Massospondylus
Massospondylus | |
---|---|
Reconstructed skeleton on display in the Royal Ontario Museum | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Dinosauria |
Clade: | Saurischia |
Clade: | †Sauropodomorpha |
Clade: | † Bagualosauria
|
Clade: | †Plateosauria |
Clade: | †Massopoda |
Family: | †Massospondylidae |
Genus: | †Massospondylus Owen, 1854 |
Species | |
Synonyms | |
List
|
Massospondylus (
The type species is M. carinatus; seven other species have been named during the past 150 years, but only M. kaalae is still considered valid. Early sauropodomorph systematics have undergone numerous revisions during the last several years, and many scientists disagree where exactly Massospondylus lies on the dinosaur evolutionary tree. The family name Massospondylidae was once coined for the genus, but because knowledge of an early sauropod relationship is in a state of flux, it is unclear which other dinosaurs—if any—belong in a natural grouping of massospondylids; several 2007 papers support the family's validity.
Although Massospondylus was long depicted as
History of discovery
The first fossils of Massospondylus were described by
Massospondylus remains have been found in the Upper Elliot Formation, the Clarens Formation, and the Bushveld Sandstone of South Africa and Lesotho, as well as the Forest Sandstone of Zimbabwe. These remains consist of at least 80 partial skeletons and four skulls, representing both juveniles and adults.[6] The report of Massospondylus from Arizona's Kayenta Formation is based on a skull described in 1985. The skull of the Kayenta specimen from Arizona is 25% larger than the largest skull from any African specimen.[7] The Kayenta specimen possesses four teeth in the premaxilla and sixteen in the maxilla. Uniquely among dinosaurs, it also had tiny, one-millimetre-(0.04 in-) long palatal teeth.[8] A 2004 restudy of African Massospondylus skulls, however, indicated that the Kayenta specimen did not pertain to Massospondylus.[9] This Kayenta skull and associated postcranial elements, identified collectively as MCZ 8893, was referred to the new genus Sarahsaurus in 2010.[10]
Massospondylus had also been reported from
Species
Many species have been named, although most are no longer considered valid. M. carinatus, named by Richard Owen, is the
M. browni, M. harriesi, and M. schwarzi were all found in the Upper Elliot Formation of
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]
Dubious names
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]
Description
Massospondylus was a mid-size sauropodomorph, around 4 metres (13 ft) in length,[24][25][26] and weighed approximately 1000 kilograms (2200 lb),[26] although a few sources have estimated its length at up to 6 metres (20 ft).[27][28] It was a typical early sauropodomorph, with a slender body, a long neck and a proportionally very small head.[6]
The vertebral column was composed of nine cervical (neck) vertebrae, 13 dorsal (back) vertebrae, three sacral (hip) vertebrae, and at least 40 caudal (tail) vertebrae. The
Cranial anatomy
The small head of Massospondylus was approximately half the length of the
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]
Classification
Basal sauropodomorph systematics continue to undergo revision, and many genera once considered classic "prosauropods" have recently been removed from the group in
Massospondylus is the
The following
The following cladogram shows the position of Massospondylus within Massopoda, according to Oliver W. M. Rauhut and colleagues, 2020:[44]
Massopoda |
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Paleobiology
As with all dinosaurs, much of the biology of Massospondylus, including its behavior, coloration, and physiology, remains unknown. However, recent studies have allowed for informed speculation on subjects such as growth patterns,[45] diet,[46] posture,[30] reproduction,[47] and respiration.[48]
A 2007 study suggested that Massospondylus may have used its short arms for defense against 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, to strip plant material from trees,[30] digging, or for grooming.[7]
Growth
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.
Diet
Early sauropodomorphs such as Massospondylus may have been
Gait and range of motion
Although long assumed to have been
Since the discovery of rudimentary and nonfunctional
Michael Cooper (1981) noted that the
Reproduction
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]
Newly hatched juveniles are known from a second sauropodomorph,
Respiratory system
Many
Paleoecology
The
It is not clear which carnivores may have preyed on Massospondylus. Most of the theropods that have been discovered in rocks of Early Jurassic age in southern Africa, such as Coelophysis, were smaller than mid-sized sauropodomorphs like Massospondylus. These smaller predators have been postulated as using fast slashing attacks to wear down sauropodomorphs, which could have defended themselves with their large hand and foot claws.[63] The 6-metre-(20 ft-) long[68] carnivorous theropod Dracovenator lived during the same period (Hettangian to Sinemurian stages) as Massospondylus and has also been found in the Elliot Formation of South Africa.[69]
References
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- ^ a b c Yates, Adam M.; Paul M. Barrett (2010). "Massospondylus carinatus Owen 1854 (Dinosauria: Sauropodomorpha) from the Lower Jurassic of South Africa: Proposed conservation of usage by designation of a neotype". Palaeontologia Africana. 45: 7–10.
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- ^ a b c d e f g Gow, Christoper E.; J. W. Kitching; Michael K. Raath (1990). "Skulls of the prosauropod dinosaur Massospondylus carinatus Owen in the collections of the Bernard Price Institute for Palaeontological Research". Palaeontologia Africana. 27: 45–58.
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Further reading
- Chinsamy, A. (1992). "Ontogenetic growth of the dinosaurs Massospondylus carinatus and Syntarsus rhodesiensis". In: Abstracts of papers. Society of Vertebrate Paleontology, fifty-second annual meeting. Royal Ontario Museum Toronto, Ontario". Journal of Vertebrate Paleontology. 12 (3): 23A. .
- Gow, C. E. (1990). "Morphology and growth of the Massospondylus braincase (Dinosauria, Prosauropoda)". Palaeontologia Africana. 27: 59–75.
- Hinic, S. (2002). "The cranial anatomy of Massospondylus carinatus Owen, 1854 and its implications for prosauropod phylogeny". Journal of Vertebrate Paleontology. Abstracts of papers. Society of Vertebrate Paleontology, 22, Supplement to number 3, 65A.
- Martínez, R. (1999). "The first South American record of Massospondylus (Dinosauria: Sauropodomorpha)". Journal of Vertebrate Paleontology, Abstracts of papers. Society of Vertebrate Paleontology, 20–23 October, 19, Suppl. 3, 61A.
- Martínez, R.N. (1999). "Massospondylus (Dinosauria: Sauropodomorpha) in northwestern Argentina". Abstracts VII International Symposium on Mesozoic Terrestrial Ecosystems, Buenos Aires, 40.
External links
- Locomotion and Ontogeny of Massospondylus carinatus from the University of Toronto at Missisauga.
- Massospondylidae from Palaeos.com (technical).
- Massospondylus from PrimeOrigins.co.za (for children).
- "Oldest known dinosaur embryos identified" University of the Witwatersrand.
- "Rare fossil embryos reveal growth". Archived from the original on May 3, 2007. Retrieved October 26, 2007. National Geographic.