Daonella

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 through- out 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.

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.

Abstract: Introductory. The following abstract gives the latest results of the examination of collections of fossils, chiefly made by the United States Geological Survey, which have passed through my hands within the past few years. The descriptions of the species mentioned and of others previously noted in my paper, “ Jura and Trias at Taylorville, California,”* will be published by the United States Geological Survey when the proper illustrations can be prepared. American and Sailors Canyons, California. THEIR FAUNAS. Monotis Beds .—The presence of the Trias above the Carboniferous in American and Sailors canyon south of Cisco was at first considered certain from the presence of the Monotis beds containing two new species, M. semiplicata and symmetrica , but further information is essential before this can be admitted. According to the observations of Dr Curtice these beds lie between the Carboniferous and the Daonella beds. Daonella Beds .—The Daonella beds contain several . . .