THE ORIGINS OF THE VERTEBRAL STRUT IN GOPHER TORTOISES IN THE GOPHERUS POLYPHEMUS LINEAGE, WITH COMMENTS ON AN IMPORTANT NEW FOSSIL FROM THE OLIGOCENE OF SOUTH CAROLINA AND ITS BEARING ON THE EVOLUTION OF THE GROUP (TESTUDINES: TESTUDINES)
This research is a collaboration with Al Sanders at the Charleston Museum in Charleston, South Carolina. This investigation focuses on a new strut-bearing fossil tortoise that was collected in the Chandler Bridge Formation in Charleston County, South Carolina. The fossil is the anterior part of a carapace showing the nuchal bone, neurals I-IV, and accompanying costals and peripherals. Its presence in the Chandler Bridge Formation indicates a late Oligocene age (early Arikareean, 27 million years old). The formation where the fossil was collected consisted of marine sediments that were rich in marine mammals, birds, crocodilians, fishes, mollusks, corals, and nannoplankton. The presence of soft-shelled turtles and terrestrial plant material in this formation indicates an inshore or near-shore depositional environment, possibly a lagoon, associated with coastal islands. These coastal island habitats in Georgia and Florida today maintain large gopher tortoise populations.
Al Sanders, Curator at the Charleston Museum
SUMMARY. The gopher tortoise genus Gopherus is considered a North American endemic, with fossil and recent species occurring in the southeastern and western United States, and in northern Mexico. In recent reviews, the genus Gopherus includes nine recognized fossil species and four living species. Gopherus is considered monophyletic with origins in the early Tertiary in western North America, probably from Hadrianus-like tortoise ancestors. Fossil and recent species in the genus Gopherus are grouped into the polyphemus (gopher tortoises and their relatives) and agassizii (desert tortoises and their relatives) lineages. These lineages have been treated variously as genera and subgenera (Gopherus and Xerobates, respectively). The polyphemus lineage is considered the oldest group, and the late Eocene Gopherus laticuneus (Cope) from the White River beds in northern Great Plains, the earliest representative (35 Ma), although this arrangement was recently questioned with the description of a new monotypic subgenus (Oligogopherus Hutchinson 1996) for G. laticuneus. The origin of the agassizii lineage is unclear, with the oldest “secure records” no older than 0.5 million years.
Examinations of previously unreported fossil Gopherus specimens from the Oligocene to Pleistocene of Florida, South Carolina, and several southwestern states show that the phylogeny of this group is more complex than previously thought. These new fossils indicate a need to shift from rather simplified phylogenetic hypotheses to one that records the dramatic morphological shift that occurred in the mid to late Tertiary in a group of newly-emerging specialized burrowers. The most novel innovation is the development a vertebral strut that extends from the first dorsal vertebra onto the underside of the nuchal bone. It is absent in Hesperotestudo, agassizii lineage, and early members of the polyphemus lineage, except for a unique tortoise from the late Oligocene of South Carolina. This strut apparently strengthens the connection between the final cervical vertebra and the first shell vertebrae to allow for more effective burrowing using its head and neck to maintain position while digging. The strut and other Polyphemus-related morphological features are reviewed here.
The discovery of a new fossil gopher species with a vertebral strut from the Chandler Bridge Formation, on the bank of Dorchester Creek, Charleston County, South Carolina, by Steven T. Miller, in January 1988, challenges previously held ideas concerning the monophyly of the polyphemus lineage. This new fossil (to be latter named in honor of Shelley Franz) is amongst the earliest known gopher tortoises and the oldest documented occurrence for Gopherus-like tortoises in eastern North America. This curious new tortoise shares shell shape and carapacial scute features with early members of the Polyphemus group, but has the vertebral strut scar on the nuchal plate found in shells of recent Gopherus polyphemus, Gopherus flavomarginatus, and more modern strut-bearing fossils.
We interpret the blend of characters in the new species to imply ancient affiliations with both the earliest Gopherus fossils in the West and early experiments with vertebral struts within the polyphemus lineage in the East. However, because of its aberrant neural pattern, we believe that the new fossil should be considered a sister taxon to the strut-bearing polyphemus evolutionary line. We further suggest that the Gopherus polyphemus line is polyphyletic and that there were early attempts in this lineage to build a better burrower. Other strut-bearing Polyphemus-like tortoises are known from the late Miocene and early Pliocene of Florida. This group also appears in the West during the mid to late Pliocene. Because of the early appearance of strut-bearing gopher tortoises in the Southeast, we suggest that these tortoises and their South Carolina sister taxon evolved in the southeastern United States and later moved westward with the establishment of the Gulf Coast Corridor in the Pliocene.Posted September 14, 2010.