Sauropods from the Early Jurassic of South America and the Radiation of Eusauropoda

Abstract

Eusauropods are large-bodied and long-necked dinosaurs that dominated the role of large herbivores in terrestrial ecosystems since at least the late Early Jurassic (Pliensbachian-Toarcian). Their early diversification is best recorded in South America where the best-preserved eusauropods and close relatives from this period of time have been found. The earliest sauropod from the Jurassic of South America is Amygdalodon patagonicus from the Cerro Carnerero Formation (Pliensbachian?early Toarcian), and its fragmentary remains suggest a position at the base of Gravisauria or as closely related to this clade. The Cañadón Asfalto Formation (middle late Toarcian) has provided three named sauropods, although a higher diversity of sauropods may have existed. These are the basal eusauropod Patagosaurus fariasi, known from multiple specimens, the much more incompletely known early sauropod Volkheimeria chubutensis, and Bagualia alba that is known from multiple specimens and includes fairly complete craniomandibular remains. These taxa provide the earliest evidence of ecological predominance by large-bodied sauropods and are therefore significant for understanding the rise and success of this group in the Jurassic Period. The current knowledge of these sauropods from the late Early Jurassic of South America indicates that the evolutionary radiation of Eusauropoda occurred at least by the mid-Toarcian, subsequent to a large-scale volcanic event in the Southern Hemisphere that has been linked to global climatic change and the rise of conifers as the predominant components of Jurassic seasonal forests.