Taxonomic and paleoclimatic significance of Late Cretaceous wood from the San Juan Basin, New Mexico.

Fossil wood has been demonstrated to be a powerful tool for studying past environments and biotas. Anatomical structure of silicified woods from Late Cretaceous sediments in northwestern New Mexico was used to conduct a taxonomic survey of plants with secondary growth and tree stature, as well as estimate paleoclimate. A total of 50 specimens compromising both gymnospermous and angiospermous remains were surveyed from deposits within the upper Fruitland and lower Kirtland formations of the San Juan Basin. Anatomical characters were used to designate distinct xylotypes, which were then linked to modern orders or families. Paleoclimate was estimated using three quantitative methods: the Mean Sensitivity statistic (MS), the Vulnerability Index (VI), and regression equations that link eudicot/magnoliid wood to mean annual temperature (MAT). The taxonomic results indicate an abundant but low diversity coniferous flora and less abundant by higher diversity angiosperm flora with xylotypes that have potential links to eight modern orders. These orders ranged from ancestral to derived within the angiosperm phylogeny, and two of the xylotpes represented new records of taxa. The paleoclimate results confirmed a warm, wet environment for the Late Cretaceous. VI values were comparable to modern day tropical trees, implying that water was not a limited resource in the floodplain. The regression equations generated an average MAT of 23.13 ± 5.22oC, a value statistically similar to MAT calculated by leaf physiognomy at 26.8 ± 2.24oC (α = 0.05, p \u3c 0.001). Growth ring sequences analyzed by MS produced an average value of 0.47 ± 0.09, indicating that some of the trees experienced irregular growth. Due to the warm MAT and high VI values, the irregular growth recorded in the conifer wood was probably the result of environmental disturbances, such as flooding or volcanic events, capable of disrupting wood production. This study is the first comprehensive examination of wood from the Late Cretaceous San Juan Basin, and enhances our knowledge of the paleoecology of the region