| Mussaurus | |
|---|---|
 | |
| Fossil juvenile skeleton | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | †Sauropodomorpha |
| Family: | †Mussauridae Bonaparte & Vince, 1979 |
| Genus: | †Mussaurus Bonaparte & Vince, 1979 |
| Type species | |
| †Mussaurus patagonicus Bonaparte & Vince, 1979 | |
Mussaurus (meaning "mouselizard") is agenus ofherbivoroussauropodomorphdinosaur that lived in southernArgentina during theSinemurian stage of theEarly Jurassic. 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, sinceMussaurus is now known from adult specimens, the name is something of a misnomer.
In its early stages of life,Mussaurus was a smallquadrupedal herbivore, walking on all four legs. As it grew up, the changes in body proportions may have led its centre of mass to move backwards towards itspelvis. Adults would have been medium-sizedbipedal herbivores, measuring up to 8 metres (26 ft) long and weighing up to 1.2–1.6 metric tons (1.3–1.8 short tons).
Numerous specimens of varying age found in a single locality suggest thatMussaurus is one of the earliest dinosaurs to have lived in a gregarious lifestyle. With its possible origin from theTriassic, this complex social behaviour may have given rise to the sauropods' early success as the largest herbivores on land.Mussaurus also possessed anatomical features that suggest a close, possibly transitional evolutionary relationship with true sauropods.
Infant and juvenile fossils ofMussaurus were first discovered by an expedition led byJose Bonaparte during the 1970s to theLaguna Colorada Formation, where the team found fossilized eggs and hatchlings, which added insight into the reproductive strategies ofMussaurus and other sauropodomorph dinosaurs.[1] The age of the formation is estimated between 192.78 ± 0.14 Ma and 192.74 ± 0.14 Ma.[2] The first adult specimens ofMussaurus were described in 2013, although some of these specimens had first been described in 1980 and were originally attributed to the genusPlateosaurus.[3]
Previous to the discovery of adult specimens ofMussaurus, thephylogenetic position of this taxon was difficult to establish. Infant and juvenile fossils are known to show morebasal traits than adult specimens of the same taxon. Furthermore, the recently discovered of one subadult and three adult specimens assigned toMussaurus 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 ofMussaurus, included information only from adult specimens. The followingcladogram is simplified after their analysis (relationships outsidePlateosauria are not shown).[3]
Mussaurus specimens have been found in association with nests that are believed to contain multiple eggs apiece. The skeletons ofMussaurus infants were small, measuring about 20 centimetres (7.9 in) long and weighing about 53.3–76.5 grams (1.88–2.70 oz).[4][5] 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, juvenileMussaurus had tall skulls with short snouts and large eyes. 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 earlysauropodomorphs.[1] Ignacio Cerda and Diego Pol reported putative evidence of medullary bone tissue from a specimen ofMussaurus in 2013,[6] but both authors with Anusuya Chinsamy subsequently argued in 2014 that this tissue most likely represents a pathologically formed tissue instead based on histological features.[7]
As of 2021,Mussaurus represents the earliest unequivocal evidence of complex social behaviour in dinosaurs, with over 100 eggs and skeletal specimens of 80 individuals ranging from embryos to adults found in the same locality. This discovery predates the previous records of herd-living dinosaurs by at least 40 million years. It is thought that this behaviour has been originated from the Triassic period, leading them to become successful as large terrestrial herbivores.[2]
A study published in May 2019 shows that in its first year of life, during which it weighed 6.5–10.2 kilograms (14–22 lb),M. patagonicus probably a wasquadruped, walking on all four limbs. Changes in the relative proportions of its body during growth (ontogeny) may have caused itscentre of mass to move backwards towards itspelvis, resulting in the animal adopting a two-legged (bipedal) stance later in life. AdultMussaurus had a tail length of 3.13 metres (10.3 ft) and weighed up to 1.2–1.6 metric tons (1.3–1.8 short tons), significantly larger than subadults which weighed about 106.2–557 kilograms (234–1,228 lb).[5][2] Individuals of such size would have measured up to 8 metres (26 ft) in total body length.[8] It is estimated thatMussaurus would have been sexually mature at 23 to 31 years of age, and reached somatic maturity after 14 years.[9]
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