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How did the Diamondback Terrapin Evolve?

Baby diamondback terrapin - Species like these benefit from the work of the CIB.The turtles of Delaware are a pretty diverse group.  We have about 10 species of solely freshwater turtles and 4 species of sea turtles that spend almost their entire life in the ocean.  But there is only one species that spends its life in the great mixing zones of Delaware’s estuaries: the diamond back terrapin. 

This interesting fact is what recently led a curious volunteer to inquire about the evolutionary origins of terrapins. Recognizing that terrapins share features of both fully freshwater and fully marine turtles, he wanted to know from which group the terrapins evolved from as they adapted into their estuarine niche. He asked this while holding a huge female terrapin who had recently been run over by a truck — more on that later. 

Luckily for terrapin lovers everywhere, Barbara Brenessel former Professor of Biology at Wheaton College, wrote the book on this estuarine icon in 2006 — and did not leave out its evolutionary origins. As it turns out, diamondback terrapins belong to a large family of freshwater turtles that include local favorites such as painted turtles.

“We are fa-mi-ly!” – "Northern Map Turtle" by Dger, Wikimedia Commons  |  Terrapin Photo by Del. Seashore State Park  |  “Female Painted turtle” by Jmalik, Wikimedia Commons

“We are fa-mi-ly!” – “Northern Map Turtle” by Dger, Wikimedia Commons | Terrapin Photo by Del. Seashore State Park | “Female Painted turtle” by Jmalik, Wikimedia Commons


The closest relative of the terrapin is the map turtle, a completely freshwater turtle that gets its names from the contour-line like pattern on its skin.  Researchers used both fossil and DNA evidence to conclude that terrapins and map turtles evolved from a common ancestor about 7 to 11 millions years ago. Around this time, terrapins developed characteristics such as salt glands that adapted them to estuaries where they have persisted since with little evolutionary change.     

Where the terrapin is going evolutionarily is another question. The turtle that spawned this question was likely returning to the Bay from its nesting grounds in the ocean dunes when it was run over on Route 1.Terrapin

Thankfully for the next generation, it had already laid its eggs when it was hit. The evolutionary pressures acting on this generation will be much different than those that came before it. Those terrapins that have the best success negotiating traffic and those that can successfully lay eggs in bayside backyards will have the best chance of passing on their genes. 

We can help terrapins have a fighting chance at survival by doing a few things:

  • First, build far away from the Bay: a wide buffer of natural lands between development and the estuary is needed for the animals and plants of the estuary to survive and reproduce.
  • Second, never harden a bay shoreline with bulkheads or rip-rap, terrapins need natural or living shorelines to access the uplands they use to lay eggs.
  • Third, slow down, watch out, and, when safe, help turtles cross the road during their summer migrations to and from their nesting grounds.        

Learn more about this fascinating species »

About the Author

Chris Bason

Chris Bason

Chris Bason is the Executive Director for the Center for the Inland Bays.

With the Center, he has been responsible for assessing the health of the Bays and synthesizing environmental research to educate the public and decision makers. He also conducts and coordinates research and water quality improvement demonstration projects.

Chris has a life-long passion for the environment of Delaware, and enjoys spending time outdoors surfing, fishing, kayaking, and hiking.


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