Trinisaura is a genus of ornithopod dinosaur that lived during the late Campanian stage of the Upper Cretaceous, around 73 to 72 million years ago in what is now James Ross Island off the coast of northern Antarctica near Patagonia.

It remained undescribed in the collections of the Museo de La Plata until its description by Coria and colleagues in 2013, being the basis of the novel genus and species Trinisaura santamartaensis.

During Trinisaura's existence, Antarctica had just recently broken off from the rest of Gondwana, causing it to evolve distinct traits due to geographic isolation.

It was found in the lower layers of the Gamma Member of the Snow Hill Island Formation, which preserves only one other named dinosaur, the ankylosaur Antarctopelta, in addition to a variety of aquatic animals like sharks and cephalopods.

Fossils of Trinisaura were first collected in 2008 by Juan Moly and Rodolfo Coria, members of the Antarctic Summer Campaign that had been mounted by Instituto Antártico Argentino to the fossilferous exposures of the Santa Marta Cove on James Ross Island, Antarctica.

The layers date to the Upper Campanian stage of the Late Cretaceous, coming from the base sandstone deriving from the Gamma Member of the Snow Hill Island Formation.

[1] The ornithopod specimen was then deposited at the Museo de La Plata under catalogue number MLP-III-1-1, later the holotype, and briefly described by Coria et al (2008),[2] but received a complete description in 2013.

[7][8][1] Isolated unguals in addition to a tibia and astragalus coming from larger, unnamed ornithopods have also been described from the Snow Hill Formation of James Ross Island.

[9][10] Trinisaura was originally found to be an indeterminate basal ornithopod related to the genera Gasparinisaura and Talenkauen, though this phylogenetic analysis was small in scope compared to other papers.

[11][8] Later phylogenetic analyses have found Trinisaura within the clade Elasmaria, a group of small to medium-sized herbivorous ornithopods characterized by their lithe builds and bipedal posture.

[14][8] However, a 2022 phylogenetic analysis by Karen Poole of the ornithischian groups Iguanodontia and basal Neornithischia found no Elasmaria clade, instead with Trinisaura in a branch of Rhabdodontoidea containing the Patagonian genera Talenkauen and Anabisetia and the South African genus Kangnasaurus.

[15] It was theorized by Poole (2022) that this group of Gondwanan genera branched off from the ancestors of Rhabdodontidae 145 million years ago during the Late Jurassic,[16] which overlaps with the separation of Laurasia from Gondwana via the North Atlantic Ocean.

All of these taxa were from the supercontinent Gondwana, but while Talenkauen, Anabisetia and Trinisaura all come from the Upper Cretaceous,[14][18] Kangnasaurus dates to the earlier part of the period.

[8][1] Based on the phylogenetic position of the genus within Elasmaria and the known material, Trinisaura was herbivorous, lightly built, with a long tail for balance in addition to lithe hindlimbs and short forelimbs.

The ischium is unique compared to other elasmarians, with a slight curve along its entire length that is an example of convergent evolution with marginocephalians, the group that contains families like ceratopsids and pachycephalosaurids.

[3] Based on the hindlimb anatomy of Trinisaura and other elasmarians, notably the slim metatarsus, it is thought they were very capable runners, distinguishing them from other herbivorous dinosaurs they lived alongside.

The expanded chevrons of Trinisaura in addition to ossified intercortal plates along the bottom of the torso preserved in related taxa would have further aided their cursorial abilities by improving tail balance and musculature control as well as breathing efficiency.

Ornithischian dinosaurs living in polar regions in the Northern Hemisphere (such as Edmontosaurus and Pachyrhinosaurus typically show different growth rates than relatives from lower latitutes, owing to the extreme temperatures of their environments, but studies have failed to find the same signs in Antarctic animals.

[8] This contrasted with existing recognition of links between the flora and marine invertebrate fauna found in Patagonia, Antarctica, Australia, and New Zealand, termed the Weddellian Bioprovince.

[25] The recognition of Morrosaurus and Trinisaura as Antarctic elasmarians closely related to animals from South America constituted important evidence of such ties existing in dinosaurs on these continents as well.

[27][28] These discoveries of connected endemic Gondwanan ecosystems have overturned the traditional view of the southern continents, acting as a refugium for animals more successful elsewhere earlier in the Cretaceous.

[34][4] The Gamma Member of the formation has yielded several other fossil remains, such as the armored ankylosaur Antarctopelta,[32] a vertebral centrum of a lithostrothian sauropod,[35] an aquatic elasmosaurid,[4] and the carnivorous tylosaurine mosasaurs Taniwhasaurus and Hainosaurus.