The neck posture and function in sauropods have been widely studied during the last decades. The cartilaginous neutral pose (CNP) method is commonly used in biomechanical reconstructions by positioning the vertebrae with the joint surfaces aligned to obtain the ‘neutral pose’. However, few studies have analysed the posture and function of the tail of sauropods. The published data suggest that most sauropods maintained their tails in a horizontal position. We test the neutral pose hypothesis in the anterior caudal vertebrae (C4 to C9) of Aeolosaurus maximus (Titanosauria, Sauropoda), a member of Aeolosaurini endemic of South America, by using the following methodologies: CNP, and range of movement (RoM). The results show that the tail of A. maximus possibly presented in a sigmoidal format which differs from the horizontal position commonly inferred to other sauropods. A. maximus is the first sauropod with caudal vertebrae presenting in situ protonic posture, which corroborates the neutral pose hypothesis and demonstrates that this characteristic is possibly a feature of the Aeolosaurini clade.