GROUND SLOTHS MEGALONYX WHEATLEYI, NOTHROTHERIOPS SHASTENSIS AND PARAMYLODON HARLANI FROM THE MIDDLE IRVINGTONIAN FAIRMEAD LANDFILL LOCALITY, MADERA COUNTY, CALIFORNIA

Located in Madera County, California, Fairmead Landfill records a diverse middle Irvingtonian, 0.78 Ma to 0.55 Ma, biota consisting of 56 taxa (1 fish, 2 amphibians, 3 reptiles, 4 birds, 27 mammals, 1 bivalve, 1 gastropod, 1 plant macrofossil, 16 diatoms). The fossils occur in sediments representing distal alluvial fan channel, distal fan overbank flood or sheetflood, and marsh/lacustrine deposits of the upper unit of the Turlock Lake Formation. Among the fauna are three ground sloth species; Megalonyx wheatleyi, Nothrotheriops shastensis and Paramylodon harlani. In the summer of 1993, remains of all three sloth species were recovered from a relatively narrow stratigraphic zone (12.2 m to 12.8 m below the surface) and from close horizontal proximity to each other (within 90 m). During the year 2006 excavation, dozens of Paramylodon specimens representing at least 6 individuals were collected from an alluvial fan channel deposit 14.6 m to 15.8 m below ground surface in a 15 m x 40 m area. Between excavation years 1993 and 2008, paleontological monitors recovered another 30 sloth specimens from Fairmead Landfill, all Paramylodon except a Nothrotheriops cranium. Of those 30 specimens, 16 associated elements recovered in 2003 appear to represent one individual. Co-occurrence of these three sloth taxa in a single fauna is rare in both the Irvingtonian and Rancholabrean, a reflection of the different ecological requirements of each species. This is only the fourth documented Irvingtonian fauna to contain all three sloth genera. Typically, in areas of co-occurrence one taxon is more common than others. At Fairmead Landfill, Paramylodon, a grazer/mixed feeder, dominates the sloth assemblage with multiple individuals ranging from neonates to adults. The browsers Megalonyx and Nothrotheriops are represented by a minimum of one adult and two adults and a juvenile, respectively. The high frequency of the grazing species is consistent with composition of the assemblage as a whole, which is dominated by presumed open-country grazers Equus sp., Camelops sp., and Mammuthus columbi. Evolutionarily the Fairmead Landfill sloths exhibit morphological, size and proportional characters that suggest they represent transitional populations between older and younger members of their respective lineages.