Posts Tagged ‘graptemys genus’

An Extinct Map Turtle (Graptemys kerneri) and Pleistocene Sea Level Fluctuations

March 15, 2018

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.

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Barbour’s map turtle is the closest living relative of the extinct Kerner’s map turtle.

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Barbour’s map turtle range.  Note how it is restricted to 1 river system.

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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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Pleistocene Terrapins (Malaclemys terrapin)

March 20, 2017

Until recently, there was little fossil evidence of diamond-backed terrapins. This species inhabits salt marshes and mangrove swamps from the Gulf of Mexico to Cape Cod, Connecticut.  For most of the past 2 million years, sea level has been much lower than it is today due to the larger ice caps of long-lasting Ice Ages.  This means many potential fossil sites where the remains of terrapins might be found are submerged deep underwater and difficult to access.  Sea level has been the same or higher than it is today probably for less than 20% of the last million years, and this reduced the chances easily accessible fossil sites developed in salt marsh zones.  However, the remains of terrapins dating to the Pleistocene have been excavated from  3 sites in Florida, 1 in Georgia, and 1 in South Carolina.  These specimens weren’t described in the scientific literature until 2012.

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The diamond-backed terrapin is adapted to living in salt marshes.

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Diamond-backed terrapin habitat–a salt marsh.

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Diamond-backed terrapin range map.

The 3 sites in Florida where Pleistocene-age terrapin remains were discovered are Page-Ladson, Aucilla River, and Wekiva River.  Terrapin material turned up at Edisto Beach, South Carolina, and fossil hunters found terrapin bones in spoil piles dumped on Andrews Island, Georgia.  (All of Andrews Island is manmade, consisting of spoil piles dredged from the South Brunswick River, aka Fancy Bluff Creek. The Army Corps of Engineers periodically dredges the river to keep it deep enough for safe shipping. Plants have taken root there and it is an haven for wildlife.) The specimens are thought to be Pleistocene in age because they are associated with bones of other species that lived then.  The 3 sites in Florida and the 1 at Edisto Beach commonly yield bones of extinct Pleistocene mammals.  The spoil piles on Andrews Island contained the remains of snapping turtles (Chelydra serpentina), yellow-bellied cooters (Trachemys scripta), and the extinct giant tortoise (Hesperotestudo crassicutata).  These species all lived during the late Pleistocene.  The presence of these 3 species along with the terrapin indicates the local environment at the time of deposition was a brackish marsh bordering an open grassy savannah. Snapping turtles and yellow-bellied cooters are fresh water species that can tolerate brackish conditions, and giant tortoises preferred dry land environments.

Terrapins are not closely related to sea turtles.  Morphological and genetic evidence suggests they are most closely related to freshwater turtles in the Graptemys genus.  In North America this genus includes 10 species of map turtles and saw backs. Terrapins are the only turtle species uniquely adapted to live in salt marshes.  They have lachrymal salt glands that help them get rid of excess salt.  These are absent on all species of fresh water turtles.  Terrapins are also able to drink the layer of rain water that temporarily floats on top of salt water.  Terrapins feed upon shellfish–periwinkle snails are their favorite but they consume shrimp, crabs, and bivalves as well.

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The salt marsh periwinkle (Littorina irrorata) is the diamond-backed terrapin’s favorite food.

Terrapins were formerly so abundant they constituted the main source of protein for coastal slaves during the 18th and 19th century.  But a faddish craze for turtle soup circa 1900 greatly reduced their numbers.  All of the finest restaurants served turtle soup, and it was the most expensive item on the menu.  I’ve only had the opportunity to eat turtle meat once.  Turtle meat is very delicious, tasting like lobster.  Because terrapins feed on shellfish, their flesh likely reflects their diet.  Terrapins are presently a protected species but are still considered threatened.  Real estate development destroys their habitat, they drown in crab traps, cars run over them, and there are people who still eat them.  Egg-eating raccoons flourish as well, since most large predators that kept their population in check no longer exist on the east coast.  If I get the urge to eat turtle again, I’ll stick with the common snapping turtle which as their name suggests are still common.

Reference:

Ehret, Dana; and Benjamin Atkinson

“The Fossil Record of the Diamond-backed Terrapin, Malaclemys terrapin (Testudines: Emydidae)”

Journal of Herpetology 46 (3) September 2012