Chakisaurus Explained

Chakisaurus (meaning "elder guanaco lizard") is an extinct genus of elasmarian ornithopod dinosaur from the Late Cretaceous Huincul Formation of Argentina. The genus contains a single species, C. nekul, known from multiple partial skeletons belonging to individuals of different ages. Chakisaurus represents the first ornithischian species to be named from the Huincul Formation.

Discovery and naming

The Chakisaurus fossil material was discovered in sediments of the Huincul Formation in Pueblo Blanco Natural Reserve (previously known as the Violante Farm locality) near Ezequiel Ramos-Mexía Lake in Río Negro Province, Argentina. The holotype specimen, MPCA Pv 816, consists of several partial dorsal vertebrae, a partial sacrum, twelve caudal vertebrae, an incomplete haemal arch, partial left femur and fibula, partial right tibia and calcaneus, and two toe bones from the fourth digit. Three additional paratype specimens were also assigned to Chakisaurus, found in a group about 500m (1,600feet). The first is MPCA Pv 822, which belongs to a juvenile individual, including five dorsal vertebral centra, a left humerus, and the bottoms of both femora. The second is MPCA Pv 823, another juvenile individual consisting of the top of a right ulna. The third is MPCA Pv 813, which includes eight dorsal vertebral centra, two partial ribs, two partial haemal arches, the bottom of a right radius, a toe bone of digit four, and a toe claw of digit two or four. An additional cervical vertebra (possibly the fourth), MPCN Pv 846, was also referred to Chakisaurus.^[1]

In 2024, Alvarez Nogueira et al. described Chakisaurus nekul as a new genus and species of ornithopod based on these fossil remains. The generic name, "Chakisaurus", combines "Chaki", an Aonikenk word meaning "elder guanaco"—, referring to the species Lama guanicoe—with the Greek "σαῦρος" ("sauros"), meaning "lizard". The specific name, "nekul", is a Mapudungun word meaning "swift".^[1]

Chakisaurus represents the tenth basal ornithopod named from South America.^[1]

Description

Chakisaurus has been described as a "medium-sized" elasmarian ornithopod, similar in size to taxa such as Anabisetia, Notohypsilophodon and Trinisaura, but smaller than taxa such as Talenkauen, Mahuidacursor, and Isasicursor. Analysis of the forelimb bones preserved for the species finds no adaptations towards some level of quadrupedal locomotion, suggesting that some other elasmarians developed these traits independently.^[1]

When the anterior caudal vertebrae were articulated, this likely resulted in a protonic posture, with the base of the tail curving downward. This feature has only otherwise been observed in titanosaurs, including the aeolosaurin Arrudatitan. Like other elasmarians, the tail shares similar adaptations towards cursoriality as with some coelurosaur theropods. ^{[1] [2] [3]}

Classification

In their phylogenetic analyses, Alvarez Nogueira et al. (2024) recovered Chakisaurus as an elasmarian ornithopod within the iguanodontian clade Dryomorpha. They note that due to the fragmentary nature of the Chakisaurus fossil material, their tree was not well-defined. Their results are shown in the cladogram below:^[1]

Palaeoenvironment

Chakisaurus is known from the Late Cretaceous Huincul Formation of Río Negro Province, Argentina. Many saurischian dinosaurs, including rebbachisaurids (Cathartesaura, Limaysaurus, and Sidersaura),^{[4] [5]} titanosaurs (Argentinosaurus, Bustingorrytitan, Chucarosaurus, and Choconsaurus),^[6] carcharodontosaurids (Mapusaurus, Meraxes, and Taurovenator),^[7] a megaraptoran (Aoniraptor), abelisaurids (Skorpiovenator, Tralkasaurus, and Ilokelesia), an elaphrosaurine (Huinculsaurus),^[8] a paravian (Overoraptor), and the unusual avetheropod Gualicho have also been named from the formation.^{[9] [10]} Remains of an unnamed unenlagiid have also been reported.^[11] The non-dinosaurian fauna includes fossil fish, sphenodonts, indeterminate squamates, chelid turtles, and eusuchian crocodilians.

Chakisaurus is the first ornithischian from the formation to receive a scientific name; the ungual of an indeterminate ornithopod was the only ornithischian bone previously recovered here.

Notes and References

1. Alvarez-Nogueira . Rodrigo . Rozadilla . Sebastián . Agnolín . Federico L. . Garcia Marsà . Jordi A. . Motta . Matias J. . Novas . Fernando E. . 2024-03-11 . A new ornithopod from the Upper Cretaceous (Huincul Formation) of Northwestern Patagonia, Argentina. Implications on elasmarian postcranial anatomy . . 159 . In press . 105874 . 10.1016/j.cretres.2024.105874. 2024CrRes.15905874N .
2. Vidal . Luciano da Silva . Pereira . Paulo Victor Luiz Gomes da Costa . Tavares . Sandra . Brusatte . Stephen L. . Bergqvist . Lílian Paglarelli . Candeiro . Carlos Roberto dos Anjos . 2021-09-02 . Investigating the enigmatic Aeolosaurini clade: the caudal biomechanics of Aeolosaurus maximus (Aeolosaurini/Sauropoda) using the neutral pose method and the first case of protonic tail condition in Sauropoda . . en . 33 . 9 . 1836–1856 . 10.1080/08912963.2020.1745791 . 2021HBio...33.1836V . 0891-2963.
3. Osteology of Ornithopod Macrogryphosaurus gondwanicus (Dinosauria, Ornithischia) from the Upper Cretaceous of Patagonia, Argentina . Rozadilla . Sebastián . Cruzado-Caballero . Penélope . Calvo . Jorge O. . Cretaceous Research . 2020 . 108 . 104311 . 10.1016/j.cretres.2019.104311. 2020CrRes.10804311R . 213679041 .
4. Calvo . Jorge O. . Salgado . Leonardo . Rebbachisaurus tessonei sp. nov. a new Sauropoda from the Albian-Cenomanian of Argentina; new evidence on the origin of the Diplodocidae . Gaia . 11 . 1995 . 13–33 . https://web.archive.org/web/20210923134140/http://www.dinochecker.com/papers/Calvo+Salgado-%5B1995%5D-Rebbachisaurus-tessonei.pdf . 23 September 2021 .
5. Lerzo . Lucas Nicolás . Gallina . Pablo Ariel . Canale . Juan Ignacio . Otero . Alejandro . Carballido . José Luis . Apesteguía . Sebastián . Makovicky . Peter Juraj . 2024-01-03 . The last of the oldies: a basal rebbachisaurid (Sauropoda, Diplodocoidea) from the early Late Cretaceous (Cenomanian–Turonian) of Patagonia, Argentina . . en . 1–26 . 10.1080/08912963.2023.2297914 . 0891-2963.
6. Simón . M. E. . Salgado . L. . 2023 . A new gigantic titanosaurian sauropod from the early Late Cretaceous of Patagonia (Neuquén Province, Argentina) . Acta Palaeontologica Polonica. 10.4202/app.01086.2023 . free .
7. Canale . Juan I. . Apesteguía . Sebastián . Gallina . Pablo A. . Mitchell . Jonathan . Smith . Nathan D. . Cullen . Thomas M. . Shinya . Akiko . Haluza . Alejandro . Gianechini . Federico A. . Makovicky . Peter J. . New giant carnivorous dinosaur reveals convergent evolutionary trends in theropod arm reduction . Current Biology . July 2022 . 32 . 14 . 3195–3202.e5 . 10.1016/j.cub.2022.05.057 . 35803271 . 250343124 . free . 2022CBio...32E3195C .
8. Baiano . Mattia A. . Coria . Rodolfo A. . Cau . Andrea . A new abelisauroid (Dinosauria: Theropoda) from the Huincul Formation (lower Upper Cretaceous, Neuquén Basin) of Patagonia, Argentina . Cretaceous Research . June 2020 . 110 . 104408 . 10.1016/j.cretres.2020.104408 . 2020CrRes.11004408B . 214118853 .
9. Cerroni . M.A. . Motta . M.J. . Agnolín . F.L. . Aranciaga Rolando . A.M. . Brissón Egli . F. . Novas . F.E. . 2020 . A new abelisaurid from the Huincul Formation (Cenomanian-Turonian; Upper Cretaceous) of Río Negro province, Argentina . Journal of South American Earth Sciences . 98 . 102445 . 10.1016/j.jsames.2019.102445 . 2020JSAES..9802445C . 213781725.
10. Matías J. Motta . Federico L. Agnolín . Federico Brissón Egli . Fernando E. Novas . 2020 . New theropod dinosaur from the Upper Cretaceous of Patagonia sheds light on the paravian radiation in Gondwana . The Science of Nature . 107 . 3 . Article number 24 . 2020SciNa.107...24M . 10.1007/s00114-020-01682-1 . 32468191 . 11336/135530 . 218913199. free .
11. Agnolin . Federico L. . Gonzalez Riga . Bernardo J. . Aranciaga Rolando . Alexis M. . Rozadilla . Sebastián . Motta . Matías J. . Chimento . Nicolás R. . Novas . Fernando E. . 2023-02-02 . A new giant titanosaur (Dinosauria, Sauropoda) from the Upper Cretaceous of Northwestern Patagonia, Argentina . Cretaceous Research . 146 . en . 105487 . 10.1016/j.cretres.2023.105487 . 2023CrRes.14605487A . 0195-6671.