Late Jurassic and early Cretaceous Ostreidae in Arctic seas of Siberia

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 are 1) reduction of attachment scar size, and presumably of duration of attachment during ornogeny; 2) elongation and upward curving of the posterior shell margin — change from oval to falcate outline; 3) development and subsequent loss of a dorsoposterior auricle. Elongation of the valve takes place only with reduction of attachment scar size in all species. These trends are adaptations to changing marine environments within the Anabar- Khatanga basin after its geographic and environmental restriction during the Late Jurassic. The lineage appears endemic to the basin. The adaptive value of many morphologic features is discussed at length.—E. J. Kauffman.