Silurian nuculoid and modiomorphid bivalves from Sweden

The nuculoids and modiomorphids from the Silurian ofSwedenare reviewed with respect to their taxonomy, functional morphology, ecology, and stratigraphical distribution. SeveraI thousands of specimens, in many cases excellently preserved, have been studied. Twenty-one species of nuculoids, ofwhich Praenuculafaba and Ekstadia kellyi are new, assigned to 10 genera, are referred to the families Praenuculidae and Nuculidae ofthe Nuculacea; Malletiidae and Nuculanidae ofthe Nuculanacea. The extensive nuculoid material available enabled ontogeny and intraspecific variation to be recorded for some of the taxa. Three nuculoid associations have been studied, in which tiering and life habits ofthe different speeies have been reconstructed. The nuculoid species have a stratigraphical range from the uppermost Llandovery to uppermost Ludlow. Some 25 modiomorphidspeeieshave been recognized, ofwhich 18 arenew, belonging to 7 genera, ofwhich Aleodonta and Mimerodonta are new. The family Modiomorphidae is subdivided into the new subfamilies Modiomorphinae and Modiolopsinae. The speeies recorded exhibit a muscular morphology pattern intermediate between those ofhighly effective burrowers and Recent myti- lids. Most speeies also show indications of byssal attachment. Thus, it appears as if the Silurian modiomorphids examined retained the ability of effective burrowing. Several of them also had a byssus for maintaining a stable life position. These bivalves have been found in various types of sediments, from fine-grained limestones and siltstones to coarse-grained oolitic limestones and sandstones, indicating a variety of habitats. The speeies have a stratigraphical range from the uppermost Llandovery to Pridoli. Three of the seven genera seem to have been endemie to Baltoseandia in Silurian times.