elasmaria Elasmaria is a clade of ornithopods known from Cretaceous deposits in South America, Antarctica, and Australia that contains many bipedal ornithopods that were previously considered "hypsilophodonts".Madzia, Daniel; Boyd, Clint A.; Mazuch, Marti ...

ornithopod Ornithopoda () is a clade of ornithischian dinosaurs, called ornithopods (), that started out as small, bipedal running grazers and grew in size and numbers until they became one of the most successful groups of herbivores in the Cretaceous wo ...

Chorrillo Formation The Chorrillo Formation, also named as Chorillo Formation,Río Leona
at Santa Cruz Province in

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...

. The type and only species is ''Isasicursor santacrucensis''. It was a contemporary of the sauropod '' Nullotitan'' which was described in the same paper.Novas, F., Agnolin, F., Rozadilla, S., Aranciaga-Rolando, A., Brissón-Eli, F., Motta, M., Cerroni, M., Ezcurra, M., Martinelli, A., D'Angelo, J., Álvarez-Herrera, G., Gentil, A., Bogan, S., Chimento, N., García-Marsà, J., Lo Coco, G., Miquel, S., Brito, F., Vera, E., Loinaze, V., Fernandez, M., & Salgado, L. (2019)
Paleontological discoveries in the Chorrillo Formation (upper Campanian-lower Maastrichtian, Upper Cretaceous), Santa Cruz Province, Patagonia, Argentina
Revista del Museo Argentino de Ciencias Naturales, 21(2), 217-293.

Discovery and naming

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 José Fernando Bonaparte (14 June 1928 – 18 February 2020) was an Argentine paleontologist who discovered a plethora of South American dinosaurs and mentored a new generation of Argentine paleontologists . One of the best-known Argentine paleo ...

. 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 . 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 Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ' meaning 'lizard') comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Maastricht on ...

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 their 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

Chorrillo Formation The Chorrillo Formation, also named as Chorillo Formation,Río Leona
at

Size and distinctive features

The describers were able to identify five distinctive features. They are autapomorphies, unique derived properties. The sacrum is curved downwards. The first and second sacral vertebra are connected to each other via a pin-and-hole connection. The ''crista cnemialis'' of the tibia is thickened and highly upward excellent. On the upper surface of the tibia, the outer rear lobe has an additional protrusion that faces forward and faces. The second metatarsal has a well on the outside for the joint capsule that is higher than normal.

Skeleton

The cervical vertebra is keeled and flattened on the side. The amphicoelous vertebrae show no special details. The sacrum has six sacral vertebrae and is fairly robust. It is exceptional that the sphere curves downwards. The keeled first sacral vertebra is greatly widened sideways. The second sacral vertebra has a keel that extends into a protrusion that fits into a hole in the oval back facet of the first vertebra; their contact is normally flat. The fourth vertebra has a longitudinal trough on the underside with only a low ledge at the front. The front facet is square, the back facet hexagonal. The fifth vertebra has the same morphology from the bottom. It is again oval on the front but rectangular on the back. The sixth vertebra also sticks with a pin in the fifth. The tail vertebrae are slightly amphicoel and have length trough. Lateral ridges increase towards the rear, so that eventually a hexagonal cross-section is created. With the shoulder blade, the cavity above the shoulder joint is round and shallow, as with ''

Gasparinisaura ''Gasparinisaura'' (meaning "Gasparini's lizard") is a genus of herbivorous ornithopod dinosaur from the Late Cretaceous. The first fossils of ''Gasparinisaura'' were found in 1992 near Cinco Saltos in Río Negro Province, Argentina. The type sp ...

'' and '' Trinisaura'' . The front leg is relatively long but shorter than the rear leg. The corridor was probably optional two-legged: at high speed the animal walked on only the hind legs. The fact that the front leg was not used for a powerful sale is clear from the delta-detectoral comb of the humerus that is reduced to a low ledge. The outside thereof is intersected by numerous roughenings for tendons. The humerus bends outwards. Both of these characteristics are typically elasmaric. The pubis apparently did not touch the process obturatorius of the Ischium which means that the foramen obturatum is not closed but open downwards. For the femur, the trochanter major at the top is convex and on the inside separated by a groove from the front narrow trochanter minor. The front groove is not well developed between the lower joint nodules. The inner nodule is not widened outwards. With the tibia, the upward widening of the crista cnemialis, above the joint surface, gives it a triangular profile. This comb is also thickened across, with a round top. The crista lateralisis also thick, with a straight front edge. The triangular outer lobe of the upper surface has a special obliquely forward projection. The lower outer leg style has an elongated comb that forms an elevation in the middle. The inner post is not bent forward. In the young animals, all these characteristics are less pronounced on the lower part of the tibia. The second metatarsal is flattened at the top and thickened from front to back, a

synapomorphy In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to hav ...

of the Elasmaria. However, the thickening remains limited. With the third metatarsal, the innermost lower joint bump is the largest. With the remarkably robust fourth metatarsal leg, the upper outer edge forms a sharp ledge; at ''

Talenkauen ''Talenkauen'' is a genus of basal iguanodont dinosaur from the Campanian or Maastrichtian age of the Late Cretaceous Cerro Fortaleza Formation, formerly known as the Pari Aike Formation of Patagonian Lake Viedma, in the Austral Basin of Sa ...

'', ''

Morrosaurus ''Morrosaurus'' is an extinct genus of herbivorous ornithopod dinosaur member of the Elasmaria, that lived in the late Cretaceous in the Antarctica. The only known species is the type ''Morrosaurus antarcticus''. Discovery In 2002 the Argent ...

'' and the Styracosterna this piece is completed. The bottom surface forms a parallelogram whose rear edge protrudes more inwards but is curled outwards. The toe legs are sturdy and short, also from front to back, a distracted feature. The foot claws are not hoof-shaped but sharp, albeit with a flat bottom.

Phylogeny

''Isasicursor'' was placed in the

Elasmaria Elasmaria is a clade of ornithopods known from Cretaceous deposits in South America, Antarctica, and Australia that contains many bipedal ornithopods that were previously considered "hypsilophodonts".Madzia, Daniel; Boyd, Clint A.; Mazuch, Marti ...

, although without a precise cladistic analysis, so its exact relationships remain unclear. If this placement is correct, the animal is the youngest known

iguanodont Iguanodontia (the iguanodonts) is a clade of herbivorous dinosaurs that lived from the Middle Jurassic to Late Cretaceous. Some members include ''Camptosaurus'', ''Dryosaurus'', ''Iguanodon'', '' Tenontosaurus'', and the hadrosaurids or "duck-bil ...

, and by extension, ornithopod, known from the Southern Hemisphere.

Palaeobiology

Chorrillo Formation Dinosauria Size Comparison

''Isasicursor'' was a herbivore. The describers consider it likely that the Elasmaria lived together with the relatively larger

Hadrosauroidea Hadrosauroidea is a clade or superfamily of ornithischian dinosaurs that includes the "duck-billed" dinosaurs, or hadrosaurids, and all dinosaurs more closely related to them than to ''Iguanodon''. Their remains have been recovered in Asia, Europ ...

, with the two groups belonging in different niches, though hadrosauroids were not found at the site. The presence of ''Nullotitan'' suggests

niche partitioning In ecology, niche differentiation (also known as niche segregation, niche separation and niche partitioning) refers to the process by which competing species use the environment differently in a way that helps them to coexist. The competitive exclu ...

with giant sauropods. ''Isasicursor'' was probably hunted by ''

Maip ''Maip'' is a genus of large Megaraptora, megaraptorid Theropoda, theropod dinosaur from the Late Cretaceous (Maastrichtian) Chorrillo Formation of Santa Cruz Province, Argentina, Santa Cruz, Argentina. The genus contains a single species, ''M. ma ...

'', possibly the largest Patagonian megaraptorid. The describers saw the remains of young and old individuals being found together as evidence that ''Isasicursor'' lived in herds.

References

{{Taxonbar, from=Q77814552 Ornithopods Campanian life Maastrichtian life Late Cretaceous dinosaurs of South America Cretaceous Argentina Fossils of Argentina Fossil taxa described in 2019 Ornithischian genera