Bajadasaurus: the sauropod with forward-curving neck spines

Bajadasaurus pronuspinax was an Early Cretaceous dicraeosaurid sauropod from northern Patagonia, Argentina. One holotype preserves much of the skull, both lower jaws and several bones from the front of the neck. Its most striking fossil is a cervical vertebra with a pair of thin, forward-curving bony spines about 58 centimetres long.

The bones are real; a complete row of matching horns, a keratin sheath and a defensive purpose are not directly preserved. The neck body, limbs and tail are also absent. The animal's familiar outline therefore combines a small amount of genus-specific anatomy with comparison to better-known relatives.

Quick facts

Discovery in Patagonia

The site was investigated during field research in Neuquén Province that began in 2010. Small teeth were visible at the surface, but the bones were fragile. Researchers excavated the find as a block so that the delicate skull and neck elements could be prepared in the laboratory rather than broken apart in the field.

Pablo Gallina, Sebastián Apesteguía, Juan Canale and Alejandro Aluza named Bajadasaurus pronuspinax in 2019. The holotype is catalogued as MMCh-PV 75 and is kept in the Municipal Ernesto Bachmann Museum in Villa El Chocón. Its single-specimen basis is important: the diagnostic skull and neck belong to one mature individual, not to an average assembled from a herd.

The genus name refers to the Bajada Colorada locality and the Greek word for lizard. The species epithet combines words meaning forward-inclined spine. The name describes the conspicuous orientation of the fossil process rather than proving that the living animal carried a naked spike or a horn.

Classification and close relatives

Bajadasaurus is a sauropod within Diplodocoidea and Flagellicaudata, and its placement in Dicraeosauridae is well supported. Dicraeosaurids were generally smaller and shorter-necked than many diplodocids. Several developed tall, bifurcated neural spines along the neck. The exact relationships among Bajadasaurus, Amargasaurus, Dicraeosaurus, Brachytrachelopan and Pilmatueia shift among phylogenetic analyses.

That uncertainty does not challenge the family-level identification. It means that the available skull and a handful of vertebrae do not resolve every branch inside Dicraeosauridae. The Amargasaurus profile offers a useful comparison with another Patagonian dicraeosaurid, while the dinosaur classification guide explains why a stable family name can coexist with a less settled position inside that family.

What the holotype preserves

MMCh-PV 75 includes elements from the skull roof, braincase and palate, the left maxilla, both dentaries and parts of the lower jaw. The atlas region is represented by proatlantal and atlantal bones. There is a nearly complete axis and another cervical vertebra, often interpreted as a probable fifth neck vertebra. Its precise serial position is inferred by comparison and remains tentative.

The bones show a mature individual, but they do not give an exact age in years. The torso, sacrum, tail, shoulder and hip girdles, and all limbs are missing. For these regions, palaeontologists use other dicraeosaurids to construct a plausible body plan. This is why the skull can be discussed in detail while length, mass and limb posture remain comparatively broad estimates.

How big was it?

There are no complete limb or trunk bones with which to calculate a precise body size. Estimates commonly place Bajadasaurus around nine metres long, with a cautious range of roughly 8–11 metres. A mass near 3–6 tonnes is sometimes proposed by comparison with relatives. These values indicate an approximate scale, not a direct measurement of the holotype.

As a dicraeosaurid, it is reconstructed as a four-legged sauropod with a long tail and a neck shorter than those of many diplodocids. The shoulders, hips and limb proportions are borrowed from related taxa. The exact tail length, body height and number of neck spines are not known. A single vertebra cannot establish that every segment along the neck had an identical pair.

The forward-curving neck spines

The most remarkable cervical vertebra is about four times taller than it is long. Its neural spine divides into two narrow rods approximately 58 centimetres long. In cross-section they are oval; the ends flare somewhat, and the rods curve forward. These are extensions of a vertebra, not separate horns attached to the skull.

Only one pair of such extreme processes is preserved in the holotype. Reconstructions often show several pairs down the neck because Amargasaurus preserves a more complete sequence with elongated spines. That comparison makes a row plausible, but the number, length and curvature of the processes in Bajadasaurus are not directly known. The axis, for example, has a single upright process rather than the extraordinary pair.

The first description proposed that keratin sheaths could have extended the bones and that the structures might have helped defend the neck. A keratin covering is not fossilised here. Keratin could in principle protect a thin process, but that mechanical possibility is not evidence that a sheath existed. The similar but more complete neck spines of Amargasaurus help test the comparison, without proving the soft tissues of Bajadasaurus.

A 2022 histological study of Amargasaurus did not support the proposed horn-like keratin sheaths in that genus. Instead, its findings were consistent with skin and interspinous ligaments. This is relevant comparative evidence, not a direct test of Bajadasaurus: it is another animal with a different specimen. Possible appearances include skin-covered projections or soft tissue between the spines, but the surface covering remains unresolved.

Defence is only one suggested function. The spines may have supported ligaments, contributed to display or had more than one role. No fossil preserves a predator attack on these processes, a healed fracture proving combat, or the soft tissues needed to model all forces. Calling them proven weapons or horns would overstate what is known.

Skull, jaws and teeth

The skull was lightweight and contained large openings separated by thin bones. The J-shaped lower jaws carried teeth only near the front of the mouth. The left maxilla has eight visible tooth positions. A nearby row of 24 upper crowns was recovered, although it had become detached from the bone. Each dentary has twelve teeth. These observations suggest roughly 48 active teeth, but the detached maxillary row makes the exact count less certain than the jaw positions alone.

The crowns were narrow and peg-like, straight or slightly curved inward. They were suited to cropping plants rather than grinding them thoroughly. Wear patterns differ along the tooth row, and the replacement cycle has been estimated at about 40.7 days on average. This is a calculated average based on dental growth, not a timetable observed in a living animal.

Researchers have used muscle reconstructions to explore small movements between skull elements during feeding. Such modelling can test whether a mechanism is mechanically possible; it does not record a feeding action. The more secure interpretation is that Bajadasaurus plucked vegetation with the front teeth and swallowed it with limited oral processing.

Environment and diet

The Bajada Colorada Formation records a continental setting in the Neuquén Basin. Sandstones, conglomerates and finer deposits have been interpreted in relation to river channels, floodplains and developing soils. The regional vegetation included gymnosperms, ferns and horsetails. Flowering plants were already present in the Early Cretaceous, but their existence does not identify what this animal ate.

Its narrow anterior teeth and relatively short neck support a likely emphasis on low vegetation. It may have reached shrubs or lower branches by changing head and neck position; it should not be described as feeding exclusively at ground level. No stomach contents identify specific plants, and height estimates depend on missing vertebrae.

Other sauropods, including Leinkupal and Ninjatitan, are known from the formation, together with fragmentary theropod material. The same named formation can span time and area, so its fossils do not guarantee that every animal shared one site or directly interacted. Broad context is available in the Cretaceous Period overview and the dinosaur catalogue.

Behaviour and reconstruction limits

No trackway, nest, egg, skin impression or group assemblage has been securely assigned to Bajadasaurus. The specimen does not establish herding, migration, parental care, courtship, sound or colour. It provides no direct clue that the spines were used to display to a mate or to repel a predator.

Popular images sometimes show a uniform line of enormous horns from the head to the torso. That scene extrapolates beyond the single extreme pair preserved. A responsible reconstruction should make clear which vertebra is known and which parts of the neck come from related taxa. Likewise, a keratin-covered spear can be a visual hypothesis, but not the only scientifically defensible result.

The holotype is valuable even with these gaps. Its skull refines knowledge of dicraeosaurid jaw shape, tooth positions, palate and braincase. The neck vertebra documents a striking anatomical feature in a Patagonian lineage. A new specimen could revise both its familiar silhouette and its relationship to Amargasaurus, while leaving the currently preserved skull and spines just as informative.