In the fall of 2001, Bob and Sonya Athen discovered the remains of an adult Jefferson’s ground sloth, Megalonyx jeffersonii, in the West Tarkio Creek behind their home, south of Shenandoah, IA. The Athens brought several specimens to the University of Iowa, where they generated considerable excitement. Megalonyx has been found in Iowa since 1862 (Hall and Whitney 1862), but only isolated bones on sand bars or in other secondary deposits.
Only five (5) semi-complete Megalonyx skeletons are known: American Falls, ID – complete head to pelvis (Hopkins, et al. 1969); Orton, OH – 70 elements (Claypole 1891); Big Bone Cave, TN – 40 elements (Mercer 1897); SeaTac, WA – 36 elements (McDonald 1977, 1998), and; Darke County, OH – 93 elements (Mills 1975). Another, found in Henderson, KY (Leidy 1855) is missing (Lane 2000) and only eight elements remain.
A survey of the Athen’s property, and the bones they had recovered suggested the animal was located within a small area in the creek, possibly extending under the bank to the north, onto the property of the adjoining landowners, Dean and Loreta Tiemann of Lincoln, NE, who were contacted and also agreed to the excavation.
National Science Foundation (NSF) funding was secured and the US Army Corps of Engineers authorized “dewatering” (partial diversion) of the creek. A sand bag levee was constructed to protect the fossil-bearing deposits.
In the spring of 2006, the remains of a juvenile Megalonyx jeffersonii were discovered intermingled with the adult. Astonishingly, that fall the scapula from another juvenile was found, half the size of the first. All three animals are clearly contemporaneous. A semi-complete skeleton of an adult intermingled with two juveniles, in primary context, with associated paleoecological data is unprecedented.
About half of the target area in the creek was excavated before high water returned in 2007. 104 elements from the adult have been recovered. 41 bones apparently belonging to the larger juvenile have been found, making the “toddler” the second-most complete juvenile Megalonyx on record. Confirmed remains of juvenile #2, the “baby,” are limited to the single scapula.
An international team of scholars has been assembled with specialties in molecular biology, palynology, stable isotope geochemistry, plant macrofossils, microvertebrates, vertebrate morphology, archaeology and Quaternary geology. All have specialized in the Quaternary and are eager to analyze the specimens and their geologic environs, and address the many mysteries of sloths in general and these three individuals in particular.
Claypole, E. W. 1891. Megalonyx in Holmes County, Ohio. American Geologist 7:122-132.
Hall, J. and J.D. Whitney. 1862. Report of a geological survey of the upper Mississippi lead region. Geological Survey of Wisconsin 1:135.
Hopkins, M. L., R. Bonnichsen and D. Fortsch. 1969. The stratigraphic position and faunal associates of Bison (Gigantobison) latifrons in southeastern Idaho, a progress report. Tebiwa 12: 1-7.
Lane, N. G. 2000. The mystery of the missing Megalonyx. In Geology at Indiana University. Department of Geological Sciences, Indiana University of Indiana, Bloomington.
Leidy, J. 1855. A memoir on the extinct sloth tribe of North America. Smithsonian Contributions to Knowledge, 7.
McDonald, H. G. 1977. Description of the Osteology of the Extinct Gravigrade Edentate Megalonyx with observations on its ontogeny, phylogeny and functional anatomy. University of Florida Masters thesis.
McDonald, H. G. 1998. The sloth, the president, and the airport. Washington Geology 26: 40-42.
Mercer, C. H. 1897. The finding of the remains of the fossil sloth at Big Bone Cave, Tennessee, in 1896. Proceedings American Philosophical Society 36: 36-70
Mills, R. S. 1975. A ground sloth, Megalonyx, from a Pleistocene site in Darke Co., Ohio. The Ohio Journal of Science, 75(3): 147-154.