THE STRATIGRAPHY AND PALEOECOLOGY OF PLEISTOCENE DEPOSITS ASSOCIATED WITH GIANT GROUND SLOTHS (MEGALONYX JEFFERSONII), PAGE COUNTY, IOWA

An adult and two juvenile individuals of the giant ground sloth Megalonyx jeffersonii were recovered in 2001 from along West Tarkio Creek, Page County, Iowa. The recovery of 3 individuals of differing ages makes this excavation unique. Initial estimates placed the demise of these giant ground sloths at approximately 11-12,000 years B.P.

Two sediment cores were taken in order to examine the stratigraphy and paleoecology of the sediments associated with the sloth remains. Analysis of the cores included detailed soil and sediment descriptions and identification of paleosols. Particle size analysis performed on the lower sections of the cores showed a prevalence of silt, punctuated by sand-rich intervals, in fining-upward sequences that are indicative of a meandering stream system.

Superjacent to the sloth material, carbon isotope values of -21 to -24 δ¹³C were indicative of C₃ woody vegetation. Recovered phytoliths and plant macrofossils were indicative of woody deciduous vegetation with subordinate amounts of sedge.

Radiocarbon analysis yielded two dates. The upper sample produced an age of 150 (+/- 40) years B.P. indicating that the upper half of the sediment core was deposited in historic time. The lower sample, from directly above the sloth matrix, yielded a date >43, 500 years B.P. indicating that the sloths were deposited prior to the late Wisconsinan. Two OSL ages on quartz sand from the sloth-bearing interval suggest deposition before 100,000 years ago.

Dating results indicate the age of the M. jeffersonii at this locality, in excess of 100,000 years B.P., is significantly greater than once assumed. Associated sediments and plant fossils portray a forested interglacial landscape in which the sloths were buried during the flooding of a meandering stream system.