper Choristoceras Zone), consists of shallow restricted marine or peritidal carbonates of the Chonchodon Formation dominated by barren lime mudstone and dolostone, algal laminites, and oolitic grainstone. The Zu-Conchodon transition predates the Triassic-Jurassic boundary and represents the first, and most severe, diversity decline for the Lombardian fauna corresponding to a fall in sea-level. Where observed, the upper and lower contacts of the Conchodon Formation are conformable and do not constitute sequence boundaries as suggested by some workers. The Lower Jurassic (?Psiloceras Zone) Sedrina Limestone marks the beginning of the third phase with the onset of transgression and return of normal marine conditions. Typical microfacies include molluscan, echinoderm, and sponge wackestone and packstone with abundant anomuran microcoprolites. The second diversity decline occurred at, or just above, the Triassic-Jurassic boundary at the Conchodon-Sedrina transition, where the remaining restricted marine forms disappeared with the transgression. Anoxia was not a factor in this decline in diversity, although other mechanisms in addition to sea-level change cannot be discounted.

Macrofaunal Response to the End-Triassic Mass Extinction in the West-Tethyan Kössen Basin, Austria

Abstract Bivalves are the most common macrofauna present in marine sequences spanning the end-Triassic mass extinction and document the initial ecological response to the crisis. In the west-Tethyan Kössen Basin, marine bivalves occur within distinctive low diversity episodic shell beds at the time of initial crisis and &dgr;13C minimum, and continue for 1 m (<∼20 kyr) upward into the peak extinction phase devoid of macrofauna. The paleoecology, shell mineralogy, and paleobiogeographic context of these well-preserved bivalves suggest they are part of eurytopic opportunistic paleocommunities flourishing in a time of crisis and are consistent with some, but not all, of the paleoenvironmental scenarios hypothesized in the context of synchronous volcanic activity. The best model-to-data fit is found for an ocean acidification scenario, which predicts an increased extinction risk for taxa with thick calcareous skeletons and aragonite mineralogy due to reduced CaCO3 saturation of seawater for a period of <20 kyr. Other kill mechanisms, such as climatic change and reduced salinity in nearshore marine settings, are less well supported but not fully incompatible with our data and might have acted in concert with ocean acidification.

A primitive Halobia (Bivalvia: Halobioidea) from the Triassic of northeast British Columbia

Abstract Halobia daonellaformis new species is described from the lowermost Carnian of northeast British Columbia. Halobia daonellaformis n. sp. is regarded as a primitive Halobia characterized by external ornamentation similar to Daonella lommeli, but with a poorly developed anterior auricle. Morphologic characters of H. daonellaformis n. sp. suggest that Halobia may be not a natural taxon but a polyphyletic group with one or more ancestors from Daonella and Aparimella and/or other posidoniid(s). The sudden appearance of Halobia throughout the marine Triassic suggests a rapid dispersal mechanism following a Ladinian origin. Larval shell morphology indicates a planktotrophic developmental strategy for H. daonellaformis n. sp., and by extension to other halobiids, which may explain the widespread distribution of many halobiid species.