Federico Abel Gianechini

El área Cerro Overo-La Invernada (Patagonia norte) ha provisto un rico registro de tetrápodos continentales cretácicos en las últimas dos décadas, principalmente de la Formación Bajo de la Carpa (Cretácico Superior, Santoniano). La fauna de dinosaurios de esta área es diversa, incluyendo nuevos taxones, siendo los terópodos abelisáuridos particularmente abundantes. Recientemente, una vértebra caudal aislada (MAU-Pv-CO-598) fue recuperada aquí. Aunque fragmentaria, muestra características que la diferencian de otros restos de abelisáuridos previamente hallados en esta área. Es considerada una vértebra caudal media y está caracterizada por un surco longitudinal ventral en el centro; un proceso transverso escasamente elevado con un extremo distal anteroposteriormente expandido y lateralmente cóncavo; una cresta longitudinal anteriormente extendida desde el borde anterior de la espina neural; y tubérculos pequeños extendidos desde los extremos de las zigapófisis, considerados como estructuras accesorias entrelazadas entre las vértebras. Algunos caracteres, como la cresta dorsal longitudinal y los tubérculos accesorios de las zigapófisis, están presentes en el abelisáurido no braquirostro Majungasaurus. Sin embargo, el proceso transverso es similar al de las vértebras caudales de braquirostros no furileusaurios, como Ilokelesia, Ekrixinatosaurus y Skorpiovenator. Un análisis filogenético agrupa a MAU-Pv-CO-598 con los últimos tres taxones. MAU-Pv-CO-598 proviene de niveles más bajos de la Formación Bajo de la Carpa que los furileusaurios Viavenator y Llukalkan. La presencia de un espécimen con afinidades no furileusaurianas en esta formación indica un posible reemplazo de braquirostros no furileusaurios a furileusaurios ocurrido durante el Santoniano, luego del propuesto recambio faunístico del Turoniano.

Abelisaurids are among the most abundant and diverse Patagonian Late Cretaceous theropods. Here, we present a new furileusaurian abelisaurid, Llukalkan aliocranianus gen. et sp. nov., represented by cranial remains from the Bajo de la Carpa Formation (Santonian) at La Invernada fossil area, northwestern Patagonia. Features characterizing this taxon include a possible caudal tympanic recess posterior to the columellar recess, a T-shaped lacrimal with jugal ramus lacking a suborbital process, and large foramina for caudal middle cerebral veins widely separated from the median supraoccipital crest. In addition to this, a bulge on the anteromedial border of the supratemporal fossa, tall and posteriorly projected paroccipital processes, basal tubera interconnected distally, a triangular basisphenoid recess, and a single foramen for the sphenoidal artery on the basisphenoid, differentiate Llukalkan from Viavenator exxoni. The latter is the other furileusaurian taxon from the same area and stratigraphic unit. Although the holotype of Llukalkan probably corresponds to a sub-adult—as the lacrimal morphology suggests— the possibility that it represents a juvenile of V. exxoni is discarded based mainly on the presence of a caudal tympanic recess (which is absent in V. exxoni). The probable coexistence of two abelisaurid taxa demonstrates that the abelisaurids were one of the most important—and likely the main—predator component of the ecosystems, not only in this area, but also in all of Patagonia, during the Late Cretaceous.

Tetrapod limbs morphology is a reliable proxy of locomotor capacities. Beyond this, other aspects of life habits, such as predation abilities, can also be relevant to determine main morphofunctional appendicular properties, which ultimately reflect a compromise between different factors of the biological role. Dromaeosauridae is a dinosaur clade belonging to Theropoda, a group of bipedal predators. Dromaeosaurids represent an interesting study case, in which the hindlimbs have been proposed to be involved in both locomotion and predation activity. A peculiar feature characterizing all dromaeosaurids is a modified second pedal digit, which is typically related to predation. This theropod group is closely related to birds and diversified during the Cretaceous Period, mainly in the Northern Hemisphere (Laurasia). However, a subclade of dromaeosaurids, the Unenlagiinae, was recently recognized for Gondwana. Nevertheless, there are morphological differences between derived Laurasian dromaeosaurids (eudromaeosaurs) and unenlagiines. Such differences are observed in the proportions between hindlimb bones and in the presence of a subarctometatarsalian condition in unenlagiines, which is mainly characterized by a proximally constricted metatarsal III. To evaluate the function of these divergent morphologies, we conducted morphometric analyses and comparisons of qualitative morphological aspects, encompassing unenlagiines, other dromaeosaurids, as well as taxa from other theropod groups, including extant birds. The former approach consisted of two phylogenetic principal component analyses, one based on the main measurements of the hindlimb, and the other focused on the lengths of the pedal phalanges. The first analysis drew the unenlagiines close to taxa with long tibiae, as well as long and slender metatarsi. Instead, eudromaeosaurs are closer to taxa with shorter tibiae, and shorter and wider metatarsi. The second analysis showed that eudromaeosaurs and unenlagiines have similar phalangeal proportions, including the elongation of distal phalanges. However, the shorter second phalanx of the pedal digit II of eudromaeosaurs could have increased the force generated by this digit, which was the main predatory tool of the autopodium. This, together with a shorter and wider metatarsus, and a marked hinge‐like morphology of the articular surfaces of metatarsals and phalanges, possibly allowed eudromaeosaurs to exert a great gripping strength and hunt large prey. Conversely, the longer and slender subarctometatarsus, and less well‐marked hinge joints of unenlagiines possibly gave them greater cursorial capacities. Additionally, the longer second phalanx of digit II allowed unenlagiines fast movements of this digit to hunt smaller and elusive prey. Thus, the distinctive morphological evolutionary pathways of these two dromaeosaurid clades seem to have been influenced by the particular locomotor and predatory specializations that characterized each of these lineages.