Massospondylus

''Massospondylus'' ( ) is a genus of sauropodomorph dinosaur from the Early Jurassic of southern Africa. It was described by Sir Richard Owen in 1854 from remains discovered in South Africa, and is thus one of the first dinosaurs to have been named. Although the original fossils were destroyed in London during a bombing raid in World War II, 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 from the Hettangian to Pliensbachian ages, ca. 200–183 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'' Assemblage Zone, is named after it.

The type species is ''M. carinatus''; seven other species have been named during the past 150 years, but among these, only ''M. kaalae'' is still considered valid. Originally, ''Massospondylus'' and similar dinosaurs have been regarded as theropods, but are now classified as basal ("primitive") members of Sauropodomorpha, a group that also includes sauropods. Within sauropodomorphs, ''Massospondylus'' is often classified in the family Massospondylidae. The genus was long, and had 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).

It was probably a plant eater (herbivore), although some have speculated that early 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 with 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 (ca. 0.1 mm), unlike the much thicker eggshells in later dinosaurs. The embryos had longer arms than adults and a proportionally 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.

History of discovery

Owen's 1854 description and loss of the type material

The first fossils of ''Massospondylus'' were described by paleontologist Sir 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, and was donated to the Hunterian Museum at the Royal College of Surgeons in London, of which Owen was curator. Owen erected 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 ) is derived from the Greek (massōn) and (spondylos) ; 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 , while ''Pachyspondylus'' means . Among the bones in Orpen's collection were vertebrae from the neck, back, hip, and tail; bones of the pelvis; the humerus; and parts of the hindlimb including the femur, tibia, and some foot bones. Orpen believed that more fossils would be found if the site were excavated. All these bones were probably found (not in their original anatomical compound), 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'', seemingly "somewhat arbitrarily". In the decades after Owen's publication, the three species were neglected by other scholars, which has been speculated to be 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''. He also mentioned very similar fossils he had previously described from the "Maleri beds" of India, which he assigned to ''Massospondylus''. Lydekker furthermore proposed that Owen's description was invalid and that his own publication should instead be considered the source of the name ''Massospondylus carinatus'', and selected a neck vertebra and a toe bone as types (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 in 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 of the museum's comparative anatomy collection survived. The losses included many specimens that have been pivotal in the history of science, as well as ''Massospondylus'', ''Pachyspondylus'', and ''Leptospondylus'', of which only illustrations and plaster casts remain.

Later discoveries and the neotype specimen

By 1976, ''Massospondylus'' was the most widespread sauropodomorph known from southern Africa thanks to continued discoveries in South Africa, Lesotho, and Zimbabwe. In 1981, Michael Cooper published a comprehensive monograph on the Zimbabwean material, describing the entire skeleton apart from the skull, of which no material was available. He also discussed the paleobiology 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 synonymized 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 ''Massospondylus carinatus'' (the set of distinguishing traits).

The five neck vertebrae on which ''Massospondylus carinatus'' was originally based on (the syntype series) do not show diagnostic features and cannot serve as a basis for comparison. Consequently, Yates and Paul Barrett, in 2010, proposed to designate a different specimen, BP/1/4934, as the neotype specimen (representative specimen). The skull and postcranium (the skeleton apart from the skull) of this specimen has been described in detail in 2018 and 2019, respectively. 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 sauropodomorph specimen discovered in Africa. Since 1990, it has been on public exhibit in the Evolutionary Studies Institute in Johannesburg. 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 in weight. In the 2000s, the specimen was extensively renovated 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.

Misidentified specimens

During the 21st century, two additional massospondylid genera were identified in southern Africa, raising the question of whether all of the previous identifications of ''Massospondylus'' specimens were correct. The first of these genera, '' Ignavusaurus'', was described in 2010 from a young specimen. In 2011, Yates and colleagues considered it to be a probable synonym of ''Massospondylus'', but cladistic analyses led by Kimberley Chapelle in 2018 and 2019 recovered it as a distinct taxon of massospondylid. The second genus, ''Ngwevu'', was described in 2019 by Chapelle and colleagues based on a complete skull with a fragmentary skeleton that had been discovered in 1978 and was previously assigned to ''Massospondylus carinatus''.

''Massospondylus'' has been previously recognised from outside of Africa, namely from the Upper Maleri Formation of India, the Kayenta Formation of the US, and the Cañón del Colorado Formation of Argentina. Cooper, in his 1981 study, even suggested that the Chinese genera '' Yunnanosaurus'' and '' Lufengosaurus'' were synonyms of ''Massospondylus'', which would have expanded its range to China. All of these specimens are no longer assigned to ''Massospondylus'', and Cooper's suggestion of synonymy was not followed by subsequent authors. Material from India was first assigned to ''Massospondylus'' by Lydekker in 1888 and 1890. Other ''Massospondylus'' fossils were mentioned in 1987 by T.S. Kutty and colleagues but have instead been assigned to the family Guaibasauridae in 2011. The Argentinian material, consisting of several partial skeletons described in 1999, has been assigned to the new and closely related genus '' Adeopapposaurus'' in 2009, while the material from the US, which consists of a nearly complete skull with skeleton described in 1985, was assigned to the new and closely related genus '' Sarahsaurus'' in 2010–2011.

Species

Two species are currently recognized: the type species ''M. carinatus'', described by Owen in 1854, and ''M. kaalae''. ''M. kaalae'' is known from a single partial skull (SAM-PK-K1325) from the Upper Elliot Formation near Herschel, South Africa. This skull had been collected in 1966 by Gow and others but since then remained undescribed in the Iziko South African Museum in Cape Town. In 2004, Paul 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.

Multiple other species have been named but were considered as indeterminate by recent reviews. In 1890, Lydekker described the species ''M. hislopi'' and ''M. rawesi'' from fossils found in India. ''M. hislopi'' was based on a single fragmentary vertebra from the Lower Maleri Formation in Andhra Pradesh, while ''M. rawesi'' was based on a single tooth found by Mr Rawes in the Upper Cretaceous Takli Formation in Maharashtra. In 1906, von Huene believed that both species were not dinosaurs. ''M. hislopi'' was listed as an indeterminate sauropodomorph in the latest review, but ''M. rawesi'' is an indeterminate theropod. In 1895, Seeley named another species, ''M. browni'', and tentatively assigned it to ''Massospondylus''. This species was based on two neck, two back, and three tail vertebrae as well as femora and toe bones that were discovered north of the Witteberg by Mr. Alfred Brown. In 1906, von Huene concluded that the vertebrae and femora of ''M. browni'' belonged to two separate species. He therefore restricted ''M. browni'' to just the femora, and moved the species into the genus '' Thecodontosaurus'', as ''T. browni''. He assigned the vertebrae as well as additional fossils stored in Vienna to another species, '' Hortalotarsus skirtopodus'', which he also moved into ''Thecodontosaurus'', as ''T. skirtopodus''. In 1920, Egbert Cornelis Nicolaas van Hoepen assigned a partial skeleton to ''M. browni'', which was later assigned to ''M. harriesi'' and is now listed as a specimen of ''M. carinatus''. ''M. browni'' was considered an indeterminate sauropodomorph in the most recent review.

Two more species were described in the first half of the twentieth century: ''M. harriesi'' and ''M. schwarzi''. ''M. harriesi'' was named by Robert Broom in 1911 based on parts of a forelimb and hind limb discovered near Fouriesburg. Broom suggested that the original material described by Owen included a second species besides ''M. carinatus'' which can possibly be assigned to his ''M. harriesi''. Several additional specimens were assigned to ''M. harriesi'' in 1911 and 1924, and in 1976, Galton and Michael Albert Cluver synonymized a number of other sauropodomorph species with it. ''M. schwarzi'' was named by Sydney H. Haughton in 1924 based on an incomplete foot found near Tlokoeng by Professor Schwarz. Both species were considered as indeterminate sauropodomorphs in the most recent review. In 1981, Cooper used the name ''M. huenei'', a combination derived for '' Lufengosaurus huenei'', as he considered ''Lufengosaurus'' and ''Massospondylus'' to be synonyms. This synonymy has not been accepted by subsequent authors.

Previously synonymized species

Several other species from South Africa have been previously synonymized with ''Massospondylus''. These include the above-mentioned ''Leptospondylus capensis'' and ''Pachyspondylus orpenii'', as well as '' Hortalotarsus skirtopodus'', '' Aetonyx palustris'', '' Gryponyx africanus'', '' Gyposaurus capensis'', '' Gryponyx transvaalensis'', '' Thecodontosaurus minor'', '' Aristosaurus erectus'', '' Dromicosaurus gracilis'', '' Thecodontosaurus dubius'', and '' Gryponyx taylori''.

''Hortalotarsus skirtopodus'' was erected by Seeley in 1894, based on a specimen found in the Clarens Formation near Barkly East. Most of the original skeleton was destroyed in an attempt to remove it from the rock matrix, and only a fragmentary hind limb remains. ''Aetonyx palustris'', ''Gryponyx africanus'' and ''Gyposaurus capensis'' were all erected by Broom in 1911. ''Aetonyx palustris'' and ''Gryponyx africanus'' are each based on a fragmentary skeleton without skull found in the upper Elliot Formation near Fouriesburg, while ''Gyposaurus capensis'' is based on a fragmentary skeleton without skull discovered in the Clarens Formation near Ladybrand. One year later, in 1912, Broom erected ''Gryponyx transvaalensis'' from two foot bones (an ungual and a metatarsal) discovered in the Bushveld Sandstone Formation in Limpopo Province. ''Thecodontosaurus minor'' was named by Haughton in 1918 based on a neck vertebra, a tibia, and an ilium found in the Elliot Formation near Maclear. Haughton originally erected the taxon as a species of '' Thecodontosaurus''. ''Aristosaurus erectus'' was named by van Hoepen in 1920 based on a nearly complete skeleton of a small and potentially juvenile individual found by quarry workers in the Clarens Formation near Rosendal. In another paper from the same year, van Hoepen also named ''Dromicosaurus gracilis'' from a fragmentary skeleton he had discovered near Bethlehem. In 1924, Haughton erected another species of ''Thecodontosaurus'', ''T. dubius'', as well as ''Gryponyx taylori''. ''Thecodontosaurus dubius'' is based on a specimen comprising much of the skeleton from the Clarens Formation near Ladybrand, South Africa. ''Gryponyx taylori'' is based on a fragmentary pelvis from the Upper Elliot Formation near Fouriesburg.

Galton and Cluver, in 1976, assigned ''Aetonyx palustris'', ''Gryponyx africanus'', ''Gryponyx taylori'', ''Dromicosaurus gracilis'', and ''Thecodontosaurus dubius'' to ''Massospondylus harriesi''. Cooper, in 1981, instead considered all taxa as synonyms of ''Massospondylus carinatus''. Galton, in a 1990 review, only listed ''Aetonyx palustris'', ''Gryponyx africanus'', ''Gyposaurus capensis'', and ''Gryponyx taylori'' as synonyms of ''M. carinatus'', while the latest review, by Galton and Paul Upchurch in 2004, considers all species to be indeterminate sauropodomorphs rather than synonyms of ''Massospondylus''.

Description

''Massospondylus'' was a mid-sized sauropodomorph. In a 2024 popular book, Gregory S. Paul estimated ''Massospondylus'' to be in length, while Barrett and colleagues estimated the particularly large neotype specimen at around in length, in 2019. Paul gave a body weight of , while Frank Seebacher, in 2001, gave an estimate of while assuming a body length of . It was a typical early sauropodomorph, with a slender body, a long neck, and a proportionally very small head. It had a slighter build than '' Plateosaurus'', an otherwise similar but larger dinosaur. Early sauropodomorphs probably had air sacs and possibly a bird-like flow-through lung. In theropods and sauropods, these air sacs invaded bones, in particular the vertebrae ( postcranial skeletal pneumaticity). Although some early sauropodomorphs such as ''Plateosaurus'' show incipient pneumaticity in their vertebrae, those of ''Massospondylus'' are apneumatic (lack any signs of pneumaticity).

Skull

The small skull of ''Massospondylus'' was approximately half the length of the (upper thigh bone). Several openings, or fenestrae, in the skull reduced its weight and provided space for muscle attachment and sensory organs. At the front of the skull was the large (nostril), which was roughly half the size of the (eye socket). The posterior margin of the external naris was semicircular, while the anterior part of the opening was triangular. The orbit was circular and proportionally larger than that of ''Plateosaurus''. The antorbital fenestra, situated between the orbit and the external naris, was triangular and smaller than that of ''Plateosaurus''. At the rear of the skull were the two temporal fenestrae: the immediately behind the orbit that was shaped like an inverted "T", and the supratemporal fenestra on the top of the skull. A small fenestra also penetrated the mandible. Traditionally, the skull was thought to be wider and shorter than that of ''Plateosaurus'', but these proportions may be the result of deformation during fossilisation. Some features of the skull are variable between individuals; for example, the thickness of the upper border of the orbits and the height of the posterior (rear) portion of the , the main tooth-bearing bone of the upper jaw. These differences may be due to sexual dimorphism or individual variation. A single autapomorphy (distinguishing feature) is found in the skull: the , a pair of bony extensions that braces the against the , form an angle of ca. 35°, whereas this angle is much larger in all other basal sauropodomorphs in which it can be measured. The species ''M. kaalae'' differs from ''M. carinatus'' in the morphology of the braincase; the proportionally longer premaxillary tooth row that accounted for more than 30% of the upper tooth row; and a better developed ridge on the upper edge of the .

Tooth count was variable between individuals and increased with skull size. The , the front bone of the upper jaw, showed the constant number of four teeth per side in all known skulls. In the maxilla, the tooth count varied from 14 to 22. There were 26 teeth on each side of the lower jaw in the largest known skull. The upper halves of the tooth crowns had serrated edges, with three to eight on each side. The teeth were proportionally longer and slenderer than those of ''Plateosaurus''. Their height decreased from front to back in the upper jaw, but was more or less constant in the lower jaw. The rear edges of the teeth did partially overlap the front edges of the teeth behind them, forming a continuous cutting edge. The teeth were throughout life, as in other dinosaurs. This resulted in neighbouring teeth being of different heights, giving the tooth row the profile of a saw. The lack of pronounced tooth wear suggests that tooth replacement was rapid; a 2013 study estimated that a tooth remained in use for only 17 to 30 days. The teeth varied in shape in different regions of the jaw. This heterodonty was more pronounced than that present in '' Plateosaurus'', although not as pronounced as the specialization of teeth in '' Heterodontosaurus''. Teeth closer to the front of the snout had round cross-sections and pointed tips, unlike the posterior teeth that were spatulate with oval cross-sections.

As with other early sauropodomorphs, it has been proposed that ''Massospondylus'' had fleshy cheeks, as there were few but large holes for blood vessels on the surfaces of the jaw bones, unlike the numerous small holes present on the jaws of cheekless reptiles. The cheeks would have prevented food from spilling out when ''Massospondylus'' ate. In 1986, Crompton and Attridge described skulls of ''Massospondylus'' as possessing pronounced overbites and suggested the presence of a horny beak on the tip of the lower jaw to make up the difference in length. However, the difference in length may be a misinterpretation and caused by crushing of the skull in a top–bottom plane, and more recent studies consider the possession of a beak as unlikely. Gow, in 1990, suggested that the articulation between the premaxilla and the maxilla allowed for some degree of movement (cranial kinesis), and that the same was true for the articulation between the and the ( streptostyly). However, a later study found these articulations to be relatively rigid.

Vertebrae, girdles, and limbs

The vertebral column was composed of nine cervical (neck) vertebrae, 13 dorsal (back) vertebrae, three sacral (hip) vertebrae, and at least 40 tail vertebrae. The neck was proportionally long, and the (vertebral bodies) of the anterior (front) cervicals were more than seven times as long as they were tall, more elongated than in any other basal sauropodomorph and therefore an autapomorphy. In the anterior cervicals, the neural spine, the bony keel that formed the top of each vertebra, was elongated, and its anterior end had a hook-like protuberance that constitutes another autapomorphy.

The shoulder blade had an expanded distal (upper) end. This expansion was more symmetrical than in related genera, in which the dorsal (front) expansion was much larger than the ventral (rear) expansion, and is an autapomorphy. In the hip, the pubis faced forward, as with most saurischians. The distal (front) end of the pubis was expanded in side view, but this expansion affected only its posterior margin, while the bone was flat anteriorly, which is an autapomorphy. The forelimbs were only half the length of the hind limbs but quite powerful, as indicated by a broad flange on the upper portion of the called the , which provided attachment areas for a large arm musculature. This crest was longer than in related genera, extending for ca. 60% of the length of the bone, which is an autapomorphy. The lower arm was proportionally short, with the measuring around 60% of the length of the humerus, which is another autapomorphy. Like ''Plateosaurus'', it had five on each hand and foot. The hand was short and wide, with a large sickle-shaped thumb claw used for feeding or defence against predators. The thumb was the longest finger in the hand, while the fourth and fifth digits were tiny, giving the hands a lopsided look. The phalangeal formula of the hand, which states the number of finger bones from the thumb to the fifth digit, was 2-3-4-3-2. The foot also had a large claw on the first digit, which was curved and flattened laterally (side-to-side). The claws on the middle three digits instead were smaller, less curved, and flattened dorsoventrally (top to bottom). The fifth toe only had a single toe bone and no claw. The phalangeal formula of the foot was 2-3-4-5-1.

Classification

''Massospondylus'' was one of the first named dinosaurs from the southern hemisphere, a group that Owen himself had named twelve years earlier, in 1842. Owen, however, did not recognize the finds as dinosaurian, and instead attributed them to "large extinct carnivorous Reptiles" of uncertain classification, noting similarities with crocodilians, lizards, and dinosaurs. In an 1860 book, Owen classified ''Massospondylus'' as a crocodilian but did not explain this decision. In 1888, Lydekker demonstrated that ''Massospondylus'' is a genus of dinosaur, and classified it in the family Anchisauridae, within Theropoda. Seeley, in his 1895 revision, also found ''Massospondylus'' to be a theropod (a " megalosaurian saurischian"). Huene, in his 1906 redescription of the Orpen fossils, classified ''Massospondylus'' as a member of Thecodontosauridae, together with '' Thecodontosaurus'' and '' Anchisaurus''. In 1914, von Huene instead erected a new family, Massospondylidae, to accommodate ''M. carinatus'', ''M. harriesi'', and ''Aetonyx palustris'', but in 1932 returned to his former classification of ''Massospondylus'' within Thecodontosauridae.

In 1920, von Huene erected the new group Prosauropoda to unite the early long-necked dinosaurs including ''Massospondylus''. In 1932, he also erected the new group Sauropodomorpha, which contained his Prosauropoda as well as Sauropoda, abandoning the traditional classification of ''Massospondylus'' and related genera within Theropoda. Yet, the relationships between Theropoda and Prosauropoda remained contested until 1965, when Alan J. Charig argued that Prosauropoda is more closely related to Sauropoda. Charig divided Prosauropoda into three families, following earlier studies: the small-sized Thecodontosauridae, which still included ''Massospondylus''; the medium-sized Plateosauridae; and the large-sized Melanorosauridae. This division of Prosauropoda into three families was generally uphold during the following decades, although a number of studies classified ''Massospondylus'' within the Plateosauridae or within another family, the Anchisauridae. In the 1990 encyclopedia The Dinosauria, Galton recognised additional prosauropod families including a resurrected Massospondylidae that contained ''Massospondylus'' as its only member. The majority of studies have since classified ''Massospondylus'' within Massospondylidae. The name "Prosauropoda" meanwhile fell into disuse as some prosauropods were closer to sauropods than to other prosauropods. Massospondylidae has since been classified within the group Plateosauria, which also includes the Plateosauridae, the Riojasauridae, and, depending on the study, may or may not include sauropods. Adam Yates, in 2007, erected the group Massopoda as a sub-group of Plateosauria that includes Massospondylidae and sauropods but not Plateosauridae.

Besides ''Massospondylus'' itself, Massospondylidae typically includes ''Adeopapposaurus'', ''Coloradisaurus'', '' Leyesaurus'', and ''Lufengosaurus''. ''Glacialisaurus'' has also been consistently recovered as a member of Massospondylidae, and '' Pradhania'' and '' Ignavusaurus'' might also be members. The massospondylid ''Ngwevu'' was described in 2019 based on a skull previously assigned to ''Massospondylus''. The following cladogram shows the position of ''Massospondylus'' according to Oliver Rauhut and colleagues, 2020:

Paleobiology

As with all dinosaurs, much of the biology of ''Massospondylus'', including its behavior, coloration, and physiology, remains unknown. However, numerous studies have allowed for informed speculation on subjects such as growth patterns, diet, posture, reproduction, and locomotion.

Diet

Early sauropodomorphs such as ''Massospondylus'' may have been herbivorous or omnivorous. As recently as the 1980s, paleontologists debated the possibility of carnivory in the "prosauropods". However, the hypothesis of carnivorous "prosauropods" has been discredited, and all recent studies favor a herbivorous or omnivorous lifestyle for these animals. In 2004, Galton and Upchurch found that cranial characteristics (such as jaw articulation) of most basal sauropodomorphs are closer to those of herbivorous reptiles than those of carnivorous ones, and the shape of the tooth crown is similar to those of modern herbivorous or omnivorous iguanas. The maximum width of the crown was greater than that of the root, resulting in a cutting edge similar to those of extant herbivorous or omnivorous reptiles. In 2000, Barrett proposed that basal sauropodomorphs supplemented their herbivorous diets with small prey or carrion. Gastroliths (gizzard stones) have been found in association with three ''Massospondylus'' fossils from the Forest-Sandstone in Zimbabwe. In one of these specimens, the preserved gastroliths made up ca. 1% of the body mass. Until recently, scientists believed that these stones functioned as a gastric mill to aid ingestion of plant material, compensating for their inability to chew, as is the case in many modern birds. However, Wings and Sander showed in 2007 that the polished nature and the abundance of those stones precluded their use as an effective gastric mill in most non- theropod dinosaurs, including ''Massospondylus''.

Posture, range of motion, and function of thumb claw

Although long assumed to have been quadrupedal (four-legged), Matthew Bonnan and Phil Senter questioned this in 2007, arguing that the animal was restricted to bipedal (two-legged) gaits. Neither could the forelimbs swing forward and behind in a fashion similar to the hindlimbs, nor could the hand be pronated (rotated so that the fingers face forwards when the forelimb is vertical). This inability to pronate the hand is also supported by findings of (still-connected) arms that always show palmar surfaces facing each other. The study also ruled out the possibility of knuckle-walking that would make effective locomotion possible without the need to pronate the hand. Bonnan and Senter suggested that some bipedal fossil track, trackways of the ichnogenus ''Otozoum'' might have been produced by ''Massospondylus'' or similar bipedal sauropodomorphs.

It was often assumed that the large thumb claw of ''Massospondylus'' and other basal sauropodomorphs was used in defence against predators. In their 2007 study, Bonnan and Senter questioned this because the humerus could not swing forwards past the vertical. However, using the thumb claw for defence would have been possible if the animal reared up by using the tail as a third "leg". Although the hand could not reach the mouth, the thumb claw was probably used for feeding, such as for disrooting vegetation or, when rearing up, for pulling down branches.

Since the discovery of rudimentary and nonfunctional clavicles in ceratopsians, it was assumed that these shoulder bones were reduced in all dinosaurs that did not have true furculae. In 1987, Robert Bakker suggested that this would have allowed the scapula, shoulder blades to swing with the forelimbs in quadrupedal dinosaurs, increasing their functional forelimb length. This would have reduced the discrepancy of length between fore- and hindlimbs in a quadrupedal ''Massospondylus''. However, a recent discovery shows that ''Massospondylus'' possessed well-developed clavicles that were joined in a furcula-like arrangement, acting like a clasp between the right and left shoulder blades and prohibiting any rotation of these bones. This discovery indicates that the clavicle reduction is limited to the evolutionary line leading to the ceratopsians. It also indicates that the furcula of birds is derived from clavicles.

In 1981, Michael Cooper noted that the zygapophyses of the neck vertebrae were inclined, prohibiting significant horizontal movement of the neck, so that "consequently any significant movement in this direction must have been accomplished by a change in the position of the entire body". This was contradicted in a recent study, noting that only the basalmost cervicals show inclined zygapophyses, allowing sufficient horizontal movement of the neck as a whole.

Eggs and embryos

In 1976, a clutch of seven 190-million-year-old ''Massospondylus'' eggs was found in Golden Gate Highlands National Park in South Africa by James Kitching, who identified them as most likely belonging to ''Massospondylus''. It was nearly 30 years before extraction was started on the fossils of the 15-centimetre- (6 in-) long embryos. They are amongst the oldest dinosaur embryos ever found. By early 2012, at least 10 egg clutches from at least four fossiliferous horizons had been found, with up to 34 eggs per clutch. This indicates that this nesting site may have been used repeatedly (site fidelity), by groups of animals (colonial nesting); in both cases, these represent the oldest evidence of this behaviour. Sedimentary structures indicate that the nesting area was in the vicinity of a lake. There are no hints that ''Massospondylus'' constructed nests; however, Reisz and colleagues suggested in 2012 that the arrangement of the eggs in tight rows indicates that the eggs were pushed in this position by the adults.

''Massospondylus'' eggs are amongst the oldest known amniote eggs from the fossil record, together with the roughly contemporaneous eggs of ''Lufengosaurus'' and ''Mussaurus''. The eggshell consisted of a thick membrane covered by a layer of calcite that was very thin, about 0.1 mm, and had a rugged surface with few, unevenly distributed pores. Later dinosaurs had much thicker calcite layers, which must have evolved convergent evolution, independently in the different groups. According to a 2019 study by Koen Stein and colleagues, the eggs were probably rigid rather than flexible despite their thin calcitic layers. This is because the calcite crystals interlocked with each other, and because the fossil eggshells retained their original shape. In contrast, Mark Norell and colleagues argued in 2020 that the eggshell of early dinosaurs such as ''Massospondylus'' was soft. The thin shells allowed for gas exchange even in a low-oxygen and carbon dioxide-rich environment, which indicates that the eggs were at least partly buried in the substrate.

The embryos probably represented near-hatchlings. While the skeletal features were similar to those of the adults, the body proportions were very dissimilar. The head was big with a short snout and very large orbits, whose diameter amounts to 39% of the entire skull length. The neck was short, in contrast to the very long neck in the adults. Girdle bones and tail vertebrae were relatively tiny. The forelimbs were of equal length to the hindlimbs. The manus was not pronated, and the head proportionally very large, indicating that effective locomotion was not possible for newly hatched ''Massospondylus''. Also, the near-hatchlings had no teeth, suggesting they had no way of feeding themselves. Based on the lack of teeth and the ineffective locomotion, scientists speculate that postnatal care might have been necessary.

Possible postural shift

In 2005, Robert Reisz and colleagues suggested that newly hatched ''Massospondylus'' must have been quadrupedal due to their long forelimbs and large heads, unlike the bipedal adults. The quadrupedality of the hatchings suggests that the quadrupedal posture of later sauropods may have evolved from retention of juvenile characteristics in adult animals, an evolutionary phenomenon known as paedomorphosis. Hatchlings are known from a second sauropodomorph, ''Mussaurus''; these remains resemble those of the embryonic ''Massospondylus'', suggesting that quadrupedality was present in newly hatched ''Mussaurus'' and presumably other basal sauropodomorphs as well. In 2012, Reisz and colleagues suggested that tracks from at the same site at which the embryos were discovered were produced by juvenile ''Massospondylus''. These tracks include both pes (hindfoot) and manus (hand) tracks, with the manus tracks rotated outwards, suggesting a quadrupedal trackmaker with an unpronated manus. Only the base of the thumb is impressed, suggesting that the enlarged thumb claw was held clear off the ground. The tracks vary in size but reach up to 15 mm in length, larger than the estimated 7 mm of a freshly hatched individual. Reisz and colleagues therefore speculated that ''Massospondylus'' hatchlings remained at the nest sites at least until they had doubled in size.

In 2019, James Neenan and colleagues tested the postural shift hypothesis by analysing the bony labyrinth of the inner ear based on X-ray microtomography, computed microtomography scans of eight skulls covering the size spectrum from embryo to adult. The labyrinth houses the sense of balance, and one of its three canals (the lateral semicircular canal) is roughly horizontal when the head is in an alert posture in modern animals. Since Neenan and colleagues did not observe a change in orientation of this canal during growth, no support for a postural shift was found. In a 2020 study, Chapelle and colleagues also presented evidence against the postural shift hypothesis. These authors argued that limb length ratios cannot reliably predict posture, and instead proposed an alternative method based on the minimum circumferences of the humeri and femora. Although the analysed ''Massospondylus'' embryo was found to be ambiguous, the smallest hatchling was found to be bipedal. This hatchling would have been the size of a newly hatched individual, suggesting that ''Massospondylus'' was bipedal at all ages. ''Mussaurus'', in contrast, was confirmed to show a postural shift from juvenile to adult.

Growth

Growth in dinosaurs can be reconstructed using thin sections of bones that reveal growth rings, similar to the Dendrochronology, growth rings in trees. The first such study for ''Massospondylus'' was conducted by Anusuya Chinsamy-Turan in 1993, who, based on femora of different sizes, found that growth was cyclically, with one growth ring formed each year. Because an external fundamental system (tightly packed growth rings that indicate that growth had stopped) was absent, she suggested that ''Massospondylus'' had indeterminate growth (growth throughout life). This suggests that the Dinosaur physiology, physiology of ''Massospondylus'' was intermediate between ectothermy and endothermy. In a 2001 study, Gregory M. Erickson and colleagues indicated that ''Massospondylus'' grew at a maximum rate of 34.6 kg per year and was still growing at around 15 Age determination of dinosaurs, years of age.

A 2005 study by Martin Sander and Nicole Klein indicated that the related ''Plateosaurus'' adjusted its growth according to environmental conditions: when food was plentiful or when the climate was favorable, ''Plateosaurus'' exhibited accelerated growth. Consequently, body size is only weakly correlated with age. This pattern of growth is called "developmental plasticity", and is common in ectothermic but uncommon in endothermic animals. According to Sander and Klein, developmental plasticity is absent ''Massospondylus'', which they found to grow along a specific growth trajectory with little variation in the growth rate and ultimate size of an individual. In 2022, Chapelle and colleagues analysed multiple different bones from 27 ''Massospondylus'' specimens and instead found substantial growth variation as in ''Plateosaurus'', suggesting that developmental plasticity was widespread in early sauropodomorphs. These authors also argue that ''Massospondylus'' had determinate growth, contradicting the 1993 study of Chinsamy that suggested that growth was indeterminate. Growth had stopped in the oldest analysed specimen, which was estimated at 20 years old.

Paleoecology

Distribution and abundance

Most ''Massospondylus'' remains have been found in the upper Elliot Formation and the Clarens Formation of South Africa and Lesotho, which are part of the Stormberg Group within the Karoo Supergroup. These Geological formation, formations were deposited during the Hettangian, Sinemurian, and Pliensbachian ages of the Early Jurassic, ca. 200–183 million years ago. ''Massospondylus'' has also been found in the Forest Sandstone and the Mpandi Formation of Zimbabwe, which are thought to be contemporaneous with the upper Elliot and Clarens formations.

''Massospondylus'' is known from numerous almost complete skeletons, including at least 13 reasonably complete skulls and thousands of isolated bones and fragments. This wealth of material makes ''Massospondylus'' one of the best-known sauropodomorphs from the Early Jurassic. Because of its great abundance, it is used as the defining taxon of a biozone, the ''Massospondylus'' Assemblage Zone, which is the uppermost such zone in the Karoo Supergroup. The presence of its fossils is used as evidence of Biostratigraphy, determining the relative ages of sedimentary rocks.

Paleoenvironment

During the deposition of the Stormberg Group, the climate became gradually dryer. The oldest part of the group (the Molteno Formation) consists of sandstones that were Fluvial sediment processes, deposited by rivers and show evidence of heavy rainfall. As the Elliot Formation was deposited, the climate became Semi-arid climate, semi-arid and the environment was characterised by short-lived rivers and lakes that deposited sandstones and mudstones. During the later half of the Elliot Formation (the upper Elliot Formation), Aeolian processes, aeolian (wind) erosion and sedimentation created depressions in which playa lakes formed. The overlying Clarens Formation consists mostly of aeolian sandstones deposited by dunes in a desert environment.

The faunas and floras of the Early Jurassic were similar worldwide, with Pinophyta, conifers adapted for hot weather becoming the common plants; basal sauropodomorphs and theropods were the main constituents of a worldwide dinosaur fauna. African ''Massospondylus'' was a contemporary of temnospondyli; turtles; a sphenodontia, sphenodont; rauisuchids; early crocodylomorpha, crocodylomorphs; tritylodontidae, tritylodontid and trithelodontidae, trithelodontid therapsids; and morganucodontid mammals. Predatory dinosaurs included the small theropod ''Megapnosaurus'' as well as the 6-metre-long theropod ''Dracovenator'', which might have preyed on the sauropodomorphs. Several genera of early ornithischians are known, such as ''Lesothosaurus'' and the heterodontosauridae, heterodontosaurids ''Abrictosaurus'', '' Heterodontosaurus'', ''Lycorhinus'' and ''Pegomastax''. Until recently, ''Massospondylus carinatus'' was regarded as the only known sauropodomorph from the upper Elliot and Clarens formations. However, newer finds revealed a diverse contemporary sauropodomorph fauna with at least six additional species from the upper Elliot Formation, including ''Aardonyx celestae'', ''Antetonitrus ingenipes'', ''Ignavusaurus rachelis'', ''Arcusaurus pereirabdalorum'', ''Pulanesaura eocollum'', as well as a second species of ''Massospondylus'', ''M. kaalae''. In 2023, Chapelle and colleagues concluded that a humerus from the upper Elliot Formation, which had been provisionally assigned to ''Massospondylus'', belonged to a new sauropodomorph species that was unusually small.

Notes

References

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Massospondylidae
Dinosaur genera
Early Jurassic dinosaurs
Elliot Formation
Dinosaurs of Africa
Fossil taxa described in 1854
Taxa named by Richard Owen
Taxa with lost type specimens