CIRCOPOROXYLON FROM THE UPPER JURASSIC MORRISON FORMATION OF CENTRAL MONTANA

Two specimens of the fossil wood Circoporoxylon were discovered in the Upper Jurassic Morrison Formation of central Montana. The first specimen (TUFA1) consists of a partial log incorporated in a Morrison-aged tufa deposit on the southern flank of Spindletop Dome, a Laramide anticline. The second specimen, a tracheidoxyl (PW14), is located 2.75 kilometers to the northwest. TUFA1 is stratigraphically 46 meters above the underlying Swift Formation and PW14 is 44 meters stratigraphically above the same formation.

In transverse section growth rings are expressed as a gradual transition from earlywood to latewood. The latewood bands are narrow and separated by numerous false rings. Earlywood tracheid cells are squarish polygons but are round or obround in latewood. Tracheids in earlywood have an average perimeter of 146 μm with an average cell area of 1294 μm². The tracheid cells of latewood have an average perimeter of 65 μm and an average area of 225 μm². Latewood cells display a reduction in perimeter of 54% and area of 82%. Axial parenchyma is present distributed throughout the earlywood. In tangential section, medullary rays are slightly heterocellular, uniseriate, occasionally biseriate never triseriate. Specimen TUFA1 ray heights vary 1‒20 cells whereas, PW14 ray heights vary 1‒52 cells. Bordered pits on the tracheid tangential walls are uniseriate, separate, and circular. In radial section, the pitting on the tracheid radial walls is abietinian pitting composed of uniseriate (66%) to biseriate (34%) round to elliptical bordered pits. The median bordered pit width is 17.9 μm with a median height of 15.0 μm yielding a median height/width (h/w) ratio of 0.84. Cross-field pitting consists of 1‒4 circopores per cross-field. These features are equivalent to those of woods assigned to Circoporoxylon (Kräusel, 1949).

Circoporoxylon likely had an affinity for temperate climates, however, is stratigraphically interspersed with several specimens of Xenoxylon (Richmond et al., 2017) and has a close stratigraphic proximity to Piceoxylon (Richmond et al., 2019). Both boreal woods Xenoxylon and Piceoxylon suggest a cool wet climate for the Morrison Formation in central Montana. Numerous growth rings and false rings suggest Circoporoxylon may have been susceptible to variable climatic conditions at higher latitudes.