Archelon

Archelon is an extinct genus of marine turtle from the Late Cretaceous period. It holds the record as the largest turtle ever documented in the fossil record. This massive animal belonged to the extinct family Protostegidae. While it lived in the ocean, it was quite different from the sea turtles we see today. Scientists study Archelon to understand how ancient marine reptiles adapted to life in the open sea.

The anatomy of Archelon was specialized for a life of swimming and feeding. Unlike most turtles with heavy, solid shells, Archelon possessed a leathery carapace. This carapace consisted of bony plates called neuralia and pleuralia. These plates were connected by irregular, finger-like sutures. Because these bony portions were thin, the animal likely had thick, strong skin covering them. This skin provided support for the shoulder girdle. The carapace might have also featured a row of seven ridges along the midline.

Archelon's skeletal structure helped it navigate the deep ocean. It possessed osteosclerotic structures, which are areas of very dense, heavy bone. These acted as ballast to help the turtle stay stable in the water, similar to how whales use weight. Its head was long and narrow, featuring a distinct hooked beak. This beak was likely covered in a sheath, much like the beak of a bird of prey. The jaws were designed for a hammering motion. This allowed the turtle to crush hard-shelled prey like mollusks or crustaceans.

The turtle's movement was powered by massive foreflippers. The humeri, or upper arm bones, were proportionally very large. The radii and ulnae, which make up the forearm, were short and compact. These limbs could spread between 2.4 and 3.1 meters wide. Such large flippers provided the powerful strokes needed for open-ocean travel. They also helped the animal escape from marine predators.

Researchers have identified several important specimens that define this genus. The holotype, catalogued as YPM 3000, was found in 1895 near the Cheyenne River in South Dakota. In 1992, a massive specimen nicknamed "Brigitta" was discovered in Oglala Lakota County, South Dakota.

Brigitta is the largest specimen known to date. It measured approximately 4.6 meters from head to tail. In life, this individual likely weighed about 2,270 kilograms. Another partial skeleton was discovered in 2002 in North Dakota.

The history of Archelon's classification has changed as more fossils were found. American paleontologist George Reber Wieland named the genus in 1896. He based his description on the South Dakota skeleton. Originally, the genus included species like A. marshii and A. copei. However, modern science has reassigned these to other genera, such as Protostega and Kansastega.

Scientists once believed the leatherback sea turtle was the closest living relative of Archelon. We now know that the family Protostegidae is a completely separate lineage from any living sea turtle.

Archelon lived in the northern Western Interior Seaway. This was a mild to cool temperate environment. The waters were filled with other large animals, including mosasaurs and plesiosaurs. Eventually, Archelon went extinct. Several factors may have caused this disappearance. The seaway began to shrink, and the global climate became rapidly cooler. Additionally, higher rates of predation on eggs and hatchlings on land, along with infant mortality in the sea, likely contributed to their end.