Dino of the Week: Mussaurus

Type Species: Mussaurus patagonicus

Classification: Dinosauria - Saurischia - Sauropodomorpha - Prosauropoda

Time Period: Early Jurassic

Location: South America

Diet: Herbivore

Jose Bonaparte made a hell of a discovery in the 1960s with Riojasaurus, and he made another in the 1970s: “the Mouse Lizard”. Mussaurus gets its name from its incredibly small size: the largest skeleton was only eight inches long! These Lower Jurassic herbivores had large heads, big eyes, and short necks. These bug-eyed critters were so small they could fit in the palm of your hand. Their skeletons evoked images of mice, and the media (and science) introduced the world to “the mouse lizard.” Hailed as the “Smallest Dinosaur Ever Found!” at a mere ¾ of a foot long, Mussaurus showed up (rather ironically, if you ask me) on all the Big 10 lists. Even notable science fiction author Michael Crichton included the mouse-sized Mussaurus in his sequel to Jurassic Park. 

But in all the excitement, a critical component of the Mussaurus dig site didn’t receive the attention it deserved: these eight-inch long dinosaurs were found amidst what turned out to be fragmented eggshells. Before being trampled, the eggs would’ve been about one inch long. As for Mussaurus, the bones speak for themselves: tall skulls with short snouts and big eyes are common features of vertebrate infants. This wasn’t the Smallest Dinosaur Ever; it was just a baby. It became clear that Bonaparte had stumbled across a newly-hatched nest forever frozen in time. The adults, however, were conspicuously absent—at least for a while. 

Later digs revealed adult specimens in the same rock formations as the infant Mussaurus. That discovery opened a treasure trove of specimens that we didn’t know we had: it turns out that the remains of several adult Mussaurus had already been found; they had just been “mislabeled” as variants on Plateosaurus. The adults reached up to ten feet in length (still fairly small for the prosauropods we’ve seen, but a titan compared to the eight-inch infants), and it had the look and feel of your basic prosauropod: a long neck and long tail, a small head with an elongated snout, and large, five-fingered hands sporting large thumb claws. Mussaurus was bipedal, but it may have been able to run on all four legs if the need arose. 

Mussaurus may not have been as small as a mouse, but more discoveries have shed further light not just on this dinosaur, but on its society as well. The paternal care (or lack thereof) among dinosaurs is a subject that remains, in some ways at least, hotly debated; and within this cacophony, Mussaurus has a booming voice. First, the fact that the babies remained at their nests after hatching—rather than scurrying fast and hard for the undergrowth, as vulnerable infants instinctively do—implies that someone was looking after them. Second, the anatomical proportions of an infant Mussaurus are reminiscent not only of infant vertebrates but, more poignantly, to species that exhibit parental care over their young while they’re vulnerable. Third, a further discovery of an intact nest with seven juvenile specimens—neither babies nor adults—crowded around it implies that, for a significant amount of time, the animals stayed around the nest. Without parental care, hovering around the nest would be tantamount to infanticide. Someone would find out about it at some point, and it’d be a bloodbath. Fourth, and finally, the first adult specimens of Mussaurus have been recognized as recently as 2013, and these specimens were found near the nests. These South American bone-beds give us a snapshot of a (rather unfortunate) day-in-the-life of a prosauropod nesting site—and at 215 million years old, it’s the oldest known nesting site in the geological record.