Isasicursor

In 1980, geologist Francisco E. Nullo noticed the presence of sauropod bones on a hillside of the Estancia Alta Vista, south of the Centinela River in the Santa Cruz province of Argentina. He reported these finds to then-prominent paleontologist José Bonaparte. Bonaparte dug up a large sauropod cervical vertebra in 1981. The old site was relocated and new excavations were carried out between 13 and 17 January and 14 to 19 March 2019, and a new site was discovered on the Estancia La Anita. A whole new fauna came to light on an area of 2,000 square kilometres (770 square miles). The sauropod finds were named as the new genus Nullotitan but bones from a new ornithopod were also discovered by technician Marcelo Pablo Isasi, which laid close to some mosasaur teeth.

In 2019, the type species Isasicursor santacrucensis was named and described by Fernando Emilio Novas, Federico Lisandro Agnolin, Sebastian Rozadilla, Alexis Mauro Aranciaga-Rolando Federico Brisson-Egli, Matias Javier Motta, Mauricio Cerroni, Martin Dario Ezcurra, Agustín Guillermo Martinelli, Julia S D´Angelo, Gerardo Alvarez-Herrera, Adriel Roberto Gentil, Sergio Bogan, Nicolás Roberto Chimento, Jordi Alexis García-Marsà, Gastón Lo Coco, Sergio Eduardo Miquel, Fátima F. Brito, Ezequiel Iganacio Vera, Valeria Susana Perez Loinaze, Mariela Soledad Fernández and Leonardo Salgado. The large number of authors is a consequence of the fact that the article described the entire fauna in which every expert contributed his part. The genus name honors Isasi and connects its name with a Latin cursor, "runner". The species designation refers to its provenance from Santa Cruz province.

The holotype, MPM 21525, was found in a layer of the upper Chorillo Formation that dates from the Maastrichtian. It consists of the upper side of a left shin.

Different bones from the same location were designated as paratypes. These were MPM 21526, a piece of a cervical vertebra; MPM 21527: two middle vertebrae; MPM 21528: a sacrum minus the third sacral vertebra; MPM 21529: thirty anterior, middle and posterior vertebrae of different individuals; MPM 21530: the bottom of a right shoulder blade; MPM 21531: the lower end of a left humerus; MPM 21532: the main body of a left pubic bone; MPM 21533: a set of young animals of which the upper side of a right thigh bone, three upper sides of a left thigh bone, three lower sides of a left thigh bone and two shafts of a thigh bone; MPM 21534: the lower end of a tibia; MPM 21535: the ends of a second metatarsal; MPM 21536: the lower end of a third left metatarsal leg; MPM 21537: the ends of a fourth metatarsal; MPM 21538: the fourth metatarsal bone of a young animal; MPM 21539L: the first foot of the first toe, first foot of the second toe and the second and third foot of the fourth toe; MPM 21540: six foot claws; and MPM 21541: the second and third toes of a young animal.

The finds represent at least four individuals. They are found in a layer of 5 metres (16 ft) long and 0.5 metres (1.6 ft) thick. The fossils are part of the collection of the Museo Regional Provincial Padre Molina.

Isasicursor was placed in the Elasmaria , although without a precise cladistic analysis, so its exact relationships remain unclear. If this placement is correct, the animal is the youngest known iguanodont, and by extension, ornithopod, known from the Southern Hemisphere.

Isasicursor was a herbivore. The describers consider it likely that the Elasmaria lived together with the relatively larger Hadrosauroidea, with the two groups belonging in different niches, though hadrosauroids were not found at the site. The presence of Nullotitan suggests niche partitioning with giant sauropods. Isasicursor was probably hunted by an as yet unnamed member of the Megaraptoridae whose bones were found on the site.

The describers saw the remains of young and old individuals being found together as evidence that Isasicursor lived in herds.