two ends of the T flatten out, getting broader and thinner, as they get farther from the center. A thick, continuous ridge connects the hyoplastron, hypoplastron, and xiphiplastron. The hyoplastron features a large number of spines projecting around the circumference. The hyoplastron is slightly elliptical, and grows thinner as it gets farther from the center, before the spines erupt. The spines grow thick and narrow towards their middle portion. The seven to nine spines projecting towards the head are short and triangular. The six middle spines are long and thin. The last 19 spines are flat. There are no marks indicating contact with the entepiplastron. The hypoplastron is similar to the hyoplastron, except it has more spines, a total of 54. The xiphiplastron is boomerang-shaped, a primitive characteristic in contrast to the straight ones seen in more modern turtles. Paleobiology Archelon was an obligate carnivore. The thick plastron indicates the animal probably spent a lot of time on the soft, muddy seafloor, likely a slow-moving bottom feeder. According to American paleontologist Samuel Wendell Williston, the jaws were adapted for crushing, implying the turtle ate large mollusks and crustaceans. In 1914, he suggested that the abundant, thin-shelled, bottom-dwelling Cretaceous bivalves–some exceeding in diameter–would have easily been able to sustain Archelon. However, these were probably absent in the central Western Interior Seaway by the Early Campanian. Conversely, the beak may have been adapted for shearing flesh. It might have been able to target larger fish and reptiles, as well as, similar to the leatherback sea turtle, soft-bodied creatures such as squid and jellyfish. However, it is possible the sharp beak was used only in combat against other Archelon. The nautilus Eutrephoceras dekayi was found in great number near an Archelon specimen, and may have been a potential food source. Archelon may have also occasionally scavenged off the surface water. Archelon probably had weaker arms, and thus less swimming power, than the leatherback sea turtle, and so did not frequent the open ocean as much, preferring shallower, calmer waters. This is indicated by the similarity of the humerus/arm and hand/arm ratios of it and cheloniids, which are known to have poor development of the limbs into flippers and a preference for shallow water. Conversely, the large flipper-to-carapace ratio of protostegids and the similarly large flipper spread, like that of the predatory cheloniid loggerhead sea turtle (Caretta caretta), combined with a broad body, indicate