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Categories: Dinosaurs
Massospondylus (from Greek, meaning "elongated vertebra"), is a genus of prosauropod dinosaur from the early Jurassic Period and possibly the Late Triassic (?Carnian to Sinemurian ages, around 220 to 190 million years ago). It was probably a plant eater (herbivore), although it is speculated that the prosauropods may have been omnivorous. Massospondylus was described by Sir Richard Owen in 1854 from remains found in South Africa. It is thus one of the first dinosaurs to have been named. Fossils have since been found in Lesotho, Zimbabwe, and other parts of South Africa. Further material from Arizona's Kayenta Formation, India, and Argentina has been ascribed to this genus, but is dubious. The type, and only universally recognized species, is M. carinatus, although many other species have been named in the past 150 years. The family name Massospondylidae was once erected for the genus, but because of the spotty fossil record in strata from the Late Triassic and Early Jurassic, it is not clear which other dinosaurs, if any, belong in a natural grouping of massospondylids. Long depicted as quadrupedal, a recent study indicates Massospondylus was a biped. The animal had a long neck and tail with a small head and slender body. It bore a sharp thumb claw on each of its forefeet, used in defense or feeding. Recent studies indicate Massospondylus grew steadily, possessed air sacs similar to those of birds, and may have cared for its young. Massospondylus was a mid-sized prosauropod at around 4 meters (13 ft) long and weighed around 135 kilograms (300 lbs).[1] Although long assumed to have been quadrupedal, a recent anatomical study of the forelimbs has questioned this, arguing that their range and motion precludes effective habitual quadrupedal gait.[2] The animal was a typical prosauropod in most other respects. It possessed a slender body and long neck; it had a slighter build than that of Plateosaurus.[3] Like Plateosaurus, however, it had five digits on each foot, with a large thumb claw used for feeding or defense against predators. The fourth and fifth digits of the forepaws were tiny, giving the forepaws a lopsided look. The 2007 study indicated that Massospondylus held its manus (hands) in a semi-supinated ("prayer-like") orientation, with the palmar surfaces facing one another; the wrist was never found rotated in articulated (still-connected) fossil specimens. It may have used its short arms in defense from predators ("defensive swats"), in intraspecies combat, or in feeding, although its arms were too short to reach its mouth. Scientists speculate that Massospondylus could have used its large pollex (thumb) claw in combat or to strip plant material from trees. [2] The small head of Massospondylus was approximately half the length of the femur. The skull bore two almost circular eye sockets. Based on the presence of holes for blood vessels near the coronoid eminence, it has been proposed that Massospondylus had cheeks. The cheeks would have prevented food from spilling out when Massospondylus ate. The jaw of this dinosaur was slightly more primitive than that of Plateosaurus. Teeth in the lower jaw were not as long as those in the upper jaw. There also seems to be some variation of tooth morphology, based upon the tooth's position in the jaw, although this is not as pronounced as the specialization of teeth in Heterodontosaurus. Teeth occupying a position closer to the front of the snout were rounder than the more oval-shaped back teeth. Massospondylus is occasionally depicted with a beak, but this is not consistent with presence of teeth in the front of the lower jaw. Interestingly, the skull of the Arizona specimen is 25% larger than the largest skull from any African specimen. The first skeleton of Massospondylus was described by paleontologist Sir Richard Owen in 1854;[4] the name derived from the Greek term sphondylos/σφονδυλος 'vertebra'.[5] Massospondylus remains have been found in the Upper Elliot Formation, the Clarens Formation, and the Bushveld Sandstone of South Africa and Lesotho; the Forest Sandstone and the Upper Karroo Sandstone of Zimbabwe; and the Kayenta Formation of Arizona. These remains consist of at least 80 partial skeletons and four skulls, representing both juveniles and adults.[3] Many species have been named, although most are no longer considered valid. M. carinatus, named by Richard Owen, is the type species.[6] Other named species include: M. browni (Seeley, 1895),[7] M. harriesi (Broom 1911),[8] M. hislopi (Lydekker, 1890),[9] M. huenei (Cooper, 1981),[10] M. rawesi (Lydekker, 1890),[9] and M. schwartzi (Haughton, 1924).[11] A number of dinosaurs are often considered synonymous with Massospondylus. These include Aristosaurus, Dromicosaurus, Gryponyx, Hortalotarsus, and Pachyspondylus, which are dubious names of little scientific value.[10] Under the rules of zoological nomenclature, these names are junior synonyms. They were named after Massospondylus was described in a scientific paper; thus the name Massospondylus takes priority. Massospondylus is a prosauropod, a grouping of early saurischian dinosaurs which lived during the Triassic and Jurassic, but which had died out by the end of the Jurassic. Other members of the group include Plateosaurus,[3] Yunnanosaurus,[3] and Riojasaurus.[12] Basal sauropodomorph systematics continue to undergo revision, and many genera once considered classic prosauropods have recently been removed from the group in phylogenetic nomenclature, on the grounds that their inculsion would not constitute a clade (a natural grouping containing all descendants of a single common ancestor). Exactly which animals constitute a monophyletic (natural grouping) of prosauropods is unclear. Galton and Upchurch (2004) included Ammosaurus, Anchisaurus, Azendohsaurus, Camelotia, Coloradisaurus, Euskelosaurus, Jingshanosaurus, Lessemsaurus, Lufengosaurus, Massospondylus, Melanorosaurus, Mussaurus, Plateosaurus, Riojasaurus, Ruehleia, Saturnalia, Sellosaurus, Thecodontosaurus, Yimenosaurus and Yunnanosaurus in a monophyletic Prosauropoda.[3] Wilson (2005) considered Massospondylus, Jingshanosaurus, Plateosaurus, and Lufengosaurus a natural group.[13] Massospondylus is the type genus of the proposed family Massospondylidae, to which it gives its name. The Massospondylidae family may also include Yunnanosaurus[14] although Lu et al. (2007) placed Yunnanosaurus in its own family.[15] As with all dinosaurs, much of the biology of Massospondylus, including its behavior, coloration, ecology, and physiology, remains unknown. However, recent studies have allowed for some informed speculation on subjects such as growth patterns,[16] diet,[17] posture,[2] reproduction,[18] and respiration.[19] 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 not seen in other dinosaurs, including Massospondylus, despite the close relationship between Plateosaurus and Massospondylus. The study indicated that Massospondylus grew along a specific growth trajectory with little variation in the growth rate and ultimate size of an individual.[16] Prosauropods such as Massospondylus may have been either herbivorous or possibly omnivorous. Most recent studies favor a herbivorous lifestyle for these animals. Gastroliths (gizzard stones) have been found in association with Massospondylus fossils,[17] indicating Massospondylus swallowed stones to aid in digestion. In the 1970s, six 190-million year old Massospondylus eggs were found, in Golden Gate Highlands National Park in South Africa by James Kitching, who identified them as belonging most likely to Massospondylus. It took 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. Notably, the near-hatchlings had no teeth, suggesting they had no way of feeding themselves. Scientists speculate that after-birth care might have been necessary based on the body proportions and the lack of teeth. The four legs of the near-hatchlings were of equal length, indicating that newly-hatched Massospondylus were quadrupedal. The skull and eye were proportionately over-sized. This suggests a different pattern of development for dinosaurs from that which had previously been thought.[18] Many saurischian dinosaurs possessed vertebrae and ribs which 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 avians, 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. This complex method of respiration is very efficient.[20] Prosauropods were unusual as the only major group of saurischians which lacked an extensive system of pneumatic foramina. Although possible pneumatic indentations have been found in Plateosaurus and Thecodontosaurus, these hollows were very small. One study in 2007 concluded that it was likely that prosauropods like Massospondylus had abdominal and cervical air sacs because of the good evidence for them in sister taxa (theropods and sauropods). The study concluded that it was not possible to determine whether prosauropods had a bird-like flow-through lung, but that the air sacs were almost certainly present.[19]
Contents
[edit] Description
[edit] Discovery and species
[edit] Classification
[edit] Paleobiology
[edit] Growth
[edit] Diet
[edit] Reproduction
[edit] Respiratory system
[edit] See also
[edit] References
[edit] External links
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