The morphology and relationships of Thuja polaris sp.nov. (Cupressaceae) from the early Tertiary, Ellesmere Island, Arctic Canada

Abstract

Fossil foliage and seed cones of Thuja (Cupressaceae) have been discovered in early Tertiary (Paleocene) sediments of the Eureka Sound Group on Ellesmere Island, Canadian Arctic Archipelago. Vegetative remains of the fossil species, Thuja polaris sp.nov., bear alternately branched, moderately divided, flattened, and pinnatelike sprays with scale-like, decussate leaves. Seed cones are oblong, bearing 8 – 9 pairs of thin, probably leathery cone scales with distinct, reflexed umbos. Fossil cones and foliage resemble closely those of extant Thuja plicata. However, fossil seed cones have twice as many pairs of scales as do extant species of Thuja. A review of the fossil record indicates that most Thuja-like vegetative remains which have been reported from Cretaceous and Tertiary deposits in the Northern Hemisphere are best assigned to form taxa and not to extant genera. Reproductive material from four Tertiary localities can be assigned to Thuja on the basis of seed cone structure. The evolutionary history of the genus, based on fossil and extant seed cone morphology, appears to include a reduction in the number of cone scales. Extant species form a closely related, natural group and, with the exception of T. sutchuenensis, may have arisen from an ancestor similar to T. polaris. Although Thuja was widespread in the Northern Hemisphere during much of the Tertiary, the genus is now confined to northeastern and northwestern North America, and to Japan, Korea, and central China.