NEW YORK, NY.- They are some of the rarest of rare artifacts: fossil dinosaur eggs with the embryo still inside. And they are prized for what they can tell paleontologists about the adults that laid them.

The exhibition The World’s Largest Dinosaurs features a scale model of a nest found at Auca Mahuevo, Argentina, one of the largest known dinosaur nesting sites in the world. While it isn’t always possible to figure out which dinosaur laid a particular egg, in this case, an embryo within an egg found at Auca Mahuevo site allowed scientists to identify these eggs as those of titanosaurs, a group of sauropods that included such species as Ampelosaurus and Saltasaurus. Herds of female titanosaurs are thought to have laid the thousands of eggs — 15 to 40 at a time — in shallow nests dug out with their huge feet in dry mud and sand over miles of ground at Auca Mahuevo.

Titanosaurs are among the biggest sauropods, the group of saurischian dinosaurs featured in this exhibition. Titanosaur fossils have been found on every continent except Antarctica, and some of the biggest titanosaurs have been discovered in South America. These include the massive Argentinosaurus, which greets visitors at the entrance to the exhibition. In life, an adult Argentinosaurus could weigh up to 90 tons.

Size is a curious part of the story of dinosaur eggs. One might think such huge creatures would have equally super-sized eggs. But consider that the extinct elephant bird, which weighed about 880 lbs., had, on average, a 26-lb. egg compared to the average 9-lb. egg of the Ampelosaurus, which grew to about 7.7 tons. Also, there is a limit to the size any egg can be. Eggshell is very brittle, so the larger the egg, the thicker its shell must be to keep from shattering. However, the shell must also allow oxygen and water to get through to the embryo growing inside, and, above a certain size, the egg wouldn’t be both suitably strong and porous. So although the sauropod young grew big very fast, they started out relatively tiny. The hatchlings of the 13-ton female Mamenchisaurus at the center of the exhibition, for example, would have weighed about as much as a small goose.

A version of this story originally appeared in the Summer issue of Rotunda, the American Museum of Natural History Members’ magazine.