Notocolossus

Notocolossus gonzalezparejasi was a giant herbivorous titanosaur from the Plottier Formation of Mendoza Province, Argentina. It lived near the late Coniacian–early Santonian boundary, approximately 86 million years ago. One species is recognised.

Two incomplete skeletons preserve very different evidence. The giant holotype includes a 1.76-metre humerus and isolated vertebrae; a smaller referred animal provides a rare fully articulated hind foot. This combination informs both extreme body size and the load-bearing design of titanosaur feet, provided the two individuals are never confused as one skeleton.

Quick facts

Name, discovery and two localities

The genus combines Greek notos, “south”, with Latin colossus, referring to its southern Gondwanan origin and extraordinary size. The species honours Jorge González Parejas for work protecting palaeontological sites in Mendoza Province.

Bernardo González Riga discovered both specimens around Cerro Guillermo. González Riga, Matthew Lamanna, Leonardo Ortiz David, Jorge Calvo and Juan Coria described the genus in 2016. The fossils are held by the dinosaur laboratory of the National University of Cuyo.

Both individuals came from the same basal horizon of the Plottier Formation but not the same excavation. They were separated by 403 metres. They therefore cannot be treated as a single composite skeleton.

Classification

Notocolossus is securely a titanosaur. The first phylogenetic analysis placed it among lithostrotians and recovered Dreadnoughtus as its closest relative. Later, broader datasets have produced different arrangements.

Many recent trees place it in Colossosauria, a South American radiation containing several exceptionally large forms. It may sit inside Lognkosauria with Mendozasaurus, Futalognkosaurus and Patagotitan, or just outside that narrower clade. Missing skull, neck, sacrum and much of the limbs make exact resolution difficult.

The giant holotype

Holotype UNCUYO-LD 301 belonged to a very large individual. It includes an anterior dorsal vertebra, an anterior tail vertebra, a complete right humerus and the upper part of the left pubis. These bones lay within an area roughly eight metres square, on one horizon and at matching scale, supporting their association.

The right humerus is 1.76 metres long and was the longest titanosaur upper-arm bone known when described. Its proximal end is extraordinarily broad, nearly 2.9 times the width of the shaft at mid-length. A strong deltopectoral crest and medial expansion indicate a powerful shoulder apparatus.

The dorsal vertebra contains honeycomb-like pneumatic tissue that reduced bone mass while internal partitions maintained strength. Its arch bears a complex arrangement of plates and recesses. The anterior tail vertebra has robust transverse processes associated with the tail-base musculature.

The smaller specimen and its foot

UNCUYO-LD 302 is substantially smaller. It preserves seven incomplete anterior tail vertebrae with chevrons, a right astragalus and a complete right hind foot. The foot bones were articulated close to life position. Preparators made a mould before separating them, preserving the original arrangement.

The referral to Notocolossus depends mainly on shared anterior caudal features, including an unusual combination of proportions and ridges. It is a referred specimen, not part of the holotype. The famous foot therefore never belonged to the same giant individual as the humerus.

All five metatarsals are short, thick and unusually similar in length. In many neosauropods the central metatarsals are clearly longer than the first, creating a more elongated foot. In Notocolossus the third is only about 20 percent longer than the first, producing a compact and relatively symmetrical base.

This structure may have distributed loading more evenly. That is a plausible biomechanical interpretation, not a direct pressure measurement from a living animal. The foot belonged to the smaller individual, so its proportions cannot automatically be scaled without change onto the holotype.

The phalangeal formula was 2-2-2-2-0. The first four toes retained two phalanges each, while the fifth had none. Claw bones on the first three toes are short, rough and blunt. Their shared form may diagnose the genus, but pathology cannot be excluded until more feet are found.

Length and uncertain mass

Notocolossus is generally reconstructed at about 25–28 metres. That is not a complete skeletal measurement. It scales the immense humerus against proportions of better-known titanosaurs, while the full lengths of its neck and tail remain unknown.

The missing femur was estimated at approximately 2.17 metres from limb ratios. A calculation combining humeral circumference with reconstructed femoral circumference produced 60.4 tonnes. The authors emphasised a large error range; subtracting the average error of the equation lowered the figure to about 44.9 tonnes. Other giant-sauropod methods often disagree as well. Roughly 40–60 tonnes is therefore a working interval, not a record certificate.

The distinction matters in any dinosaur size comparison. One of the two values feeding the upper mass calculation comes from a reconstructed bone, and the entire body shape remains unknown.

Locomotion and body reconstruction

Like other sauropods, Notocolossus walked on four columnar limbs. The humerus demonstrates enormous forelimb loading, while the compact hind foot suggests a robust support architecture. Yet the fossils do not define shoulder height, habitual neck angle, walking speed or trackway width.

A complete skeletal mount must borrow the skull, neck, ribcage, pelvis and most limbs from related titanosaurs. It is valid as a comparative model only when those restored regions are labelled as such. No skin impressions or osteoderms are known, so armour, scale pattern and colour remain artistic choices.

Environment and feeding

The Plottier Formation records continental river channels, floodplains and developed soils during the Cretaceous Period. Red mudstones, siltstones and sandstones reflect quiet floodplain accumulation interrupted by stronger floods.

As a titanosaur, Notocolossus was herbivorous. No skull or tooth reveals muzzle form, bite or preferred plant. The long neck and large fermenting gut in reconstructions come from general sauropod anatomy, not direct remains of this genus.

Two individuals from one horizon do not demonstrate a herd. They lay hundreds of metres apart and may have been buried at different times. Nesting, parental care, migration, colour, sound and lifespan are unknown.

Claims that exceed the fossils

Notocolossus is often advertised as the heaviest dinosaur. Its bones prove immense size, but the 60-tonne result rests partly on an inferred femur, while a conservative treatment approaches 45 tonnes. Comparisons with other giants vary with the selected equation and missing proportions.

The giant arm and complete foot must not be illustrated as one discovered skeleton. They belong to separate animals linked by tail anatomy and geological context. The blunt toe claws could be a normal feature or a pathology, and their keratin coverings did not fossilise. A precisely shaped head, exceptionally long neck, armour or fixed colour pattern are likewise unsupported.

Why Notocolossus matters

The genus adds a rare articulated foot to the sparse record of giant titanosaur extremities. It shows that sauropod feet could vary more than once assumed and that enormous weight could be supported by shortening and strengthening the existing bones rather than adding more of them.

It also captures the central challenge of fragmentary giants. Body size, family placement and even claw form remain open to testing, while the preserved bones securely establish a powerful shoulder and an unusually compact hind-foot design.

Across sauropod evolution the number of toe bones generally declined, reaching an extreme in derived titanosaurs. Notocolossus preserves that reduced count together with robust proportions, giving researchers a rare anatomical endpoint for comparison. Future articulated feet can test whether the blunt claws and near-equal metatarsals characterised the genus, a broader lineage or only one individual.

Evidence, inference and reconstruction