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Hypselosaurus (meaning 'highest lizard', from Ancient Greek ὑψηλός meaning 'high' or 'lofty' and σαυρος meaning 'lizard') is a nomen dubium genus of sauropod that lived in southern France during the Late Cretaceous, approximately 70 million years ago in the early Maastrichtian. Hypselosaurus was first described in 1846, but was not formally named until 1869, when Phillip Matheron named it under the binomial Hypselosaurus priscus. The holotype specimen includes a partial hindlimb and a pair of caudal vertebrae, and two eggshell fragments were found alongside these bones. Because of the proximity of these eggshells to the fossil remains, many later authors, including Matheron and Paul Gervais, have assigned several eggs from the same region of France all to Hypselosaurus, although the variation and differences between these eggs suggest that they do not all belong to the same taxon. Hypselosaurus has been found in the same formation as the Variraptor and Pyroraptor, the ornithopod Rhabdodon, and the Rhodanosaurus, as well as indeterminate bones from other groups.
Since its original description, several specimens have been referred to Hypselosaurus. One of these, originally described in 1957 by Albert de Lapparent, includes a caudal vertebra, as well as another vertebra described in 1960 by Bataller. In 1993, a review of European sauropods stated that Hypselosaurus was a nomen dubium, and that its holotype could not be verifiably distinguished from other sauropods in the same, and many other, regions. Based on this, the material that was referred to Hypselosaurus by several other authors would have to be considered an intermediate titanosaur.
In addition to the holotype skeleton, Matheron described two fragments of a spherical or ellipsoid fossil in 1869. These fragments were studied for some time by Matheron and his contemporary French paleontologists, and the only probable conclusion was that they were fragments of eggshells. The eggs were proposed to have been significantly larger than those of Aepyornis when complete, and Matheron suggested that they either were a very large bird egg, or belonged to Hypselosaurus.
When originally named, Matheron proposed that Hypselosaurus was an aquatic crocodile, as was suggested for similar taxa like Pelorosaurus, Cetiosaurus, Streptospondylus and Steneosaurus by their describers. Matheron noted that the long bones, particularly the femur, lacked medullary bone, and thus proposed that it could not have been terrestrial like Iguanodon.
The discoveries of new material from the Rognac area of France has led to the suggestion that Hypselosaurus may be the same taxon as Atsinganosaurus, which requires further material with overlap to confirm or deny.
As preserved, the left tibia is largely incomplete, with only a small section of the shaft, just proximal to the distal condyles, known. Like the femur, there is no medullary canal present, and the spongey internal bone is asymmetrically dense. Few other features can be identified, among those the dimensions and cross-section of the bone. The tibia is ovoid in cross-section, with the anterior diameter long, and wide.
The fibula, at in length, corresponds well with the size of the femur and tibia. The fibula is approximately an equilateral triangle in cross-section, with the inner face slightly concave and outer face convex. The flatter surface of the fibula is wide, and opposite ridge protrudes from this.
Eggs with abnormally thin shells have been attributed to Hypselosaurus priscus. Some experts have speculated that this was the cause of the species' extinction, with vegetation changes, climatic change and overcrowding being the original impetus for the shell thinning. However, there are alternative explanations for the thin eggshell not dependent on pathology. Later researchers found evidence that the eggs in question successfully hatched. Some researchers postulated that the thinner " Hypselosaurus priscus" eggshells came from different taxa than the thicker eggshells, and subsequent researchers have come to support this idea. Another potential explanation for variation in eggshell thickness is that the thinner eggs were laid by younger individuals than the thicker shell eggs laid by older individuals or that it was a consequence of natural variations of eggshell thickness within a single species.
Most Hypselosaurus egg are in collections and museums in France, but an egg is on display in the Life on Earth gallery at Leeds City Museum, United Kingdom, and a rock slab with twelve eggs is on display at the Museum of Evolution in Knuthenborg Safaripark, Denmark.
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