Maximiliano Fabianelli, Rubén Darío Juárez Valieri, Rafael Lindoso, Jorge Meso, Guillermo Salinas, José Haro, Albert Prieto-Marquez

Returning the dragon to its cave: the spinosaurid nature of the purported ornithopod materials from the ‘Middle’ Cretaceous Alcântara Formation, northeastern Brazil

Introduction

Returning the dragon to its cave: the spinosaurid nature of the purported ornithopod materials from the ‘middle’ cretaceous alcântara formation, northeastern brazil. Dinosaur caudal vertebrae from Brazil's Alcântara Formation, previously identified as ornithopod, are reclassified as spinosaurid (Spinosaurinae), negating ornithopod presence in the region.

Abstract

We have conducted a reassessment of dinosaur caudal vertebrae from the Cenomanian Alcântara Formation in the Maranhão State, northeastern Brazil. These elements were originally described as belonging to the coeval African theropod Sigilmassasaurus. Subsequently, they were reassigned to a large ornithopod similar to the slightly older Ouranosaurus, representing the first occurrence of a basal hadrosauriform in South America. Here we review the anatomy of the caudal vertebrae from the Alcântara Formation, emphasizing comparisons with both spinosaurid theropods and hadrosauriform ornithopods to re-evaluate their taxonomic assignment between these two distantly related groups. Characters such as centra with subrectangular lateral walls, ventral surface bearing a longitudinal depression, low base of the neural arches, rod-shaped neural spines and transverse processes not reaching the anterior and posterior borders of the centra, among others, allow referral of these vertebrae to Spinosaurinae.The only nominal spinosaurid described for the Alcântara Formation is Oxalaia quilombensis. However, the absence of overlapping material precluded the referral of the Maranhão caudal vertebrae to this species. Furthermore, the existence of multiple dental morphotypes in the Alcântara Formation that suggests a greater diversity of spinosaurids opens the possibility of those vertebrae belonging to a different species. Nevertheless, the reassignment of the Maranhão caudal vertebrae to Spinosauridae nullifies the evidence of ornithopod remains in the Alcântara Formation and limits its record in northeastern South America to footprints from Cenomanian strata.

Review

This paper presents a rigorous reassessment of dinosaur caudal vertebrae from the Cenomanian Alcântara Formation, northeastern Brazil, which have a complex taxonomic history. Originally attributed to *Sigilmassasaurus*, these elements were subsequently controversially reassigned to a large ornithopod, resembling *Ouranosaurus*, and posited as the first occurrence of a basal hadrosauriform in South America. Through meticulous comparative anatomical analysis, focusing on key differences between spinosaurid theropods and hadrosauriform ornithopods, the authors identify a suite of diagnostic characters within the vertebrae. Features such as subrectangular lateral walls, a distinct ventral longitudinal depression, low neural arch bases, rod-shaped neural spines, and transverse processes that do not extend to the anterior and posterior borders of the centra, collectively provide compelling evidence for their re-referral to Spinosaurinae. This re-assignment carries significant paleontological implications, fundamentally altering our understanding of the Alcântara Formation's dinosaur faunal composition. Most notably, it nullifies the purported evidence of hadrosauriform ornithopod remains in the formation, consequently limiting their known record in northeastern South America to footprints from Cenomanian strata. Conversely, the study strengthens the evidence for a robust spinosaurid presence in the region. While the absence of overlapping material precludes a definitive assignment of these specific vertebrae to *Oxalaia quilombensis*, the sole nominal spinosaurid described from the Alcântara Formation, the authors prudently acknowledge the existence of multiple spinosaurid dental morphotypes, suggesting a potentially greater diversity of these aquatic theropods. The strength of this study lies in its thorough and detailed re-evaluation of previously ambiguous material, highlighting the critical importance of precise anatomical comparisons in resolving complex taxonomic issues for isolated fossil elements. The authors provide a strong, character-based argument for their revised classification, correcting a significant aspect of the regional faunal list. Moving forward, the inability to definitively link these vertebrae to *Oxalaia quilombensis* or other potential spinosaurid species underscores the need for further discoveries of more complete skeletal material. This will be crucial not only for fully elucidating the spinosaurid diversity within the Alcântara Formation but also for definitively confirming or refuting the presence of hadrosauriform body fossils in this important South American Cretaceous locality.

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