Morphology and relationships of Mesocyparis umbonata sp.nov.: fossil Cupressaceae from the Late Cretaceous of Alberta, Canada

Abstract

Late Cretaceous silicified Cupressaceae remains, recently recovered from the Horseshoe Canyon Formation of Alberta, Canada, include three-dimensional leafy twigs of the Cupressinocladus interruptus type with attached seed cones, seeds, pollen cones, pollen, and woody stems. The abundance and excellent preservation of the remains allows detailed description of most aspects of the plant, including foliage, reproductive organs, epidermal features, and wood. Foliage of the fossil is frondlike and flattened, with an opposite, less commonly alternate, branching pattern. Seed cones, borne in decussate pairs, are globose, woody, about 4.0 mm in diameter, and bear four approximately equal-sized cone scales with small umbos near the scale center. Each seed has two broad, semicircular wings. Pollen cones are typical for the Cupressaceae; pollen grains are structurally comparable, but small, for the family. Fertile branches and seed cones are not identical to any extant member of the Cupressaceae, but they are sufficiently similar to those of the Paleocene cedar Mesocyparis borealis from Western Canada to be referred to that genus. Dissimilarities justify assignment to Mesocyparis umbonata sp.nov. Seed cone structure of Mesocyparis umbonata resembles most closely that of the northern hemispheric taxon Chamaecyparis nootkatensis; the leaves resemble those of Thuja and Chamaecyparis. Other features, including the number of seed cone scales, branching pattern, and wood anatomy, are more comparable to those of taxa of the southern hemispheric Libocedreae. New information, based on these remains, indicates that the relationship between some northern and southern hemispheric taxa is closer than previously proposed, supporting the premise that present systems of classification separating these taxa do not reflect phylogeny. Key words: Cupressaceae, Mesocyparis, Late Cretaceous, fossil, seed cones, evolution, phylogeny.