Sauroposeidon (meaning "earthquake god lizard", after the Greek god Poseidon) is a genus of sauropod dinosaur known from several incomplete specimens including a bone bed and fossilized trackways that have been found in the American states of Oklahoma, Wyoming, and Texas. The fossils were found in rocks dating from near the end of the Early Cretaceous (Aptian–early Albian), a time when sauropod diversity in North America had greatly diminished. It was the last known North American sauropod prior to an absence of the group on the continent of roughly 40 million years that ended with the appearance of Alamosaurus during the Maastrichtian.
While the holotype remains were initially discovered in 1994, due to their unexpected age and unusual size they were initially misclassified as pieces of petrified wood. A more detailed analysis in 1999 revealed their true nature which resulted in a minor media frenzy, and formal publication of the find the following year.
Paleoecological analysis indicates that Sauroposeidon lived on the shores of the Gulf of Mexico, in a river delta. Extrapolations based on the more completely known Brachiosaurus indicate that the head of Sauroposeidon could reach 18 m (59 ft) in height with its neck extended, which would make it the tallest known dinosaur. With an estimated length of up to 34 m (112 ft) and a mass of 50–60 t (55–66 short tons), it also ranks among the longest and heaviest. However, this animal may not be as closely related to Brachiosaurus as previously thought, so these estimates may be inaccurate.
While initially described as a brachiosaurid closely related to Brachiosaurus and Giraffatitan, the discovery of additional remains in the Cloverly Formation of Wyoming suggested that it was in fact more closely related to the titanosaurs, in the group Somphospondyli. Analysis of these remains and comparison with others from Texas supported this conclusion, and demonstrated that the more completely known sauropods from the Twin Mountains Formation (including a partial skull and fossil trackways) previously named Paluxysaurus jonesi also belonged to Sauroposeidon.
The press release in 1999 immediately garnered international media attention, which led to many (inaccurate) news reports of "the largest dinosaur ever!". While it is true that Sauroposeidon is probably the tallest known dinosaur, it is neither the longest nor the most massive. Argentinosaurus is a better candidate for the title "World's Largest Dinosaur" (presuming to ignore, as is conventional, Amphicoelias), though weak fossil evidence makes an exact ranking impossible.
The original Sauroposeidon find was composed of four articulated, mid-cervical vertebrae (numbers 5 to 8), with the cervical ribs[clarification needed] in place. The vertebrae are extremely elongated, with the largest one having an overall length of 1.4 m (4.6 ft), making it the longest sauropod neck vertebra on record. Examination of the bones revealed that they are honeycombed with tiny air cells, and are very thin, like the bones of a chicken or an ostrich, making the neck lighter and easier to lift. The cervical ribs were remarkably long as well, with the longest measurable rib (on vertebra 6) measuring 3.42 m (11.2 ft) – about 18% longer than the longest rib reported for Giraffatitan, but exceeded in length by the cervical ribs of Mamenchisaurus.
Estimates of Sauroposeidon's size are based on a comparison between the four Sauroposeidon vertebrae and the vertebrae of the HM SII specimen of Giraffatitan brancai, located in the Humboldt Museum in Berlin. The HM SII is the most complete brachiosaur known, though since it is composed of pieces from different individuals its proportions may not be totally accurate. Comparisons to the other relatives of Sauroposeidon are difficult due to limited remains. The neck length of Sauroposeidon is estimated at 11.25–12 m (37–39 ft), compared to a neck length of 9 m (30 ft) for the HM SII Giraffatitan. This is based on the assumption that the rest of the neck has the same proportions as Giraffatitan, which is a reasonably good conjecture.
Sauroposeidon was probably able to raise its head 17–18 m (56–59 ft) above the ground, which is as high as a six-story building. In comparison, Giraffatitan could probably raise its head 13.5 m (44 ft) into the air.
Sauroposeidon's shoulder height has been estimated at 6–7 m (20–23 ft) based on an interpretation of the animal as a brachiosaurid. Estimates of its total possible length have ranged from 28 m (92 ft) to 34 m (112 ft).
The mass of Sauroposeidon is estimated at 50–60 t (55–66 short tons). While the vertebrae of Sauroposeidon are 25–33% longer than Giraffatitan', they are only 10–15% larger in diameter. This means that while Sauroposeidon probably has a larger body than Giraffatitan its body is smaller in comparison to the size of its neck, so it did not weigh as much as a scaled-up Giraffatitan. By comparison, Giraffatitan might have weighed 36–40 t (40–44 short tons). This estimate of the Giraffatitan is an average of several different methodologies.
However, Sauroposeidon has a gracile neck compared to Giraffatitan. If the rest of the body turns out to be similarly slender, the mass estimate may be too high. This could be similar to the way the relatively robust Apatosaurus weighs far more than the longer but much slimmer Diplodocus. In addition, it is possible that sauropods may have had an air sac system, like those in birds, which could reduce all sauropod mass estimates by 20% or more.
Sauropods, which include the largest terrestrial animals of all time, were a very wide ranging and successful group. They first appeared in the Early Jurassic and soon spread across the world. By the time of the late Jurassic, North America and Africa were dominated by the diplodocids and brachiosaurids and, by the end of the Late Cretaceous, titanosaurids were widespread (though only in the southern hemisphere). Between these periods, in the Early Cretaceous, the fossil record is sparse. Few specimens have been found in North America from that time and those specimens that do exist are often fragmentary or represent juvenile members of their species. Most of the surviving sauropods at the time were also shrinking in size to a mere 15 m (49 ft) in length, and maybe 10–15 t (11–17 short tons), which makes the discovery of an extremely specialized super-giant like Sauroposeidon very unusual.
Sauroposeidon lived on the shores of the Gulf of Mexico, which ran through Oklahoma at that time, in a vast river delta similar to the Mississippi delta today. This paleoenvironment, which has been preserved in the Antlers Formation, also stretches from southwest Arkansas through southeastern Oklahoma and into northeastern Texas. This geological formation has not been dated radiometrically. Scientists have used biostratigraphic data and the fact that it shares several of the same genera as the Trinity Group of Texas, to surmise that this formation was laid down during the Albian stage of the Early Cretaceous Period, approximately 110 mya. The area preserved in this formation was a large floodplain that drained into a shallow inland sea. Several million years later, this sea would expand to the north, becoming the Western Interior Seaway and dividing North America in two for nearly the entire Late Cretaceous period. The paleoenvironment of Sauroposeidon consisted of tropical or sub-tropical forests, river deltas, coastal swamps, bayous and lagoons, probably similar to that of modern-day Louisiana. There were few predators which could attempt to attack a full-grown Sauroposeidon, but juveniles were likely to be preyed on by the contemporary Acrocanthosaurus atokensis  (a carnosaur slightly smaller than a Tyrannosaurus), which likely were the apex predators in this region, and the small coelurosaur Deinonychus antirrhopus. Sauroposeidon also shared its paleoenvironment with other dinosaurs, such as the sauropod Astrodon (Pleurocoelus) and the most common dinosaur in this region, the ornithopod Tenontosaurus. Other vertebrates present during this time included the amphibian Albanerpeton arthridion, the reptiles Atokasaurus metarsiodon and Ptilotodon wilsoni, the crurotarsan reptile Bernissartia, the cartilaginous fish Hybodus buderi and Lissodus anitae, the ray-finned fish Gyronchus dumblei, the crocodilians Goniopholis and Paluxysuchus, and the turtles Glyptops and Naomichelys. Possible indeterminate bird remains are also known from the Antlers Formation. The fossil evidence suggests that the gar Lepisosteus was the most common vertebrate in this region. The early mammals known from this region included Atokatherium boreni and Paracimexomys crossi.