Abelisauroidea is the best known carnivorous dinosaur group from Gondwana. The clade was erected by the legendary paleontologist Jose Bonaparte with the description of Abelisaurus Comahuensis. These theropods exhibit spectacular cranial ornamentation in the form of horns and spikes, and strongly reduced forelimbs. Abelisauroidea has been divided into two main branches: the Noasauridae and the Abelisauridae. The Noasauridae are known from Cretaceous beds in northern Argentina, Madagascar, India, and Niger. They are small and slender with sizes that range from 1 to 3 metres in length. The Abelisaurids are medium to large, robust animals, such as the Carnotaurus and the Majungasaurus of Madagascar. The group exhibits short, round snouts; thickened teeth; short, stocky arms; and highly reduced forearms. The Argentinean record of abelisauroid theropods begins in the Middle Jurassic with Eoabelisaurus mefi, and spans most of the Late Cretaceous. The clade includes Carnotaurus sastrei, Aucasaurus garridoi, Ekrixinatosaurus novasi, Skorpiovenator bustingorryi, Tralkasaurus cuyi, Viavenator exxoni, Niebla antiqua, and Llukalkan aliocranianus. All of them are from the Cretaceous of Patagonia.
Guemesia ochoai is the first definitive abelisaurid theropod from Northwestern Argentina. The holotype (IBIGEO-P 103) is represented by a nearly complete braincase, with parietals, frontals, supraoccipital, basioccipital, exoccipital-opisthotic complex, basisphenoid-parasphenoid, prootics, laterosphenoids, and orbito-sphenoids. It was recovered from Los Blanquitos Formation (Campanian, Late Cretaceous), near Amblayo town, Salta province, Argentina. The name honours General Martin Miguel de Güemes who defended northwestern Argentina during the War of Independence, and Javier Ochoa, technician of the Museo Regional “Florentino Ameghino,” who discovered the specimen. The braincase of Guemesia is small compared to other abelisaurids. Unfortunately, the absence of postcranial elements makes impossible to perform histological analysis and to corroborate if the specimen reached somatic maturity. The cranial endocast has a total length of 73 mm from the base of the olfactory tract to the foramen magnum, and the volume is ∼47.6 cm3 (almost 70% smaller than the cranial endocasts of Carnotaurus and Viavenator). But the most striking feature of Guemesia is a row of foramina close to the midline of the fused frontals that is unknown in other abelisaurids. These rows may be linked to a zone of thermal exchange.
Federico L. Agnolín et al, First definitive abelisaurid theropod from the Late Cretaceous of Northwestern Argentina, Journal of Vertebrate Paleontology (2022). DOI: 10.1080/02724634.2021.2002348