Pycnonemosaurus

Discovery and naming

The only known Pycnonemosaurus specimen, DGM 859-R, was dug from a red conglomeratic sandstone near Paulo Creek in Mato Grosso State, part of the Cachoeira do Bom Jardim Formation. In the early 1950s, Llewellyn Ivor Price visited a farm called Roncador, where owner Max de Barros Erhart and his workers had found dinosaur bones. The most significant discovery was a partial skeleton of a large abelisaurid theropod, lying close to titanosaurid sauropod remains. The genus name combines Greek words for dense, pastures, and woods, referencing Mato Grosso, while the species name honors the late Dr. Iedo Batista Neves, who encouraged paleontology. Kellner and Campos formally described Pycnonemosaurus nevesi in 2002.

Description

The remains of Pycnonemosaurus are fragmentary, with no well-preserved elements and abraded bone surfaces indicating partial exposure before collection. The type specimen, housed at the Earth Sciences Museum in Rio de Janeiro, includes five incomplete teeth, parts of seven caudal vertebrae, the distal part of a right pubis, a right tibia, and the distal articulation of the right fibula. A small pubic foot and hatchet-shaped cnemial crest of the tibia distinguish this species among abelisaurs.

The caudal vertebrae show abelisaurid features such as fan-shaped transverse processes and cranial projections, though the awl-like projections diminish more toward the distal caudals than in related forms like Aucasaurus. The specimen is from an immature individual, probably a subadult. Initial size estimates gave 6–7 m in length and 1.2 t, but later analyses suggest about 8.9 m long, making it the largest formally described abelisaurid; another estimate gives 9 m and 3.6 t.

Paleoecology

Brazil’s best-known abelisaurid is Pycnonemosaurus, a theropod from a region where most dinosaur remains are just teeth or footprints. Its fossils were first linked to the Bauru Group, then the Cambambe Formation, but now belong to the Campanian-age Cachoeira do Bom Jardim Formation. That formation formed in a semi-arid setting, as the middle parts of alluvial fans. This predator likely hunted titanosaurs from the same area. Its teeth were small but sharp, letting it grip prey while shaking and tearing them apart.