Palaeoecology of the Triassic black shale bivalve Daonella-new insights into an old controversy

Several modes of life have been proposed for bivalves of the genus Daonella. Taphonomic studies revealed that most taphocoenoses containing lumachellas (shellbeds) and butterflied preserved specimens are allochthonous. Articulated specimens with closed valves and well-orientated sagittal planes are interpreted to be in situ. Autochthonous assemblages show a population structure dominated by subadults and adults. Monospecific and paucispecific taphocoenoses with daonellids are not due to preservational biases but reflect the former biocoenoses. Most species show a dependence on facies. Some are restricted to dysoxic milieus, others are only known from normoxic facies, but all preferred soft, soupy sediments. Morphological examination does not support the presence of byssus gland in adult stages of daonellids, which suggests that they could not be attached to floats. Proposed life habits such as semi-infaunal mud-stickers, epibenthic chemosymbionts, holonekton, holoplankton or byssal attached pseudoplankton conflict with taphonomical constraints, distribution and morphology. Daonellids are herein interpreted as epibethic, pleurothetic bivalves, showing adaptive features to dysoxic milieus like subcircular, flat, thin shelled valves. Daonellids were well adapted to soft, soupy substrates. The use of lighter calcite in place of aragonite, the thin shell and the general form of the shell qualified them to float on soft sediments (snow-shoe strategists).