Sauropods were the biggest dinosaurs — and the biggest land animals — ever to stomp across the planet. Their long-necked group included apatosaurus, brontosaurus, camarasaurus and the even more massive titanosaurs, whose leg bones were longer than a person is tall.
But each of their first steps on Earth were teensy. These great beasts came from little packages, hatching out of eggs no bigger than grapefruits or soccer balls. They must have had "a ridiculous growth rate," said D. Cary Woodruff, director of paleontology at the Great Plains Dinosaur Museum in Montana.
Woodruff knows how small these animals began — along with a team of dinosaur experts, Woodruff describes the smallest diplodocus skull ever found in a new study in the journal Scientific Reports. The skull, from a diplodocus the scientists nicknamed Andrew, could fit in Woodruff’s cupped palms.
Sauropod skulls are rare. Immature skulls, tiny and fragile, are rarer still. Paleontologists can glean lots of information from skulls: The orientation of ear canals tells researchers how the animal held its head. Fossilized teeth are markers of what it ate. This skull was about nine inches long. Andrew had oversize eyes, a short muzzle and unusual teeth.
Skulls are particularly valuable to experts who study sauropod growth, too, because other developmental characteristics are comparatively rare, Woodruff said. Dinos like triceratops had frills and horns, which scientists can track though various ages of the animals' life. Not so for a sauropod.
The newly described skull fills critical gaps in the understanding of sauropod size and development, Woodruff said. Adult diplodocuses had teeth like wooden pegs. They were grazers, like cattle, nuzzling up to soft ferns with their long snouts. Other sauropods, like camarasaurus, had spoon-shape teeth, better to munch on tougher vegetation.
Andrew, surprisingly, had both types of teeth: pegs in the front, spoons in the back. This, Woodruff predicts, would have allowed Andrew to chow down on all sorts of food, nipping at soft ferns but also crunching through more fibrous stuff.
"It would be tough to imagine that sauropods ate the same things throughout their lives given the size disparity as they aged," said Macalester College paleontologist Kristina Curry Rogers, who was not involved with this research but has studied baby sauropods from fossils found in Madagascar. "There is certainly no way that juvenile sauropods could feed at the same browse heights as adults."
The skull and two vertebrae were collected from a quarry in Montana. Woodruff estimated the animal would have been about 2 to 4 years old, about 20 feet long and about chest-height. That's tiny for an animal that, had it survived, would have grown to about 90 feet long and 13 tons in the span of two decades.
The nickname Andrew came from industrialist Andrew Carnegie, who funded excavations to dig up dinosaur fossils and has a namesake sauropod — Diplodocus carnegii. The study authors aren’t exactly sure what the species is, but they know it is a diplodocid, meaning a member of the same family as diplodocus. (The paleontologists have no idea whether the animal was male or female.)
Andrew was found among a jumble of other young sauropods, Woodruff said. He said that this probably represented an "age segregated herd," young animals within a similar age range that found food and shelter in a thick forest. In this view, diplodocuses were the opposite of helicopter parents. He suspects the animals were like sea turtles: A mother's duty ends at laying eggs, leaving the hatchlings to fend for themselves.
The Swiss-army teeth, Woodruff said, is a sign that the young sauropods did not rely on adults to feed them ferns. "If that's the case, why do they have different kinds of teeth?" he said.
Curry Rogers was not sure the teeth were so revealing. "I don't see such an obvious argument when it comes to the link between differential feeding strategies and a lack of parental care," she said. She said that hypothesis needs more data, including anatomical features beyond a skull and a few vertebrae.
There's still plenty more to study. "I want to find sauropods smaller than Andrew," Woodruff said. "There's still so much more we can learn."
Even younger sauropods could shape the idea of how they lived. "As adults, sauropods are so giant that they can almost seem like biological impossibilities - it is really challenging to understand how something so weird could work out so well in an evolutionary sense," Curry Rogers said. "Sometimes, studying sauropods is like studying aliens."
The public will be able to study Andrew’s bones up close beginning Nov. 11, Woodruff said, when the skull is unveiled at the Cincinnati Museum.