New remains of ankylosaurs in Río Negro (Argentina) found while revisiting collections in museums expand their presence to new late Cretaceous localities.

BECERRA, M.G.; JUÁREZ VALIERI, R.D.; MESO, J. G.; GUZMÁN, F.A.; HEREDIA, J.

Corrientes Capital

Jornada; XXXVII Jornadas Argentinas De Paleontología De Vertebrados; 2024

Asociación Paleontológica Argentina

The fossil record of ankylosaurian dinosaurs in SouthAmerica has been markedly expanded over the past decade, revealing new speciesand reshaping our understanding of their phylogenetic affinities, evolutionaryhistory, and diversification in Gondwana. Currently, Patagopelta cristata from Salitral Moreno (Allen Formation, middleCampanian – lower Maastrichtian, Rio Negro) is the only recognized Argentinianankylosaurian, though depicted in varying phylogenetic positions as anodosaurid, or as a parankylosaurian, together with other Gondwanan taxa (Stegouros elengassen, Antarctopelta oliveroi, and Kunbarrasaurusieversii). Additional ankylosaurianremains from Puerto Yerúa (Late Cretaceous, Entre Ríos), Chorrillo (upperCampanian – Maastrichtian; Santa Cruz), and La Colonia (Maastrichtian, Chubut) formationshint at potential new discoveries. This study introduces new isolated fossilremains from Museo Provincial Carlos Ameghino (MPCA - Cipolletti) and MuseoJorge H. Gerhold (MJHG – Ing. Jacobacci), whose morphology relate them toAnkylosauria. At MPCA, we found an isolated osteoderm (MUCP-RN-13) from theSalitral Moreno locality, and a rib fragment (provisionally cataloged as α60)from the Salitral Ojo de Agua locality, both from Allen Formation. At MJHG, wediscovered a cranial and a rib fragment (provisionally cataloged as MJHG 77-33and MJHG 271B-17, respectively), from Bajo Colorado locality, AngosturaColorada/Coli Toro formations (Campanian-Maastrichtian). The osteodermMUCP-RN-13 is thin and strongly concave ventrally, with a decentered, low and roundeddome, and coarsely ornamented with pits and shallow furrows. The morphologicalsimilarities and same provenance of Patagopeltaand MUCP-RN-13 allow to consider them as part of the same species. The cranialfragment from Bajo Colorado comprises an osteoderm fused to a laminar bone viaexostosic tissue, which overgrows the level of the bone medially whereas iscovered by the osteoderm externally. The osteoderm is ornamented by pits andfurrows following the main orientation of the supporting bone, akin to the jugal-quadratojugalof the La Colonia ankylosaur, but with coarser ornamentation, in a similar way tothe dermal plates of Patagopelta. Thefossil ribs from Salitral Ojo de Agua and Bajo Colorado are T-shaped in cross-section,a feature observed in other Gondwanan parankylosaurians and most euankylosaurians,but absent in any other contemporaneous terrestrial vertebrates. This material permitsto document the previously unknown presence of ankylosaurians at Salitral Ojode Agua and Bajo Colorado localities. These findings highlight the criticalneed for ongoing paleontological research in South America. Future studies areessential to uncover new fossil sites and further enhance our understanding of ankylosauriananatomy, evolution, and biogeography in southern continents.