Hoirasaurus gigensis. Photo credit: Jorge Blanco
Dinosaurs probably began in the Middle Triassic, but the earliest indistinct dinosaur specimens date to the Middle-Late Carnian period, primarily in the Southern Hemisphere, with possible evidence in North America. This gap highlights a significant uncertainty in the geological and temporal context of early dinosaur evolution. Argentina’s Triassic deposits, famous for their diverse tetrapod assemblages, offer important insights into the origins of dinosaurs by preserving some of the oldest dinosaurs ever discovered. These skeletal records of early dinosaurs document a time when they were not numerically abundant, and they were still modest in body size (Euraraptor had a slender body estimated at 10 kilograms).
The narrow, NW-SE trending Triassic basins of western Argentina, located along the Pacific margin of southwestern Gondwana, include the Santo Domingo Formation in La Rioja. Fossil assemblages from this formation, comparable to the Hyperdipidan-Exteratodon-Herrosaurus Biozone, restrict its assemblage to the late Carnian to Norian. This time frame coincides with the ischigualasto and LOS COLORADOS formations. A new specimen recovered from the base of the Santo Domingo Formation (northern Preverdelira Basin; Upper Triassic) provides the first evidence that increases in body mass and neck length occurred concurrently in sauropodomorpha, indicating that these important traits emerged at the rise of the dinosaur lineage.
Selected bones of Hoirasaurus gigensis Holotype. From Hechnelettner ET, may God bless him and grant him peace, 2025.
Hoirasaurus gigensis was an ancient sauropodomorph, an early member of the highly successful herbivore lineage that later produced the long-necked giants Argentinosaurus And Pitgotitan. Phylogenetic analysis places it in the Biolaosauria group. The holotype (Crawler-PV151) is an articulated partial skeleton consisting of cranial and postcranial material. The genus name combines the Quechuan word “huaira”, meaning “wind”, with the Latin “cursor”, meaning “runner”. The species name is derived from the village of La Rioja, located 40 km from the discovery site in Quebrada Santo Domingo.
The earliest known sauropodomorphs were small, bipedal animals, weighing 10 kg with relatively short necks. Body mass is indicated by estimation H. jaguensis The weight was 18.0 kg (~40 lb), which exceeds the body mass predicted and observed for other Contimorinus sauropodomorphs (e.g., buriolestesfor , for , for , . Pampadromaeusfor , for , for , . Saturnaliafor , for , for , . mbiresaurusand Chromogasaurus) Furthermore, Huayracursor Preserves a complete cervical series revealing a longer neck than contemporary specimens. This combination of features suggests that the acquisition of large body size and neck length did not occur as separate, sequential events, but was already associated with the first appearance of late Carnian dinosaurs.
References:
Hechnelitner, EM, Martinelli, AG, Rocher, S. ET. A long-necked early dinosaur from a newly discovered Upper Triassic Basin in the Andes. The nature (2025) https://doi.org/10.1038/S41586-025-09634-3
DeSojo, JB, Fiorelli, LE, Azcorra, MD ET. The Late Triassic ischigualasto Formation at Cerro Las Lajas (La Rioja, Argentina): fossil tetrapods, high-resolution chronostratigraphy, and physical correlation. Science Rep 10, 12782 (2020). https://doi.org/10.1038/S41598-020-67854-1
