Gallina et al., 2019
Gallina et al., 2019
Bajadasaurus is a genus of sauropod dinosaur from the Early Cretaceous epoch (late Berriasian to Valanginian stages) 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 comparatively small and short-necked sauropods that lived from the Early or Middle Jurassic to the end of the Early Cretaceous.
Bajadasaurus sported bifurcated, extremely elongated extending from the neck vertebrae. Similar elongated spines are known from the closely related and more completely known Amargasaurus. Various possible functions have been proposed for these spines in Amargasaurus, with the 2019 description of Bajadasaurus suggesting that they could have served as passive defense against predators in both genera. The skull was gracile and equipped with around 44 teeth that were pencil-shaped and restricted to the front of the jaws. The eye openings of Bajadasaurus were exposed in top view of the skull, possibly allowing the animal to look forwards while feeding. Bajadasaurus was discovered in sedimentary rocks of the Bajada Colorada Formation, and its environment resembled a braided river system. It shared its environment with other dinosaurs including the sauropod Leinkupal and different theropods.
Discovery and naming
The only specimen was excavated in 2010 by palaeontologists of the CONICET, the science agency of the Argentinian government. The site of discovery, the Bajada Colorada locality, is located 40 kilometres (25 mi) south the town of Picún Leufú, near the western banks of Limay River, in Patagonia. 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 wrapped in plaster. Preparation of the block in the laboratory then revealed most of the skull as well as the first two and probably the fifth neck vertebrae of a new genus of dinosaur. The site of discovery 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. 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 the holotype of 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 is derived from the Latin pronus ("bent over forward") and the Greek spinax ("spine"), referring to the long and anteriorly curved (spinous processes) of the neck.
Bajadasaurus is classified as a member of the sauropod family Dicraeosauridae. As all sauropods, dicraeosaurids were large, four-legged herbivores with a long neck and tail and proportionally very small head. They were, however, small in comparison with most other sauropods, roughly reaching sizes of present-day Asian Elephants, and their neck was comparatively short. Long bifurcated neural spines were a common feature of the group, although only extremely elongated in Bajadasaurus and the closely related Amargasaurus.
The skull includes most of the and , the bones of the , as well as the lower jaws and parts of the upper jaws, and is therefore the most complete skull of a dicraeosaurid known to date. The middle section of the skull is not preserved. Its overall built was gracile.
All bones that surround the (eye opening) are preserved, except for the , which would have formed the lower margin of the opening. The , 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), unlike the larger foramen seen in Dicraeosaurus. The upper-front corner of the orbit was formed by the . The contribution of the prefrontal to the orbit was, however, smaller than in Dicraeosaurus and Amargasaurus; the bone was also smaller and less robust than in the latter genera. The upper rim of the orbit was formed by the , which was fused to the behind; together, these bones formed most of the rear part of the skull roof. In top view, 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, 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 . 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 well developed. This suggests that it was connected to the at the lower rear edge of the skull, although the articulation itself is not preserved. This probable articulation is 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 , 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 condition seen in diplodocids.
The braincase is mostly hidden from view by overlaying bones, with only the (rear part) being 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 high. Its rear surface was not wider than its neck, 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 slenderer than in Dicraeosaurus and Amargasaurus, being more than six times longer than wide. The left and right pterygoids, the only elements preserved of the palate, featured a smooth crest that received the basipterygoid processes.
The teeth were restricted to the front parts of the jaws and were pencil-shaped, with their narrow crowns nearly straight or slightly curved 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 44 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 typical for dicraeosaurids. The front of the dentary had a hook-like "chin" projecting downwards, as seen in other flagellicaudatans. The angular bone of the hind part of the lower jaw was very elongated and longer than the surangular bone, unlike in diplodocids.
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 long, while its (or vertebral body) was twice as long as 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.
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 pair of rod-like elements measures 58 cm (23 in) in length and made the vertebra four times taller than long. Among sauropods, it was only comparable to those of the related Amargasaurus, but, unlike in the latter, the spine was not directed backwards but curved toward the front. Their base was 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 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 they were covered by a horny sheath as well. The exact position of the vertebra in the neck is unclear, however. Its morphology is comparable to the fifth neck vertebra of Dicraeosaurus, the probably 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 tall and narrowed into a longitudinal keel on the underside; this keel was broader and concave in other dicraeosaurids.
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 tail characteristic for the group. In their 2019 description of Bajadasaurus, Gallina and colleagues recognised seven additional dicraeosaurid 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 US; 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 phylogenetic analysis, Gallina and colleagues recovered Bajadasaurus as an intermediate member of Dicraeosauridae, more derived (advanced) than Suuwassea and Lingwulong, but less so than Pilmatueia, Amargasaurus, Dicraeosaurus, and Brachytrachelopan. Amargatitanis was removed from the analysis due to its unstable position. The referral of Bajadasaurus to the Dicraeosauridae was supported by six synapomorphies (anatomical features shared with other members of the group). 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 spine of second neck vertebra; and elongated, forward curved neural spines in the neck).
Function of neural spines
Elongated and deeply bifurcated neural spines were common in dicraeosaurids. In Dicraeosaurus and Brachytrachelopan, they were inclined toward the front but remained much shorter than in Bajadasaurus. Only the spines of Amargasaurus were similarly elongated. The spines of Amargasaurus led to much speculation about their possible life appearance and function. As hypothesised by separate authors, they could have supported a sail or horny sheaths, and could have been used for display, defense, or thermoregulation. Daniela Schwarz and colleagues, in 2007, found that the double-row formed by the bifurcated neural spines along the spine of dicraeosaurids would have enclosed an air sac, the so-called supravertebral diverticulum, that would have been connected to the lungs as part of the respiratory system. In Dicraeosaurus, this air sac would have occupied the entire space between the left and right parts of the spines, while it would have been restricted to the lower third of the spines in Amargasaurus. The upper two thirds would likely have been covered by a horny sheath, as is indicated by longitudinal striations on their surface.
Gallina and colleagues, in 2019, considered this the most reasonable interpretation that may likewise be applied to Bajadasaurus. 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. Fracturing of the spines might have been a critical threat, as the bases of the spines roofed the spinal chord. The protection of the sheath would have been further enhanced if it would have extended past its bony core. Schwarz and colleagues reconstructed Amargasaurus with horny sheaths that did not reach far beyond their bony core; the same is true for most modern reptiles. In some modern even-toed ungulates, however, the horny sheath can be double the length of the horn core, and the exquisitely preserved ankylosaur Borealopelta was found with horny sheaths that extended the length of its spines by 25%, demonstrating that substantial horny extensions may occur in dinosaurs as well. Gallina and colleagues suggested that the spines of Amargasaurus and Bajadasaurus might have been 50% longer than indicated by their bony core. Their bending would have further increased their resistance, as is the case in modern bighorn sheep.
Gallina and colleagues further speculated that the spines in both Amargasaurus and Bajadasaurus might have been used for defense. Due to its forward bent, the bifurcated neural spine of the supposed fifth neck vertebra would have reached past the head, acting as what was compared to a fence to deter predators. Similar, even larger spikes were postulated for the following neck vertebrae. Moderate damages would result in the break-off of the horny tips, leaving the bony spine intact. Amargasaurus lived around 15 million years later than Bajadasaurus, indicating that elongated neural spines were a long-lasting defense strategy.
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. Palaeontologist Paulina Carabajal Carballido inferred that Amargasaurus would have had its snout facing downwards. 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. These researchers furthermore speculated that this feature could have allowed for stereoscopic vision.
Bajadasaurus was recovered from the Bajada Colorada Formation, a geological formation of the Mendoza Group that is exposed in northern Patagonia. 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. These sediments were mostly deposited by braided rivers, as is evident by well-preserved river channels with cross bedding. Paleosols are present in the formation. The Bajada Colorada Formation overlies the Quintuco and Picún Leufú Formations. At its top, it is separated by the overlying Agrio Formation by an unconformity (sedimentation hiatus) that has been dated at 134 mya. Bajadasaurus stems from the Bajada Colorada locality, the type locality of the formation. The locality yielded the remains of another sauropod, the diplodocid Leinkupal laticauda, as well as of several species of theropod that can be ascribed to basal tetanurans and possibly to abelisauroids and deinonychosaurians.
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