LATE PLEISTOCENE LIFE ON THE COLORADO PLATEAU: FAUNAL AND FLORAL EVIDENCE FROM THE NATIONAL PARKS, ARIZONA AND UTAH

The Colorado Plateau is renowned for its mountains, mesas, and canyons. Extremes of topography (1500 m to 3400 m elevation) have dictated the mosaic structure of the modern and past biotic communities. The Colorado Plateau provides a wealth of data about the Pleistocene, even though lake, wetland, or marsh deposits are rare, and rock shelters and caves are randomly distributed across the region. Biotic remains typically lost to decay, such as feces, hair, and flesh, are preserved in these unique taphonomic settings. Dry localities also provide a detailed vegetation record preserved in fossil packrat middens and have produced a paleoclimate record spanning the past 50 ka yr.

Faunal and floral data allow insight into the biotic response to the end of the Wisconsinan Glaciation and the beginning of the Holocene. ¹⁴C analysis of dung samples of extinct species such as Shasta ground sloth (Nothrotheriops shastensis), mammoth (Mammuthus), Harrington’s mountain goat (Oreamnos harringtoni), shrubox (Euceratherium collinum) and bison (Bison) has yielded detail on the last occurrence of these taxa. Extirpation of Oreamnos harringtoni, Mammuthus, Euceratherium collinum, and Nothrotheriops shastensis occurred between 11.8 and 11.0 ka ¹⁴C yr BP (13.9-12.8 ka cal yr BP) on the Colorado Plateau.

Assembly of the modern plant communities and ascendance of dominant plant species on the Colorado Plateau are often a recent phenomenon. Pollen and packrat midden data demonstrate that today’s ponderosa pine (Pinus ponderosa) forests were occupied by mixed conifers (P. flexilis, Abies concolor, and Picea engelmannii) until as late as 12 ka ¹⁴C yr BP (14 ka cal yr BP), and ponderosa pine did not arrive until after 10 ka ¹⁴C yr BP (11.5 ka cal yr BP). Likewise, Colorado pinyon (P. edulis) today a common co-dominant with Utah juniper (Juniperus osteosperma) across mid-elevations, was apparently restricted to central New Mexico during the Pleistocene, and only migrated into its present distribution over the past 7-2 ka yr. However, new midden data suggests that Pleistocene pinyon distribution is still poorly understood, as newly analyzed middens from north of the Grand Canyon and in Canyonlands National Park contain pinyon fossils dating to 23 ka and 32 ka ¹⁴C yr BP, respectively.