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.
The diamond-backed terrapin is adapted to living in salt marshes.
Diamond-backed terrapin habitat–a salt marsh.
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.
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
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