Sauropodomorpha (/ˌsɔːrəˌpɒdəˈmɔːrfə/ SOR-ə-POD-ə-MOR-fə; from Greek, meaning "lizard-footed forms") is an extinct clade of saurischian dinosaurs that includes the long-necked, herbivorous sauropods and their ancestral relatives. Early, more basal sauropodomorphs (traditionally termed prosauropods) were bipedal, and the earliest show evidence of omnivorous or carnivorous diets. Over time, sauropodomorph evolution resulted in a shift to herbivorous diets, larger body sizes, and quadrupedal locomotion. The sauropods themselves generally grew to very large sizes, had long necks and tails, and became the largest animals to ever walk the Earth. The sauropods were the dominant terrestrial herbivores throughout much of the Mesozoic Era, from their origins in the Late Triassic (approximately 230 Ma) until their decline and extinction at the end of the Cretaceous.

Gigantic bones of sauropods have been known for thousands of years and become part of legends and cultures but the beginning of their scientific study was in the 1830s. Most of the early taxonomy was based on incomplete and disarticulated material as relatively complete skulls or skeletons were not discovered until closer to the end of the 19th century. The French anatomist Georges Cuvier studied large fossils from the Jurassic of Stonesfield Slate in England, believing them to have belonged to ancient relatives of whales up to his death in 1832. However, British paleontologist Richard Owen instead classified these large bones, which he gave the name Cetiosaurus in 1841, as marine whale-like crocodilians. This was the second genus of sauropod to be named, following his earlier description of a single isolated tooth as the taxon Cardiodon. At the same time, the first early sauropodomorphs were being described, with Thecodontosaurus named in 1836 for material from England and Wales, and Plateosaurus named in 1837 for material from Germany. Owen would even name the early sauropodomorph Massospondylus in 1854 for partial specimens from southern Africa, though all these early taxa were considered carnivorous theropods rather than relatives of sauropods. Owen's interpretation of Cetiosaurus as a marine crocodilian, which he specifically excluded from Dinosauria when he named it in 1842, began to be disputed with additional discoveries of limb bones of both Cetiosaurus and the new sauropod Pelorosaurus, with the classification of "cetiosaurs" as dinosaurs firmly established by 1874 by which time many the other genera including Aepisaurus and Astrodon had been named.

The limited samples of material of sauropods and sauropodomorphs were followed by abundant discoveries in the late 19th and early 20th centuries from North America and east Africa. In the United States, American paleontologists Othniel Charles Marsh and Edward Drinker Cope began substantial excavations and competition that would be termed the Bone Wars. From this, Marsh and Cope would name and describe exemplars of the first complete sauropod skulls and skeletons, as well as the mostly complete skull and skeleton of the sauropodomorph Anchisaurus. Many of the most iconic sauropods-Apatosaurus, Brontosaurus, Camarasaurus, and Diplodocus-were all named during this time. In 1859 Owen had named the group Opisthocoelia to unite the "cetiosaurs" Cetiosaurus and Streptospondylus as crocodilians, followed by the naming of Ceteosauria by Harry Govier Seeley in 1874 for "cetiosaurs" as dinosaurs. Marsh ignored both of these older names to create the group Sauropoda in 1878 (from the Ancient Greek words sauros and podus as "saurian/lizard feet"), which became the accepted name following his work on their classification throughout the end of the 19th century and is now the term used. The early forms of sauropodomorphs were united within Prosauropoda by German paleontologist Friedrich von Huene in 1920 as a primitive stock to give rise to both theropods and sauropods, with Sauropodomorpha being named in 1932 by Huene to unite prosauropods and sauropods as the sister to Carnosauria.

The earliest and most primitive sauropodomorphs—animals like Buriolestes and Pampadromaeus—were small bipedal animals that were in the range of 1–2 m (3.3–6.6 ft) in length and weighed around 2–5 kg (4.4–11.0 lb). These earliest forms were either fully carnivorous or omnivorous, but herbivory quickly became the dominant feeding strategy of sauropodomorphs. Over the course of the Triassic Period, they increased in size, leading to the evolution of animals like Plateosaurus and Gresslyosaurus, which could reach 7–8 m (23–26 ft) long and weighed around 2-2.5 tons. During this period, all sauropodomorphs were obligate bipeds, which was the ancestral condition for dinosaurs. The largest bipedal sauropodomorph known from substantial remains was Lishulong. Only the head and neck of Lishulong are preserved, but this was enough to estimate a total length of about 13 m (43 ft), which is about twice the size of the related Yunnanosaurus, which is known from more complete remains and weighed about 3 tons. However, there is a large but relatively incomplete sauropodomorph (specimen BP/1/5339) discovered in South Africa that has not yet been fully described. Andrew Yates and Matthew Wedel have suggested that the morphology of its arm bones meant it was probably an obligate biped. Scaling based on the sympatric genus Aardonyx and femur allometry, this bipedal sauropodomorph would have weighed between 10-15 tons, making it comparable in size to Diplodocus and possibly one of the largest bipedal animals ever.

The evolution of obligatory quadrupedality enabled the true sauropods and their closest relatives to achieve very large sizes. The oldest confidently quadrupedal sauropodomorph, Melanorosaurus is not known from very complete remains, and Paul Barrett and Jonah Choiniere declined to suggest a mass estimate in their osteology of Melanorosaurus published in 2024. However, Gregory S. Paul estimated Melanorosaurus to have been about 8 m (26 ft) long and weighed around a ton, which is comparable to many bipedal sauropodomorphs. By the end of the Triassic, the 7 ton Lessemsaurus had evolved, marking the origin of the oldest true sauropods. The early Jurassic saw the evolution of the even larger Ledumahadi, which weighed around 12 tons.

The final anatomical bottleneck on the size of the true sauropods was the anatomy of their limbs. Columnar limbs evolved at some point in the early Jurassic. The oldest sauropod known to have had columnar limbs was Vulcanodon, which lived in what is now Zimbabwe around 199-188 million years ago. Earlier sauropods may have had columnar limbs, but their remains are too incomplete to determine if this is the case. Vulcanodon has been estimated to have weighed up to 10 tons, making it one of the largest terrestrial animals of its time. Most sauropods during this period—such as Barapasaurus, Rhoetosaurus, and Patagosaurus—weighed between 7-10 tons. True gigantism emerged at the beginning of the Late Jurassic; genera like Turiasaurus and Mamenchisaurus were up to 20–25 m (66–82 ft) long and may have weighed up to 30 tons. During the Late Jurassic and throughout the Cretaceous Period, true gigantism evolved independently several times in distantly related sauropod groups. Giants like Brachiosaurus, Dreadnoughtus, and Ruyangosaurus are believed to have exceeded 30–35 m (98–115 ft) in length and weighed in excess of 50 tons, making them the largest land animals of all time. Higher mass estimates have been made for very poorly-known taxa such as Maraapunisaurus and Bruhathkayosaurus, but these remain controversial.

Sauropods reached a variety of different body proportions, so the "largest" individual species will vary based on the measurement concerned. The longest known sauropod was probably Supersaurus, which may have exceeded 40 m (130 ft) in length. The tallest sauropodomorph was probably Sauroposeidon, which had a relatively erect posture and may have been able to reach a height of between 16.5–18 m (54–59 ft). The longest neck of any sauropod known from complete remains is that of Xinjiangtitan, which had a neck 14.9 m (49 ft) meters long. Very large isolated cervical vertebrae from taxa like Hudiesaurus and Mamenchisaurus have been found, but the incompleteness of these remains makes it difficult to assess the total length of their necks. The most massive sauropodomorph known from relatively complete remains is generally accepted to be Argentinosaurus, which may have exceeded 70 tons in weight.

The smallest sauropodomorphs were the oldest and earliest-diverging taxa. Genera like Pampadromaeus and Eoraptor may have been 1.5 m (4.9 ft) long or shorter and weighed as little as 10 kg (22 lb). Sauropods were generally much larger, but several sauropods are believed to have been examples of insular dwarfism. Magyarosaurus, Europasaurus, and Petrustitan are the smallest sauropods known from adult remains; they were between 3–5 m (9.8–16.4 ft) long and weighed less than a ton. Other small sauropods existed throughout the Mesozoic including Haplocanthosaurus, Bonatitan, and Ohmdenosaurus, which were each between 1-2 tons.