Discovery of Joaquinraptor casali: A New Apex Predator from the Late Cretaceous of Patagonia

A recently described, well-preserved megaraptoran dinosaur, Joaquinraptor casali, from the Late Cretaceous of Patagonia provides significant insights into the evolution, dietary habits, and late-stage survival of this enigmatic clade. Discovered in the uppermost strata of the Lago Colhué Huapi Formation in south-central Chubut Province, Argentina, this specimen represents one of the most complete megaraptorids found to date.

Introduction

Recent paleontological discoveries have greatly expanded our understanding of Megaraptora, a clade of large-clawed theropods that were dominant predators across Cretaceous Asian, Australian, and South American landscapes. Despite these advances, much of their evolutionary history and morphological development has remained obscure due to the fragmentary nature of many remains. This report details the identification of a previously unknown megaraptoran genus and species recovered from the Maastrichtian deposits of Argentina.

Systematic Palaeontology

The new taxon, Joaquinraptor casali gen. et sp. nov., is classified within the derived Megaraptoridae subclade. The holotype, cataloged as UNPSJB-PV 1112, consists of a partially articulated partial skeleton. Recovered only meters below the top of the formation, its stratigraphic position confirms Joaquinraptor as one of the latest-surviving members of its group, likely persisting until the Cretaceous-Paleogene (K-Pg) boundary. Intriguingly, a crocodyliform humerus was found preserved between the specimen’s dentaries, offering rare evidence regarding the dietary preferences of these theropods.

Morphological diagnosis confirms its placement within Megaraptoridae through shared synapomorphies. However, Joaquinraptor casali is distinguished by several autapomorphies, including transversely compressed parietals that are narrower than the frontals, a distinct sigmoid contour on the anterior margin of the supratemporal fossa, and a rugose atlantal intercentrum characterized by a rectangular ventral contour.

Osteological and Histological Analysis

As one of the most complete megaraptorans known, Joaquinraptor casali allows for rigorous osteological comparison with taxa such as Megaraptor, Murusraptor, and Australovenator. Notable features include a humerus that is significantly more massive than those of its relatives and an ulna with a proportionally elongated olecranon process.

Histological examination of the tibia indicates that the holotype individual was sexually mature, with an inferred age of at least 19 years based on lines of arrested growth (LAGs). However, the absence of an external fundamental system (EFS) suggests the animal had not yet reached somatic maturity. Further analysis of the dorsal ribs revealed 13 LAGs, though the study noted that the growth record in megaraptoran long bones is generally better preserved than in ribs due to extensive remodeling and pneumaticity in the latter.

The research also investigated Postcranial Skeletal Pneumaticity (PSP). While pneumatic cavities were observed, the internal fibers did not align perfectly with previously defined “pneumosteon” tissue, suggesting that PSP in megaraptorans may be more histologically diverse than previously hypothesized.

Phylogenetic Implications

Phylogenetic analyses nested Megaraptora within Coelurosauria as the sister group to Tyrannosauroidea. The analysis identified a new lineage, Megaraptorid Clade 1 (MC1), representing a group of derived South American megaraptorids. Joaquinraptor is positioned as the earliest diverging member of this clade.

These findings reinforce the hypothesis that South American megaraptorids reached significantly larger body sizes than their Australian counterparts. With an estimated mass exceeding 1,000 kg, Joaquinraptor exemplifies the trend toward gigantism in South American theropods leading up to the end-Cretaceous mass extinction.

Palaeobiology and Ecological Role

Megaraptorids functioned as apex predators in the ecosystems of central and southern Patagonia during the latest Cretaceous. The association of the crocodyliform humerus (UNPSJB-PV 1113) with the holotype suggests that Joaquinraptor was a generalist or opportunistic predator, capable of preying upon or scavenging large crocodyliforms that shared its environment.

Reference

Ibiricu, L. M., Lamanna, M. C., Alvarez, B. N. et al. Latest Cretaceous megaraptorid theropod dinosaur sheds light on megaraptoran evolution and palaeobiology. Nature Communications 16, 8298 (2025). https://doi.org/10.1038/s41467-025-63793-5

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Skeletal reconstruction of Joaquinraptor casali in left lateral view. The blue highlighted regions indicate the preserved elements of the holotype (UNPSJB-PV 1112), demonstrating the comprehensive nature of this find and its significance in understanding late Maastrichtian megaraptoran morphology.