Articulated osteoderms on a titanosaur tail from Cerro Overo – La Invernada (Bajo de la Carpa Formation), Upper Cretaceous, northern Patagonia Argentina: paleobiological and paleoecological implications

FILIPPI, LEONARDO S.; BELLARDINI, F.; PAULINA-CARABAJAL, A.; CRUZADO-CABALLERO, P.; GONZÁLEZ-DIONIS, J.; MÉNDEZ, ARIEL H.; GIANECHINI, FEDERICO A.; ULLOA-GUAIQUIN, K.S.; GARRIDO, ALBERTO C.; MANIEL, I.J.; LEE, Y.-N.; DO-KWON, K.

General Roca, Río Negro

Congreso; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2023

The osteoderm fossil record of titanosaurian sauropods is widespread, with evidence from Africa, Australia, Europe, Pakistan, India, and, especially, South America. However, the major part of these occurrences is represented by osteoderms associated with titanosaurian carcasses or isolated elements indirectly referred to as titanosaurian specimens. Here, we describe a new partial articulated titanosaurian tail (MAU-Pv-CO-726) (Bajo de la Carpa Formation, Santonian) of Cerro Overo – La Invernada (CO-LI), Neuquén, Argentine Patagonia. The tail is composed of 25 caudal vertebrae, 11 haemal arches, and two osteoderms articulated on the lateroventral surfaces of most anterior caudal vertebrae. Moreover, another isolated osteoderm was found in proximity to the tail. The caudal centra have strongly procoelic articulations, low neural arches on the anterior half of the centrum, long and anteriorly directed prezygapophysis, and posterodorsally inclined neural spines. The two articulated osteoderms are bulbous and oval (101-106 mm long, 56-57 mm broad, 81-123 mm height, including spine), with a medial ridge and a prominent and tapered spine on the lateral half of the bone. These osteoderms were found on both sides of the posterior-most anterior caudal vertebra of MAU-Pv-CO-726, covering the lateroventral portions of the caudal centrum and the dorsal rami of the haemal arches. Conversely, the isolated osteoderm is bulbous, elliptic, and dorsoventrally compressed, with a shallow crest on the convex surface (124 mm long, 81 mm broad, 51 mm height). Different paleoecological interpretations have been proposed about the function of the osteoderms in sauropod dinosaurs, such as defense, sexual display, thermoregulation, and reserve of minerals, especially calcium during active growth or oogenesis. The new evidence from CO – LI allows us to infer that part of the proximal third of the titanosaurian tail possesses a set of osteoderms. This condition would support a possible defensive and/or predator repellent function, considering that the lateroventral portion of the sauropod tail is one of the most vulnerable parts of the body to predator-attacks. When the preliminary phylogenetic analysis was carried out, MAU-Pv-CO-726 was-recovered as a sister taxon of Rinconsaurus, suggesting, for the first time, the presence of osteoderms in Rinconsauria. MAU-Pv-CO-726 represents not only the first evidence of titanosaurian osteoderms in anatomical position but also the first record of sauropod osteoderms in the ventral portion of the tail. This material adds new anatomical and morphological information with paleobiological and paleoecological implications, which contribute to reconstructing part of the evolutionary history of Upper Cretaceous titanosaurians of southwestern Gondwana.