A Florida State University professor thinks he's solved one of the "greatest riddles" about non-avian dinosaurs: Did their eggs incubate slowly like those of lizards, or quickly like those of birds? The former, found Gregory Erickson and his team, and they came to their conclusion by studying embryonic teeth. The team looked at both ends of the size spectrum: fossilized embryos of the sheep-sized Protoceratops, whose eggs weighed 6.8 ounces; and those of the massive duck-billed Hypacrosaurus, whose volleyball-sized eggs weighed nearly 9 pounds. After running the jaws through a CT scanner and actually extracting some of the teeth, they were able to count lines—called lines of von Ebner, reports NPR, and likened to an annual tree ring, but in this case, formed daily—in the dentine of the teeth in order to determine age.
For the Protoceratops, it was about three months; for the Hypacrosaurus, about six. That's outside the average range of bird eggs, which Phys.org reports hatch in about a week and a half to just shy of three months. It's not just a neat-to-know fact, but one that could have contributed to their demise, per the study. As the Verge reports, dinosaur parents could have been tethered to their eggs for six months, making it tougher to migrate and easier to fall susceptible to floods, droughts, and predators. And "after the asteroid hit" 65 million years ago, Erickson tells NPR, dinosaurs had the double whammy of a dearth of resources and lengthy incubation periods that left them ill-positioned to survive. He'd next like to track down specimens of carnivorous embryos—no easy feat, as fossilized embryos are rare—to see if the timing holds true for them, too. (This dinosaur fossil is one of the "saddest" ever found.)