| Bajadasaurus | |
|---|---|
 | |
| Skeletal elements | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | †Sauropodomorpha |
| Clade: | †Sauropoda |
| Superfamily: | †Diplodocoidea |
| Family: | †Dicraeosauridae |
| Genus: | †Bajadasaurus Gallinaet al.,2019 |
| Species: | †B. pronuspinax |
| Binomial name | |
| †Bajadasaurus pronuspinax Gallinaet al., 2019 | |
Bajadasaurus is agenus ofsauropoddinosaur from theEarly Cretaceousepoch (late Berriasian toValanginianstages, between 145 and 132.9million years ago) of northernPatagonia,Argentina. It was first described in2019 based on a single specimen found in 2010 that includes a largely complete skull and parts of the neck. The only species isBajadasaurus pronuspinax. The genus is classified as a member of theDicraeosauridae, a group of relatively small and short-necked sauropods.
Bajadasaurus sportedbifurcated (two-pronged), extremely elongatedneural spines extending from the neck. Similarly elongated spines are known from the closely related and more completely knownAmargasaurus. Several possible functions have been proposed for these spines inAmargasaurus; the 2019 description ofBajadasaurus suggested they could have served as a passive defense against predators in both genera. The skull wasslender and equipped with around 48 teeth that were pencil-shaped and restricted to the front of the jaws. The eye openings ofBajadasaurus were exposed in top view, possibly allowing the animal to look forwards while feeding.Bajadasaurus was discovered insedimentary rocks of theBajada Colorada Formation, which weredeposited bybraided rivers. It shared its environment with other dinosaurs including the sauropodsLeinkupal andNinjatitan and differenttheropods.
The only specimen of thedinosaurgenusBajadasaurus was excavated in 2010 by palaeontologists of theCONICET (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 ofPicún Leufú, near the western bank of theLimay River, inPatagonia. The site is part of theBajada Colorada Formation, a succession ofsedimentary rocks in theNeuquén Basin that is dated to the lateBerriasian toValanginianstages of theEarly Cretaceous (~145–132.9million years ago[1]). 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 wasprepared in a laboratory, revealing most of the skull as well as the first two and probably the fifth neck vertebrae.[2][3] The specimen is now curated by theMuseo Municipal Ernesto Bachmann inVilla El Chocón,Neuquén Province, under the specimen number MMCh-PV 75.[3]
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 theSpanish wordBajada ("downhill") in reference to the Bajada Colorada locality, and theGreeksaurus ("lizard"). Thespecific namepronuspinax is derived from theLatinpronus ("bent over forward") and the Greekspinax ("spine"), referring to the long and forward-curvedneural spines (spinousprocesses) of the neck.[3] Because of the spectacular spines, the discovery ofBajadasaurus was widely reported on by international news media.[4] In 2023, Juan Garderes and colleagues published a comprehensive description of the skull based on a digital 3D reconstruction obtained fromCT scans of the bones.[5]
Bajadasaurus is classified as a member of theDicraeosauridae, afamily ofsauropod dinosaurs. As with all sauropods, dicraeosaurids were large, four-leggedherbivores 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-dayAsian elephants, and their neck was comparatively short.[6][7] Longbifurcated (two-pronged) neural spines were a common feature of the group, although they were only extremely elongated inBajadasaurus and the closely relatedAmargasaurus.[3] Bone fusion in the braincase and vertebral column indicates that the holotype represented an adult individual.[5]
As in otherflagellicaudatans (the group to which dicraeosaurids and diplodocids belong), the skull was elongated with an overallslender built.[5] The preserved skull includes thepterygoid bones of thepalate, most of theskull roof andbraincase, 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.[3] The holotype skull is affected by crushing (brittle deformation) that occurred duringdiagenesis, which resulted in a number of fractures. In addition, the braincase is deformedplastically (without causing fractures).[5]
All bones that surround theorbit (eye socket) are preserved, except for thejugal bone, which would have formed the lower margin of the opening. Thelacrimal bone, which formed the front margin of the orbit, had a straight ridge on its upper half that was similar to that ofDicraeosaurus. It was pierced by a smallforamen (opening), similar to the larger foramen seen inDicraeosaurus. The upper-front corner of the orbit was formed by theprefrontal bone. The contribution of the prefrontal to the orbit was smaller than inDicraeosaurus andAmargasaurus; the bone was also smaller and less robust than in these genera. The upper rim of the orbit was formed by thefrontal, which was fused to theparietal 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 exceptLingwulong. At the back, the frontal also formed a small part of thesupratemporal fenestra, a major opening on the rear part of the skull roof. The rear margin of the orbit was formed by thepostorbital bone. Typically in dinosaurs, this bone featured a rearwards extending process, the posterior process. InBajadasaurus,Dicraeosaurus, andAmargasaurus, this process was reduced and indistinct. The downward projecting process of thesquamosal, 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 thequadratojugal bone at the lower rear edge of the skull, although thearticulation itself is not preserved. This probable articulation was absent indiplodocids, and has not previously been documented in dicraeosaurids. Below the orbit and framed by the squamosal, postorbital, quadratojugal, and jugal was thelateral temporal fenestra, another major skull opening. InBajadasaurus, 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 ofBajadasaurus.[3][5]
The braincase is mostly hidden from view by overlying bones; only theoccipital region (rear part) is exposed. The uppermost bone of the occipital region is thesupraoccipital, which inBajadasaurus was completely fused to theexoccipital-opisthotic bone below and featured a distinct and narrow longitudinal ridge, thesagittal nuchal crest. Theposttemporal fenestrae, a pair of openings between the parietal and the occipital region, were extended medially (towards the mid-plane of the skull), which is anautapomorphy ofBajadasaurus (a unique feature not found in closely related genera). Theoccipital condyle, 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 fromAmargasaurus andDicraeosaurus. Thebasisphenoid, which formed part of the underside of the braincase, had a pair of gracile bony extensions, thebasipterygoid processes, 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 inDicraeosaurus andAmargasaurus, 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.[3]
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.[5] The teeth were restricted to the front parts of the jaws, were pencil-shaped and tilted forwards; their narrowcrowns were nearly straight or curved slightly inwards. Of the upper jaw, only the front section of the leftmaxilla (the largest bone of the upper jaw) is preserved. It preserves eightalveoli (tooth sockets), a count similar toSuuwassea, but less than inDicraeosaurus, 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 thelower jaw, where the teeth are still anchored within the left and rightdentaries (the only tooth-bearing bones of the lower jaw).Bajadasaurus thus likely had 48 teeth in total. The dentary was slender, similar toSuuwassea but unlike the deep dentary ofDicraeosaurus. 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, theangular bone was very elongated and longer than thesurangular bone, unlike in diplodocids.[3][5]
Bothproatlases—small, triangular bones located between the first neck vertebra and the skull—were found in articulation with the skull. Of the first neck vertebra (theatlas), only the upper elements, theatlantal neurapophyses, are preserved. These were triangular and wing-like inBajadasaurus. The second neck vertebra, theaxis, is nearly complete. As inDicraeosaurus, it was twice as high as it was long, while itscentrum (or vertebral body) was twice as long as it was high. Thediapophyses (sidewards projecting processes) were small and directed backwards as inSuuwassea rather than downwards as inDicraeosaurus andAmargasaurus. 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.[3]
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 relatedAmargasaurus, 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 inAmargasaurus. InAmargasaurus, the spines show striations on their surface that indicate that akeratin (horn) sheath was present in life. Although similar striations cannot be observed on the spines ofBajadasaurus 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: itsmorphology is comparable to the fifth neck vertebra ofDicraeosaurus, the probable sixth ofBrachytrachelopan, and the seventh ofAmargasaurus; 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.[3]
Dicraeosaurids are one of the three principal families comprising theDiplodocoidea, a major subdivision of sauropod dinosaurs. Within Diplodocoidea, dicraeosaurids form thesister group of theDiplodocidae, while the third family, theRebbachisauridae, 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.[8] The number of genera classified within Dicraeosauridae varies between studies. Gallina and colleagues, in their 2019 description ofBajadasaurus, recognised seven additional genera. The earliest isLingwulong from the lateEarly to earlyMiddle Jurassic of China,[9] while three genera are known from theLate Jurassic—Brachytrachelopan from Argentina;Suuwassea from theUnited States; and the eponymousDicraeosaurus fromTanzania. Early Cretaceous dicraeosaurids includeBajadasaurus as well asAmargatitanis,Pilmatueia, andAmargasaurus, all from Argentina.[10][11]
In theiranalysis of evolutionary relationships, Gallina and colleagues recoveredBajadasaurus as an intermediate member of Dicraeosauridae, more derived (more recentlydiverging from acommon ancestor) thanSuuwassea andLingwulong, but less so thanPilmatueia,Amargasaurus,Dicraeosaurus, andBrachytrachelopan. The referral ofBajadasaurus to the Dicraeosauridae was supported by sixsynapomorphies (anatomicalfeatures distinguishing the group from relatedtaxa). A subsequent analysis by John Whitlock and Jeffrey Wilson Mantilla, in 2020, foundBajadasaurus to be the mostbasal member of a clade that also containsLingwulong 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 foundKaatedocus andSmitanosaurus to be basal members of Dicraeosauridae.[12] A 2022 study led by Guillermo Windholz analyzed multiple datasets and found conflicting results, with one of their analyses findingBajadasaurus to be most closely related toAmargatitanis andPilmatueia 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 onbiogeographic grounds.[13]
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).[3]
Cladogram by Gallina and colleagues (2019)[3] | Cladogram by Whitlock and colleagues (2020)[12] | Cladogram by Windholz and colleagues (2022) showing their favored result of a clade of South American dicraeosaurids[13]
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Elongated and deeply bifurcated neural spines were common in dicraeosaurids. InDicraeosaurus andBrachytrachelopan, they were inclined toward the front but remained much shorter than inBajadasaurus. Only the spines ofAmargasaurus were similarly elongated. The spines ofAmargasaurus 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 fordisplay, defense, orthermoregulation.[3] 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 anair sac, the so-called supravertebral diverticulum, that would have been connected to the lungs as part of therespiratory system. InDicraeosaurus, 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 inAmargasaurus. The upper two thirds would likely have been covered by a horny sheath, as is indicated by longitudinal striations on their surface.[14][3]
Gallina and colleagues, in 2019, considered this the most reasonable interpretation that may likewise be applied toBajadasaurus. 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 thespinal cord. The protection of the sheath would have been further enhanced if it would have extended past its bony core. Schwarz and colleagues reconstructedAmargasaurus with horny sheaths that did not reach far beyond their bony core, as is the case for most modern reptiles. In some moderneven-toed ungulates, the horny sheath can be double the length of the horn core, and the exquisitely preservedankylosaurBorealopelta 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 speculated that the spines ofAmargasaurus andBajadasaurus might have been more than 50% longer than indicated by their bony core.[3] Their bending would have further increased their resistance, as is the case in modernbighorn sheep.[3][15]
Gallina and colleagues further speculated that the spines in bothAmargasaurus andBajadasaurus might have been used for defense. 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. Similar, even larger spikes were postulated for the following neck vertebrae. Moderate damage would result in the break-off of the horny tips, leaving the bony spine intact.Amargasaurus lived around 15 million years later thanBajadasaurus, indicating that elongated neural spines were a long-lasting defense strategy.[3]
In 2022, a detailed study was published by Cerda et al. It analyzed the external morphology, internal microanatomy and bone microstructure of the hemispinous processes for the first time from the holotype ofAmargasaurus and an indeterminate dicraeosaurid. Proximal, mid and distal portions of both cervical and dorsal hemispinous processes reveal that the cortical bone is formed by highly vascularized fibrolamellar bone interrupted with cyclical growth marks. Both anatomical and histological evidence does not support the presence of a keratinized sheath (i.e. horn) covering the hyperelongated hemispinous processes ofAmargasaurus, and either, using a parsimonious criterion, in other dicraeosaurids with similar vertebral morphology. Osteohistology of the spines suggests that they were likely, if not exclusively, covered in a sail of skin. The spines are also highly vascularized and bear cyclical growth marks, adding credence to this theory. This could have applied to the structures possessed byBajadasaurus as well.[16]
The orientation of thesemicircular canals, ring-like structures in theinner ear that house thesense 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.[17][18] Palaeontologist Paulina Carabajal Carballido, in 2015, inferred thatAmargasaurus would have had its snout facing downwards.[19] Assuming a similar head orientation inBajadasaurus, 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 further speculated that this feature could have allowed forstereoscopic vision.[3] Such stereoscopic vision would only have come into effect when the snout was downturned.[20]
In 2023, Garderes and colleagues suggested thatBajadasaurus might have had abeak-like structure supporting the tooth-bearing portions of the muzzle. As beaks are made out ofkeratin, which rarely fossilises, their presence can only be indirectly inferred. InBajadasaurus, such indirect evidence includes, amongst other features, a step in the side surface of the maxilla, which is often present in beaked animals. A beak could have reduced stress on the jaws during feeding and provided an additional cutting edge. However, definitive evidence for the presence of a beak-like structure is missing, and future research is needed to confirm its assumed functional advantages.[5]
In 2025, Garderes estimated how frequently teeth wereshed and replaced (tooth replacement rate) as well as the time required for teeth to form (tooth formation time) inBajadasaurus.[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.[22] Garderes estimated that tooth formation time varied from tooth to tooth, and that a tooth was replaced after an average of 40.7 days, with a lower number of replacement teeth than in other diplodocoids. He also suggested thatBajadasaurus was a specialist inbrowsing lower vegetation, based on the shape of its teeth.[21]
Bajadasaurus was recovered from theBajada Colorada Formation, ageological formation of theMendoza Group that isexposed in northern Patagonia. The formation is composed of red and green-brownsandstones andconglomerates of fine to coarsegrain size together with bands of reddishclaystones and light brownsiltstones. These sediments were mostly deposited bybraided rivers, as is evident by well-preserved river channels withcross bedding.Paleosols are present in the formation. The Bajada Colorada Formation overlies theQuintuco andPicún Leufú Formations.[23][3][24] At its top, it is separated from the overlyingAgrio Formation by anunconformity (sedimentation hiatus).Bajadasaurus stems from the Bajada Colorada locality, thetype locality of the formation. BesidesBajadasaurus, the locality has yielded the remains of other sauropods, including the diplodocidLeinkupal laticauda and the early titanosaurNinjatitan zapatai, as well as those of several species oftheropod that can be ascribed to basalTetanurae and possibly toabelisauroids anddeinonychosaurians.[25][23][3]
