Triunfosaurus

Did You Know?

• More than 535 individual dinosaur trackways are known from the Triunfo Basin, some possibly made by Triunfosaurus.
• The environment of the Triunfo Basin likely consisted of alluvial fans and river systems.
• Fragments of crocodylomorph bones have been found at the Poço County locality.

Description

Triunfosaurus is distinguished from other titanosaurs by two autapomorphies of the ischium: the part contacting the pubis is half the height of the entire bone, and the shaft forms an angle of less than 70° with the acetabular portion. The vertebrae from near the front of the tail have short, robust prezygapophyses that are slightly angled upward. The neural spines are small and bear processes along their midlines that expand near the top. Transverse processes are placed near the back of the vertebrae and are also slightly angled upward.

The back half of the sides of the centra are strongly concave, while the pleurocoels are small; the internal textures are not camellate. In the haemal arches from near the front of the tail, the haemal canals are narrow, the arches are relatively straight, and their bottom ends are rounded and expanded. Near the middle of the tail, the haemal canals are proportionally wider and shorter, the arches become very flattened and slightly concave, and the joints with the centra are closer to the back end.

Discovery and naming

The holotype, UFRJ-DG 498, was found in a sandstone layer of the Rio Piranhas Formation west of Paraíba State. It includes three middle tail vertebrae (later considered front tail vertebrae), three isolated chevrons, three isolated neural spines, and a right ischium. The fossils are housed at the Universidade Federal do Rio de Janeiro. The Rio Piranhas Formation is considered a contemporary of the Sousa Formation, which spans the Berriasian to Valanginian, possibly into the Hauterivian, based on pollen and trackway correlations. The type species Triunfosaurus leonardii was named in 2017 by Ismar de Souza Carvalho and colleagues; the generic name refers to the Triunfo Basin, and the specific name honors paleontologist Giuseppe Leonardi.

Classification

In 2017, Triunfosaurus was placed in Titanosauria based on the articulations of its haemal arches having two distinct surfaces. It was recovered in a basal polytomy with Mendozasaurus, supported by tail vertebrae with flattened back ends rather than the procoelous condition of derived lithostrotians. The short ischial shaft resembles that of Andesaurus, but straight vertical neural spines suggest a more derived position. However, the topology is poorly supported; forcing Triunfosaurus outside Titanosauria requires only two additional evolutionary steps. Subsequent work by Poropat and colleagues noted that the furrow separating the haemal arch into two surfaces is also present in non-titanosaur somphospondylans Phuwiangosaurus and Tangvayosaurus, and that the short haemal canal contradicts the definition of Titanosauria. Thus, the taxon has been reassigned to Somphospondyli.