Uncovering Los Bastos: New titanosaur specimens and palaeoecological insights from the Santonian of Patagonia, Argentina

Abstract

The Bajo de la Carpa Formation (Santonian, Late Cretaceous) of the Neuquén Basin, Argentina, represents a critical stratigraphic interval for understanding the evolution and diversity of South American titanosaurians. This study presents the description and comparative analysis of newly recovered titanosaurian remains from the Los Bastos locality, located near Senillosa (Neuquén Province). The material includes both cranial and postcranial elements belonging to distinct individuals, including an isolated cranial bone, appendicular and pelvic fragments, and some anterior to middle caudal vertebrae. Sedimentological and taphonomic data indicate a dynamic floodplain environment with episodic burial conditions conducive to the preservation of both associated and weathered remains. Detailed morphological comparisons suggest the presence of titanosaurs morphologically compatible with both Colossosauria and Saltasauridae within the same stratigraphic horizon. Notably, an associated series of caudal vertebrae (MMS-PV-75) exhibits characters diagnostic of Aeolosaurinae, while showing procoelous-distoplatyan centra and laterally expanded transverse processes. A separate pelvic specimen (MMS-PV-82), composed of an ilium, a pubis, and an ischium, displays some features of Saltasauroidea representing the earliest known occurrence of this clade in the Neuquén Basin. The coexistence of colossosaurian and saltasaurid titanosaurs within a single depositional context challenges models positing a rapid faunal replacement and instead suggests a more gradual, overlapping diversification. The presence of putative theropod feeding traces on some bones adds a rare ichnological dimension to the palaeoecological reconstruction. Collectively, these findings provide new information on titanosaurian evolution during the Santonian highlighting the importance of the Bajo de la Carpa Formation as a key window into Late Cretaceous Gondwanan dinosaur faunas.