Amargasaurus

Phylogeny: Dinosauria > Saurischia > Sauropodomorpha > Plateosauria > Massopoda > Sauropodiformes > Sauropoda > Gravisauria > Eusauropoda > Neosauropoda > Diplodocoidea > Dicraeosauridae

Overview: In the mid-1980’s, a team of Argentine paleontologists came across the partial skeleton of a highly unusual sauropod. This same expedition would also yield the fossils of the well known abelisaurid Carnotaurus, though the two genera didn’t live together and were separated by many tens of millions of years (Carnotaurus lived towards the end of the Cretaceous). The sauropod came from rocks dated to the Early Cretaceous, or around one hundred and twenty-five million years ago, in Argentina’s La Amarga Formation. The formation would inspire the creature’s generic name, Amargasaurus, coined in its 1991 description. Fossils of other sauropods, a theropod, and a potential stegosaur are also known from the La Amarga. Amargasaurus likely filled the niche of a low to mid-level browsing herbivore, given its size and neck length. The original holotype specimen of Amargasaurus remains the only one found thus far, which included shoulder bones, pieces of the pelvis, limb bones, and numerous vertebrae from the tail, spine and neck. The latter set of vertebrae contained the animal’s most distinctive traits.

Similar to some of its relatives, the neural spines sticking out from the neck vertebrae were double-pronged, instead of being only one process like in most animals. However, on Amargasaurus, these prongs were particularly elongated, the longest being up to sixty centimeters. These ran all along its neck and on the upper back. Scientists at first proposed these spines supported sails of skin, the purpose of which was unclear. Later, some studies would note features that suggested this wasn’t the case, with the spines being more like horns, bearing sheathes of keratin. Recently, the original idea has come back into favor. Either way, why it had this feature is still being debated. Spines could be useful for defense and display, but twin sails could’ve had some thermo-regulating function. Its close relative, Bajadasaurus, also had similar neck spines, though its were raked forwards, while those of Amargasaurus were raked towards the back. Both dinosaurs belonged to a family called the Dicraeosauridae, itself a part of the larger sauropod superfamily Diplodocoidea. As with most dicraeosaurids, Amargasaurus wasn’t very large for a sauropod, being medium-sized at best.