Evolutionary diversification of taiwanioid conifers: evidence from a new Upper Cretaceous seed cone from Hokkaido, Japan

A single cylindrical seed cone 2 cm long, 1.1 cm wide has been found preserved in a calcium carbonate marine concretion from the Hakobuchi Formation (late Campanian-early Maastrichtian) of Hobetsu, Hokkaido, Japan. The cone, attached to a bent peduncle lacking leaves, has helically arranged bract/scale complexes that arise at right angles from the cone axis in the middle of the cone. The cone axis, ca. 1 mm wide, has a broad cylinder of secondary vascular tissue, and lacks a continuous resin canal system. Bract-scale complexes are laminar, cordate-orbiculate, and upturned distally, consisting primarily of bract tissue with no visible scale tip. The vascular trace to the bract/scale complex originates as a rod that divides laterally into several traces at the level of seed attachment. A single resin canal originates at the base of the bract-scale complex abaxial to the vascular strand, but more distally there are up to ca. 15 large resin canals that form a single row. Two to three inverted winged seeds are attached adaxially near the cone periphery. Cone structure and vascularization are most similar to those in the Cupressaceae, Subfamily Taiwanioideae, differing from living Taiwania cryptomerioides by having up to three seeds/scale rather than two, an abruptly upturned bract tip, in details of bract/scale vasculature, and a cone peduncle lacking leaves. This cone is described as Mukawastrobus satoi Stockey, Nishida and Rothwell. Together with previously described Early to Late Cretaceous taiwanioid seed cones from Mongolia and Hokkaido the new species demonstrates that the taxonomically diagnostic characters of such conifers are as subtle as those of Cretaceous and Cenozoic sequoioid Cupressaceae. This realization emphasizes that evolutionary diversification and turnover among taiwanioid conifers during the Cretaceous and Paleogene are probably far greater than currently recognized.

Keywords: Anatomy; Conifer seed cone; Cupressaceae; Hokkaido; Late Cretaceous; Taiwanioid diversity.