Pycnonemosaurus (meaning ‘thick forest lizard’) is a genus of carnivorous theropod dinosaur that belonged to the family Abelisauridae. It was found in the Upper Cretaceous Bauru-type red conglomerate sandstone, Mato Grosso, Brazil, and it lived about 70 million years ago during the Late Cretaceous (Maastrichtian stage). Initial size estimations put this animal at 7 metres (23 ft) in length, but later analyses have found that it was likely larger, being about 8.9 metres (29.2 ft) long. This new size estimate currently makes Pycnonemosaurus the largest formally described member of the Abelisauridae thus far.
Thus far, the remains of Pycnonemosaurus have been fragmentary. No elements were well preserved, and the bone surface is well abraded that indicates the elements were partially exposed at the discovery location before being collected. The type specimen (DGM 859-R), housed at the Earth Sciences Museum, Rio de Janeiro, consists of 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. The small pubic foot and hatchet-shaped cnemial crest of the tibia distinguishes this species within the abelisaurs. The caudal vertebra has distinct abelisaurid features, such as a fan-shaped transverse process and a cranial projection. However, these awl-like projections are somewhat unlike related abelisaurids, such as Aucasaurus, in that they diminish more towards the distal caudals. All remains were found associated and are presently regarded as belonging to the same individual.
Initial size estimates put the animal at 7 metres (23 ft) in length and 1.2 tonnes in weight (1.3 short tons), but later analyses have found that it was likely larger, being about 8.9 metres (29.2 ft) long. This new size estimate currently makes Pycnonemosaurus the largest formally described member of the Abelisauridae thus far. Another estimation gave a length of 9.3 meters (30.5 feet) and a weight of 3.6 tonnes (~4 short tons).
Pycnonemosaurus is the best known abelisaurid from Brazil, where most theropod material is presently rare besides preserved teeth and footprints. Even though only a few species are known from Brazil, it is one of the most informative countries concerning the Lower Cretaceous period.