Turtles and tortoises (Testudines)
The largest turtle ever was Archelon ischyros at 4 m (13 ft) long, 4.9 m (16 ft) wide and 2200 kg (4,850 lbs).
The first specimen of Archelon (YPM 3000) was collected from the Campanian-age Pierre Shale of South Dakota (a geological formation dated to 80.5 million years ago)by Dr. G.R. Wieland in 1895 and described by him the following year (Wieland, 1896). The largest Archelon fossil, found in the Pierre Shale of South Dakota in the 1970s, measures more than 4 metres (13 ft) long, and about 4.9 metres (16 ft) wide from flipper to flipper. It was a marine turtle, whose closest living relative in the present day is the leatherback sea turtle. Archelon lived at a time when a shallow sea covered most of central North America. Most of the known remains have been found in South Dakota and Wyoming. Though anatomically similar to the earlier species Protostega gigas, it was much larger.
Unlike most modern turtles, Archelon did not have a solid shell, but instead had a skeletal framework supporting a leathery or bony carapace. Other distinguishing features include a pointed tail, a narrow skull, a relatively narrow, high-vaulted shell, and a pronounced overbite.The live weight of an Archelon ischyros is estimated at more than 2200 kg (4,850 lb). They probably had a very strong bite, and were optimized for feeding on pelagic mollusks such as squid.The specimen exhibited by the Museum of Natural History in Vienna is estimated to have lived to be a century old, and may have died while brumating on the ocean floor.
Although a turtle, Archelon has become a staple species that appears in almost every book about dinosaurs and other prehistoric animals. This almost mandatory inclusion comes from the fact that Archelon is the largest turtle ever known to exist and it lived at the end of the Cretaceous. For the most part dinosaurs were probably not a threat, as assuming Archelon laid eggs like turtles do today, it would only ever venture onto land to lay eggs. Conversely however these eggs would eventually hatch and the young may have had to run a gauntlet of predators on their way to the ocean. Back in the Cretaceous this could mean everything from small beach combing dinosaurs, primitive seabirds similar to Ichthyornis to even pterosaurs.
Archelon is usually envisioned as being very similar to the leatherback turtle, and likely had a similar preference for eating jellyfish and cephalopods. The horned beak of the mouth which had a clear overbite would have been very effective at snipping soft bodied animals like these into bite sized portions that could be swallowed. Another similarity between these two turtles is that neither one has a solid shell. Instead a series of bony struts, that in Archelon are actually the ribs, create a framework of bone that an outer and relatively thin carapace sits on top of.
Perhaps the easiest explanation for Archelon’s shell design is that of neutral buoyancy. This is where a marine animal’s body adapts so that it neither floats to the surface, nor sinks to the bottom. In order to achieve this effect the body changes along such lines as denser thick bones that counter the lifting effect of the air in the lungs so that the animal does not bob around on the surface. Good examples for marine adaptation in reptiles can be seen in the placodonts from the Triassic which were actually negatively buoyant so that they could sink to the bottom where their preferred food was.
Archelon however was not a bottom feeder, it had to stay active in the mid to upper surface areas where its prey was most abundant, and as such it needed to be neutrally buoyant so that it could adapt to different depths wherever its prey was. As a large turtle Archelon obviously had a large shell, but if this shell was solid bone with a horn covering like in smaller turtles it would be extremely heavy. It’s feasible that a heavy solid shell would have tipped the scale into negative buoyancy so that Archelon would have had to spend a greater amount of energy just to stay up in the areas its food supply was, as well as rising up to breathe. This would not suit any animal, especially large ones like Archelon.
A shell composed of a framework with a relatively thin covering however would actually be very lightweight as well as reducing the body density so that Archelon did not readily sink as well as a more solid creature. This in turn would allow Archelon to cruise the oceans at a more leisurely pace and conserve energy so that it would not need as high a calorie intake as it would have done if the shell were solid.
The presence of large mosasaurs such as Tylosaurus as well as possibly even sharks like Cretoxyrhina suggest that Archelon probably was not invulnerable to predators, particular from attacks to the flippers. Despite this the sheer physical size of the shell may have still been enough to prevent some predators from being able to close their jaws around the body. At the very least this would have made a fully grown Archelon a difficult prey item compared to other softer bodied marine reptiles. Aside from this large size four star-shaped plates are found on the underside of the shell which seems to reinforce it. These may appear on the underside as a form of additional defence from predators that hit it from below. It’s perhaps not implausible that such attacks may have been initiated by mistake by predators that confused Archelon for a different kind of animal, possibly explaining the presence of these plates only on the underside.
The next largest was Protostega at 3 m (9.8 ft).
Protostega ('first roof') is an extinct genus of marine turtle containing a single species, Protostega gigas. Its fossil remains have been found in the Smoky Hill Chalk formation of western Kansas (Hesperornis zone, dated to 83.5 million years ago) and time-equivalent beds of the Mooreville Chalk Formation of Alabama. Fossil specimens of this species were first collected in 1871, and named by Edward Drinker Cope in 1872. With a length of 3 metres (9.8 ft), it is the second-largest sea turtle that ever lived, second only to the giant Archelon, and the third-largest turtle of all time behind Archelon and Stupendemys. Growing more than 10 feet (3 meters) long, Protostega was among the largest turtles to ever live. Unlike most turtles, whose shells are made of expanded and fused bones that form a relatively solid dome, widely spaced bones that looked more like the rafters of a roof held up Protostega's leathery shell. Though the shell design provided less protection, the lighter load combined with powerful, flipper-like front legs made Protostega a strong, inexhaustible swimmer. Females likely migrated hundreds of miles to lay eggs on sandy beaches, much like sea turtles do today.
Movement onshore, however, was difficult. An adult female Protostega may have weighed a ton or more, a hefty load to drag out of the ocean to lay eggs. But lay eggs on the beach they did by the dozens, a reproductive strategy of safety in numbers that helped at least a few survive to adulthood. In fact, marine turtles were the only seagoing reptiles to escape extinction at the end of the Cretaceous, 65 million years ago.
A large and pointed head with a sharp beak and strong jaws probably helped the ancient turtles feed on slow-moving marine creatures such as jellyfish and shellfish as well as seaweed and floating carcasses—much as their descendents do today. Shark teeth embedded in Protostega bones housed at a museum in Chicago suggest the turtles were sometimes a meal themselves.
Atlantochelys was first named in 1849 and based upon the description of half a humerus (analogous to your upper arm bone). For well over one hundred and fifty years that was it, but then in 2012 the missing half of this bone was actually matched up to the original specimen. Although Atlantochelys may be considered dubious because of still a lack of overall fossil remains, the humerus of Atlantochelys is still identified as coming from a protostegid turtle, while at the same time not like other known specimens. Scaling the size of the humerus to relative genera has yielded an approximate estimate that Atlantochelys grew to about three meters in length. This large size would have been the best protection for Atlantochelys given that large marine reptiles called mosasaurs as well as large sharks with teeth especially adapted for cutting bone and shell like those of Cretoxyrhina were all swimming in the oceans at the same time as Atlantochelys.
Atlantochelys has been classed within the Protostegidae group of turtles, and so speculated reconstructions of Atlantochelys are usually based upon other members of this group such as Protostega and Archelon, both of which seem to have been larger than Atlantochelys.
The newly discovered turtle, named Ocepechelon bouyai, is one of the biggest marine turtles that ever lived on Earth.
The generic name Ocepechelon derives from an acronym for the Groupe Office Chérifien des Phosphates, the mining company exploiting phosphatic deposits in Morocco, and from the Greek word chelone (turtle).
Ocepechelon is known from an almost complete 28-inch-long (70 cm) skull found in Sidi Chennane area in Morocco’s Khouribga Province.
Its striking anatomy suggests the turtle was adapted for suction feeding, never seen in known turtles.
Ocepechelon shares interesting resemblances with beaked whales. “Ocepechelon and most beaked whales are large animals, and, as in all other extant cetaceans and Ocepechelon, the nostrils of beaked whales are posterodorsally shifted. They also share similarities on their feeding apparatus: elongated jaws and a small gape.
The mouth gape of Ocepechelon can be inferred from the diameter of the tube – about 2.5 inches (6 cm). It is worth emphasizing the disproportion between the tiny size of the mouth opening and the large size of the Ocepechelon’s skull.
The snout diameter shows that Ocepechelon was a small-prey hunter and may have fed on small fishes, cephalopods and jellyfishes.