Early ontogeny of Jurassic hiatellids from a wood-fall association: implications on phylogeny and palaeoecology of Hiatellidae

We provide a detailed description of exceptionally well-preserved specimens of Hiatella (Pseudosaxicava) phaseolus (Eudes-Deslongchamps) from Middle Bathonian (Middle Jurassic) clays of the Częstochowa Ore-bearing Clay Formation at Gnaszyn (southern Poland). The juvenile shell is illustrated for the first time, allowing detailed interpretation of the ontogenetic history of these bivalves. The specimens settled on sunken driftwood that served as an attachment surface and thus acted as a ‘benthic island’ on the otherwise muddy seafloor. A review of the Mesozoic record of the bivalve family Hiatellidae shows that the group appeared in the earliest Jurassic, shortly after the end-Triassic extinction event. The Jurassic species of Hiatella (Pseudosaxicava) do not significantly differ from the Recent species of Hiatella (Hiatella) with regard to major shell characters. However, the absence of hinge teeth and the presence of multiple rows of tiny tubercles on the juvenile dissoconch of Pseudosaxicava clearly distinguish the two subgenera. Although highly variable with regard to morphology and behaviour (boring, nestling or byssal attachment), the Hiatellidae are a conservative group that have not changed significantly since the Mesozoic.