OSTEOPETROSIS IN AN AEOLOSAURINI AFF. (DINOSAURIA: TITANOSAURIA) FROM MORRO DO CAMBAMBE (UPPER CRETACEOUS), MATO GROSSO, BRAZIL

Several fossil bones closely related to Aeolosaurini were recently reported to Morro do Cambambe site (Mato Grosso state, Late Cretaceous). Among them, we selected one medium-posterior dorsal rib (DGM 198-R), which presents a macroscopically abnormal growing texture on the bone. We performed 14 paleohistological sections along its proximodistal axis to investigate the origin of this anomalous structures. The results of the cross-sections revealed a medullar cavity composed of wide cavities, occupying most of the bone in proximal samples and being reduced in distal ones. Few resorption cavities advanced through the compact bone. The medullar cavity was delimitated by a thick lamellar endosteal bone. The secondary bone was dense, with secondary osteons reaching more than three generations, and filling most of the compact bone. The primary bone occupied most of the posteromedial regions of the cross-sections. The inner cortex exhibited a fibrolamellar complex, with wide longitudinal vascular canals, indicating a continuous growth phase. The outer cortex exhibited a decreased in vascularization and was rich in parallel bone, varying to lamellar in distal samples. The canals were narrow, longitudinal and organized into rows, with few interlayered by growth marks, indicating a cyclical growth phase. Among all the samples, we observed a maximum of four Lines of Arrested Growth (LAGs). Based on the thick endosteal bone, the dense secondary bone, a centripetally decreasing of the vascularization and an alternation from continuous to cyclical bone growth, we assigned the DGM 198-R as an adult titanosaur. The microstructure of an osseous knob close to the proximal region was sectioned, and this region was characterized by an endosteal and periosteal fibrolamellar complex, rich in woven bone and wide longitudinal-radial canals. These endosteal and periosteal microstructures were observed in bone callus among several non-avian and avian dinosaurs. Such bone growth is a pathological expression of the osteopetrosis, caused by bone injuries and cracks. We conclude that the specimen DGM 198-R belonged to a small adult titanosaur and, during its life, was recovered from a bone injury in its dorsal rib. [*CAPES, 88887.336584/2019-00; **CNPq 140789/2016-2; ***CNPq; ****CNPq 311715/2017-6; *****CNPq 304780/2013-8].