Bicharracosaurus: a new macronarian from Jurassic Patagonia

Bicharracosaurus dionidei is a large sauropod from the Late Jurassic of Patagonia, formally described in April 2026. Its holotype preserves a long, partly articulated stretch of the backbone, with ribs and part of the pelvis. There is no skull or limb material. The vertebrae carry features that help test the early history of Macronaria, the sauropod branch that later included brachiosaurids and titanosaurs.

Quick facts

Found by Dionide Mesa

In March 2001, Chubut resident Dionide Mesa alerted the Egidio Feruglio Paleontological Museum to large bones on his land. Excavation began in 2002 and continued across several field seasons. Additional cervical vertebrae were recovered in 2018. The species epithet dionidei honours Mesa, while the generic name comes from the informal Spanish word bicharraco, used locally for a very large animal, combined with the Greek-derived saurus.

The holotype, MPEF-PV 1730, represents one individual. It includes articulated and closely associated middle-to-posterior neck vertebrae, a complete dorsal series, ribs, a sacrum fused to part of the right ilium, and several disarticulated tail vertebrae. The lack of duplicated vertebrae and the anatomical fit between parts support their assignment to the same skeleton. Some bones are not articulated, so the positions of missing or displaced elements have to be reconstructed carefully.

Age and geological setting

The skeleton came from the Cañadón Calcáreo Formation in Chubut Province, Argentina. Its fossil-bearing interval is assigned to the Late Jurassic, within an Oxfordian–Kimmeridgian age framework, around 155 million years ago. The exact numerical age depends on the correlation of the beds; the formation-level estimate is not a radiometric date for the dinosaur itself.

The bones occur in continental fluvial deposits. Field evidence indicates that the carcass was partly disarticulated before final burial. That is a taphonomic observation about the preservation process, not a complete story of how the animal died. The vertebral sequence remains informative despite the displacement of some elements, but gaps in the neck and tail have to be filled by comparison rather than direct measurement.

Vertebrae with a distinctive combination

The describing team identified three particularly distinctive features. In middle and posterior caudal vertebrae, a triangular pneumatic opening pierces a depression beneath the transverse process. In posterior cervical vertebrae, a small upward-pointing projection occurs at the lower end of a postzygapophyseal lamina. The articular surfaces of the sacral centra have a boat-shaped outline.

Many vertebrae also contain air spaces associated with the pneumatic system of sauropods, but the pattern is not identical across the skeleton. The ribs lack the proximal pneumatic openings seen in more derived titanosauriform sauropods. This mosaic of features is useful because it shows that pneumatic anatomy did not necessarily appear as one uniform package throughout the body.

Vertebral characters diagnose the fossil, but they do not reveal every part of the animal. Without a skull, teeth, hands or feet, we cannot compare the full range of anatomy normally used to classify sauropods. The most secure statements concern the preserved axial skeleton and the combination of traits that distinguishes it from other known taxa.

Where it fits among sauropods

Phylogenetic analyses place Bicharracosaurus within Macronaria, near the base of the group. Its precise position changes under different analytical settings: some results draw it closer to early titanosauriforms, while others leave it outside that branch. This variation does not mean that the bones change. It reflects the sparse early macronarian record and the effects of how incomplete taxa and anatomical characters are scored.

Comparisons with other Jurassic sauropods, including Tehuelchesaurus and Losillasaurus, help identify similarities and differences but do not make them close copies of one another. Patagonia preserved several sauropod lineages, and broad body shape alone is a poor guide to ancestry. The placement of Bicharracosaurus is best treated as a testable result of the published analysis rather than a settled label.

Estimated size and missing anatomy

The primary description estimated a body length of roughly 15 metres and a mass near 20 tonnes. These values are reconstructions, not measurements of a complete skeleton: the legs, skull, and portions of the vertebral column are missing. The estimate depends on restoring those parts from related sauropods and should be read as an approximate scale for the animal.

Sauropod ancestry supports a plant-eating interpretation, but this specimen has no teeth or skull. It cannot show the shape of the muzzle, the heights at which the animal fed, or which plants it selected. Nor does the single skeleton demonstrate herding, nesting, skin coverings or colour. The cover scene conveys a general macronarian body plan, not a preserved life portrait.

Bicharracosaurus is valuable because it adds a new, anatomically distinct skeleton to a poorly sampled interval in South American sauropod history. Its vertebrae already contribute to comparisons of early macronarians. More complete finds could change the branching pattern, while the distinctions visible in MPEF-PV 1730 remain the direct basis for the current account. See the dinosaur catalogue for other named taxa and the different kinds of fossils behind their reconstructions.