LATE PLEISTOCENE BISON CF. B. LATIFRONS FROM FRESNO, CALIFORNIA WITH COMMENTS ON THE AGE OF THE UPPER UNIT OF THE MODESTO FORMATION

Occurring in California’s eastern San Joaquin Valley, the Modesto Formation represents sediment deposited during aggradational events associated with the last glacial period. The formation occasionally produces fossils, such as the December 2006 discovery by Thomas C. Broach of a Bison radius from the upper unit of the Modesto Formation (36°52’51” N, 119°46’51” W) in Fresno, California. The specimen’s large size and associated radiocarbon date are significant. The radius’ measurements, 401 mm greatest length, 126.6 mm greatest proximal transverse diameter, 45.5 mm least anteroposterior width of shaft, 73 mm least transverse width of shaft, and 122.5 mm greatest distal transverse diameter, compare best with Bison latifrons.

At the Broach locality, the Modesto Formation composes part of the San Joaquin River alluvial fan and consists of a complex of channel fill deposits comprising cross-stratified sand and minor gravel, overbank sediments consisting of interbedded laminated-massive fine sand, silt and clay, and mature paleosols. Occurring 4.7 m below the outcrop surface, the bone was preserved within a massive, light gray, clay-rich, 35-55 cm thick stratum containing abundant charcoal. The fine grain size, massive character and common charcoal suggest floodplain deposition. This facies continues laterally for at least 2 km.

Charcoal associated with the bison radius yielded an AMS ¹⁴C date of 19,230 ± 80 RCYBP (Beta-244983). This compares well with the inferred 27,000 to 9000 RCYBP age range for the upper unit of the Modesto Formation. However, Marchand & Allwardt (1981) proposed a more restricted age of 14,000 to 9000 RCYBP for the upper unit based on dates of wood, 14,100 ± 200 RCYBP from the Kern River alluvial fan at Bakersfield, California and 14,060 ± 450 RCYBP from sediments 40 km to the west of the first site. The broader age range for the upper unit of the Modesto Formation is supported by the Broach locality ¹⁴C date.

Only two of the other twenty-eight Bison latifrons localities in California have associated ¹⁴C dates, Rancho La Brea and Costeau Pit. The Broach locality is an important numerical age record for the taxon in California and bolsters growing evidence that giant bison survived the 21,000 to 30,000 RCYBP terminal age range suggested by Kurten & Anderson (1980).