Ditaxocladus (extinct Cupressaceae, Cupressoideae) from the Upper Cretaceous and Paleocene of the Northern Hemisphere

An emended diagnosis of the extinct cupressoid genus Ditaxocladus S. X. GUO & Z. H. SUN (syn. Fokieniopsis MCIVER & BASINGER) and its three species is proposed based on new collections from the type locality in the Paleocene Wulonggu Formation of Altai in NW China, and other sites from China, SE Russia and western North America. Ditaxocladus planiphyllus S. X. GUO & Z. H. SUN (generitype) is now known from compound racemose fertile branches bearing several opposite pairs of globose to sub-globose seed cones composed of 6-12 decussate peltate scales, in addition to typically oppositely branched sprays. Besides D. planiphyllus, two more species are recognized based on differences in seed cone morphology. D. kivdensis KODRUL in KRASSILOV et al. from the Paleocene of Raichikha in Amur Province, Russia, is distinguished by similar but slender and pendulous cones, while D. catenulatus (W. A. BELL) S. X. GUO, KVACEK, MANCHESTER & Z. K. ZHOU comb. n. (syn. Fokieniopsis catenulata (W. A. BELL) MCIVER & BASINGER and Fokienia ravenscragensis MCIVER & BASINGER) typified by early Paleocene records in North America bears ovoid seed cones with a higher number of cone scales. All three species share the same type of bipinnately and oppositely ramified elongate sprays of cladode-like foliage and pinnately and oppositely arranged seed cones at successive nodes on compound racemose fertile branches. They differ from the living Fokienia, with which they were formerly compared, in oppositely (vs. alternately) branched leafy sprays and oppositely clustered (vs. solitary) seed cones consisting of much fewer cone scales (6-12 vs. 12-18 in Fokienia). Sterile foliage attributed to Ditaxocladus occurs in the Upper Cretaceous to Paleocene of North America, Russia and Paleocene of Spitsbergen. Tracing differentiation of Ditaxocladus populations (or species) over the Northern Hemisphere is hampered by incompleteness of most records based on quite uniform foliage without seed cones. Tetraclinis salicornioides (UNGER) KVACEK from the Eocene to Pliocene of Europe and the Oligocene to Miocene of North America, which shares very similar gross morphology of foliage, differs from Ditaxocladus in the quadrivalvate seed cones and papillate leaf epidermis with undulate anticlinal walls.