How difficult is it to reconstruct how dinosaurs raised their children? Well, consider this: until the 1920's, scientists weren't even sure if all dinosaurs laid eggs (like modern reptiles and birds) or gave birth to live young (like mammals). Thanks to some spectacular fossil discoveries, we now know the former was the case, but the evidence for child-rearing behavior is more elusive--consisting mainly of tangled skeletons, preserved nesting grounds, and analogies with the behavior of modern reptiles, birds and mammals.
One thing has become clear: different kinds of dinosaurs had different child-rearing regimens. Just as the babies of modern prey animals like zebras and gazelles are born fully mobile (so they can escape from predators), one would expect that sauropod eggs yielded "ready-to-run" hatchlings. And just as modern birds feed their newborns in specially prepared nests, one would think that at least some dinosaurs did the same--not in trees, necessarily, but in clearly marked-out birthing grounds.
Dinosaur Eggs
In practical terms, one of the main differences between viviparous and oviparous animals is that the former can only give birth to a limited number of live newborns (one at a time for large animals like elephants, seven or eight at a time for small animals like cats), while the latter can potentially lay dozens of eggs in a single sitting. Even a female Seismosaurus probably layed 20 or 30 eggs at a time (despite what you may think, the eggs of sauropods weren't any bigger than bowling balls, and usually significantly smaller).
Why did dinosaurs lay so many eggs? As a general rule, any given species will only produce as many young as are needed to assure that at least one of them reaches adulthood (and produces young herself). The gruesome fact is, out of a clutch of 20 or 30 newly hatched Stegosaurus babies, only two or three might escape being immediately gobbled up by waiting predators--just enough to ensure the perpetuation of the Stegosaurus line. (Just as many modern reptiles, like turtles, leave their eggs unattended, it's a good bet that many dinosaurs did too.)
The Good Mother Lizard
For a long time, paleontologists assumed that all dinosaurs employed this drop-your-eggs-and-run strategy, and that all newly hatched chicks were left to survive (or die) in a hostile environment. That all changed in the 1970's, when the paleontologist Jack Horner discovered, in Montana, the immense nesting ground of a duck-billed dinosaur he named Maiasaura (Greek for "good mother lizard.") Each of the hundreds of Maisaura females nesting here laid 30 or 40 eggs in a circular clutch; the site, now known as Egg Mountain, has yielded fossils not only of eggs, but of hatchlings, juveniles, and adults as well.
Finding all these Maiasauras tangled together, in all these different stages of development, was tantalizing enough. Further analysis of the fossils showed that the newly hatched chicks had immature leg muscles (and thus may not have been capable of walking, much less running), and that their teeth had evidence of wear. What this implied was that adult Maiasauras brought food back to the nest and cared for their children until they were old enough to fend for themselves--the first clear evidence of dinosaur child-rearing. A later find in China yielded similar evidence of family life for Psittacosaurus, an early, relatively small ceratopsian.
It would be a mistake, though, to conclude from these discoveries that all herbivorous dinosaurs fed and cared for their hatchlings. For instance, paleontologists believe that sauropods expressly did not look after their young, for a very simple reason: a twelve-inch-long, newborn Apatosaurus could easily have been crushed by the lumbering feet of its own mother. In those circumstances, the newborn would stand a better chance of survival on its own--even as its siblings were picked off by hungry raptors and tyrannosaurs (of whose own child-rearing habits we know virtually nothing).
Avian and Aquatic Reptiles
Ironically, the ancient reptiles most similar behaviorally to modern birds--pterosaurs--are a black hole when it comes to evidence of child-rearing. To date, only a handful of fossilized pterosaur eggs have been discovered, the first as recently as 2004, hardly a large enough sample to draw any inferences about parental care. The current state of thinking, based on analysis of fossilized juveniles, is that pterosaur chicks emerged from their eggs "fully cooked" and required little or no parental attention. There are also hints that some pterosaurs may have buried their eggs rather than incubating them inside their bodies, though the evidence is far from conclusive.
The real surprise happens when we turn to the aquatic reptiles that existed alongside dinosaurs during the Mesozoic Era. Based on firm evidence (such as tiny embryos fossilized inside the bodies of their mothers), paleontologists now believe that most, if not all, ichthyosaurs gave birth to live young rather than laying eggs--the first, and as far as we know only, reptiles ever to have done so. As with pterosaurs, the evidence for later aquatic reptiles like plesiosaurs and pliosaurs is pretty much nonexistent; some of these sleek swimmers may well have been viviparous, but they may also have returned to land seasonally to lay their eggs. Pending further fossil discoveries, we may never know for sure.
