Michael Hautmann

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.

Catastrophic ocean acidification at the Triassic-Jurassic boundary

Palaeobotanical and geochemical evidence indicate a sudden rise in atmospheric carbon dioxide (CO2) across the Triassic-Jurassic boundary, probably reflecting the combined effect of extensive volcanic degassing and thermal dissociation of marine gas hydrates. Using carbon isotopes as a geochemical marker, we found that the onset of the CO2 emissions coincided with an inter- ruption of carbonate sedimentation in palaeogeographically distant regions, suggesting that hydro- lysis of CO2 led to a short but substantial decrease of seawater pH that slowed down or inhibited precipitation of calcium carbonate minerals. The cessation of carbonate sedimentation correlates with a major marine extinction event, which especially affected organisms with aragonitic or high-Mg calcitic skeletons and little physiological control of biocalcification. These findings strengthen current concerns that ocean acidification from industrial CO2 release threatens biotopes that are dominated by such organisms, in particular tropical reef systems.

Taxonomy and phylogeny of cementing Triassic bivalves (families Prospondylidae, Plicatulidae, Dimyidae and Ostreidae)

Based on new material from the Upper Triassic Nayband Formation of east-central Iran and on type material from the Alpine Triassic, the taxonomy of the cementing bivalve families Prospondylidae, Plicatulidae, Dimyidae and Ostreidae is examined and their phylogenetic relations are discussed. The Prospondylidae are characterized by the presence of an early pectiniform stage in their Palaeozoic genera which disappeared in most later forms due to ontogenetic pre-displacement of cementation. The Plicatulidae probably evolved from an ancestor within the Prospondylidae by the formation of strong crura, which allowed the reduction of the lateral part of the ligament. Their hinge was later modified by shifting resilifer and crura in a ventral direction and by forming a secondary ligament dorsally. The emended genera EoplicatulaPseudoplacunopsis represent different early stages of this development. The species Eoplicatula parvadehensis sp. nov. and Pseudoplacunopsis asymmetrica sp. nov. from the Nayband Formation are described. The shell of some early Ostreidae is characterized by the lack of structural chambers and by the presence of an originally aragonitic inner shell layer. For such forms, the new genus Umbrostrea is proposed, and the new species Umbrostrea emamiiUmbrostrea iranica are described. The available data on shell microstructure as well as most conchological characters do not support a close relationship between Ostreidae, Plicatulidae and Dimyidae.

Early Mesozoic evolution of alivincular bivalve ligaments and its implications for the timing of the ‘Mesozoic marine revolution’

The early Mesozoic radiation of the Pteriomorphia was accompanied and furthered by the development of several new types of alivincular ligaments. These new types evolved as modifications of the primitive alivincular-areate (new term) ligament, which is characterized by an ontogenetic shift of both the central resilium and the straight lateral ligament in the direction of main shell growth. Arching of the attachment surface of the ligament led to the alivincular-arcuate (new term) ligament type, which has been realized by the Ostreidae only. By contrast, a replacement of the lateral ligament by hinge teeth, limiting the (primary) ligament to a central groove (alivincular-fossate, new term), has evolved independently in three families (Dimyidae, Plicatulidae and Spondylidae). Functionally, both kinds of modification effectively impede shearing of the valves and are interpreted as an antipredatory adaptation advantageous in the cemented habit of these families. The alivincular-alate (new term) ligament of the Entoliidae and Pectinidae differs from the other types of alivincular ligaments by different growth directions of resilium and lateral ligament, which result in an internal position of the resilium suitable for fast and powerful opening of the valves. This arrangement is an important prerequisite for effective swimming, which, in its turn, is a behaviour chiefly used to escape from predator attacks. The simultaneous early Mesozoic appearance of different antipredatory adaptations within independent clades hints at increased predator pressure as a stimulant and may therefore point to a contemporaneous proliferation of durophagous predators. Hence, an important aspect of the ‘Mesozoic marine revolution’ might have started earlier than previously thought.

Palaeoecology of the Spathian Virgin Formation (Utah, USA) and Its Implications for the Early Triassic Recovery

The Spathian (late Early Triassic) Virgin Formation of south-western Utah (USA) yields a comparatively diverse benthic fauna that flourished ∼2 Ma after the end-Permian mass extinction. In this study, we present quantitative palaeoecological data, which are analysed in the context of depositional environments. This integrated approach helps to discriminate between effects of the end-Permian mass extinction event and local environmental factors on alpha diversity and ecological structure of the Virgin Fauna. Shallow subtidal environments yield the highest species richness and lowest dominance values as recorded in two benthic associations, the Eumorphotis ericius Association and the Protogusarella smithi Association, both of which contain 20 benthic species (bivalves, gastropods, brachiopods, echinoderms, and porifers). Tidal inlet deposits yield a low diverse fauna (Piarorhynchella triassica Association) with a very high dominance of filter feeders adapted to high energy conditions. Another comparably low diverse fauna is recorded by the Bakevellia exporrecta Association, which occurs in deposits of the offshore transition zone, most likely reflecting unconsolidated substrates. A single sample containing five bivalve species (Bakevellia costata Assemblage) is recorded from a marginal-marine setting. The Virgin fauna yields a bulk diversity of 30 benthic species (22 genera) of body fossils and 14 ichnogenera and, thus, represents the most diverse marine bottom fauna known so far from the Early Triassic. Our results suggest that oceanographic conditions during the early Spathian enabled ecosystems to rediversify without major abiotic limitations. However, taxonomical differentiation between habitats was still low, indicating a time lag between increasing within-habitat diversity (alpha diversity) and the onset of taxonomical differentiation between habitats (beta diversity). We suggest that taxonomical habitat differentiation after mass extinction events starts only when within-habitat competition exceeds a certain threshold, which was not yet reached in the Spathian of the investigated area. This interpretation is an alternative to previous suggestions that the prevalence of generalistic taxa in the aftermath of mass extinction events reflects protracted environmental stress. The onset of increasing beta diversity is a potential criterion for distinguishing two major recovery phases, the first ending with habitat saturation and the second ending with the completion of ecosystem differentiation.

A New Paleoecological Look at the Dinwoody Formation (Lower Triassic, Western USA): Intrinsic Versus Extrinsic Controls on Ecosystem Recovery After the End-Permian Mass Extinction

Abstract The Dinwoody Formation of the western United States represents an important archive of Early Triassic ecosystems in the immediate aftermath of the end-Permian mass extinction. We present a systematic description and a quantitative paleoecological analysis of its benthic faunas in order to reconstruct benthic associations and to explore the temporal and spatial variations of diversity, ecological structure and taxonomic composition throughout the earliest Triassic of the western United States. A total of 15 bivalve species, two gastropod species, and two brachiopod species are recognized in the study area. The paleoecological analysis shows that the oldest Dinwoody communities are characterized by low diversity, low ecological complexity and high dominance of few species. We suggest that this low diversity most likely reflects the consequences of the mass extinction in the first place and not necessarily the persistence of environmental stress. Whereas this diversity pattern persists into younger strata of the Dinwoody Formation in outer shelf environments, an increase in richness, evenness and guild diversity occurred around the Griesbachian–Dienerian boundary in more shallow marine habitats. This incipient recovery towards the end of the Griesbachian is in accordance with observations from other regions and thus probably represents an interregional signal. In contrast to increasing richness within communities (alpha-diversity), beta-diversity remained low during the Griesbachian and Dienerian in the study area. This low beta-diversity reflects a wide environmental and geographical range of taxa during the earliest Triassic, indicating that the increase of within-habitat diversity has not yet led to significant competitive exclusion. We hypothesize that the well-known prevalence of generalized taxa in post-extinction faunas is primarily an effect of reduced competition that allows species to exist through the full range of their fundamental niches, rather than being caused by unusual and uniform environmental stress.

Bivalves from the Olenekian (Early Triassic) of south-western Utah: systematics and evolutionary significance

The recovery from the end-Permian mass extinction event was a key interval in the history of life, but few modern studies provide systematic data on benthic marine faunas from the epoch immediately following the crisis. Here, the bivalve fauna from the early Spathian (Olenekian, late Early Triassic) Virgin Limestone Member of the Moenkopi Formation is comprehensively documented for the first time. The new genus Sementiconcha (Myophoricardiidae), type species Sementiconcha recuperator sp. nov., and the new species Leptochondria nuetzeli, Eumorphotis ericius, E. virginensis and Pleuromya prima, are described. Leptochondriidae is placed in synonymy with Asoellidae, which is revised. With 27 species belonging to 18 genera, the Virgin Limestone Member records the highest bivalve diversity reported so far from this time interval, questioning previous claims that the recovery from the end-Permian mass extinction was delayed until the Middle Triassic. The two bivalve subclasses (Pteriomorphia and Heteroconchia) that are present in the Virgin Limestone Member clearly differ in their evolutionary contexts. Pteriomorphs of the Virgin Limestone are nearly exclusively composed of genera that survived the end-Permian mass extinction event, whereas heteroconchs are highly dominated by genera that evolved in the Early Triassic. This contrasting evolutionary background probably reflects differential effects of the end-Permian mass extinction event and subsequent crises on these two subclasses, possibly related to differences in filter feeding efficiency and shell mineralogy. The high proportion of infaunal heteroconchs, including deep-infaunal Pholodomyoida, is an additional indicator of a relatively advanced recovery stage, further corroborating that recovery of benthic organisms was well underway during the late Early Triassic. http://zoobank.org/urn:lsid:zoobank.org:pub:EC9802EA-F1D5-4066-A608-2B8DECE7C10D

Extinction-Recovery Pattern of Level-Bottom Faunas Across the Triassic-Jurassic Boundary in Tibet: Implications for Potential Killing Mechanisms

Abstract Scarcity of fossiliferous boundary sections makes the Triassic-Jurassic biotic crisis the most enigmatic of the five major Phanerozoic faunal turnovers. We report a bivalve-dominated level-bottom fauna from the Triassic-Jurassic transition in southern Tibet, which is unique for two reasons: (1) it documents the faunal turnover across the system boundary without major facies changes, and (2) it provides paleobiological data from the immediate postextinction interval. In the extinction pattern, selectivity against burrowing suspension feeders and taxa with completely aragonitic shells emerges, but no major clades or ecological groups disappeared. The earliest postextinction fauna differs in three significant ways from typical survival faunas of other mass extinction events: (1) it was surprisingly diverse, (2) it was heterogeneous (Simpson D = 0.1078) without dominance of disaster taxa or opportunists, and (3) there was a prevalence of highly specialized, morphologically complex forms reaching normal growth sizes. In spite of these unusual features, extinction of typical Triassic taxa occurred comparatively sharply within the underlying beds. These findings support scenarios of relatively short environmental disturbances triggered by the volcanic activity of the Central Atlantic Magmatic Province, which led to rapid extinction of taxa but did not impose ongoing environmental stress on the survival fauna. The nearly instantaneous recovery of level-bottom faunas is in sharp contrast to a prolonged reef eclipse, which probably indicates both higher extinction rates of reef-organisms and intrinsic limitations to the tempo of recovery due to their high level of co-evolution.

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.

Climatic oscillations at the onset of the Mesozoic inferred from palynological records from the North Indian Margin

The beginning of the Mesozoic, the Early Triassic, is characterized by several ecological perturbations following the end-Permian mass extinction. They are reflected in multiple C-isotope excursions coupled with climatic changes. Here we present palynological data from two accurately dated sections from the North Indian Margin (Pakistan and South Tibet). The climate of the Early Triassic was controlled by persistent monsoon circulation. The spore/pollen ratios, used as a proxy for humidity changes, indicate several significant climatic changes coinciding with C-isotope excursions. Comparison with published climate model simulations reveals that the climatic shifts were induced by orbital forcing and probably represent eccentricity cycles. Humidity peaks indicate an insolation forced shift of the intertropical convergence zone towards the North Indian Margin. Comparison with palynological data from Norway and other proxies reveal that the profound climatic change from humid to drier climate across the Smithian–Spathian boundary represents a global event, which affected southern and northern mid-latitudes and coincided with major ammonoid and conodont extinction events. This implies that increased greenhouse gas concentrations owing to recurring volcanic pulses increased the climate system sensitivity, resulting in climatic changes in distant parts of the world. Our data strongly support a link between C-isotope excursions, climatic changes and biotic responses.