FIRST REPORT OF THE FOSSIL WOOD GENUS PICEOXYLON FROM THE NORTH AMERICAN JURASSIC (MORRISON FORMATION, CENTRAL MONTANA)

Piceoxylon Gothan is a fossil wood first described from the Paleogene of California, USA. This genus is also reported from the Jurassic (France) and Early Cretaceous (Canada, NW Europe, Russian Arctic and Asia). Piceoxylon is characterized by gradual early-latewood transition, abietinian radial pitting composed of uniseriate to biseriate round bordered pits, Abietineentüpfelung (rounded pits occurring on the end walls of ray cells), cross-fields with 1-4 pits, fusiform rays, and small-diameter axial resin ducts lined with epithelial cells. The axial resin ducts are generally isolated but may occur in tangentially aligned clusters.

A tracheidoxyl with pith assignable to Piceoxylon was discovered in the Upper Jurassic Morrison Formation 25 meters stratigraphically above the underlying Swift Formation. In radial section, the majority of bordered pits are uniseriate, uniformly divided between round and noncontiguous to slightly obround and contiguous. Biseriate pits are arranged with either opposite or alternate spacing. Cross-field pits are cupressoid. The tangential section displays fusiform rays that are uniseriate, biseriate, or triseriate. Triseriate rays are characterized by horizontal resin ducts with epithelial cells. Axial resin ducts are uncommon, and clusters may signify trauma. This wood is most similar to Piceoxylon antiquius Gothan but tyloses are absent in both axial and horizontal resin ducts.

Piceoxylon is grouped by Oh et al. (2015) as belonging to the Protocedroxylon-group, a collection of closely allied genera exhibiting Abietineentüpfelung that have affinities to Pinaceae. In terms of climate reconstruction, members of this group have boreal distributions and very frequently co-occur with Xenoxylon Gothan. The distribution and paleoecological contexts of Xenoxylon suggest a strong affinity for cool/wet climates. We (Richmond et al., 2017) discovered the fossil wood X. meisteri stratigraphically ~20 meters above Piceoxylon. The co-occurrence of Piceoxylon and Xenoxylon is suggestive of a cool/wet climate for a period of local Morrison deposition. Resolving the temporal separation between specimens is of great interest. Future study will determine whether these specimens indicate independent paleoclimate intervals or a prolonged paleoclimatic regime.