Dinosaurs came in many shapes and sizes, and so did their babies. The smallest eggs found were just a few centimeters long. One chicken-sized dinosaur, Sinosauropteryx, was found fossilized with unlaid eggs in her abdomen, researchers reported in 1998 in the journal Nature. The eggs were about 1.5 inches (4 centimeters) long and just over 1 inch (2.5 centimeters) wide, which is a little smaller than a chicken egg.
But even large dinosaurs had small babies. In a 2005 Science paper, researchers reported the discovery of an embryo that was 6 inches (15 centimeters) long, curled in an egg just 2.5 inches (6 centimeters) long. The species was Massospondylus, a plant-eater with an adult length of about 16 feet (5 meters).
Maiasaurus, the "good mother lizard," was as long as a school bus when grown, but only 1 foot (30 centimeters) long when coming out of its egg. And the long-necked Camarasaurus grew from a 3-foot (1-meter)-long hatchling to a behemoth with a length of 59 feet (18 meters).
To attain their huge bulk, these prehistoric giants had some impressive growth spurts. Using data from growth plates in bones, researchers estimated that to reach an adult weight of 57,094 pounds (25,952 kilograms), a baby Apatosaurus would pack on more than 30 pounds (14 kilograms) per day about the same rate as modern whales, according to a paper in Nature in 2001.
That might seem like a lot, but when your largest predator is the toothy, 17-foot-tall Allosaurus, there's no lack of motivation to get big and strong.