Astigmasaura

MEANING: Signless lizard

PERIOD: Late Cretaceous

CONTINENT: South America

Astigmasaura is a medium-sized rebbachisaurid sauropod dinosaur from the Late Cretaceous period of what is now Argentina. Rebbachisaurids had highly pneumatized bones that helped them to hold their weight on their four pillar-like legs, and extremely long tails. Astigmasaura was likely a ground-level browsing herbivore.

Abstract from paper: Rebbachisaurids are medium to large-sized, non-selective and ground-level browser diplodocoid sauropods, and they are characterised by highly specialised skulls, widely pneumatized axial elements and gracile appendicular skeletons. Known from the Early Cretaceous to the early Late Cretaceous, the rebbachisaurid fossil record is particularly diversified in Gondwana, with several specimens found in North Africa and South America. Notably, Patagonia has yielded over more than half of all known Rebbachisauridae, including the most basal forms and the youngest rebbachisaurid remains to date. Herein, we describe a new species of Rebbachisauridae from the Huincul Formation (upper Cenomanian) of the Neuquén Basin (Patagonia, Argentina): Astigmasaura genuflexa gen. et sp. nov. New laboratory work provided the complete osteology of the specimen MAU-Pv-EO-629, complementing the previously published hind limb anatomy (Bellardini et al., 2024). Thus, the type material of Astigmasaura comprises the posterior portion of an articulated, postcranial skeleton of a single, morphologically adult individual. Astigmasaura shares different conditions with other Rebbachisauridae, including anterior caudal vertebrae with tall neural spines and tetraradiate neural laminae, asymmetric middle haemal arches, mediolaterally compressed proximal tibiae, and femur with medially inclined distal condyles. Furthermore, Astigmasaura shows a unique combination of diagnostic features that distinguish it from all other sauropods. This new record not only provides new morphological information about the caudal and pelvic girdle anatomy of Rebbachisauridae, which is poorly known to date, and suggests a greater taxonomic diversification within the family during the last stages of its evolutionary history than known before.

Astigmasaura 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.

Astigmasaura 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.