The Herpetological Journal

pp. 73-86

Authors: Lauren E. Brown, Don Moll & Evan S. Brown

Abstract: The Illinois Episode was the most extensive Quaternary glaciation in North America and extended deep into the central USA, further south than any other glacial episodes. It was followed by a period of mild climate termed the Sangamon Interglacial Episode. Relatively few reptile fossil sites have been found along the Illinois-Sangamon boundary. Thus, the 1986 report of isolated fossil remains of a giant tortoise (Hesperotestudo) near the boundary is of particular significance. We re-examined this important fossil because of inconsistencies and misinterpretations of prior researchers. The three morphological characters used for prior species identification of the tortoise are faulty and unreliable. Lack of additional, pertinent, diagnostic fossil elements presently prevents positive species identification. We critically appraised the pollen-based analysis of climate and environment at the tortoise strata by prior researchers. Their data suggest a transitional area between forest and prairie, or savannah, but the prior researchers misinterpreted their own data, concluding the vegetation was “relatively xeric grassland”. Consequently, the climate and environment at the tortoise stratum are yet to be determined. We present several zoogeographical scenarios pertaining to the origin and movement of the tortoise to the collection site. The most likely is perhaps northward movement from the central Gulf Coast along the Mississippi River floodplain before the major meltdown of the Illinois glaciation. East of St. Louis, the glacier met the Mississippi River floodplain and as the meltdown progressed, the tortoise could have travelled on the till plain north-east to the collection site. Hesperotestudo likely had considerable coldadaptation and thus may have tracked the Illinois glacier relatively closely as it melted northward.

Key words: giant fossil tortoises, Hesperotestudo, climate, zoogeography