Biochronology of Triassic bivalves

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. Following the end-Permian mass extinction, benthic marine ecosystems underwent a remarkable reorganization associated with the post-extinction recovery and subsequent adaptive radiation during the Mesozoic. Among shelly benthos, this reorganization resulted in the diminishment of typical Late Palaeozoic brachiopod, pelmatozoan, and stenolaematid bryozoan dominated level-bottom marine communities in favor of communities characterized by bivalve and gastropod molluscs, bony fishes, gymnolaemate bryozoans, echinoids and certain crustaceans, representing an increase in trophic variety ranging from deep infaunal suspension feeders to active nektonic carnivores (Vermeij 1977; Sepkoski 1981). A significant component of this essentially modern fauna are bivalve molluscs, which in the Triassic dominate many level-bottom, reefal, and pelagic settings (McRoberts 2001; Fraiser & Bottjer 2007). Among the bivalves of the Triassic radiation, those belonging to the genera Claraia, Peribositria, Enteropleura, Daonella, Aparimella, Halobia and Monotis, are arguably the most abundant macrofauna in deep-water marine facies of the Triassic (Fig. 1). Often referred to as ‘flat clams’ because of their very thin shell width and narrow valve convexity, these bivalves generally have widespread distributions and very high species turnover rates making them excellent biochronological macrofossils of the Triassic. Their biochronological value was recognized soon after their discovery, and within the past 30 years they have become standard components of biochronological schemes (e.g. De Capoa Bonardi 1984; Polubotko 1986; McRoberts 1997; Zakharov et al. 1997; Yin 2003). Even though bivalve zonations have been provided for specific regions and or limited portions of the Triassic, this paper provides the first global summary of bivalve zonation for the entire Triassic (Figs 1, 3, 5, 6). Taxonomic and nomenclature issues Perhaps the largest problem with the use of Triassic bivalves in biochronological studies is a combination of the multitude of available species names, many of which are in open nomenclature, and a high degree of uncertainty in species and higher taxonomy of this heterogeneous grouping of bivalves. These issues are due to several factors including: poor understanding of significant morphological traits and their variation in natural populations, their frequently poor preservation resulting in a misinterpretation of such traits, and palaeogeographical/palaeooceanographical provincialism that is in many cases unrecognized. As construed herein, Triassic ‘flat clams’ represent a morphologically and taxonomically diverse group belonging to at least three separate bivalve From: LUCAS, S. G. (ed.) The Triassic Timescale. Geological Society, London, Special Publications, 334, 201–219. DOI: 10.1144/SP334.9 0305-8719/10/$15.00 # The Geological Society of London 2010. clades distributed across four family-group taxa: the Posidonidae, Halobiidae, Monotidae and Pterinopectinidae. Further compounding the taxonomic clutter has been the introduction of lower-level groups (genera and subgenera) which have proliferated in recent years but which have often been constructed on limited morphological evidence without regard to natural morphological variation within species or populations or phylogenetitc considerations. The primary genus-group taxa used here are broadly construed (sensu lato) and include Claraia, Ellesmerella, Enteropleura, Daonella, Halobia, Eomonotis, and Monotis. Brief summaries of these accepted taxa and comments on group membership and affinities are provided in Appendix 1 and representative species are illustrated in Figures 4 and 7. The sheer number of species in some of these genera is another subject of concern. For example, following Mojsisovics’ (1874) impressive monograph, Kittl (1912) alone recognized 66 species of Daonella and 116 species of Halobia. More recent workers have not fared much better and the pace of Fig. 1. Temporal distribution of Triassic flat clam bivalve genera with biochronological utility. C. A. MCROBERTS 202