The Eastern Box Turtle is one of the most familiar and beloved turtles of eastern North America, yet its story begins far beyond the forests, fields, and backyards where we encounter it today. To understand this small, domed, gold-mottled turtle, we have to travel through deep time to the first shell-less turtle ancestors, to the Triassic world of Pangaea, to the rise of North American pond turtles, and eventually to the emergence of box turtles on a changing continent. This post begins that journey with the evolution of the Eastern Box Turtle, the first in a series that will later explore its distribution, subspecies and regional forms, habitats, behavior, diet, reproduction, conservation, and natural history writ large.
The turtle lineage began shell-less, nearly unrecognizable to us as a turtle. Before evolving shells bit by bit, stage by stage; turtle ancestors resembled broad, stout-limbed lizards that were adapted to digging. The shell as we know it probably began from the bottom up, plastron (lower shell) first. That lower shell likely derived from a fusion of big belly bones called gastralia, which braced and protected the belly, as seen in a fossil species from the middle Triassic in modern Germany. The carapace (top shell) evolved from a reorganization of the dorsal ribs, gradually transforming the turtle’s ribs, backbone and dorsal bones into a protective dome.
Mind-blowingly, early turtles also didn’t have recognizable beaks. Instead, they had teeth! Between 260 to 220 Ma, the fossil record only yields toothed “turtles”: Eunotosaurus (260 Ma), Pappochelys (240 Ma), and Odontochelys (220 Ma) all were beakless and had mouths filled with little teeth. It turns out that teeth are fragile and destabilizing, while beaks can handle scraping and crushing of tough foods, while protecting the skull and jaw. While we don’t know what pre-historic turtles ate, modern turtles have vastly varied diets, with some species specializing in crushing mollusks and other in grazing seagrass, some exhibiting carnivory and others herbivory. For example, a study of Alligator Snapping Turtles in the American South found that they eat crayfish, mollusks, other turtles, insects, and even nutria (Elsey 2006) while southeastern Gopher Tortoises eat over 300 species full of high-fiber foliage (USFWS nd).
Turtles—shelled, beaked, and recognizable to us—have been present in the geologic record for about 200 million years, appearing during the Triassic revolution, when dinosaurs, coniferous trees, insects, and other major terrestrial plant and animal lineages diversified and flourished. This diversification came after a period of intensification of temperature and acid rain, known as the Carnian Pluvial Episode (233-232 Ma), triggered in part by massive volcanic eruptions. After the eruptions ended, the planet became more arid, spurring plant and animal diversification and the creation of modern ecosystems (Benton and Wu 2022).
This is the period of Pangaea, when the continents we recognize today were a fused land mass, with future North America, Europe and Asia (Laurasia) to the north and the other future continents located to the south as Gondwana, all surrounded by the immense Panthalassa Ocean. At this time, during the late Triassic, shelled turtles were found in patches across much of the Pangaea super continent, as evidenced by partially-shelled (plastron only) or fully shelled (both plastron and carapace) turtle fossils found in modern southwest China (Odontochelys semitestacea, ~220 Ma, Li et al. 2008), Germany and Switzerland (Proganochelys quenstedtii, ~210 Ma, Gaffney 1990; Scheyer et al. 2022), North American New Mexico (Chinlechelys tenertesta; Joyce et al. 2009; Lichtig & Lucas 2021), and South America (Rougier et al. 1995; Sterli et al. 2007). Note that Odontochelys was beakless and Proganochelys appears to be beaked with denticles, or reduced, tooth-like bumps.
Fast forward 200 million years to the Eocene (56-42 Ma), when the continents had split apart due to the widening gulf of the Atlantic Ocean separating North America from Europe and Africa. During this time, North America was crisscrossed with lowlands and drainage systems left behind by the retreating Western Interior Seaway and still developing the mountains of the western United States. The lowlands and drainage systems, plus warm temperatures, created favorable conditions for the diversification of a group of North American turtles, the Emydidae. This family of pond turtles eventually gave rise to the genus Terrapene that encompasses North American box turtles.
The Terrapene genus first appeared in the Miocene (23-5.3 Ma), evidenced by Terrapene cf ornata (ie specimens that look like modern T. ornata) fossils from 14.5 to 13 Ma found in Nebraska and T. cf carolina fossils from 10 to 9 Ma found in Kansas (Holman and Fritz 2005). The climate continued to shift, cooling and drying through the Pleistocene. During this time, some box turtle lineages remained or differentiated in the grasslands of Great Plains and southwestern United States, while the species we know as Eastern Box Turtles (T. carolina) became associated with mesic forested habitats, and likely moved eastward, its range contracting and expanding with shifting distributions of riparian and mesic hardwood forests.
The Eastern Box Turtle carries an astonishing amount of history in its body: widened ribs transformed into a shell, teeth replaced by a beak, aquatic ancestry reshaped into terrestrial life, and a lineage that moved through shifting climates and continents. In future posts, we will bring this story closer to the present, tracing where Eastern Box Turtles live today, how their subspecies and regional forms differ, and what their daily lives reveal about forests, connectivity, seasons, and survival.
References
Benton, M. J., & Wu, F. (2022). Triassic revolution. Frontiers in Earth Science, 10, Article 899541. https://doi.org/10.3389/feart.2022.899541
Elsey, R. M. (2006). Food habits of Macrochelys temminckii (Alligator Snapping Turtle) from Arkansas and Louisiana. Southeastern Naturalist, 5(3), 443–452. https://doi.org/10.1656/1528-7092(2006)5%5B443:FHOMTA%5D2.0.CO;2
Gaffney, E. S. (1990). The comparative osteology of the Triassic turtle Proganochelys. Bulletin of the American Museum of Natural History, 194, 1–263.
Holman, J. A., & Fritz, U. (2005). The box turtle genus Terrapene (Testudines: Emydidae) in the Miocene of the USA. Herpetological Journal, 15, 81–90.
Joyce, W. G., Lucas, S. G., Scheyer, T. M., Heckert, A. B., & Hunt, A. P. (2009). A thin-shelled reptile from the Late Triassic of North America and the origin of the turtle shell. Proceedings of the Royal Society B: Biological Sciences, 276(1656), 507–513. https://doi.org/10.1098/rspb.2008.1196
Li, C., Wu, X.-C., Rieppel, O., Wang, L.-T., & Zhao, L.-J. (2008). An ancestral turtle from the Late Triassic of southwestern China. Nature, 456(7221), 497–501. https://doi.org/10.1038/nature07533
Lichtig, A. J., & Lucas, S. G. (2021). Chinlechelys from the Upper Triassic of New Mexico, USA, and the origin of turtles. Palaeontologia Electronica, 24(1), Article a13. https://doi.org/10.26879/886
Rougier, G. W., de la Fuente, M. S., & Arcucci, A. B. (1995). Late Triassic turtles from South America. Science, 268(5212), 855–858. https://doi.org/10.1126/science.268.5212.855
Scheyer, T. M., Klein, N., Evers, S. W., Mautner, A.-K., & Pabst, B. (2022). First evidence of Proganochelys quenstedtii (Testudinata) from the Plateosaurus bonebeds, Norian, Late Triassic, of Frick, Canton Aargau, Switzerland. Swiss Journal of Palaeontology, 141, Article 25. https://doi.org/10.1186/s13358-022-00260-4
Sterli, J., de la Fuente, M. S., & Rougier, G. W. (2007). Anatomy and relationships of Palaeochersis talampayensis, a Late Triassic turtle from Argentina. Palaeontographica Abteilung A, 281, 1–61.
U.S. Fish and Wildlife Service. (n.d.). Gopher tortoise. https://www.fws.gov/project/gopher-tortoise
Sense of Place 
Leave a comment