Massospondylus carinatus Owen, 1854 isn't one of those flashy dinosaurs that has a lot of popular appeal to the crowds, but as you can see from the name, it has been known for a long time (first described by Richard Owen in 1854), and many specimens of various ages have been found—if you aren't familiar with what this beastie looks like, here are some photos of a fossilized adult and a reconstruction. It's basically your standard early prosauropod, but from the numbers of specimens found, it must have been a particularly successful species.

Now we know even more about it's life history, though, because several beautiful specimens of unhatched embryos have been discovered in Lower Jurassic strata from South Africa.

Isn't this little guy gorgeous?

An articulated embryonic skeleton of Massospondylus (BP/1/5347A). an, angular; c3 to c10, cervical vertebrae; ca, caudal vertebra; ch, hemal arch; co, coracoid; cp?, cultriform process?; de, dentary; d1 to d14, dorsal vertebrae; f, frontal; fe, femur; fi, fibula; h, humerus; il, ilium; is, ischium; l, lacrimal; m, maxilla; mc, metacarpals; mt1 to mt3, metatarsals; n, nasal; p, parietal; pal, palatine; ph, phalanges; pm, premaxilla; po, postorbital; prf, prefrontal; pt, pterygoid; pu, pubis; q, quadrate; r, rib; ra, radius; s1 to s3, sacral vertebrae; sc, scapula; scl, scleral ring; sr1 to sr3, sacral ribs; sq, squamosal; su, surangular; t, tibia; u, ulna.

It's estimated to be a bit more than 8cm from snout to vent (the tail is not well preserved, unfortunately), and the authors think from its size within the egg and the degree of ossification that it was probably close to hatching. If alive, it would be small enough to hold in your hand, and would grow to about 5m long at adulthood. There are a few other things that can be determined from the skeleton.

The combination of the body proportions and poorly developed dentition suggest that the hatchlings may have required parental care. The diminutive ventral elements of the pelvic girdle, small caudal vertebrae, and relatively enormous head of the Massospondylus embryos suggest it would have been difficult for the hatchlings to move around efficiently. The virtual absence of teeth in these embryos is another indicator of altricial behavior. Only a single possible tooth fragment is preserved in the two skulls, whereas other delicate, loosely attached elements were preserved largely undisturbed. Even if most of the teeth were poorly mineralized or lost postmortem, they were not well suited for feeding. If this interpretation is correct, these embryos provide early evidence of altricial behavior in a nonavian dinosaur.

So, it would have been a clumsy, big-headed, big-eyed baby…or, as the Minnesotans say, "Oh, for cute!"

Another interesting observation is that the adults are clearly animals that can walk either quadrupedally or bipedally—they have hindlimbs that are significantly longer than their forelimbs, and would have run along on just their hindlegs, at least when they were in a hurry. The embryos, though, have fore- and hind-limbs of equal length, and would have had to walk quadrupedally at all times.

Reconstructions of Massospondylus embryos. (A) An articulated skeleton in lateral view; the horizontally held neck is shown at maximum dorsiflexion. The total length of the tail could not be determined; the estimated minimum length is shown. Estimated snout-vent length of the embryo is 8.1 cm. (B) An embryonic skull in dorsal and lateral views.

Enough specimens have been found to allow an analysis of relative growth rates of various bones in this species, and they've put together a lovely developmental picture of the animal. From these more lizard-like proportions, everything in the animal is going to grow rapidly, but the hindlimb and neck grow fastest of all to produce the long-necked, long-legged adult form.

Reisz RR, Scott D, Sues H-D, Evans DC, Raath MA (2005) Embryos of an Early Jurassic Prosauropod Dinosaur and Their Evolutionary Significance. Science 309:761-764.