Sidersaura

MEANING: Star lizard

PERIOD: Late Cretaceous

CONTINENT: South America

Rebbachisaurids like Sidersaura are the last known diplodocoids. They lived alongside the titanosaurs until fairly late in the Cretaceous before disappearing from the fossil record, presumably going extinct. They had long tails and short necks, by sauropod standards. Sidersaura was large for a rebbachisaurid, growing to about 20 m in length.

Abstract from paper: Rebbachisauridae is mainly recorded during the early Late Cretaceous in Gondwana, and in South America in particular. Sidersaura marae gen. et sp. nov. was found in rocks of the Huincul Formation (upper Cenomanian – Turonian) of Neuquén Province, Argentina. Sidersaura has notable characteristics that allow recognising it as a new species and defining its phylogenetic relationships. First, it has a frontoparietal foramen, as in dicraeosauridae. It bears fourteen caudal vertebrae with neural arches displaced closer to the anterior margin of the centrum and a ventral longitudinal hollow on middle of the centra, both characters previously considered as titanosaurian synapomorphies, and here recognised in these diplodocoid sauropods. Sidersaura shows a peculiar tarsal condition with an unusual calcaneum morphology which resembles that of basal sauropods. The haemal arches have a stellate morphology with two sets of projections comparable to the specimen MMCh-PV 47 from the Candeleros Formation (Cenomanian), previously described as a titanosaurian. The phylogenetic analysis retrieves Sidersaura as a basal Rebbachisauridae more closely related to Zapalasaurus than to Limaysaurinae. The presence of a basal taxon at Cenomanian – Turonian times, so close to the extinction of the group, implies that the evolutionary history of rebbachisauridae was more complex than previously thought.

Sidersaura is from the Late Cretaceous. The Cretaceous is the third and final geological period of the Mesozoic Era, with the Late Cretaceous making up roughly the second half of it, lasting from about 100 to 66 million years ago. It was a time of significant evolutionary change, with dinosaurs reaching their greatest diversity before the mass extinction at the end of the Cretaceous.

The Cretaceous was a period with a relatively warm climate, though the Late Cretaceous experienced a global cooling trend, caused by falling levels of carbon dioxide in the atmosphere. The continents were nearing their present positions, but high sea levels flooded low-lying regions, turning Europe into an archipelago, and forming the Western Interior Seaway in North America. These seas were home to a variety of marine reptiles, including mosasaurs and plesiosaurs, while pterosaurs and birds shared the skies.

On land, dinosaurs continued to thrive and diversify during the Late Cretaceous, producing many of the most well-known goups, including tyrannosaurs, hadrosaurs, and pachycephalosaurs. Established Cretaceous dinosaur clades like the ceratopsians, ankylosaurs, and dromaeosaurs continued to flourish. Sauropod species consisted almost exclusively of titanosaurs, which seemed to be confined to the Southern Hemisphere for much of the Late Cretaceous. Flowering plants and grasses diversified and spread, becoming the dominant flora similar to what we see today.

The Cretaceous (along with the Mesozoic) ended with the Cretaceous-Paleogene (K-Pg) extinction event, a large mass extinction in which many groups, including non-avian dinosaurs, pterosaurs, and large marine reptiles, died out. This event, likely triggered by an asteroid impact, is marked by the abrupt K-Pg boundary, a distinct geologic layer separating the Mesozoic and Cenozoic Eras. In its aftermath, mammals and avian dinosaurs rapidly diversified, becoming the dominant land animals of the Cenozoic Era.

Sidersaura is a Rebbachisaurid. Rebbachisauridae is a distinctive group of sauropod dinosaurs within the larger clade Diplodocoidea, known primarily from the Cretaceous. They are characterized by several key anatomical traits, including lighter skeletal construction. Some rebbachisaurids also retained unusual, toothy adaptations such as elongated, spoon-shaped or spatulate teeth confined to the front of the jaws, likely used for cropping vegetation. These feeding adaptations suggest they may have specialized in a different range of plant material than other sauropods of their time.

Rebbachisaurids are best known from the southern hemisphere, particularly South America and Africa, but their fossils have also been found in Europe. This widespread distribution implies they were successful and adaptable herbivores in a variety of Cretaceous environments. Rebbachisaurids were important members of their ecosystems, often among the last surviving diplodocoid lineages before sauropod diversity narrowed to the titanosaurs in the Late Cretaceous.