Archelon 

When: Late Cretaceous (~75 to 65 million years ago)

Where: The shallow sea that covered Central North America during the late Cretaceous.  

What: Archelon is an extinct sea turtle. It is the biggest sea turtle known; the largest specimen measuring 13 feet (~4 meters) long and 16 feet (~5 meters) wide, including the flippers. Archelon’s weight is estimated at almost 5,000 lbs (~2,200 kg). The closest living relative of this massive reptile is the leatherback sea turtle, Dermochelys.  As a comparison, the largest living leather back on record was 10 feet (~3 meters) long and weighed over 2,000 lbs (~900+ kg). As in the leatherback today, Archelon did not have a completely solid shell, but rather had large open areas in the bones, which were covered by thick skin. This results in a far lighter shell than if the structure was solid. This reduction in bone mass is part of a suite of adaptations that allowed these turtles to grow so large. Archelon is thought to have had a long life span, based on growth lines on long bones, several specimens have estimated ages of over a century. 

Archelon swam though the Cretaceous Seaway, a shallow sea that covered central North America in the late Cretaceous. It shared these waters with many large reptilian predators, such as the mosasaurs, an example of which is seen behind the mounted skeleton in the top image above.  As you can see these carnivores were plenty large enough to easily make a meal out of this huge turtle. Archelon and all members of the family Protostegidae vanish from the record at the end of the Cretaceous period.