Cariocecus

MEANING: War god

PERIOD: Early Cretaceous

CONTINENT: Europe

Cariocecus is a basal hadrosauroid ornithopod, and the first iguanodontian discovered in Portugal with a skull. The fossil remains preserve interesting soft tissue samples, including the most detailed inner-ear of any dinosaur to date. Cariocecus was a medium-sized herbivore at approximately 5-6 m in length.

Abstract from paper: In Portugal, iguanodontian dinosaurs are mostly known from the Late Jurassic of the Lourinhã Formation and are represented by dryosaurids and basal styracosternans. The Early Cretaceous record of iguanodontians in Portugal is scarce in comparison, with scattered and uninformative remains referred to Styracosterna indet. Here, we describe SHN.832, the first iguanodontian skull from Portugal, named Cariocecus bocagei gen. nov. sp. nov. The specimen was found in Praia do Areia do Mastro (Cabo Espichel, Sesimbra) in the Papo Seco Formation (lower Barremian), and comprises the right side of the skull, part of the skull vault and a nearly complete basicranium. Cariocecus bocagei is diagnosed based on autapomorphies such as the co-ossified maxillo-jugal complex and the trilobated shape of the supraoccipital. The phylogenetic analysis retrieves C. bocagei as a basal hadrosauroid in a clade with Comptonatus chasei and Brighstoneus simmondsi. Our biogeographical analysis emphasizes the effects of insular endemism during the Hauterivian–Aptian range in the European regions. We show that Iguanodontia originated in South America, and through a dispersal event towards North America during the Upper Jurassic, expanded their latitudinal range. Cariocecus and other early-diverging iguanodontians emerged during an eastward dispersal event in the Lower Cretaceous. We reconstructed the endocast, cranial nerves and inner ear of Cariocecus via segmentation of micro-computed tomography scanning, showing similarities with other Hadrosauriformes such as Iguanodon and Proa. We propose the most detailed inner ear soft-tissue reconstruction for a dinosaur so far, including the macula and sub-branches of the vestibulocochlear nerve, supported by the extant phylogenetic bracket. Based on the unossified suture of the cranial elements, we hypothesize that SHN.832 had not yet reached full skeletal maturity, and our restoration suggests a total skull length of about 45 cm. The supraorbital membrane was reconstructed based on the well-preserved supraorbital bone and comparison with modern taxa.

Cariocecus is from the Early Cretaceous. The Cretaceous is the third and final geological period of the Mesozoic Era, with the Early Cretaceous making up roughly the first half, lasting from about 143 to 100 million years ago. The poles were ice-free, due to the relatively warm climate, and forests extended into high latitudes. The continued breakup of the continents created new coastlines and isolated landmasses, influencing the evolution of distinct dinosaur faunas.

It was a time of transition, as many groups of animals and plants began to take on more modern forms while others declined or disappeared. Pterosaurs continued to thrive, though early birds were becoming more diverse and widespread. Mammals remained small but adapted to a variety of ecological niches. In the oceans, ichthyosaurs and plesiosaurs were common, and early mosasaurs began to appear.

Dinosaurs remained the dominant land animals, with groups like iguanodontians, spinosaurids, and carcharodontosaurids rising to prominence. While sauropods declined in some regions, they remained abundant in the Southern Hemisphere. The first true ceratopsians appeared, and ankylosaurs replaced stegosaurs in their niche. Dromaeosaurs and other small theropods diversified. During this time, the first flowering plants evolved, gradually changing global ecosystems by providing new food sources for herbivores.

Cariocecus is an iguanodontian. Iguanodontia was an important branch of the ornithopoda, a group of herbivorous dinosaurs that would become one of the most successful clades of the Cretaceous. These dinosaurs first appeared in the Late Jurassic as small bipedal grazers, but diversified through the Early Cretaceous, gradually increasing into larger more robust forms. While the more derived hadrosaurs of the Late Cretaceous became highly specialized for chewing tough plant material, basal iguanodontians retained a mix of primitive and advanced traits. They were among the first large-bodied ornithopods to spread widely across the Northern Hemisphere, with fossils found in Europe, North Africa, and Asia.

Early iguanodontians possessed powerful hind limbs for efficient bipedal movement but could also walk on all fours when foraging. One of their most distinctive features was their unique hand structure, which included a stiff, spike-like thumb that may have been used for defense or breaking apart tough vegetation. Although they lacked the fully developed dental batteries of later hadrosaurs, their teeth were already adapted for efficient plant processing, giving them an evolutionary advantage as herbivores. These adaptations laid the groundwork for the eventual dominance of hadrosaurs, which would expand into even more diverse habitats and continue to thrive until the end of the Cretaceous.