VIAVENATOR

Abelisaurid theropods are a stiff lot, thanks to a suite of tongue-twisting joints that reinforce their vertebral column. Hypertrophied cervical epipophyses in the neck, hyposphene-hypantrum structures in the back, a fused sacral complex in the hip and transverse processes in the tail all combine to limit the mobility of the spine. But Viavenator took stiffening to the next level with the same but much more of said joints, plus an extra one that was previously undocumented in theropod dinosaurs, abelisaurid or otherwise: An "interspinal articulation" formed by two bony projections on the spine of each back vertebra that "embrace" the spine of the preceeding vertebra. With a backbone as stiff as a board from the neck to the first third of the tail, up and down movement was likely restricted to pivoting at the pelvic girdle. However, this stiffness wasn't necessarily a hindrance, and perhaps allowed Viavenator to take advantage of gravity-assisted acceleration during the pursuit of prey. Modern athletes similarly exploit gravity by leaning forward to harness downward momentum during sprints.

CT scans of the holotype braincase allowed the most complete neuroanatomical reconstruction of a South American abelisaurid and proved that Viavenator was closer to Argentina's Aucasaurus than it was to Madagascar's Majungasaurus. Compared to the latter, it had a larger flocculus of the cerebellum — the part of the brain that tells the VOR (vestibulo-ocular reflex) to make adjustments to eye position during head movement — which would ensure stable vision as it bobbed up and down, like a seesaw with teeth. It also had a higher olfactory ratio (olfactory lobe compared to total brain size), meaning it had a superior sense of smell.

The Santonian age of Viavenator goes some way to filling a void in the Late Cretaceous South American fossil record between the basal brachyrostran abelisaurids (such as Xenotarsosaurus, Ekrixinatosaurus and Skorpiovenator) of the Cenomanian-Turonian, and the derived forms (such as Carnotaurus, Quilmesaurus and Pycnonemosaurus) from the Campanian-Maastrichtian, as it appears to sit somewhere between those two groups, both in features and in position on the theropod dinosaur family tree. In 2016, Leonardo Filippi and colleagues found Viavenator to be the oldest member of a new abelisaurid family called Fusilersauria — the stiff-backed lizards — which was a South America-exclusive club at that point, and included all of that continent's brachyrostrans from the Santonian onwards. However, several discoveries across Europe, including Caletodraco cottardi from France, have expanded their geographic distribution drastically.

Viavenator is the prime suspect behind bite and scrape marks found on the isolated back vertebra (MAU-Pv-CO-651) of a titanosaurian sauropod from the Bajo de la Carpa Formation of Argentina's Neuquén province.