Prosauropod

The taxon Prosauropoda was erected by the German paleontologist Friedrich von Huene (1875–1969) in 1920 to accommodate a group of Late Triassic and Early Jurassic saurischian dinosaurs that he interpreted as the ancestral stock from which the giant Sauropoda arose. Huene initially subdivided the group into an omnivorous/carnivorous subgroup (Palaeosauria) and a herbivorous subgroup (Plateosauria). The Palaeosauria concept was later abandoned when it became clear that many of the supposedly carnivorous specimens were based on jaw fragments of rauisuchian archosaurs that had been erroneously associated with sauropodomorph postcranial material, particularly in the Middle Stubensandstein of Germany and the Lower Lufeng Formation of China.

The modern study of basal sauropodomorphs was catalyzed by Peter Galton's 1976 comprehensive review of North American prosauropod material. In the decades that followed, numerous new genera were described from South America (Riojasaurus, Coloradisaurus, Mussaurus, Lessemsaurus, Unaysaurus), Africa (Melanorosaurus, Massospondylus), Asia (Lufengosaurus, Jingshanosaurus, Yimenosaurus, Yunnanosaurus), and Europe (Efraasia, Ruehleia), dramatically expanding the dataset available for phylogenetic analysis. The discovery of Saturnalia tupiniquim from the Santa Maria Formation of Brazil (Langer et al. 1999) provided the first tangible evidence for the very roots of the sauropodomorph radiation, near the Middle–Late Triassic boundary at approximately 228 million years ago.

The monophyly versus paraphyly of Prosauropoda has been one of the most persistently debated issues in dinosaur systematics. Traditionally, prosauropods were treated as a monophyletic sister group to Sauropoda within Sauropodomorpha. Paul Sereno (1998, 1999) formally defined Prosauropoda as all taxa closer to Plateosaurus engelhardti than to Saltasaurus loricatus, and several workers including Galton and Upchurch (2000, 2004) supported monophyly based on shared characters such as a proportionately broad proximal metacarpal I, an enlarged distal carpal 1, and a very broad proximal metatarsal IV.

However, beginning with Adam Yates's influential phylogenetic analyses (2003, 2004, 2006, 2007), evidence accumulated that the traditional prosauropod genera form a paraphyletic grade on the stem lineage leading to Sauropoda. In this interpretation, forms such as Thecodontosaurus and Efraasia are more basal, the 'core prosauropods' (Plateosaurus, Massospondylus, Lufengosaurus, Riojasaurus) occupy intermediate positions, and the melanorosaurids and other 'near-sauropods' are successively closer to the sauropod crown. Sereno (2007) acknowledged this tension, noting that future resolution would likely fall somewhere between the two polar viewpoints, depending on continued clarification of anatomical details.

Thomas Holtz's teaching materials at the University of Maryland (updated through 2025) follow the paraphyletic-grade hypothesis, distinguishing three informal tiers: basalmost sauropodomorphs, core prosauropods, and near-sauropods. The prevailing consensus in the field treats Prosauropoda as not a valid cladistic taxon, and the informal term 'basal sauropodomorphs' is used instead. Nevertheless, some analyses continue to recover a monophyletic core Prosauropoda, and the question cannot be considered fully settled.

The dinosaurs traditionally grouped as prosauropods share a suite of morphological features that distinguish them from both their more basal relatives and the derived Sauropoda.

Skull and dentition: The skull was disproportionately small relative to body size, bearing phyllodont (leaf-shaped) teeth with coarse, diagonally angled marginal serrations suited for processing plant matter. The upper tooth row enclosed the lower in a wrap-around overbite. Post-Efraasia forms appear to have possessed small fleshy cheeks, analogous to but smaller than those inferred for ornithischians. The teeth were not efficient for mastication; the jaws operated in a nutcracker-like fashion, and tooth crowns show little evidence of wear from chewing resilient vegetation. To compensate, many prosauropods employed gastric mills—gastroliths embedded in the stomach wall that mechanically ground food.

Neck and body plan: The neck comprised approximately ten elongated cervical vertebrae and was proportionately longer than in other contemporaneous saurischians, enabling access to higher vegetation. The tail was long, often constituting roughly half of total body length. The forelimbs were approximately half the length of the hindlimbs.

Locomotion: Traditional reconstructions depicted most prosauropods as facultative bipeds that could shift between bipedal and quadrupedal gaits. However, recent biomechanical studies of the forelimbs, forearms, and wrists indicate that most core prosauropods were actually obligate bipeds—their forelimbs were not used for locomotion. Riojasaurus, with its more equalized limb proportions, is an exception that was almost certainly primarily quadrupedal. In the near-sauropod grade (Melanorosauridae and allies), the transition toward obligate quadrupedality is clearly documented.

Size: The basalmost sauropodomorphs (Saturnalia, Panphagia, Eoraptor) were small, approximately 1–2.5 meters in length. Core prosauropods ranged from 3 to 10 meters, with Plateosaurus reaching up to 10 meters and an estimated 4,000 kilograms. Melanorosaurus attained approximately 12 meters. Near-sauropods reached 10–13 meters and perhaps 5 metric tons.

Basalmost sauropodomorphs: Saturnalia (Late Triassic, Brazil), Panphagia (Late Triassic, Argentina), Eoraptor (Late Triassic, Argentina—once classified as a basal theropod), Buriolestes (Late Triassic, Brazil—notable for possessing ziphodont teeth suggesting carnivory), and Mbiresaurus (Late Triassic, Zimbabwe). These are restricted to the Carnian age and were small (~1–2.5 m), bipedal, and likely omnivorous.

Core prosauropods: Plateosauridae (Plateosaurus of Late Triassic Europe), Riojasauridae (Riojasaurus of Late Triassic Argentina), and Massospondylidae (Massospondylus of Early Jurassic southern Africa, Lufengosaurus of Early Jurassic China, Glacialisaurus of Antarctica, Seitaad of North America). These were the dominant herbivores globally from the late Late Triassic through the Early Jurassic, in some communities constituting up to 95 percent of known biomass.

Near-sauropods: Anchisaurus (Early Jurassic, North America), Yunnanosaurus (Early Jurassic, China), Melanorosauridae (Melanorosaurus of Late Triassic–Early Jurassic South Africa). These transitional forms bridge the morphological and ecological gap between core prosauropods and true sauropods, and some workers classify melanorosaurids as basal sauropods.

Prosauropods achieved a global distribution during the Late Triassic, when the supercontinent Pangaea was still largely intact. Fossils have been recovered from Europe (Germany, France, Switzerland, England), South America (Argentina, Brazil), Africa (South Africa, Lesotho, Zimbabwe, Madagascar), Asia (China, India, Thailand), North America (Connecticut, Arizona, Nova Scotia), Antarctica (Mt. Kirkpatrick), and Greenland. Australia remains the only continent with no confirmed prosauropod record.

The rapid diversification of core prosauropods may be linked to the Carnian Pluvial Event, a period of enhanced rainfall approximately 234–232 million years ago. This climatic episode triggered the evolution and diversification of new tree-forming plant lineages, providing the resources that larger herbivorous dinosaurs could exploit. In the Late Triassic of Europe, prosauropods were the only common dinosaurs in some assemblages, and their dominance represents the first time in Earth's history that dinosaurs controlled a major ecological niche.

Prosauropods inhabited diverse environments ranging from floodplains (where Plateosaurus specimens in Trossingen, Germany, were trapped in quicksand) to semi-arid interiors. Their long necks enabled them to reach vegetation above the browsing height of contemporary herbivores, and evidence of gregarious behavior (mass accumulations of Plateosaurus, Massospondylus nesting sites) suggests social grouping for protection against predators.

No core prosauropods are known from after the end of the Early Jurassic. Their disappearance has been linked to the Jenkyns Event (early Toarcian, approximately 183 million years ago), a global warming episode triggered by massive volcanism in the Karoo-Ferrar Large Igneous Province. Raven et al. (2020) demonstrated through calibrated phylogenetic trees that multiple lineages of early sauropodomorphs went extinct during this interval, coinciding with vegetation turnover and climatic disruption. The derived Eusauropoda subsequently diversified in the Toarcian across South America, Africa, and Asia, filling the ecological roles vacated by their more basal relatives.

The evolutionary transition from prosauropod-grade animals to sauropods documents a correlated progression toward larger body size, proportionately smaller heads, longer necks, and obligate quadrupedality. Two selective pressures likely drove this trend: competition with other herbivores for access to higher canopy, and defense against increasingly large predatory theropods. Key anatomical changes observed in the near-sauropod to sauropod transition include the loss of fleshy cheeks, development of tooth-to-tooth occlusion, shift from leaf-shaped to spoon-shaped teeth with reduced denticles, evolution of very wide gape for bulk-browsing, and development of columnar forelimbs with reduced elbow and wrist flexure. The transition from a grasping hand to a weight-bearing, horseshoe-shaped manus and from a digitigrade to a plantigrade foot with a large fleshy pad completed the transformation into the sauropod body plan.

Prosauropods occupy a pivotal position in understanding dinosaurian evolution for several reasons. They represent the first successful radiation of large-bodied terrestrial herbivores among dinosaurs, demonstrating that herbivory evolved early within Saurischia despite the order's carnivorous origins. They document the evolutionary pathway from small, bipedal, omnivorous forms to the largest land animals in Earth's history. Their global distribution in the Late Triassic provides evidence for the ease of faunal interchange across Pangaea before continental fragmentation. Finally, the debate over their monophyly versus paraphyly has served as an important case study in the application of cladistic methods to dinosaur systematics, illustrating how traditional taxonomic groupings based on shared primitive features can be overturned by rigorous phylogenetic analysis.