Most species of freshwater turtles can travel overland and occupy new favorable habitat, promoting genetic vigor within the meta population. They often move between watersheds, and this explains why so many species have such a continuous geographic range. I’ve seen snapping turtles (Chelydra serpentina) and yellow-bellied sliders (Chrysemys scripta) a considerable distance from any water source. However, map turtles in the Graptemys genus (10 species) do not travel overland, and their ranges are usually restricted to single river drainages. So how did closely related Graptemys species colonize different river drainages even though they don’t travel overland? The dispersal of the Graptemys genus is closely related to Pleistocene sea level fluctuations.
Barbour’s map turtle is the closest living relative of the extinct Kerner’s map turtle.
Barbour’s map turtle range. Note how it is restricted to 1 river system.
Map of Florida during the Last Glacial Maximum. Different rivers in Florida isolated by sea level rise today were interconnected on land exposed on the continental shelf due to sea level fall. This allowed map turtles to colonize adjacent rivers systems where they evolved into distinct species following sea level rise and isolation of populations.
During Ice Ages sea level fell because so much of earth’s atmospheric water became locked in glacial ice. In Florida dry land habitat extended 120 miles west into the Gulf of Mexico. Several of Florida’s river systems that are isolated from each other today by sea level rise were interconnected during the Last Glacial Maximum on the land that was exposed by ocean recession. This allowed an ancestral population of Barbour’s map turtle (Graptemys barbouri), a species today restricted to the Apalachicola-Flint-Chattahoochee River System, to colonize several other rivers in Florida. Subsequent sea level rise isolated this founder population in the Suwanee, Santa Fe, and Waccasassa Rivers where they evolved into a now extinct species known as Kerner’s map turtle. Specimens of this species have been found in all 3 of the above mentioned rivers in Florida, and the type specimen (a complete skull) came from the Suwannee.
Kerner’s map turtle had a wider shorter skull than any extant species of map turtle. Morphologically, it most closely resembles Barbour’s map turtle, the extant species that has the widest shortest skull among living Graptemys turtles. There is an east-west gradient in the shape of map turtle skulls. Western species have narrower longer skulls, but map turtle species’ skulls get shorter and wider the farther east they occur. Kerner’s map turtle was the easternmost species, and it ranged into north central Florida and possibly southeastern Georgia where the Suwannee River headwaters originate. Rare earth element analysis indicates Kerner’s map turtle lived during the Rancholabrean Age (300,000 years BP-11,000 years BP). There are no known Graptemys specimens older than the mid-Pleistocene. The extinction of Kerner’s map turtle likely occurred during a dry climate stage of the mid-Holocene (~6,000 years BP). Map turtles require fast moving high water where they can disburse up and down rivers. But drought turns their habitat into stagnant isolated pools that can also be detrimental to their favorite food source–freshwater mussels. Other species of freshwater turtles and alligators can survive these conditions by moving overland until they find good habitat, but map turtles don’t travel overland. That’s why most species are restricted to major rivers that rarely, if ever, suffer sporadic flows.
The founding species in the Graptemys genus undoubtedly evolved in the Mississippi River. Almost all other species exist in other river systems that empty into the Gulf of Mexico. Pleistocene sea level fluctuations facilitated the colonization and speciation of map turtles in the Apalachicola River drainage, the Guadulupe River System, the Pascagoula River, Mobile Bay drainage, Yellow River System, and Pearl River. Different map turtle species live in each. Ocean recession allowed the rivers to become interconnected on the continental shelf, and map turtles were able to colonize adjacent river systems; then sea level rise isolated populations, causing speciation.
Reference:
Ehret, Dana; and J. Bourque
“An Extinct Map Turtle Graptemys kerneri (Testudinae, Emydidae) from the Late Pleistocene of Florida”
Journal of Vertebrate Paleontology 31 (3) May 2011
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