Abstract
Four arctic species of Liostrea related to the European L. delta (Smith) form a distinct evolving lineage in the Anabar-Khatanga basin during Late Jurassic Early Cretaceous time These are, in ascending order: Liostrea ex. gr. delta (Early Kimmeridgian), L. plastics (Trautschold) (Kimmeridgian), L. praeanabarensis n. sp. (described) (Volgian), and L. anabarensis Bodyl. (Early Cretaceous). Their morphology is reviewed. The lineage is characterized by small, oval to falcate shells with strong posterior curvature, convex left valves, and flat to slightly concave right valves. Widely, subevenly spaced concentric lamellae cover the surface. Some species develop small dorsoposterior auricles. Population analysis of two species demonstrates considerable variability in valve outline and development of the attachment scar and auricle.. Chronologically successive species show up to 10 percent morphologic overlap, demonstrating their genetic relationship. Principal evolutionary trends in progressively Younger species...
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