The holotype MLP08-III-1-1 was discovered in 2008 at the location of Santa Marta Cove in the layers of the Snow Hill Island formation, dated by the upper campaign (about 80–72 million years ago), James Ross Island , Antarctica [1] [2] . The holotype is a dissected and partial skeleton, including one incomplete dorsal vertebra, three bodies of the sacral vertebrae, seven tail vertebrae; two incomplete bodies of the dorsal ribs, one proximal hemal arch, incomplete right scapulocoracoid, incomplete right humerus, two metacarpal bones, both iliac bones, right pubic bone, right sciatic bone, right femur, distal part of the tibia, and soft, soft, sciatic bone, right femur, distal part of the right tibial bone, soft, soft, sciatic bone, right femur, distal part of the right tibial bone, soft, softness, , the first phalanx of the third toe, two phalanxes of the fourth toe and indefinite fragments.
In the single spinal cord and all caudal vertebrae, the arches completely merged with the bodies, although the bodies of the sacral vertebrae are separated and isolated. However, taking into account that the scapula and the coracoids are firmly adhered, it is assumed that Trinisaura is a semi-adult [1] .
Trinisaura differs from other ornithopods by the presence of a single autopomorphy : the acicular needle process of the scapula with a sharp lateral crest, the length of which is longer than that of other ornithopods; and a combination of the following features: rudimentary deltopektoralny comb represented as a thickening at the anterolateral edge humerus ( convergent with Talenkauen santacrucensis and Anabisetia saldiviai ) humerus body is strongly inclined in the lateral direction (converged with Talenkauen, Anabisetia and some representatives marginocephalia ) ischium slightly curved along the entire length (convergent with some marginocephals).
The length of the Trinisaura was approximately 1.5 m, which is comparable to the length of the Anabisetia or the adult specimen of Gasparinisaura cincosaltensis [1] .
To clarify the phylogenetic position of Trinisaura , an analysis was performed based on a data matrix consisting of 230 characters for 52 taxa. As a result, Trinisaura takes a sister position towards Anabisetia and the more advanced Euiguanodontia . Trinisaura is more advanced than Thescelosaurus and is placed inside a clade that includes all South American non-gullbird ornithopods known at the time of the study [1] .
A cladogram based on the analysis of Coria and colleagues, 2013:
| Ornithopoda |
| Hypsilophodon foxii |
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| Gasparinisaura cincosaltensis |
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| Trinisaura santamartaensis |
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| Talenkauen santacrucensis |
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| Anabisetia saldiviai |
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| Tenontosaurus |
| | Dryomorpha |
| Dryosauridae |
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| Ankylopollexia |
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In 2015, to determine the phylogenetic position of another Antarctic ornithopod Morrosaurus, an analysis was performed based on the same data matrix, consisting of 230 characters for 52 taxa. As a result, Trinisaura falls into the Elasmaria clade with other South American and Antarctic non-terrific ornithopods, with the exception of Gasparinisaura [3] .
A cladogram based on an analysis of Rozadilla and colleagues, 2015:
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| Hypsilophodon |
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| Thescelosaurus |
| | Iguanodontia | | Elasmaria |
| Gasparinisaura |
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| Morrosaurus |
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| Trinisaura |
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| Macrogryphosaurus |
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| Notohypsilophodon |
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| Talenkauen |
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| Anabisetia |
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