Description and Classification

The hatchling skeletons of Mussaurus are incredibly small, some measuring only about 20 to 37 centimeters (8 to 15 inches) in length, small enough to fit in a human hand. These tiny skeletons had disproportionately large heads, big eyes, and short snouts, features common in very young animals. For many years, only these juvenile fossils were known. Later discoveries of subadult and adult specimens showed that Mussaurus could grow to be about 3 meters (10 feet) long and weigh around 70 kilograms (150 pounds), though some estimates suggest it could reach up to 5 meters (16 feet) in length. As an adult, it had a relatively long neck, a small head, a bulky body, and a long tail, typical for early sauropodomorphs.

Mussaurus is classified as a sauropodomorph dinosaur. This large group includes the early sauropodomorphs (sometimes referred to as “prosauropods”) like Mussaurus and Plateosaurus, as well as the later, colossal sauropods such as Apatosaurus and Argentinosaurus. These early sauropodomorphs represent an early evolutionary stage of herbivorous, long-necked dinosaurs. While adult Mussaurus were primarily quadrupedal (walking on four legs), they might have been able to rear up on their hind legs to reach higher vegetation or for faster movement. The hands of Mussaurus had five fingers, with a large claw on the first digit (the “thumb”), which could have been used for defense or for grasping branches while feeding.

Distinguishing Features

• Fossils include exceptionally well-preserved complete skeletons of hatchlings and juveniles, some of the smallest dinosaur skeletons ever found.
• Shows significant changes in body proportions from hatchling to adult, a process known as ontogeny (growth and development).
• Adults possessed a long neck and tail, a small skull relative to body size, and robust limbs.
• The innermost finger (thumb) on its hand was equipped with a large, recurved claw.
• Unlike some later sauropods, its forelimbs were noticeably shorter than its hind limbs, suggesting it was more comfortable on four legs but could potentially balance on two.

Paleoenvironment and Diet

Mussaurus lived in what is now Patagonia, Argentina, during the Late Triassic period. The environment at that time was likely a semi-arid landscape characterized by seasonal rivers and floodplains. Plant life would have included ferns, cycads, and early conifers. Fossils of Mussaurus have been found in the El Tranquilo Formation, which has also yielded remains of other early dinosaurs and ancient mammal relatives. This indicates that Mussaurus shared its habitat with a variety of other prehistoric creatures in a complex ecosystem.

As a sauropodomorph, Mussaurus was an herbivore. Its small, leaf-shaped teeth were suited for stripping foliage from plants. The long neck would have allowed it to browse on vegetation at various heights, giving it access to a wider range of food sources than shorter-necked herbivores. Young Mussaurus, being very small, likely fed on low-growing plants. As they grew, adults could reach higher into shrubs and small trees. Its grasping hands might have also aided in pulling branches closer to its mouth to feed.

Significance and Ongoing Research

The discovery of Mussaurus is highly significant because its fossil record includes individuals at many different life stages, from tiny hatchlings still in or near their nests to subadults and adults. This provides invaluable information about the growth and development (ontogeny) of early sauropodomorph dinosaurs. The tiny hatchling skeletons, in particular, have offered unique insights into the earliest phases of dinosaur life, showing how their bodies developed and changed as they grew. The completeness of these small skeletons is rare in the fossil record and helps paleontologists understand how these dinosaurs cared for their young.

Fossil evidence, including nests with eggs and multiple juvenile skeletons found together, suggests that Mussaurus may have exhibited social behavior, such as nesting in groups and that young dinosaurs might have stayed together for a period after hatching. This points towards communal nesting grounds. Ongoing research continues to explore these social dynamics and potential parental care strategies. Detailed studies of Mussaurus bone histology (the microscopic structure of bones) help scientists determine its growth rates and how quickly it reached adult size. Further research aims to refine its exact position within the sauropodomorph family tree and illuminate its role in the Triassic ecosystem as herbivorous dinosaurs began their rise to prominence.