Trinisaura is a genus of ornithopod dinosaur that lived during the late Campanian stage of the Upper Cretaceous, around 73 million years ago in what is now James Ross Island off the coast of northern Antarctica near Patagonia. It is known from a single, incomplete postcranial skeleton that includes several vertebrae, a partial pelvis, and nearly complete right hindlimb. The fossils were collected in 2008 by paleontologists Juan Moly and Rodolfo Coria from the sandstone of the Snow Hill Island Formation. 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. The genus name is to commemorate the efforts of Argentine geologist Trinidad "Trini" Diaz and the Latin root -sauros, meaning "lizard". The species name is after Santa Marta Cove, where the fossils were collected.
Trinisaura has been classified as a basal (early-diverging or "primitive") member of the clade Ornithopoda along with other ornithischians that had lived in the former supercontinent of Gondwana, which lived during the Cretaceous. Its only known skeleton comes from a subadult, with a small size of . Trinisaura was very lightly built and bipedal, with long legs built for running and a rigid tail to aid in balance during locomotion. Trinisaura is distinct from its relatives in a variety of ways, such as the shape of the ischium (back pelvic bone) and a large deltopectoral crest on the humerus (upper arm bone). Trinisaura is also noted to have a slower growth rate than genera that lived in warmer climates, an adaptation to colder temperatures of the Antarctic.
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. The climate during the Upper Cretaceous was much warmer than it is today, and the continent was blanketed in forests.
Discovery and naming
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 fossiliferous exposures of the Santa Marta Cove on James Ross Island, Antarctica. The layers date to the Upper Campanian stage of the Late Cretaceous, approximately at least 73.4 million years ago, coming from the base sandstone deriving from the Gamma Member of the Snow Hill Island Formation. During prior field visits in 1986, material of the dinosaur genus Antarctopelta and fragmentary fossils of a sauropod. The fossils come from a small ornithopod, consisting of a semi-articulated partial skeleton including several vertebrae (backbones), incomplete pelvis, fragmentary scapulacoracoid, right humerus, and a partial right hindlimb. Later, however, more fossils including the left scapulacoracoid and additional vertebrae were noted to be known in 2019. 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),Coria, R.A., Moly, J.J., Reguero, M., Santillana, S., 2008, "Nuevos restos de Ornithopoda (Dinosauria, Ornithischia) de la Fm. Santa Marta, Isla J. Ross, Antártida", Ameghiniana 45(4), Supl., 25R but received a complete description in 2013.
An Argentine team led by Coria described the fossils as a new genus and species of basal ornithopod dinosaur named Trinisaura santamartaensis, the generic name after geologist Trinidad "Trini" Diaz and the -saura root from the Latin word for "lizard", a commonly used epithet for dinosaurs. The specific name is after the geographic location of the holotype's discovery in the Santa Marta Cove. Several other indeterminate ornithopod remains have been described from the Snow Hill Formation, including the well preserved partial skeleton of an ornithopod from Vega Island which bears similarities to Trinisaura. The much larger Morrosaurus, named in 2016, has been suggested to be a junior synonym of Trinisaura, though several characteristics differentiate the two genera. 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.Coria, R. A., Tambussi, C., Moly, J. J., Santillana, S., & Reguero, M. (2007, September). Nuevos restos de Dinosauria del Cretácico de las islas James Ross y Marambio, Península Antártica. In Vi Simposio Argentino y iii Latinoamericano sobre investigaciones Antárcticas, Dirección Nacional del Antárctico/instituto Antárctico Argentino–10 al (Vol. 14).Coria R A, O'Gorman J P, Cárdenas M, et al. (2015) Late Cretaceous vertebrates from Isla Vega, Antarctica: reports from the 2015 fieldwork. Ameghiniana, 52(Supl. 4): 12-13.
Phylogeny 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. 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. They also were herbivores with rounded heads and tiny beaks in conjugation with teeth. Elasmarians were the sister group to the clade Clypeodonta, which consists of iguanodontians and hypsilophodonts according to Herne et al. (2019). In this group, many of the Gondwanan ornithopods, including the contemporary Morrosaurus, are found in and diagnosed by several traits in the postcranial anatomy. 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., 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, which overlaps with the separation of Laurasia from Gondwana via the North Atlantic Ocean.
In 2015, it along with several other Patagonian and Antarctic ornithopods was found to be a part of the basal group of iguanodonts, Elasmaria in polytomy with other South American ornithopods and Morrosaurus.
Cladogram based in the phylogenetic analysis of Rozadilla et al. (2015):
In 2022, Karen Poole recovered it along with the Patagonian ornithopods Anabisetia and Talenkauen and Kangnasaurus from South Africa in their own clade at the base of the Rhabdodontoidea, instead of in the Elasmaria. All of these taxa were from the supercontinent Gondwana, but while Talenkauen, Anabisetia and Trinisaura all come from the Upper Cretaceous, Kangnasaurus dates to the earlier part of the period. Below is her phylogenetic hypothesis.
Description
Trinisaura is known from only a partial postcranial skeleton, leaving much of its skeletal anatomy unknown, and is from an immature individual. The size of the only specimen was estimated to be a mere in length, placing it on the lower end of ornithopod sizes. This is much smaller than the only other named Antarctic ornithopod, Morrosaurus, which was around long, differentiating the two in this aspect. 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 dorsum was short with an avian sternum and the neck was elongated, terminating in a small, beaked skull adapted for herbivory.
Vertebrae and pectoral girdle The vertebral column is incompletely known, with only single, partial dorsal (trunk) vertebra, three sacral (pelvic) centra, seven caudal (tail) vertebrae, and a haemal arch of a chevron (a bone attached to the underside of the tail vertebrae) described from the column. However, several additional undescribed vertebrae exist. The dorsal vertebra is incomplete, lacking the neural spine and most of the physes, though the postzygapophyses are short and angle posteriorly. The lateral sides of the centrum are concave and bear small foramina (small, hollow pits in bone), with a ventral keel on the bottom of the centrum. The sacral centra provide little information as well, but have a similar shallow concavity. The caudal vertebrae are better preserved, with posteriorly-projected neural spines, well-developed transverse processes, and a thick prespinal lamina. The prezygapophyses are small and have an anterodorsally projected condition. The mid-caudal vertebrae are morphologically akin to other ornithopods as the transverse processes are not expanded distally. The distal caudals have dwarfed transverse processes, lower neural arches, and proportionally elongated centra. The haemal arch is compressed transversely and distally, though it is slightly expanded anteroposteriorly at the end.
The right scapulacoracoid (breastbone) is incomplete and consists only of the proximal end, though it is noted that the scapula and coracoid are firmly fused. The acromial process (process making up the lateral edge of the shoulder joint) is spike-like, with a strong, sharp lateral crest that distinguishes it from other ornithopods, though a similar condition is found in the Australian genus Fostoria. The scapular blade is only known from fragments, but is dorsoventrally narrow and much of it articulates with the glenoid surface. Here, the scapulacoracoid touches the humerus (upper arm bone). The humerus is fragmentary, missing the proximal and distal ends, but is gracile and laterally bowed. When complete, the proximal end would be enlarged transversely whereas the distal end would have two large condyles (articulation points). Two metacarpals from the end were also found with the holotype, bearing an elongate morphology with convex dorsal sides and flat ventral sides. The tendon (tissue that connects to bone) attachments would connect to deep, lateral pits divided by medial grooves.
Pelvis and hindlimb The pelvis is incomplete, with both ilia, the right pubis, and right ischium described; the left pubis and ischium are missing. The ilium is slender, low, and elongate, a trait shared with most other basal ornithopods, with a dorsally sigmoid outline. The preacetabular process is large and makes up 50% of the iliar length and is tall, almost as tall as the pubic peduncle. The sides are flat but concave in cross-section to connect with the sacral ribs. The postacetabular process is robust and thick transversely with a concave brevis shelf. The pubis is slender and longer than the ischium, with a rod-like prepubic process that is rod-like and thin transversely. 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. The ischium's pubic process is square in shape and thin transversely. The ischium overall is fragile and thinly built, as in some other dinosaurs.