Archelon

Lore & Background

Archelon is known only from the Pierre Shale, a geological formation dating to the late Campanian of the Late Cretaceous. It inhabited the northern Western Interior Seaway, a mild to cool temperate region that was also home to plesiosaurs, hesperornithiform seabirds, and mosasaurs. The largest documented specimen, nicknamed "Brigitta," measures around 4.6 meters from head to tail and 4 meters from flipper to flipper, with an estimated body mass of about 2,200 kilograms. The skull alone could reach up to 70 centimeters in length. The head is distinctly elongated and narrow, featuring a pronounced hooked beak that was likely covered in a sheath during life, reminiscent of a bird of prey. The cutting edge of the beak is duller in the back compared to such birds. Much of the head’s length comes from elongated premaxillae and maxillae. The jugal bones are rounded rather than triangular, and the nostrils are elongated, positioned on top of the skull, and unusually horizontal. The jaw joint was probably heavily encased in cartilage, and the jaw likely moved in a hammering motion. Instead of a hard shell, Archelon had a leathery carapace that may have featured a row of small ridges. Its large, strong foreflippers were suited for powerful strokes needed for open-ocean travel and escape from predators. The jaws were adept at crushing, suggesting a diet of hard-shelled crustaceans, mollusks, and possibly sponges, while slowly moving over the seafloor. It may also have consumed jellyfish, squid, nautiloids, or fish while swimming near the surface. The beak may have been better adapted for shearing flesh. Extinction may have resulted from the shrinking of the seaway, increased infant mortality, higher egg and hatchling predation, and a rapidly cooling climate.

Reader's Guide

Archelon is significant as the largest turtle ever documented, providing insight into the evolution and ecology of ancient marine reptiles. Its leathery carapace, rather than a hard shell, and its osteosclerotic bones suggest adaptations for open-ocean travel, similar to modern leatherback sea turtles. The genus was originally placed in the extinct family Protostegidae, which is now thought to be a completely separate lineage from any living sea turtle. Archelon likely inhabited the northern Western Interior Seaway, a mild to cool temperate area dominated by plesiosaurs, hesperornithiform seabirds, and mosasaurs. Its extinction may have been caused by the shrinking of the seaway, increased infant mortality, higher predation on eggs and hatchlings, and a rapidly cooling climate. The species name ischyros means 'mighty' or 'powerful' in Ancient Greek.

Did You Know?

• Archelon had a leathery carapace instead of a hard shell, similar to the leatherback sea turtle.
• The largest specimen, nicknamed 'Brigitta', measures 4.6 m (15 ft) from head to tail and 4.0 m (13 ft) from flipper to flipper.
• The genus name comes from Ancient Greek words meaning 'first/early turtle'.
• Archelon's jaws were adept at crushing, suggesting it ate hard-shelled crustaceans, mollusks, and possibly sponges.