Amargatitanis (meaning "Amarga giant") is a genus of dicraeosaurid sauropod dinosaur (a type of large, long-necked quadrupedal herbivorous dinosaur) from the Barremian-age (Lower Cretaceous) La Amarga Formation of Neuquén, Argentina. It is known from a single, incomplete postcranial skeleton consisting of a partial hindlimb, ischium, and two vertebrae. These remains were unearthed by Argentine paleontologist José Bonaparte in 1983 during an expedition by the Museo Argentino de Ciencias Naturales and later described as a new genus and species, Amargatitanis macni by Sebastián Apesteguía. The genus name comes from the words Amarga, where the fossils were collected, and titanis meaning "titan". Its species name is in reference to the MACN (Museo Argentino de Ciencias Naturales ), where the remains are stored.
Though initially thought to be a titanosaur, recent research has found it to be a dicraeosaurid, a group of short-necked, smaller sauropods. It reached around 12 meters (39.7 feet) long and according to one estimate. Like other dicraeosaurids, it has robust limbs, a long tail, and tall . However, much of its anatomy can only be inferred due to the lack of preserved bones. It lived in a diverse ecosystem that included higher browsing titanosauriforms and smaller, lower browsing dicraeosaurids like Amargatitanis itself.
Discovery and naming
During an expedition by the Museo Argentino de Ciencias Naturales from March 11th to 16th of 1983, Argentine paleontologist José Bonaparte collected several fossils from four sites along the La Amarga stream in Neuquén, Argentina. At site number "1", an associated skeleton of a large sauropod was unearthed by Bonaparte who noted the discovery in his personal journal. Bonaparte initially recorded the specimen as being from the Pichi Picún Leufú Formation, although they were subsequently determined to be from the La Amarga Formation. These remains were taken to the Museo Argentino de Ciencias Naturales where they were cataloged under the specimen number MACN PV N53.
The skeleton remained undescribed until 2007, when Argentine researcher Sebastián Apesteguía named it Amargatitanis macni. The genus name comes from Amarga, where the holotype was found, and titanis meaning "titan". Its species name, macni, is to honor the Museo Argentino de Ciencias Naturales (MACN). Apesteguía erroneously assigned a scapula (MACN PV N34) and six caudal vertebrae (MACN PV N51) to the specimen, despite coming from other rock layers and being found in 1986. It was originally assigned to the group Titanosauria due to the anatomy of the scapula and caudal vertebra, but a 2016 reanalysis of the holotype (MACN PV N53) fossils demonstrated that Amargatitanis was instead a dicraeosaurid dipolodocoid. The study, authored by researcher Pablo Gallina, also removed MACN PV N34 and N51 from the holotype material, which were referred to an indeterminate titanosauriform, and considered only the dicraeosaurid bones to be from the genus. Since Gallina's paper, several phylogenetic studies have recovered Amargatitanis as a dicraeosaurid.
Fossil record
Amargatitanis macni is known from a single specimen, the holotype MACN PV N53, which was found in the Barremian-aged Puesto Antigual Member of the La Amarga Formation, approximately 129 to 123 million years old. This specimen represents a mature individual and consists of an incomplete anterior caudal vertebra, an incomplete middle caudal vertebra, an incomplete right ischium, and most of the right hind limb, including the femur, partial tibia and fibula, astragalus, and the distal half of the first metatarsal. Field notes indicate that two phalanges and another metatarsal of this specimen were also collected, but as of 2016 they could not be located. A pair of anterior dorsal vertebrae from a dicraeosaurid that were also found in the La Amarga Formation, MOZ-Pv 6126-1 and MOZ-Pv 6126-2, could be additional remains of Amargatitanis, or belong to the other La Amarga Formation dicraeosaurid, Amargasaurus.
Description
As a dicraeosaurid, Amargatitanis would have resembled better known members of the family such as Dicraeosaurus and Amargasaurus, which had shorter necks than most other sauropods and long neural spines extending from the vertebrae of the neck and trunk. It was a small sauropod, with one study estimating its mass at , slightly larger than the same study's estimate for Amargasaurus of . Mass estimates for Amargasaurus have been noted to vary widely depending on the method used, and studies using other methods, not yet applied to Amargatitanis, have found Amargasaurus to weigh to .