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.

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 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, 40 kilometres (25 mi) 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[1]).

[4] 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.

[6][7] 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.

The downward projecting process of the squamosal, a bone forming the upper rear corner of the skull, was elongated, distinguishing the genus from other dicraeosaurids.

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 posttemporal fenestrae, 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).

[5] 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.

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.

This vertebra sported the most prominent feature of the genus, an extremely elongated neural spine that was deeply bifurcated into a left and right rod-like element.

[10][11] In their analysis of evolutionary relationships, Gallina and colleagues recovered Bajadasaurus as an intermediate member of Dicraeosauridae, more derived (more recently diverging from a common ancestor) than Suuwassea and Lingwulong, but less so than Pilmatueia, Amargasaurus, Dicraeosaurus, and Brachytrachelopan.

A subsequent analysis by John Whitlock and Jeffrey Wilson Mantilla, in 2020, found Bajadasaurus to be the most basal member of a clade that also contains Lingwulong and the unnamed North American taxon MOR 592.

[12] A 2022 study led by Guillermo Windholz analyzed multiple datasets and found conflicting results, with one of their analyses finding Bajadasaurus to be most closely related to Amargatitanis and Pilmatueia in a clade of South American dicraeosaurids.

They argued that, despite the inconsistent results of their phylogenetic analyses, it would be reasonable to expect South American dicraeosaurids to group together on biogeographic grounds.

[3] Cladogram by Gallina and colleagues (2019)[3] Rebbachisauridae Diplodocidae Suuwassea Lingwulong Bajadasaurus Pilmatueia Amargasaurus Dicraeosaurus Brachytrachelopan

Cladogram by Windholz and colleagues (2022) showing their favored result of a clade of South American dicraeosaurids[13] Suuwassea Lingwulong Dicraeosaurus Brachytrachelopan Amargasaurus Amargatitanis Bajadasaurus Pilmatueia

These researchers further argued that horn is more resistant to impact-related fractures than bone, and that a horny sheath would therefore have protected the delicate spines from damage.

Due to its forward bend, the bifurcated neural spine of the supposed fifth neck vertebra would have reached past the head, and could therefore have been a barrier to predators.

Amargasaurus lived around 15 million years later than Bajadasaurus, indicating that elongated neural spines were a long-lasting defense strategy.

It analyzed the external morphology, internal microanatomy and bone microstructure of the hemispinous processes for the first time from the holotype of Amargasaurus and an indeterminate dicraeosaurid.

Both anatomical and histological evidence does not support the presence of a keratinized sheath (i.e. horn) covering the hyperelongated hemispinous processes of Amargasaurus, and either, using a parsimonious criterion, in other dicraeosaurids with similar vertebral morphology.

[16] The orientation of the semicircular canals, ring-like structures in the inner ear that house the sense of balance, have been used to reconstruct habitual head postures in some dinosaurs and other extinct animals, although the reliability of this method has been repeatedly questioned.

[19] Assuming a similar head orientation in Bajadasaurus, Gallina and colleagues hypothesised that the exposure of the eye openings in top view might have allowed the animal to look forward while feeding, while the sight of most other sauropods was limited to the sides.

[21] Such information can be derived by counting and measuring daily growth lines seen in cross-sections of teeth, and may help to reconstruct feeding habits in extinct animals where direct observation is not possible.

However, definitive evidence for the presence of a beak-like structure is missing, and future research is needed to confirm its assumed functional advantages.

The formation is composed of red and green-brown sandstones and conglomerates of fine to coarse grain size together with bands of reddish claystones and light brown siltstones.

Besides Bajadasaurus, the locality has yielded the remains of other sauropods, including the diplodocid Leinkupal laticauda and the early titanosaur Ninjatitan zapatai, as well as those of several species of theropod that can be ascribed to basal Tetanurae and possibly to abelisauroids and deinonychosaurians.