Fossil cedar foliage of theCupressinocladus interruptustype, with associated seeds and cones, is locally abundant in Paleocene deposits of the Ravenscrag Formation, southwestern Saskatchewan, Canada. Vegetative remains of this type occur frequently in early Tertiary plant assemblages throughout the northern hemisphere, indicating that this now extinct cedar was once widespread. For the first time this cedar can be described on the basis of both vegetative and reproductive morphology. Foliage is frond-like with a characteristic opposite branching pattern. Seed cones are globose and woody and bear four equal and decussate scales with prominent umbos. Seeds bear large, equal, semicircular wings. The fossil cedar appears most closely related to extant Cupressaceae such asThuja,Chamaecyparis, andHeyderia. Foliage closely resembles that ofThuja, while cones are most similar to those ofChamaecyparis. The fossil differs sufficiently in foliage and seed cone structure to preclude assignment to an extant genus and is here assigned toMesocyparis borealisgen. et sp. nov. Similarities among such extant genera asThuja,Chamaecyparis,Heyderia, andThujopsisand the fossilMesocyparis borealissuggest that all may belong to a single natural group. Furthermore, this group may be more closely related to the southern hemispheric generaLibocedrus,Papuacedrus, andAustrocedrusthan present classification schemes imply. Our examination of the Cupressaceae indicates that a revision of present systems of classification is required to accommodate evidence from both extant and extinct cedars.
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