A BRISTLECONE PINE FOREST FROM THE EARLY OLIGOCENE OF SOUTHWESTERN NEW MEXICO: EVIDENCE FOR VEGETATION RESPONSE TO THE EOCENE-OLIGOCENE TRANSITION

The Hermosa and Hillsboro floras of southwestern New Mexico are strongly dominated by foliage and cone scales of Pinus. The association of five-needled fascicles lacking persistent bundle sheaths along with cone scales bearing dorsally oriented umbos indicates an affinity with the subsection Balfourianae, which includes the extant bristlecone and foxtail pines. The fossil specimens are assigned to Pinus crossii, which is well known from the late Oligocene Creede flora of Colorado, and elsewhere. Modern members of the subsection Balfourinae are endemic to the western U.S. and typically inhabit cold climates at high elevations. The nearly exclusive dominance of this pine in the Hermosa and Hillsboro floras is unique and indicates a depauperate, cold, subalpine (physiognomically taiga-like) coniferous forest. This indicates a colder climate than the more northern Creede flora, which has greater diversity among angiosperms and other conifers. Angiosperm macrofossils are extremely rare in Hermosa and Hillsboro but include several specimens of Mahonia and Crataegus. The coniferous dominance is substantiated by the pollen record, which consists primarily of Pinus but also includes Picea and angiosperms such as Mahonia, Juglans, Carya, Ulmaceae, Gramineae, and abundant Sarcobatus. Mean annual temperature is estimated at about 1°C and is supported by both physiognomic and nearest-living-relative proxies. The floras occur in lacustrine shale that was deposited in association with volcaniclastics. This shale overlies volcanic sequences representing the eruptive phases of the Emory cauldron. An early Oligocene age for the floras is supported by bracketing dates of 28.1 ± 0.6 Ma and 31.7 Ma for the Hillsboro flora and an age of 33.6 ± 1.0 Ma for Hermosa. This chronology indicates that the Hermosa and Hillsboro floras lived during or immediately following the cooling of the Eocene-Oligocene transition. The floras provide important insight for understanding the response of vegetation to cooling climate at higher elevations in the continental interior of North America. Estimated paleoelevation is about 2850 meters or more, based on comparison with coeval early Oligocene sea level floras and inferred lapse rates. Caldera topography may have resulted in cold air drainage that exacerbated the cold character of the flora.