Paleohistology of two dicraeosaurid dinosaurs (Sauropoda; Diplodocoidea) from La Amarga Formation (Barremian–Aptian, Lower Cretaceous), Neuquén Basin, Argentina: Paleobiological implications

Despite the large number of paleohistological studies carried out on sauropodomorph dinosaurs, some particular groups, such as dicraeosaurids, have been little explored. The current knowledge about the osteological microanatomy of this family is limited to that from specimens of Dicraeosaurus spp. and the holotype specimen of Suuwassea emiliae . We have examined the bone microstructure of the femur, humerus, and dorsal rib of the holotype of Amargasaurus cazaui and the femur of the holotype of Amargatitanis macni . The main purpose of this study is to obtain information regarding the ontogenetic stages of these individuals and, in a more general sense, to increase our understanding about the growth dynamics of dicraeosaurid sauropods. We interpret that Amargatitanis represents a more maturational stage than Amargasaurus , due to its high degree of bone remodeling and to the greatest number of lines of arrested growth (LAGs). Sexual maturity, mainly evidenced by the relative abundance of LAGs in the outer cortex of stylopodial bones, appears to have been reached by both individuals prior to death. Nevertheless, only Amargatitanis shows somatic maturation, evidenced by the presence of an External Fundamental System (EFS). In Amargasaurus , the dorsal rib preserved more LAGs than the long bones, supporting previous hypotheses that the dorsal ribs are good elements for making age estimates.