‘Free-Range’ Dinosaur Parenting May Have Created Surprisingly Diverse Ancient Ecosystems

Picture a baby Brachiosaurus the size of a golden retriever, feeding on plants with its siblings while dodging hungry predators. Meanwhile, its parents—towering over 40 feet tall—are dozens of miles away, going about their lives completely unbothered by their offspring’s potential fate. 

Thomas R. Holtz Jr., a principal lecturer in the University of Maryland’s Department of Geology, has spent decades puzzling over how dinosaurs fit into their ancient worlds. His latest research, published last month in the Italian Journal of Geosciences, reveals that scientists may have been overlooking something important.

“A lot of people think of dinosaurs as sort of the mammal equivalents in the Mesozoic era, since they’re both the dominant terrestrial animals of their respective time periods,” Holtz said. “But there’s a critical difference that scientists didn’t really consider when looking at how different their worlds are: reproductive and parenting strategies. How animals raise their young impacts the ecosystem around them, and this difference can help scientists reevaluate how we perceive ecological diversity.”

Young mammals remain under intensive maternal care until they’re nearly full-grown. Mammal offspring occupy essentially the same ecological role as their parents—eating the same food and interacting with the same environment—because the adults do most of the heavy lifting, Holtz said. 

“You could say mammals have helicopter parents, and really, helicopter moms,” he explained. “A mother tiger still does all the hunting for cubs as large as she is. Young elephants, already among the biggest animals on the Serengeti at birth, continue to follow and rely on their moms for years. Humans are the same in that way; we take care of our babies until they’re adults.”

Dinosaurs operated very differently. While parents did provide some care, young dinosaurs were relatively independent. After just a few short months or a year, juvenile dinosaurs left their parents and roamed alone, watching out for each other.

“Dinosaurs were more like latchkey kids,” Holtz said. “In terms of fossil evidence, we found pods of skeletons of youngsters all preserved together with no traces of adults nearby.”

Holtz pointed out a similar case in crocodilians, some of the closest living analogs for dinosaurs. Crocodiles guard nests and protect hatchlings for a limited period, but within a few months, juveniles disperse and live independently, taking years to reach adult size. 

Dinosaurs’ free-range parenting style complemented the fact that they hatched eggs, forming relatively large broods at once. Because multiple offspring were born concurrently and reproduction occurred more frequently than in mammals, dinosaurs increased the chances of survival for their lineage without expending much effort or resources. 

Early separation and the size differences between parents and offspring likely led to profound ecological consequences, Holtz explained. 

“Over different life stages, what a dinosaur eats changes, what species can threaten it changes and where it can move effectively also changes,” he said. “While adults and offspring are technically the same biological species, they occupy fundamentally different ecological niches. So, they can be considered different ‘functional species.’”

For example, a juvenile Brachiosaurus can’t reach vegetation 10 meters above the ground like a grown-up Brachiosaurus. It must feed on other plants elsewhere and face threats from carnivores that would avoid fully grown adults. As a young Brachiosaurus grows—from dog-sized to horse-sized to giraffe-sized to its final enormous proportions—its ecological role shifts continuously.

That changes the equation for ecological diversity in the prehistoric world—often viewed as less diverse than today’s world because fewer species lived together, Holtz said. 

“But if we count young dinosaurs as separate functional species from their parents and recalculate the numbers, the total number of functional species in these dinosaur fossil communities is actually greater on average than what we see in mammalian ones,” he said.  

How could ancient ecosystems support all these functional roles? Holtz suggested two plausible explanations. 

First, the Mesozoic world had different environmental conditions than those of the present, such as warmer temperatures and higher carbon dioxide levels. These factors would have made plants more productive, generating more food energy to support more animals. Second, dinosaurs might have had somewhat lower metabolic rates than similarly sized mammals, meaning they needed less food to survive.

Holtz said his theories don’t necessarily indicate that dinosaur ecosystems were significantly more diverse than our own mammalian world—just that diversity might take forms scientists currently don’t recognize. He plans to continue exploring similar patterns within this framework of functional diversity across dinosaurs’ life stages to better understand the world they lived in and how it evolved into the one that humans inhabit.