Mussaurus

Mussaurus (meaning "mouse lizard") is a genus of herbivorous sauropodomorph dinosaur that lived in southern Argentina during the Late Triassic, about 215 million years ago. It receives its name from the small size of the skeletons of juvenile and infant individuals, which were once the only known specimens of the genus. However, since Mussaurus is now known from adult specimens,[1] the name is something of a misnomer; adults possibly reached 6 metres (20 ft) in length and weighed more than 1,000 kilograms (2,200 lb).[2] Mussaurus possesses anatomical features suggesting a close, possibly transitional evolutionary relationship with true sauropods.[3]

Mussaurus
Temporal range: Late Triassic, 215–203 Ma
Mussaurus patagonikus DSC 2904
Fossil juvenile skeleton
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Order: Saurischia
Suborder: Sauropodomorpha
Family: Mussauridae
Bonaparte & Vince, 1979
Genus: Mussaurus
Bonaparte & Vince, 1979
Type species
Mussaurus patagonicus
Bonaparte & Vince, 1979

Discovery

Infant and juvenile fossils of Mussaurus were first discovered by an expedition led by Jose Bonaparte during the 1970s to the El Tranquilo Formation.[3] There the team found fossilized eggs and hatchlings, which added insight into the reproductive strategies of Mussaurus and other sauropodomorph dinosaurs.[3] The first adult specimens of Mussaurus were described in 2013. Some of these specimens had first been described in 1980 and were originally attributed to the genus Plateosaurus.[1]

Phylogeny

Previous to the discovery of adult specimens of Mussaurus, the phylogenetic position of this taxon was difficult to establish. Infant and juvenile fossils are known to show more basal traits than adult specimens of the same taxon. Furthermore, the recently discovered one subadult and three adult specimens assigned to Mussaurus are more complete than other material assigned to it. Therefore, a cladistic analysis of basal sauropodomorphs performed by Otero and Pol (2013) to test the phylogenetic relationships of Mussaurus, included information only from adult specimens. The following cladogram is simplified after their analysis (relationships outside Plateosauria are not shown).[1]

 Plateosauria 

Ruehleia

 Plateosauridae 

Unaysaurus

Plateosaurus engelhardti

Plateosaurus gracilis

Plateosaurus ingens

 Massopoda 
 Riojasauridae 

Eucnemesaurus

Riojasaurus

Ignavusaurus

Sarahsaurus

 Massospondylidae 

Coloradisaurus

Glacialisaurus

Lufengosaurus

Massospondylus

Adeopapposaurus

Leyesaurus

 Sauropodiformes 

Jingshanosaurus

 Anchisauria 

Anchisaurus

Seitaad

Yunnanosaurus

Mussaurus

Aardonyx

Leonerasaurus

 Melanorosauridae 

Camelotia

Melanorosaurus

 Sauropoda 

Blikanasaurus

Antetonitrus

Lessemsaurus

Gongxianosaurus

Isanosaurus

 Gravisauria 
 Vulcanodontidae 

Tazoudasaurus

Vulcanodon

 Eusauropoda 

Barapasaurus

Patagosaurus

Shunosaurus

Omeisaurus

Mamenchisaurus

Cetiosaurus

Neosauropoda

Reproduction

Mussaurus patagonicus
Life restoration of an infant eating a Dicroidium fern

Mussaurus specimens have been found in association with nests that are believed to contain multiple eggs apiece. The skeletons of Mussaurus infants were small, only six inches long excluding the tail, but a total of 20 to 37 centimetres (7.9 to 14.6 in) long. This is about the size of a small lizard. Juveniles differed from adults in proportion in addition to size and mass. As is common for dinosaurs, juvenile Mussaurus had tall skulls with short snouts and large eyes.[3]

These proportions are common in many infant vertebrates and are often associated with species that provide parental care during the vulnerable early stages of life. Adults are expected to have longer snouts and necks, as typical in early sauropodomorphs.

Locomotion

A study published in May 2019 shows that while M. patagonicus probably walked on all four limbs (was quadrupedal) during the first year of its life, changes in the relative proportions of its body during growth (ontogeny) caused its centre of mass to move backwards towards its pelvis, resulting in the animal adopting a two-legged (bipedal) stance later in life.[2][4]

References

  1. ^ a b c Otero, A.; Pol, D. (2013). "Postcranial anatomy and phylogenetic relationships of Mussaurus patagonicus (Dinosauria, Sauropodomorpha)". Journal of Vertebrate Paleontology. 33 (5): 1138. doi:10.1080/02724634.2013.769444.
  2. ^ a b Otero, Alejandro; Cuff, Andrew R.; Allen, Vivian; Sumner-Rooney, Lauren; Pol, Diego; Hutchinson, John R. (2019-05-20). "Ontogenetic changes in the body plan of the sauropodomorph dinosaur Mussaurus patagonicus reveal shifts of locomotor stance during growth". Scientific Reports. Springer Science and Business Media LLC. 9 (1). doi:10.1038/s41598-019-44037-1. ISSN 2045-2322.
  3. ^ a b c d "Mussaurus." In: Dodson, Peter & Britt, Brooks & Carpenter, Kenneth & Forster, Catherine A. & Gillette, David D. & Norell, Mark A. & Olshevsky, George & Parrish, J. Michael & Weishampel, David B. The Age of Dinosaurs. Publications International, LTD. p. 40. ISBN 0-7853-0443-6.
  4. ^ Geggel, Laura (2019-05-28). "No, That Baby Dinosaur Didn't Crawl. But It Did Walk on 4 Legs As an Infant". Live Science. Retrieved 2019-05-28.

Further reading

  • J. F. Bonaparte and M. Vince. 1979. El hallazgo del primer nido de dinosaurios triasicos, (Saurischia, Prosauropoda), Triasico Superior de Patagonia, Argentina [The discovery of the first nest of Triassic dinosaurs (Saurischia, Prosauropoda,) from the Upper Triassic of Patagonia, Argentina]. Ameghiniana 16(1-2):173-182
  • D. Pol and J. E. Powell. 2007. Skull anatomy of Mussaurus patagonicus (Dinosauria: Sauropodomorpha) from the Late Triassic of Patagonia. Historical Biology 19(1):125-144
Anchisauria

The Anchisauria were a clade of sauropodomorph dinosaurs that lived during the Late Triassic and Early Jurassic. The name Anchisauria was first used by Galton and Upchurch in the second edition of The Dinosauria. Galton and Upchurch assigned two families of dinosaurs to the Anchisauria: the Anchisauridae and the Melanorosauridae. The more common prosauropods Plateosaurus and Massospondylus were placed in the sister clade Plateosauria.

However, recent research indicates that Anchisaurus is closer to sauropods than traditional prosauropods; thus, Anchisauria would also include Sauropoda.The following cladogram simplified after an analysis presented by Blair McPhee and colleagues in 2014.

Averostra

Averostra, or "bird snouts", is a clade that includes most theropod dinosaurs that have a promaxillary fenestra (fenestra promaxillaris), an extra opening in the front outer side of the maxilla, the bone that makes up the upper jaw. Two groups of averostrans, the Ceratosauria and the Orionides, survived into the Cretaceous period. When the Cretaceous–Paleogene extinction event occurred, ceratosaurians and two groups of orionideans within the clade Coelurosauria, the Tyrannosauroidea and Maniraptoriformes, were still extant. Only one subgroup of maniraptoriformes, Aves, survived the extinction event and persisted to the present day.

Avetheropoda

Avetheropoda, or "bird theropods", is a clade that includes carnosaurians and coelurosaurs to the exclusion of other dinosaurs.

Cerapoda

Cerapoda ("ceratopsians and ornithopods") is a clade of the dinosaur order Ornithischia.

Coloradisaurus

Coloradisaurus (meaning "Colorados [from Los Colorados Formation] lizard") is a genus of massospondylid sauropodomorph dinosaur. It lived during the Late Triassic period (Norian to Rhaetian stages) in what is now La Rioja Province, Argentina. It is known from the holotype PVL 5904, nearly complete skull. It was discovered and collected from the upper section of the Los Colorados Formation of the Ischigualasto-Villa Unión Basin.

Dinosauriformes

Dinosauriformes is a clade of archosaurian reptiles that include the dinosaurs and their most immediate relatives. All dinosauriformes are distinguished by several features, such as shortened forelimbs and a partially to fully perforated acetabulum, the hole in the hip socket traditionally used to define dinosaurs. The oldest known member is Asilisaurus, dating to about 245 million years ago in the Anisian age of the middle Triassic period.

El Tranquilo Group

The El Tranquilo Group is a geologic group dating to roughly between 228 and 200 million years ago and covering part of the Late Triassic. The El Tranquilo Group is found in the Austral Basin of Santa Cruz Province of Argentina. Fossil flora and purported fossils of the prosauropod dinosaur Mussaurus have been recovered from the El Tranquilo Sandstone. In older publications, the group was defined as a formation. The group contains the Laguna Colorada Formation.

Haya griva

Haya is an extinct genus of basal neornithischian dinosaur known from Mongolia.

Jingshanosaurus

Jingshanosaurus (meaning "Jingshan lizard") is a genus of sauropodomorph dinosaurs from the early Jurassic period.

Leonerasaurus

Leonerasaurus is a basal genus of sauropodomorph dinosaur. Currently, there is only one species known, named L. taquetrensis by Diego Pol, Alberto Garrido and Ignacio A. Cerda in 2011. The fossil, an incomplete subadult individual, was found in the Las Leoneras Formation in Argentina. This formation is probably Early Jurassic in age. Leonerasaurus was a small non-sauropod sauropodomorph, showing an unusual combination of basal and derived characters. This indicates that the evolution of early sauropodomorphs witnessed a great degree of convergent evolution.

Lessemsaurus

Lessemsaurus is an extinct genus of sauropod dinosaur belonging to Lessemsauridae.

Massospondylidae

Massospondylidae is a family of early massopod dinosaurs that existed in Asia, Africa, South America and Antarctica during the Late Triassic to the Early Jurassic periods. Several dinosaurs have been classified as massospondylids over the years. The largest cladistic analysis of early sauropodomorphs, which was presented by Apaldetti and colleagues in November 2011, found Adeopapposaurus, Coloradisaurus, Glacialisaurus, Massospondylus, Leyesaurus and Lufengosaurus to be massospondylids. This result supports many previous analyses that tested fewer taxa. However, this analysis found the two recently described North American massopods, Sarahsaurus and Seitaad, and the South African Ignavusaurus to nest outside Massospondylidae, as opposed to some provisional proposals. Earlier in 2011, Pradhania, a sauropodomorph from India, was tested for the first time in a large cladistic analysis and was found to be a relatively basal massospondylid. Mussaurus and Xixiposaurus may also be included within Massospondylidae. In 2019, a specimen previously assigned to Massospondylus from South Africa was re-examined and found to belong to a separate genus that was named Ngwevu.

Neotheropoda

Neotheropoda (meaning "new theropods") is a clade that includes coelophysoids and more advanced theropod dinosaurs, and the only group of theropods who survived the Triassic–Jurassic extinction event. Yet all of the neotheropods became extinct during the early Jurassic period except for Averostra.

Orionides

Orionides is a clade of tetanuran theropod dinosaurs from the Middle Jurassic to the Present. The clade includes most theropod dinosaurs, including birds.

Orodrominae

Orodrominae is a subfamily of parksosaurid dinosaurs from the Cretaceous of North America and Asia.

Plateosauria

Plateosauria is a clade of sauropodomorph dinosaurs which lived during the Late Triassic to the Late Cretaceous. The name Plateosauria was first coined by Gustav Tornier in 1913. The name afterwards fell out of use until the 1980s.

Plateosauria is a node-based taxon. In 1998, Paul Sereno defined Plateosauria as the last common ancestor of Plateosaurus engelhardti and Massospondylus carinatus, and its descendants. Peter Galton and Paul Upchurch in 2004 used a different definition: the last common ancestor of Plateosaurus engelhardti and Jingshanosaurus xinwaensis, and its descendants. In their cladistic analysis the Plateosauria belonged to the Prosauropoda, and included the Plateosauridae subgroup. In Galton's and Upchurch's study also Coloradisaurus, Euskelosaurus, Jingshanosaurus, Massospondylus, Mussaurus, Sellosaurus, and Yunnanosaurus proved to be plateosaurians.However, recent cladistic analyses suggest that the Prosauropoda as traditionally defined is paraphyletic to sauropods. Prosauropoda, as currently defined, is a synonym of Plateosauridae as both contain the same taxa by definition.

The following cladogram simplified after an analysis presented by Apaldetti and colleagues in 2011.

The following cladogram simplified after an analysis presented by Blair McPhee and colleagues in 2014.

Pulanesaura

Pulanesaura is an extinct genus of basal sauropod known from the Early Jurassic (late Hettangian to Sinemurian) Upper Elliot Formation of the Free State, South Africa. It contains a single species, Pulanesaura eocollum, known from partial remains of at least two subadult to adult individuals.

Xingxiulong

Xingxiulong (meaning "Xingxiu Bridge dragon") is a genus of bipedal sauropodiform from the Early Jurassic of China. It contains a single species, X. chengi, described by Wang et al. in 2017 from three specimens, two adults and an immature individual, that collectively constitute a mostly complete skeleton. Adults of the genus measured 4–5 metres (13–16 ft) long and 1–1.5 metres (3 ft 3 in–4 ft 11 in) tall. Phylogenetic analysis suggests that Xingxiulong is most closely related to its contemporary Jingshanosaurus, although an alternative position outside of both the Sauropodiformes and Massospondylidae is also plausible.

Despite their close relationship, Xingxiulong prominently differs from Jingshanosaurus - and from most basal sauropodomorphs - in having a number of sauropod-like traits. These include a sacrum containing four vertebrae; a pubis with an exceptionally long top portion; and the femur, the first and fifth metatarsals on the foot, and the scapula being wide and robust. These probably represent adaptations to supporting high body weight, in particular a large gut. Unlike sauropods, however, Xingxiulong would still have been bipedal.

Xixiposaurus

Xixiposaurus is a genus of prosauropod dinosaur which existed in what is now Lower Lufeng Formation, China during the lower Jurassic period. It was first named by Sekiya Toru in 2010 and the type species is Xixiposaurus suni.

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