PaleoBios

Hesperotestudo is a genus of giant tortoise that existed from the Oligocene to the Pleistocene of North and Central America. Recorded occurrences in the United States are plentiful; however, California seems to be an exception. Literature on Hesperotestudo in California is limited to faunal lists in papers, with few detailed descriptions. Here we review the literature on the genus, describe and identify specimens found in the upper Mehrten Formation (late Miocene-early Pliocene) exposed in the Central Valley of California at Turlock and Modesto Reservoirs, Stanislaus County, and address their implications for early Pliocene California biogeography and climate. All fossils described are from the collections of the University of California Museum of Paleontology (UCMP). The largest specimen from the Mehrten is a peripheral from an animal with an estimated carapace length over one meter. The specimens were compared first to modern material of Gopherus, the only other tortoise genus from the late Miocene-early Pliocene of California, and then to measurements from the literature of the three species of Hesperotestudo to which it could most likely be referred: H. osborniana, H. orthopygia, and H. campester. Based on characteristics and measurements of the carapace and plastron, these specimens are assigned to H. orthopygia. Hesperotestudo orthopygia is a species known primarily from the Great Plains region, so its presence in California during the late Miocene-early Pliocene indicates that it expanded west into California at this time. Large tortoises are not very tolerant of frost conditions, possibly indicating a relatively frost free climate for this area at the time. This agrees with previous estimates of annual temperature records based on plant fossils from the upper Mehrten Formation, in particular the presence of Persea, an avocado relative, which is also frost sensitive.