THE FIRST FOSSIL WOODS FROM THE UPPER JURASSIC MORRISON FORMATION OF WESTERN OKLAHOMA

Three fossil wood genera from the Upper Jurassic Morrison Formation were discovered in western Oklahoma. These woods are the first reported from Oklahoma for the formation. The stratigraphic position of the specimens is 30 m below the overlying Lower Cretaceous Cheyenne Sandstone.

The first fossil wood, Xenoxylon meisteri, is the third representation from the Morrison Formation in North America (Richmond et al., 2017). Growth rings exhibit an abrupt transition. Rays are uniseriate with a median ray height of 168 μm and a median cell count of 8. Bordered pits are contiguous and generally obround, although many are circular. The median height is 17.3 μm and the median width is 25.5 μm, yielding a height/width (h/w) ratio of 0.68. Cross-field pitting is fenestriform with a single large oopore. Xenoxylon grows generally in cool/wet climates. A southward extension of the genus may be related to climatic fluctuation.

The second fossil wood is Cupressinoxylon sp. Growth rings exhibit a gradual transition of 10-12 cells. Parenchyma cells are scattered throughout the growth rings and are resiniferous. Rays are uniseriate, occasionally biseriate. Median ray height is 85 μm with a median cell count of 5. Bordered pits are circular with a h/w ratio of 0.91. Pits are noncontiguous, separated by a median distance of 4 μm. The cross-field is cupressoid with a single, small oculipore per field. Some rings display disrupted growth, strongly suggestive of damage due to a surface fire. Extant Cupressaceae reside primarily in warm temperate regions.

A third fossil wood, Agathoxylon sp., is rare in the Morrison foreland basin. Growth rings exhibit a gradual transition of 10-11 cells. Parenchyma cells are absent. Rays are uniseriate with a median ray height of 118 μm and a median cell count of 6. Bordered pits are either contiguous uniseriate or araucarian pentagonal biseriate alternate pits. The median h/w ratio is 0.85. The cross-field is cupressoid with 4-6 alternate contiguous oculipores per field. This genus represents a lower latitude subtropical Mesozoic flora.

Due to their differing climatic preferences, Xenoxylon, Cupressinoxylon, and Agathoxylon are rarely associated in the fossil record. The occurrence of the three genera suggests the southeastern portion of the Morrison basin transitioned to a wetter climate during the Late Jurassic.