Patagopelta

Patagopelta (meaning "Patagonian shield") is an extinct genus of ankylosaurian dinosaur from the Late Cretaceous (upper Campanian–lower Maastrichtian) Allen Formation of Argentina. The genus contains a single species, P. cristata, known from a partial skeleton. While originally described as a nodosaurine, later discoveries provided support for parankylosaurian affinities for the taxon. Patagopelta is a very small ankylosaur, comparable in size to the dwarf nodosaurid Struthiosaurus, about 2 m long.

Discovery and naming
The Patagopelta fossil material was found in sediments of the Allen Formation (Salitral Moreno locality) near General Roca, Río Negro Province, Argentina. This locality is dated to the upper Campanian to lower Maastrichtian ages of the Late Cretaceous period. The first remains were described in 1996 and often appeared in the literature as the "Argentinian ankylosaur". The fossil material consists of various osteoderms, a tooth, dorsal and caudal vertebrae, and femora. The Patagopelta holotype specimen, MPCA-SM-78, is represented by a cervical half-ring element.

New remains described in 2022 allowed Patagopelta cristata to be described as a new genus and species of nodosaurine dinosaurs by Facundo Riguetti, Xabier Pereda-Suberbiola, Denis Ponce, Leonardo Salgado, Sebastián Apesteguía, Sebastián Rozadilla, and Victoria Arbour. The generic name, "Patagopelta", is derived from "Patago", a reference to the discovery of the taxon in Argentinian Patagonia, and the Greek word "pelta", meaning "shield". The specific name, "cristata", means "crested" in Latin, referring to the large crests on its cervical osteoderms and femur.

Classification
In their phylogenetic analyses, Riguetti et al. (2022) recovered Patagopelta as a member of the Nodosaurinae, within a clade of entirely North American nodosaurids from the middle of the Cretaceous period, in contrast to previous analyses that recovered it in the Panoplosaurini. In either case, these results would suggest that nodosaurids were part of a migration event of North American fauna into South America. The cladogram below displays the results of their phylogenetic analyses.

A study published in 2024 by Agnolín and colleagues reviewed vertebrate fossils from the Cerro Fortaleza Formation and similar Patagonian formations. They mentioned new findings that suggest that Patagopelta may actually represent a parankylosaurian, similar to ankylosaurs like Antarctopelta rather than nodosaurids. In their 2024 redescription of Antarctopelta, Soto Acuña, Vargas & Kaluza further elaborated on this, explaining that the discovery of additional fossil material allowed for a rescoring of its characters in their phylogenetic matrix. Based on these updates, they recovered Patagopelta within the Parankylosauria, along with other Cretaceous Gondwanan ankylosaurs. Their results are displayed in the cladogram below:

Paleoecology
Patagopelta is known from the Late Cretaceous Allen Formation of Río Negro Province, Argentina. Many other dinosaurs, including titanosaurs (Aeolosaurus, Bonatitan, Menucocelsior, Panamericansaurus, Pellegrinisaurus, and Rocasaurus), hadrosaurids (Bonapartesaurus, Kelumapusaura, and Lapampasaurus), abelisaurids (Niebla and Quilmesaurus), dromaeosaurids (Austroraptor), and alvarezsaurids (Bonapartenykus), have been named from the formation. Birds (Lamarqueavis and Limenavis), pterosaurs (Aerotitan), rhynchocephalians (Lamarquesaurus), plesiosaurs (Kawanectes), and dryolestoid and gondwanathere mammals have also been described from the formation.