Pteranodon

Pteranodon ("toothless wing") is a genus of pterosaur from the Late Cretaceous of North America.

Description
More fossil material from Pteranodon is known than from any other genus of pterosaur. As such, it is very well studied.

Size
Specimens of Pteranodon can be divided into two distinct size classes, representing male and female animals. Male Pteranodon were some of the largest known pterosaurs (after azhdarchids), with an average wingspan of 5.5 meters. Some males grew even larger, to 6.25 meters. Females were much smaller, with an average wingspan of approximately 4 meters.

Estimating the weight of Pteranodon is much more difficult. Estimates have ranged from as low as 20 kilograms to as high as 93 kilograms. A study by Mark Witton and Mike Habib indicated that the largest estimates were almost certainly unfeasible given the total volume of a Pteranodon body. They observed that most estimates were created by scaling modern birds and bats to larger sizes, despite the fact that pterosaurs had very different proportions and body structures.

Skull
The most distinctive characteristic of Pteranodon is its head crest. The crest shape differs between species, which helps to tell them apart. In all cases, females had smaller crests. Because it was larger in males, the crest was likely used for sexual display (see Crest function, below).

Skeleton
Pteranodon had narrow neural spines projecting from the vertebrae. It also had a short tail, which measured up to 25 centimeters in length.

Classification
Two species of Pteranodon are currently considered valid: P. longiceps (the type species) and P. sternbergi, which differ only in crest shape. Some researchers consider P. sternbergi to be part of a different genus, which was named Geosternbergia in 2010.

Dozens of Pteranodon species have been named, but the vast majority of them are synonymous with other genera.

History
Pteranodon was the first pterosaur found outside of Europe. Its fossil wing bones were first discovered in 1870 by Othniel Charles Marsh, along with a tooth from the Cretaceous fish Xiphactinus. Marsh mistakenly believed that the fish tooth belonged with the pterosaur, due to the fact that all pterosaurs known at the time had teeth. He named the animal Pterodactylus oweni in 1871, as part of the well-known but much smaller European genus Pterodactylus. Soon, Marsh realized that the binomial name had already been used for another species of Pterodactylus, and so he renamed his find Pterodactylus occidentalis ("Western wing finger") in 1872.

Around the same time, Marsh's rival Edward Drinker Cope had discovered specimens of the same animal. He described two new species and assigned them to the genus Ornithocheirus. However, Cope accidentally misspelled the name as "Ornithochirus", and so inadvertently created an entirely new genus for the two species (which were labeled O. umbrosus and O. harpyia). His paper naming the animal was published a mere five days after Marsh's paper on Pterodactylus occidentalis, which ignited a dispute over which name rightfully took priority. In 1875, Cope conceded that Marsh's names had priority over his own, and instead argued that Pterodactylus umbrosus, while not a distinct genus, was a distinct species that Marsh had not previously described.

The first two skulls of Pteranodon were discovered in 1876 by a fossil collector working for Marsh. Marsh recognized that this was the skull of the pterosaur he had previously named, and also noted that the skull was entirely toothless. He described the animal as Pteranodon longiceps ("wing without teeth", in reference to the toothless skull) and placed all North American species of Pterodactylus into the genus.

Fossils of Pteranodon sternbergi, including a lower jaw, were discovered by George F. Sternberg in 1952 and described by John Christian Harksen in 1966. It is older than P. longiceps, and considered by some to be the ancestor of the later species.

Paleoecology
Specimens of Pteranodon have been discovered in Kansas, Wyoming, and South Dakota. During the Cretaceous, this area was covered by a large inland sea called the Western Interior Seaway. The sea itself was populated by prey items such as ammonites, squid, and fish. More advanced animals such as sea turtles, plesiosaurs, and mosasaurs were also common. In fact, bones of Pteranodon were found inside the stomach of a plesiosaur skeleton, suggesting that the pterosaur was not immune to predation.

Diet
Pteranodon is known to have eaten fish, as several specimens have fish remains preserved in their stomach.

It is traditionally thought that Pteranodon caught fish by dipping its beak into the water while flying low above the water, but this is based on the assumption that the animal could not take off from the water surface. In fact, the head and shoulders of Pteranodon are similarly built as those of modern diving birds, and it is possible that it may have dived into the water to catch fish much like the present-day gannet.

Crest function
The exact function of the cranial crest was a subject of much debate. In 1910, George Francis Eaton proposed two possibilities for the crest's usage: as a muscle attachment point and a counterbalance for the large beak. Eaton admitted that the crest was too large to be used solely as a muscle attachment point, and it was later shown that the extravagant crest of a male P. sternbergi actually had a negative effect on the counterbalance of the head.

Dominik von Kripp suggested in 1943 that the crest may have been used as a rudder. This hypothesis was embraced by several subsequent researchers. However, even if the crest was used as a rudder, it would not nearly be as useful in this capacity as the animal's wings.

In fact, none of these previous hypotheses took female animals (whose crests are much smaller) into account. Thus, it is very likely that the crests were used in sexual display. This is consistent with the variation seen in Pteranodon fossils, where females and juveniles had small crests, while male individuals had long ones.

Flight
The wing shape of Pteranodon suggests that it would have flown much like a modern albatross. Like albatrosses, most of a Pteranodon flight would have been spent soaring using wind currents, but it would occasionally have required periods of wing flapping. Like other pterosaurs, it probably took to the air from a quadrupedal position.