Eusauropoda

Eusauropoda (meaning 'true sauropods') is a derived clade of sauropod dinosaurs. Eusauropods represent the node-based group that includes all descendant sauropods starting with the basal eusauropods of Shunosaurus, and possibly Barapasaurus, and Amygdalodon, but excluding Vulcanodon and Rhoetosaurus. The Eusauropoda was coined in 1995 by Paul Upchurch to create a monophyletic new taxonomic group that would include all sauropods, except for the vulcanodontids. Eusauropoda are herbivorous, quadrupedal, and have long necks. They have been found in South America, Europe, North America, Asia, Australia, and Africa. The temporal range of Eusauropoda ranges from the early Jurassic to the Latest Cretaceous periods. The most basal forms of eusauropods are not well known and because the cranial material for the Vulcanodon is not available, and the distribution of some of these shared derived traits that distinguish Eusauropoda is still completely clear. Eusauropods are long-necked, strictly herbivorous, obligate quadrupedals. They have a highly specialized set of skeletal adaptions due to their large size, and are graviportal. Yates and Upchurch described eusauropod evolution as moving towards a “bulk-browsing mode of feeding”. They describe the development of lateral plates on the alveolar margins of tooth-bearing bones. These plates can be used to strip foliage, the eusauropod's “U-shaped” jaws create a wide bite, and their loss of “fleshy cheeks” increased the gape. The crowns of eusauropod teeth also have “wrinkled enamel textures”, but it is unclear what this meant for their feeding habits. The skull length of the basal eusauropod, Patagosaurus is about 60 centimetres (24 in). One of the most basal eusauropods, Shunosaurus, has two characteristic features of the eusauropod elongated neck: the incorporation of the equivalent of the first dorsal vertebra into the cervical region of the spine, and the addition of two cervical vertebra in the middle of the cervical vertebrae. Other synapomorphies of Eusauropoda includes a retracted position of the external nares. Unlike prosauropods and theropods, which have a snout with smooth, unprotruding alveolar and subnarial regions, eusauropods have snouts with “stepped anterior margins”. Further distinguishing features of eusauropods include the absence of the contact between the squamosal and the quadratojugal, the absence of the anterior process of the prefrontal, and a distally elongated anterior ramus of the quadratojugal. Separating the anteroventral process of the nasal from the posterolateral process of the premaxilla, eusauropods also have a long maxilla that forms the posteroventral margin of the external naris. Eusauporoda are also hypothesized to have a semi-digitigrade foot posturem demonstrated by footprint evidence. Paleontologist Jeffrey Wilson explains that eusauropods differ from theropods and prosauropods that have digitigrade pes where their heel and metatarsals are lifted off the ground. Eusauropods show asymmetry in their metatarsal shaft diameters where metatarsal I is broader than the others, suggesting that their weight was mostly assumed by their inner feet. According to Steven Salisbury and Jay Nair, basal eusauropods retain four pedal unguals but reduce their phalangeal number in their fourth digit to three units. The metatarsus in eusauropods is less than a quarter of their tibial length, unlike sauropod outgroups that have long hindlimbs and metatarsus that are almost half of their tibial length. Eusauropods are found on all major continents, except Antarctica, with diplocodoids being widespread in the Northern Hemisphere, and titanosaurs being found in Southern Hemisphere. However, basal eusauropods that do not fall into either group are fairly well represented.