Bajadasaurus

*Bajadasaurus pronuspinax*

***Bajadasaurus*** is a genus of sauropod dinosaur from the Early Cretaceous period (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 neural spines 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 that they could have served as 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 sauropod *Leinkupal* 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, 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). 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 neural spines (spinous processes) of the neck. Because of the spectacular spines, the discovery of *Bajadasaurus* was widely reported on by international news media.

## 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*. Amargasaurus reached a size of 11 meters, so in size, with a length of 13 meters, Bajadasaurus was fractionally larger than Amargasaurus.

### Skull

The preserved skull includes the pterygoid bones of the palate, most of the skull roof and braincase, 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 skull's build was gracile (slender) overall.

All bones that surround the orbit (eye socket) are preserved, except for the jugal bone, which would have formed the lower margin of the opening. The lacrimal 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 foramen (opening), similar to the larger foramen seen in *Dicraeosaurus*. The upper-front corner of the orbit was formed by the prefrontal 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 frontal, which was fused to the parietal 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 supratemporal fenestra, a major opening on the rear part of the skull roof. The rear margin of the orbit was formed by the postorbital 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 squamosal, a bone forming the upper rear corner of the skull, was well developed. This suggests that it was connected to the quadratojugal 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. Behind the orbit and framed by the squamosal, postorbital, quadratojugal, and jugal was the lateral temporal fenestra, 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.

### Neck vertebrae

Both proatlases—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 atlas), only the upper elements, the atlantal neurapophyses, are preserved. These were triangular and wing-like in *Bajadasaurus*. The second neck vertebra, the axis, is nearly complete. As in *Dicraeosaurus*, it was twice as high as it was long, while its centrum (or vertebral body) was twice as long as it was high. The diapophyses (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.

Only a single vertebra is known from the remainder of the neck. 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. This neural spine is 58 cm (1 ft 11 in) long and made the vertebra four times taller than it was long. Among sauropods, it was only comparable to those of the related *Amargasaurus*, but the spine curved toward the front rather than being directed backwards as in that genus. The bases of the rod-like elements were triangular and compressed sideways; their cross-section along most of their length was egg-shaped. Their tips broadened slightly, unlike the acute tips in *Amargasaurus*. In *Amargasaurus*, the spines show striations on their surface that indicate that a keratin (horn) sheath was present in life. Although similar striations cannot be observed on the spines of *Bajadasaurus* due to poor preservation, Gallina and colleagues found it likely that the spines were covered by a horny sheath as well. The exact position of the single preserved vertebra in the neck is unclear: its morphology is comparable to the fifth neck vertebra of *Dicraeosaurus*, the probable sixth of *Brachytrachelopan*, and the seventh of *Amargasaurus*; based on these comparisons, it was tentatively described as the fifth neck vertebra. The centrum of this vertebra was twice as long as it was high and narrowed into a longitudinal keel on the underside; this keel was concave and broader in other dicraeosaurids.

## Classification

Dicraeosaurids are one of the three principal families comprising the Diplodocoidea, a major subdivision of sauropod dinosaurs. Within Diplodocoidea, dicraeosaurids form the sister group of the Diplodocidae, while the third family, the Rebbachisauridae, is more distantly related. Dicraeosaurids and diplodocids are united within the group Flagellicaudata, which is named after the whip-like tails characteristic of the group. The number of genera classified within Dicraeosauridae varies between studies. Gallina and colleagues, in their 2019 description of *Bajadasaurus*, recognised seven additional genera. The earliest is Lingwulong from the late Early to early Middle Jurassic of China, while three genera are known from the Late Jurassic—*Brachytrachelopan* from Argentina; *Suuwassea* from the United States; and the eponymous *Dicraeosaurus* from Tanzania. Early Cretaceous dicraeosaurids include *Bajadasaurus* as well as *Amargatitanis*, *Pilmatueia*, and *Amargasaurus*, all from Argentina. An unnamed specimen from the Itapecuru Formation of Brazil indicates that the group persisted at least until the end of the Early Cretaceous.

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*. The referral of Bajadasaurus to the Dicraeosauridae was supported by six synapomorphies (anatomical features distinguishing the group from related taxa). A subsequent analysis by John Whitlock and 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. In this analysis, *Bajadasaurus* therefore occupies a slightly more basal position within Dicraeosauridae than indicated by Gallina and colleagues. As well as the genera recognised by the latter study, Whitlock and Wilson Mantilla found *Kaatedocus* and *Smitanosaurus* to be basal members of Dicraeosauridae.

*Bajadasaurus* itself can be differentiated from other dicraeosaurids by a unique combination of features, which includes four autapomorphies (a medially extended posttemporal fenestra; slender and long basipterygoid processes; vertically oriented neural spines of the second neck vertebra; and elongated, forward-curved neural spines in the neck).

Cladogram by Gallina and colleagues (2019)

Cladogram by Whitlock and colleagues (2020)

## Appearance in other media

### *Jurassic Park*

* *Bajadasaurus* appears as one of many collectible prehistoric creatures in the video game *Jurassic World: Alive*. It is currently the most recently described genus to appear in the franchise.