Pity the hapless Tenontosaurus. A mild-mannered herbivorous dinosaur, it spent its days rummaging around for enough leafy food to sustain its two-ton bulk. But its life in the swampy lowlands of the early Cretaceous period was no walk in the park. Heavy and ungainly, it proved fairly easy pickings for fast and brainy predators such as Deinonychus (Deinonychus teeth have been found embedded in fossilized Tenontosaurus bone).
So what's a prey species to do? Life was nasty, brutish and short for critters like Tenontosaurus, and the pressure to reproduce before falling victim to a predator was heavy. Now, UC Berkeley researchers Sarah Warning and Andrew Lee (lowly grad students, yet!) have found evidence that Tenontosaurus, and probably many other dinosaur species as well, turned to teen pregnancy as a means of survival.
Like their modern descendants the birds, dinosaurs produced a special calcium-rich form of bone called medullary bone inside the hollow cavities of their leg bones when they were preparing to lay eggs. In modern birds, the medullary bone forms three to four weeks before the bird ovulates. Calcium from the medullary bone is used to form eggshells, and the bone disappears after the eggs are laid. Since 2005, three dinosaur skeletons have been examined that show evidence of medullary bone, indicating that the skeleton was that of a female nearly ready to reproduce. One of these skeletons was that of a Tenontosaurus that met with some mishap during this pre-ovulation period.
Dinosaurs produced distinct growth lines (similar to tree rings) in their bones as they grew. By counting the rings, scientists can estimate the age of a dinosaur at death: most species are believed to have had a lifespan of around 30 years. When Werning and Lee calculated the Tenontosaurus specimen's age, they discovered she had been only about eight years old: not yet full-grown, but apparently mature enough to bring little Tenonto-tots of her own into the world.
Interestingly, this pattern of rapid growth and early sexual maturity characterizes birds and mammals as well--think of a six-month-old cat having kittens, for example (or, for that matter, a thirteen-year-old girl pushing her baby's stroller through the mall).
Also (and this is purest speculation on my part), if Tenontosaurus lived in groups as its descendants the hadrosaurs (duck-billed dinosaurs) did, maybe the young mothers benefited from the presence and help of their mothers, aunties and older sisters. Just like one big episode in the neverending series of Land Before Time movies, yo.
BBC: Dinosaurs grew fast, bred young
SF Chronicle: They had sex young and then died
Laelaps: Precocious Dinosaur Sex
Abstract from PNAS: Sexual maturity in growing dinosaurs does not fit reptilian growth models