Bajadasaurus is a genus of sauropod dinosaur from the Early Cretaceous epoch (late Berriasian to Valanginian stages, between 145 and 132.9 million years ago) of northern Patagonia, Argentina. It was first described in 2019 based on a single specimen found in 2010 that includes a largely complete skull and parts of the neck. The only species is Bajadasaurus pronuspinax. The genus is classified as a member of the Dicraeosauridae, a group of relatively small and short-necked sauropods.
Bajadasaurus sported bifurcated (two-pronged), extremely elongated extending from the neck. Similarly elongated spines are known from the closely related and more completely known Amargasaurus. Several possible functions have been proposed for these spines in Amargasaurus; the 2019 description of Bajadasaurus suggested they could have served as a passive defense against predators in both genera. The skull was slender and equipped with around 48 teeth that were pencil-shaped and restricted to the front of the jaws. The eye openings of Bajadasaurus were exposed in top view, possibly allowing the animal to look forwards while feeding. Bajadasaurus was discovered in sedimentary rocks of the Bajada Colorada Formation, which were deposited by braided rivers. It shared its environment with other dinosaurs including the sauropods Leinkupal and Ninjatitan and different theropods.
Discovery and naming
The only specimen of the dinosaur genus Bajadasaurus was excavated in 2010 by palaeontologists of the CONICET (National Scientific and Technical Research Council), the science agency of the Argentinian government. The specimen was discovered in the Bajada Colorada locality, south of the town of Picún Leufú, near the western bank of the Limay River, in Patagonia. The site is part of the Bajada Colorada Formation, a succession of sedimentary rocks in the Neuquén Basin that is dated to the late Berriasian to Valanginian stages of the Early Cretaceous (~145–132.9 million years ago). The specimen, of which only some teeth were initially exposed, was found by Argentinian palaeontologist Pablo Gallina. As fossils in this area are often fragile, the specimen was not excavated bone-by-bone in the field, but extracted as a single large block of rock and bone; before extraction, the block had been wrapped in plaster for protection. The block was prepared in a laboratory, revealing most of the skull as well as the first two and probably the fifth neck vertebrae. The specimen is now curated by the Museo Municipal Ernesto Bachmann in Villa El Chocón, Neuquén Province, under the specimen number MMCh-PV 75.
The specimen was formally described as a new genus and species, Bajadasaurus pronuspinax, by Gallina and colleagues in 2019. The generic name is derived from the Spanish word Bajada ("downhill") in reference to the Bajada Colorada locality, and the Greek saurus ("lizard"). The specific name pronuspinax is derived from the Latin pronus ("bent over forward") and the Greek spinax ("spine"), referring to the long and forward-curved (spinous processes) of the neck. Because of the spectacular spines, the discovery of Bajadasaurus was widely reported on by international news media. In 2023, Juan Garderes and colleagues published a comprehensive description of the skull based on a digital 3D reconstruction obtained from CT scans of the bones. In 2026, Garderes and colleagues applied retrodeformation to this model, a method to reverse the deformation of the bones that occurred during fossilization.
Description
Bajadasaurus is classified as a member of the Dicraeosauridae, a family of sauropod dinosaurs. As with all sauropods, dicraeosaurids were large, four-legged herbivores with a long neck and tail and a relatively tiny head. They were small compared to most other sauropods, reaching roughly the size of present-day Asian elephants, and their neck was comparatively short. Long bifurcated (two-pronged) neural spines were a common feature of the group, although they were only extremely elongated in Bajadasaurus and the closely related Amargasaurus. Bone fusion in the braincase and vertebral column indicates that the holotype represented an adult individual.
Skull
As in other flagellicaudatans (the group to which dicraeosaurids and diplodocids belong), the skull was elongated with an overall slender built. The preserved skull includes the bones of the , most of the and , as well as the lower jaws and parts of the upper jaws. As of 2019, it is the most complete skull of a dicraeosaurid known. The middle section of the skull is not preserved. The holotype skull is affected by crushing (brittle deformation) that occurred during diagenesis, which resulted in a number of fractures. In addition, the braincase is deformed plastically (without causing fractures).
All bones that surround the (eye socket) are preserved, except for the bone, which would have formed the lower margin of the opening. The bone, which formed the front margin of the orbit, had a straight ridge on its upper half that was similar to that of Dicraeosaurus. It was pierced by a small (opening), similar to the larger foramen seen in Dicraeosaurus. The upper-front corner of the orbit was formed by the bone. The contribution of the prefrontal to the orbit was smaller than in Dicraeosaurus and Amargasaurus; the bone was also smaller and less robust than in these genera. The upper rim of the orbit was formed by the , which was fused to the bone behind; together, these bones formed most of the rear part of the skull roof. Viewed from above, the side margin of the frontal was S-shaped and narrowed from back to front. As a result, the eye openings were visible in top view of the skull, unlike in related genera except Lingwulong. At the back, the frontal also formed a small part of the , a major opening on the rear part of the skull roof. The rear margin of the orbit was formed by the bone. Typically in dinosaurs, this bone featured a rearwards extending process, the posterior process. In Bajadasaurus, Dicraeosaurus, and Amargasaurus, this process was reduced and indistinct. The downward projecting process of the , a bone forming the upper rear corner of the skull, was elongated, distinguishing the genus from other dicraeosaurids. This suggests that it was connected to the bone at the lower rear edge of the skull, although the articulation itself is not preserved. This probable articulation was absent in diplodocids, and has not previously been documented in dicraeosaurids. Below the orbit and framed by the squamosal, postorbital, quadratojugal, and jugal was the , another major skull opening. In Bajadasaurus, this opening was narrow and obliquely oriented. The quadratojugal formed an obtuse angle that framed the lower rear part of the lateral temporal fenestra, different from the feature seen in diplodocids. The quadrate, a column-like bone forming parts of the rear of the skull, showed an expanded ventral surface (underside) as well as a squamosal process that was elongated but not expanded at its end; both features are considered as potential autapomorphies of Bajadasaurus.
The braincase is mostly hidden from view by overlying bones; only the (rear part) is exposed. The uppermost bone of the occipital region is the , which in Bajadasaurus was completely fused to the bone below and featured a distinct and narrow longitudinal ridge, the . The , a pair of openings between the parietal and the occipital region, were extended medially (towards the mid-plane of the skull), which is an autapomorphy of Bajadasaurus (a unique feature not found in closely related genera). The , which articulated with the first vertebra of the neck, was wider than it was high. Its rear surface was not wider than its neck, which was different from Amargasaurus and Dicraeosaurus. The , which formed part of the underside of the braincase, had a pair of gracile bony extensions, the , which extended forwards and downwards to articulate with the pterygoid of the palate, bracing the braincase against the latter. An autapomorphy of the genus, these processes were longer and more slender than in Dicraeosaurus and Amargasaurus, being more than six times long than wide. The left and right pterygoids, the only elements of the palate that were preserved, featured a smooth crest that received the basipterygoid processes.
The lower jaw was ca. 80% the length of the skull, which is similar to the proportions seen in diplodocids, but potentially questions previous reconstructions of other dicraeosaurids that assume a proportionally shorter lower jaw. The teeth were restricted to the front parts of the jaws, were pencil-shaped and tilted forwards; their narrow crowns were nearly straight or curved slightly inwards. Of the upper jaw, only the front section of the left (the largest bone of the upper jaw) is preserved. It preserves eight (tooth sockets), a count similar to Suuwassea, but less than in Dicraeosaurus, which had 12 teeth in each maxilla. A seemingly complete tooth row of 24 teeth was found close to, but separated from, the left maxilla. This count corresponds to the tooth count of the , where the teeth are still anchored within the left and right (the only tooth-bearing bones of the lower jaw). Bajadasaurus thus likely had 48 teeth in total. The dentary was slender, similar to Suuwassea but unlike the deep dentary of Dicraeosaurus. In top view, the dentaries do not form the box-shaped snout seen in diplodocids, but are more rounded with a J-shaped curvature, as is typical for dicraeosaurids. The front of the dentary had a hook-like "chin" projecting downwards, as seen in other flagellicaudatans. In the hind part of the lower jaw, the bone was very elongated and longer than the bone, unlike in diplodocids. The teeth of Bajadasaurus pronuspinax had a mean replacement time of about 40.7 days.
Neck vertebrae
Both —small, triangular bones located between the first neck vertebra and the skull—were found in articulation with the skull. Of the first neck vertebra (the ), only the upper elements, the , are preserved. These were triangular and wing-like in Bajadasaurus. The second neck vertebra, the , is nearly complete. As in Dicraeosaurus, it was twice as high as it was long, while its (or vertebral body) was twice as long as it was high. The (sidewards projecting processes) were small and directed backwards as in Suuwassea rather than downwards as in Dicraeosaurus and Amargasaurus. The neural spine of the axis was narrow and not bifurcated; it differed from other sauropods in being vertically oriented (an autapomorphy of the genus), triangular in cross-section, and tapering towards its apex.