Vertebrate fossils found by Merriam within the deltaic deposits include "Equus", "Gomphotherium", and "Bison" and were assigned to Irvingtonian age (0.5 to 1.8 million years before present), dates consistent with the aforementioned capture of the lower Colorado River. Evidence of a giant anteater, "Myrmecophaga tridactyla", was found in the deltaic deposits in the southern Gran Desierto. Van Devender notes that the specimen was found in association with fossils of mammoths, sloths, and boa constrictors, a tropical faunal assemblage which supports a contention that the Colorado River delta of a previous interglacial period (>120,000 years ago) was much warmer and wetter than in the present interglacial.