Massospondylus (/ˌmæsoʊˈspɒndɪləs/ MASS-oh-SPON-di-lus) is a genus of sauropodomorph dinosaur from the Early Jurassic of southern Africa. It was described by Richard Owen in 1854 from remains discovered in South Africa, and is thus one of the first dinosaurs to have been named. The name Massospondylus means 'longer vertebra', alluding to what Owen identified as tail vertebrae; these vertebrae are now known to be from the neck. Although the original fossils were destroyed in London during The Blitz, a plethora of specimens have since been assigned to the genus, making it one of the best-known sauropodomorphs from the Early Jurassic. The genus lived during the Hettangian, Sinemurian, and Pliensbachian ages, which lasted from ca. 201 to 184 million years ago. Most fossils come from the upper Elliot and Clarens formations of South Africa and Lesotho, but the genus is also found in the Forest Sandstone and the Mpandi Formation of Zimbabwe. Material from the US, India, and Argentina was previously assigned to the genus, but the US and Argentinian specimens are now assigned to their own genera (Sarahsaurus and Adeopapposaurus). Because of their great abundance, Massospondylus fossils have been used to date rocks, and a biozone, the Massospondylus Range Zone, is named after the genus.

Two species are considered valid: the type species M. carinatus, as well as M. kaalae, which was named in 2009 and is known from a single skull. Six other species have been named during the past 150 years but are no longer recognised. Originally, Massospondylus and similar dinosaurs have been regarded as theropods, but are now classified as basal ("early diverging") members of Sauropodomorpha. This group also includes sauropods. Within sauropodomorphs, Massospondylus is often classified in the family Massospondylidae. The genus was 4–6 metres (13–20 ft) long, with a long neck and tail, a small head, and a slender body. It is distinguished from related genera by the very elongated vertebrae of the front portion of the neck, amongst other features. Although Massospondylus was long depicted as quadrupedal (four-legged), it is now considered to have been bipedal (two-legged).

This dinosaur was probably a herbivore (plant-eater), although some have speculated that basal sauropodomorphs may have been omnivorous. On each of its hands, it bore a sharp thumb claw that was probably used in feeding, possibly to uproot vegetation or to pull down branches while rearing up. Clutches of eggs have been found, some of which contained embryos; these are among the oldest eggs and embryos of an amniote in the fossil record. The eggshell was extremely thin, less than 0.1 mm (0.0039 in), unlike the much thicker eggshells in later dinosaurs. The embryos had proportionally longer arms than adults and a very large head, leading researchers to suggest that they were quadrupedal and shifted to a bipedal posture later during growth. Newer research instead suggested that Massospondylus was bipedal at all ages. Individuals accelerated or slowed down their growth depending on environmental factors such as food availability. The oldest known specimen was around 20 years of age.

The first fossils of Massospondylus were described by Richard Owen in 1854. The material, a collection of 56 bones or bone fragments, was found in 1853 or 1854 by the government surveyor Joseph Millard Orpen and his brothers on a farm in the Drakensberg mountains near Harrismith, South Africa. Their father then donated the fossils to the Hunterian Museum in London, of which Owen was curator. Owen named three new species from this material based on differences in their supposed tail vertebrae: Massospondylus carinatus, Pachyspondylus orpenii, and Leptospondylus capensis. The name Massospondylus (meaning 'longer vertebra') is derived from the Greek μάσσων (massōn) 'longer' and σπόνδυλος (spondylos) 'vertebra'; Owen stated that he chose this name "because the vertebrae are proportionally longer than those of the extinct crocodile called Macrospondylus". The specific name carinatus probably hints at the pronounced keel (carina) at the underside of the vertebrae. Leptospondylus means 'slender vertebra', while Pachyspondylus means 'thick vertebra'. Among the bones in Orpen's collection were vertebrae from the neck, back, hip, and tail; bones of the pelvis; the humerus (upper arm bone); and parts of the hindlimb including the femur (thigh bone), tibia (shin bone), and some foot bones. Orpen believed that more fossils would be found if the site were excavated. All these bones were probably found disarticulated (not connected to each other), making it difficult to determine whether or not they belonged to the same species. Yet, Owen assigned most of the bones to either Massospondylus, Pachyspondylus, or Leptospondylus, which Hans-Dieter Sues found to be somewhat arbitrary in 2004. In the decades after Owen's publication, the three species were neglected by other scholars, which Paul Barrett and Kimberley Chapelle speculated was due to Owen's "rather perfunctory descriptions", which lacked illustrations.

In 1888, Richard Lydekker studied the material and found Leptospondylus was likely a synonym of Massospondylus, though he did not mention Pachyspondylus. Lydekker furthermore proposed that Owen's description was too incomplete for the name Massospondylus carinatus to be considered valid, and that his own publication should instead be recognised as the source of the name. Consequently, he selected a neck vertebra and a toe bone as type specimens (representative specimens on which a taxon is based). This proposal was mostly ignored by later authors, and Owen's description is valid according to the International Code of Zoological Nomenclature (ICZN). In 1895, Harry Seeley revised Massospondylus and illustrated many of the fossils for the first time. Seeley argued that the putative tail vertebrae of Massospondylus were actually neck vertebrae, and that the tail vertebrae of Pachyspondylus orpenii were probably those of Massospondylus. Therefore, he concluded that most of the fossils probably belonged to a single species and assigned them to Massospondylus, but noted that they represent at least three individuals. Another redescription of the Orpen specimens was published by Friedrich von Huene in 1906, who assigned all material to Massospondylus. At the beginning of World War II, the basements of the Hunterian Museum were strengthened to protect specimens from German bombing raids, and several collections were moved to remote locations. The museum was damaged during several nearby bombings before it was directly hit by a bomb on the night between May 10 and 11, 1941, causing debris to fall into the dungeons and the building to be flooded due to heavy rain. Only 23 of 550 specimens in the museum's comparative anatomy collection survived. Many specimens that have been pivotal in the history of science were lost, as well as the entire Orpen collection, including Massospondylus, Pachyspondylus, and Leptospondylus, of which only illustrations and plaster casts remain.

By 1976, Massospondylus was the most widespread sauropodomorph known from southern Africa thanks to continued discoveries in South Africa, Lesotho, and Zimbabwe. In that year, James Kitching discovered clutches with eggs in a roadcut in Golden Gate Highlands National Park, a locality known as Rooidraai ('Red Bend'). Already in 1961, a British-South African palaeontological expedition discovered the skeleton of a hatchling (SAM-PK-K413) at Mafeteng, Lesotho, although this specimen was first misidentified as the varanopid Heleosaurus. In 1981, Michael Cooper published a comprehensive monograph on the Zimbabwean Massospondylus material, describing the entire skeleton apart from the skull, of which no material was available. He also discussed the palaeobiology of the genus in detail for the first time. Cooper concluded that none of the other gracile (slender-built) sauropodomorph species from South Africa differed substantially from M. carinatus, and consequently synonymised them all with the latter. A large number of specimens were assigned to M. carinatus as a result. Over the next two decades, new specimens of gracile sauropodomorphs from southern Africa were often assigned to M. carinatus by default and without much scrutiny because it was the only recognised species. The skull of Massospondylus was first described in detail in 1990 by Chris Gow and colleagues based on four well-preserved skulls housed at the Evolutionary Studies Institute in Johannesburg. In 2004, Hans-Dieter Sues and colleagues provided a more comprehensive description of the same four skulls, and proposed the first diagnosis of M. carinatus (the set of distinguishing features).

The five neck vertebrae on which M. carinatus was originally based (the syntype series) do not show distinguishing features and cannot serve as a basis for comparison. Consequently, Yates and Barrett proposed in 2010 to designate a different specimen, BP/1/4934, as the neotype specimen (the new representative specimen). This specimen consists of an almost complete skeleton with skull housed at the Evolutionary Studies Institute. The skull was described in detail in 2018 by Chapelle and Jonah Choiniere, while Barrett and colleagues described the remainder of the skeleton in 2019. The specimen, nicknamed "Big Momma" although its sex is unknown, was found in March 1980 on a farm near Clocolan, South Africa, by Lucas Huma and James Kitching. Other fossils were found on the same farm, including the holotype of the turtle Australochelys africanus and the cynodont Pachygenelus. "Big Momma" includes a nearly complete skull and large parts of an articulated skeleton. As of 2019, it is the largest and most complete Massospondylus specimen and probably the most complete basal (early diverging) sauropodomorph specimen discovered in Africa. Since 1990, it has been on public exhibit in the Evolutionary Studies Institute. To maintain the original positions of the bones as they had been found, the specimen was prepared from above and below, but with most bones still partly encased in the original rock matrix. However, the specimen has been divided into seven individual blocks, the heaviest of which is about 35 kg (77 lb) in weight. In the 2000s, the specimen was extensively restored after it became apparent that the fossils were deteriorating due to repeated handling. These conservation efforts included the filling of gaps and cracks in the bones, the application of a resin for hardening, and new support jackets to support the blocks.

Besides the type species Massospondylus carinatus, one additional species, Massospondylus kaalae, is currently recognised. M. kaalae is known from a single partial skull (SAM-PK-K1325) from the Upper Elliot Formation near Herschel, South Africa. This skull was collected in 1966 by Gow and others, but since then has remained undescribed in the Iziko South African Museum in Cape Town. In 2004, Barrett noted that this skull belonged to a new species, which he named M. kaalae in 2009. The species is named in honor of the museum's collections manager for the Karoo vertebrate fossils, Sheena Kaal, for her assistance to numerous scientists who have studied specimens at the museum.