Archelon

|alt=A black and white photo of the turtle skeleton and a man standing next to it for scale. The man appears to be about half the size of the display In 1895, American paleontologist George Reber Wieland discovered an almost complete skeleton of a large aquatic turtle, lacking the skull, along the Cheyenne River in South Dakota. More specifically, the find was made near Custer County, within the Pierre Shale, a geological formation dating to the late Campanian of the Late Cretaceous. The year following the discovery, this specimen, now catalogued as YPM 3000 in the collections of the Peabody Museum of Natural History, was described by Wieland under the scientific name Archelon ischyros and designated as the holotype of the taxon. The genus name from the Ancient Greek - (-) 'first/early', () 'turtle', and species name from () 'mighty' or 'powerful'. A second specimen, a skull, was discovered in 1897 in the same region. was moved to the genus Archelon as A. copei. In 1998, A. copei was moved to the new genus (originally named Microstega, but subsequently renamed Kansastega) as K. copei. In 1992, a fourth and the largest specimen to date, nicknamed "Brigitta", was discovered in Oglala Lakota County, South Dakota and resides in the Natural History Museum Vienna. In 2002, a fifth specimen, a partial skeleton, was discovered from the Pierre Shale of North Dakota along the Sheyenne River near Cooperstown. ==Description==

The holotype measures from head to tail, with the head measuring , the neck , the thoracic vertebrae , the sacrum , and the tail . and, in life, weighed around . Archelon had a distinctly elongated and narrow head. It had a defined hooked beak which was probably covered in a sheath in life, reminiscent of the beaks of birds of prey. However, in the back, the cutting edge of the beak is dull compared to such animals. Much of the length of the head derives from the elongated premaxillae–which is the front part of the beak in this animal–and maxillae. The jugal bones, the cheek bones, due to the elongate head, do not project as far as they do in other turtles. The nostrils are elongated and rest on the top of the skull, slightly posited forward, and are unusually horizontal compared to sea turtles. The jugal bones (cheekbones) are rounded as opposed to triangular in sea turtles. The articular bone, which formed the jaw joint, was probably heavily encased in cartilage. The jaw probably moved in a hammering motion. Five neck vertebrae were recovered from the holotype, and it probably had eight in total in life; they are X-shaped, procoelous–concave on the side towards the head and convex on the other–and their thick frame indicates strong neck muscles. Ten thoracic vertebrae were found, increasing in size until the sixth then rapidly decreasing, and they have little connection with the carapace. The three vertebrae of the sacrum are short and flat. It probably had eighteen tail vertebrae; the first eight to ten (probably in the same area as the carapace) had neural arches, whereas the remaining did not. whose juveniles have an average growth rate of per year. The carapace, in life, probably featured a row of ridges along the midline over the chest region, perhaps totaling in seven ridges, with each ridge peaking at either . This leathery carapace is also seen in the leatherback sea turtle. The spongy makeup is similar to the bones seen in open-ocean going vertebrates such as dolphins or ichthyosaurs, and was probably also an adaptation to reduce overall weight. Plastron at the North American Museum of Ancient Life|alt=The underside of a suspended skeleton A turtle plastron, its underside, comprises (from head-most to tail-most) the epiplastron, the entoplastron, which is small and wedged in between the former and the hyoplastron; then is the hypoplastron and finally, the xiphiplastron. The plastron, as a whole, is thick, In protostegids, the epiplastron and entoplastron are fused together, forming a single unit called an "entepiplastron" or a "paraplastron." This entepiplastron is T-shaped, as opposed to the Y-shaped entoplastrons in other turtles. The top edge of the T rounds off, except at the center which features a small projection. The outward side is slightly convex and bends somewhat, away from the body. The 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. ==Classification and evolution==

was previously thought to be the closest living relative.|alt=A leatherback sea turtle leaving a beach, possibly after laying eggs. In its original 1896 description, Wieland placed Archelon into the family Protostegidae, which included at the time the smaller Protostega and Protosphargis, In 1953, Swiss paleontologist Rainer Zangerl split Protostegidae into two subfamilies: Chelospharginae and Protosteginae; to the former was assigned Chelosphargis and Calcarichelys, and the latter Archelon and Protostega. }}}} '', the type genus of the Protostegidae ==Paleobiology==

may have been a food source. It might have been able to target larger fish and reptiles, 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. 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 they could have pursued active prey, though they probably could not have sustained high speeds. Overall, it may have been a moderately-good swimmer, capable of open-ocean travel. ==Paleoecology==

Archelon inhabited the shallow Western Interior Seaway; The Late Cretaceous Dakotas were submerged in the Northern Inland Subprovince, an area characterized by moderate to cool temperatures, with an abundance of plesiosaurs, hesperornithiform seabirds, and mosasaurs, particularly Platecarpus. There is no fossil evidence for vertebrate migration between the northern and southern provinces. Though sharks were generally more common in the southern province, several sharks are known from the Pierre Shale, including Squalus, Squalicorax, Pseudocorax, and Cretolamna. the squid-like belemnites, and nautilus. which other turtles were able to survive due to some thermoregulatory capabilities. Average water temperature may have decreased to depending on estimated CO2 levels. However, some Maastrichtian-age Kansas Pierre Shale fossils may have been eroded millions of years ago, and it is possible Archelon survived well into the Maastrichtian. ==See also==