Massospondylus

Fossils identified by Broom in 1911 as Gryponyx, M. carinatus, M. harriesi, and Aetonyx, all now thought to be M. carinatus

Many species have been named, although most are no longer considered valid. M. carinatus, named by Richard Owen, is the type species.[13] Other named species include: M. browni Seeley, 1895,[14] M. harriesi Broom 1911,[4] M. hislopi Lydekker, 1890,[15] M. huenei Cooper, 1981,[2] M. kaalae Barrett 2009,[16] M. rawesi Lydekker, 1890,[15] and M. schwarzi Haughton, 1924.[17]

M. browni, M. harriesi, and M. schwarzi were all found in the Upper Elliot Formation of Cape Province, South Africa. All three are based on fragmentary material, and were regarded as indeterminate in the most recent review.[6] M. browni is based on two cervical, two back, and three caudal vertebrae and miscellaneous hind limb elements.[14] M. harriesi is known from a well-preserved forelimb and parts of a hindlimb.[4] M. schwarzi is known from an incomplete hind limb and sacrum.[17] M. hislopi and M. rawesi were named from fossils found in India.[15] M. hislopi is based on vertebrae from the Upper Triassic Maleri Formation of Andhra Pradesh, whereas M. rawesi is based on a tooth from the Upper Cretaceous Takli Formation of Maharashtra.[18] M. hislopi was tentatively retained as an indeterminate sauropodomorph in the latest review,[6] but M. rawesi may be a theropod[19] or nondinosaur.[5] M. huenei is a combination derived by Cooper for Lufengosaurus huenei, as he considered Lufengosaurus and Massospondylus to be synonyms.[2] This synonymy is no longer accepted.[6]

M. kaalae was described in 2009 on the basis of a partial skull from the Upper Elliot Formation in Eastern Cape of South Africa. The species is known from the same time and region as some specimens of M. carinatus. It differs from the type species in the morphology of the braincase, as well as in several other characters of the skull such as the proportions of the premaxilla.[16]

Several genera have been previously synonymized with Massospondylus. These include the above-mentioned Leptospondylus and Pachyspondylus, as well as Aristosaurus, Dromicosaurus, Gryponyx taylori and Hortalotarsus, which are dubious names of little scientific value.[2] Together with Massospondylus carinatus, Owen named Leptospondylus capensis and Pachyspondylus orpenii for vertebrae from the same unit and location. Aristosaurus erectus was named by E.C.N. van Hoepen in 1920 based on a nearly complete skeleton, and Hoepen also named Dromicosaurus gracilis, which consisted of a partial skeleton. Gryponyx taylori was named by Sidney H. Haughton in 1924. It consists of hip bones. All of the above fossils come from the Hettangian or Sinemurian faunal stages of South Africa, where Massospondylus has been found.[20] Under the rules of zoological nomenclature, these names are junior synonyms. Massospondylus was described in a scientific paper; therefore the name Massospondylus takes priority.

Ignavusaurus, known from a young specimen, was considered by Yates et al. (2011) to be a probable synonym of Massospondylus.[21] Cladistic analyses by Chapelle & Choiniere (2018) and Chapelle et al. (2019) agreed with Yates et al. (2011) in demonstrating the massospondylid nature of Ignavusaurus, but nevertheless recovered it as a distinct taxon of massospondylid.[22][23]

The small head of Massospondylus was approximately half the length of the femur. Numerous openings, or fenestrae, in the skull reduced its weight and provided space for muscle attachment and sensory organs. These fenestrae were present in pairs, one on each side of the skull. At the front of the skull were two large, elliptical nares, which were roughly half the size of the orbits. The orbits were proportionally larger in Massospondylus than in related genera, such as Plateosaurus. The antorbital fenestrae, smaller than those seen in Plateosaurus, were situated between the eyes and the nose. At the rear of the skull were two more pairs of temporal fenestrae: the lateral temporal fenestrae immediately behind the eye sockets, which were shaped like an inverted "T" in Massospondylus,[9] and the supratemporal fenestrae on top of the skull. Small fenestrae also penetrated each mandible.[6] The shape of the skull is traditionally restored as wider and shorter than that of Plateosaurus, but this appearance may be due just to differential crushing experienced by the various specimens.[6] Some features of the skull are variable between individuals; for example, the thickness of the upper border of the orbit and the height of the posterior maxilla. These differences may be due to sexual dimorphism[31] or individual variation.[32]

Diagram of the skull of Massospondylus, showing the various skull openings.

Tooth count is variable between individuals and increases with skull size.[31] The premaxilla shows the constant number of four teeth per side in all known skulls; however, in the maxilla, the tooth count ranges from 14 to 22. There are 26 teeth in each side of the lower jaw in the largest known skull.[9][31] The height of the teeth crowns decreases from front to back in the upper jaw, but was more or less constant in the lower jaw.[9][31] The lack of pronounced tooth wear and the variable height of the crowns suggests that the teeth were replaced by succeeding new ones in relatively short time intervals.[31] Notably, there was variation in the tooth morphology based on the position of the teeth in the jaw.[31] The heterodonty present in Massospondylus is greater than that present in Plateosaurus, although not as pronounced as the specialization of teeth in Heterodontosaurus.[7] Teeth closer to the front of the snout had round cross-sections and tapered to points, unlike the back teeth, which were spatulate and had oval cross-sections.[6][7]

As with other early sauropodomorphs, it has been proposed that Massospondylus had cheeks. This theory was proposed because there are a few 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.[6] Crompton and Attridge (1986) 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 and account for tooth wear on the teeth at the tip of the snout.[33] However, the difference in length may be a misinterpretation based on crushing in a top–bottom plane, and the possession of a beak is considered unlikely in recent studies.[9][31][34]

Skulls casts of adult and juvenile M. carinatus

A 2005 study indicated that Massospondylus' sister taxon, Plateosaurus, exhibited growth patterns affected by environmental factors. The study indicated that, when food was plentiful or when the climate was favorable, Plateosaurus exhibited accelerated growth. This pattern of growth is called "developmental plasticity". It is unseen in other dinosaurs, including Massospondylus, despite the close relationship between the two. The study indicated that Massospondylus grew along a specific growth trajectory, with little variation in the growth rate and ultimate size of an individual.[45] Another study of age determination indicated that Massospondylus grew at a maximum rate of 34.6 kg (76.3 lb) per year and was still growing at around 15 years of age.[49]

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".[2][8] However, the hypothesis of carnivorous "prosauropods" has been discredited, and all recent studies favor a herbivorous or omnivorous lifestyle for these animals. Galton and Upchurch (2004) 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.[6] Barrett (2000) proposed that basal sauropodomorphs supplemented their herbivorous diets with small prey or carrion.[50] Gastroliths (gizzard stones) have been found in association with three Massospondylus fossils from the Forest-Sandstone in Zimbabwe,[51] and with a Massospondylus-like animal from the Late Triassic of Virginia.[46] Until recently, scientists believed that these stones functioned as a gastric mill to aid ingestion of plant material, compensating for its inability to chew, as it is the case in many modern birds. However, Wings and Sander (2007) showed that the polished nature and the abundance of those stones precluded a use as an effective gastric mill in most non-theropod dinosaurs, including Massospondylus.[29][52]

Mounted Massospondylus skeleton cast at the Natural History Museum, London, showing an outdated quadrupedal pose[53]

Although long assumed to have been quadrupedal, a 2007 anatomical study of the forelimbs has questioned this, arguing that their limited range of motion precluded effective habitual quadrupedal gait. Neither could the forelimbs swing fore and behind in a fashion similar to the hindlimbs, nor could the hand be rotated with the palmar surfaces facing downwards. This inability to pronate the hand is also supported by in-situ finds of articulated (still-connected) arms that always show unrotated hands with palmar faces facing each other. The study also ruled out the possibility of "knuckle-walking" and other forms of locomotion that would make an effective locomotion possible without the need to pronate the hand. Although its mass suggests a quadrupedal nature, Massospondylus would have been restricted to its hind legs for locomotion.[30]

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.[54] Robert Bakker (1987) suggested that this would have allowed the shoulder blades to swing with the forelimbs in quadrupedal dinosaurs, increasing their functional forelimb length.[55] 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.[54]

Michael Cooper (1981) 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".[2] 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.[34]

Cast of eggs and an embryo, Royal Ontario Museum

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.[56] It was nearly 30 years before extraction was started on the fossils of the 15-centimetre- (6 in-) long embryos. They remain the oldest dinosaur embryos ever found.[47] 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.[57] Sedimentary structures indicate that the nesting area was in the vicinity of a lake.[57] The eggshells were very thin (about 0.1 mm), allowing 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.[57] There are no hints that Massospondylus constructed nests; however, the arrangement of the eggs in tight rows indicates that the eggs were pushed in this position by the adults.[57]

The embryos probably represented near-hatchlings.[56] 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 39% of the entire skull length. The neck was short, contrasting to the very long neck in the adults. Girdle bones and caudals were relatively tiny.[47] The forelimbs were of equal length to the hindlimbs, indicating that newly hatched Massospondylus were quadrupedal, unlike the bipedal adults.[47] However, the reliability of these results has been questioned.[58] The discovery of hatching footprints with manus impressions confirmed their quadrupedality. These impressions show that the hand was not pronated, with palm faces facing each other and the thumb facing forwards.[57] The unpronated manus and the big head indicate that an effective locomotion was not possible for newly hatched Massospondylus.[30] Notably, the near-hatchings had no teeth, suggesting they had no way of feeding themselves.[47] Based on the lack of teeth and the ineffective locomotion, scientists speculate that postnatal care might have been necessary.[30][47] This is further supported by evidence that the hatchings remained at the nest sites until they had doubled in size.[57]

Restoration of a juvenile M. carinatus, shown here as a quadruped

Newly hatched juveniles 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.[30][56] 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.[47] This discovery therefore "sheds some light in the evolutionary pathways through which the peculiar adaptations of giant dinosaurs were attained", stated French paleontologist Eric Buffetaut.[59] However, a recent study, which evaluated locomotion in extinct taxa based on limb robusticity, found that previous conclusions that Massospondylus transitioned from quadrupedality to bipedality through ontogeny were based on unreliable allometric comparisons between limb lengths, but a model based on the circumferences of the humerus and femur supported bipedality throughout ontogeny.[58]

Many saurischian dinosaurs possessed vertebrae and ribs that contained hollowed-out cavities (pneumatic foramina), which reduced the weight of the bones and may have served as a basic 'flow-through ventilation' system similar to that of modern birds. In such a system, the neck vertebrae and ribs are hollowed out by the cervical air sac; the upper back vertebrae, by the lung; and the lower back and sacral (hip) vertebrae, by the abdominal air sac. These organs constitute a complex and very efficient method of respiration.[60] "Prosauropods" are the only major group of saurischians without an extensive system of pneumatic foramina. Although possible pneumatic indentations have been found in Plateosaurus and Thecodontosaurus, the indentations were very small. One study in 2007 concluded that basal sauropodomorphs like Massospondylus likely had abdominal and cervical air sacs, based on the evidence for them in sister taxa (theropods and sauropods). The study concluded that it was impossible to determine whether basal sauropodomorphs had a bird-like flow-through lung, but that the air sacs were almost certainly present.[48]