Scientists compared dinosaurs to mammals for decades but missed this key difference

Thomas R. Holtz Jr., a principal lecturer in the University of Maryland's Department of Geology, has spent years studying how dinosaurs functioned within their ecosystems and how those systems differed from today's world. In research published in the Italian Journal of Geosciences, he argues that scientists may have overlooked a crucial factor when comparing dinosaurs with modern mammals.

"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."

Dinosaur Parenting vs. Mammal Parenting

Most mammals provide extended care for their young. Offspring typically remain with their mothers until they are nearly adult size. Because adults supply food and protection, young mammals tend to share the same diet and habitat as their parents, filling similar ecological roles.

"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 followed a different pattern. Although some species likely guarded nests or protected hatchlings briefly, young dinosaurs became independent relatively quickly. Within months or about a year, juveniles separated from adults and formed groups with others their age.

Holtz notes that modern crocodilians offer a useful comparison. Crocodiles defend nests and newly hatched young for a short time, but juveniles soon disperse and live on their own, taking years to reach full size.

"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. These juveniles tended to travel together in groups of similarly aged individuals, getting their own food and fending for themselves."

How Juvenile Dinosaurs Filled Different Ecological Niches

Dinosaurs laid eggs and often produced sizable clutches at once. Because many offspring hatched together and reproduction occurred more frequently than in mammals, this strategy increased the odds that at least some would survive, without requiring prolonged parental investment.

"The key point here is that this early separation between parent and offspring, and the size differences between these creatures, 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. While adults and offspring are technically the same biological species, they occupy fundamentally different ecological niches. So, they can be considered different 'functional species.'"

Take Brachiosaurus as an example. A juvenile about the size of a sheep could not reach leaves 10 meters above the ground the way an adult could. It would have fed on lower vegetation, occupied different spaces, and faced predators that would not risk attacking a fully grown giant. As the animal grew from dog sized to horse sized to giraffe sized and eventually to enormous proportions, its place in the ecosystem shifted at each stage.

"What's interesting here is that this completely changes how scientists view ecological diversity in that world," Holtz said. "Scientists generally think that mammals today live in more diverse communities because we have more species living together. 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."

Rethinking Mesozoic Ecosystem Productivity

How could ancient environments sustain so many distinct ecological roles? Holtz suggests two possible explanations.

First, environmental conditions during the Mesozoic were different, with warmer global temperatures and higher carbon dioxide levels. These factors likely boosted plant growth, increasing the energy available at the base of the food chain. Second, dinosaurs may have had somewhat lower metabolic demands than similarly sized mammals, meaning they required less food overall.

"Our world might actually be kind of starved in plant productivity compared to the dinosaurian one," Holtz suggested. "A richer base of the food chain might have been able to support more functional diversity. And if dinosaurs had a less demanding physiology, their world would've been able to support a lot more dinosaur functional species than mammalian ones."

Holtz emphasizes that this does not necessarily mean dinosaur ecosystems were categorically more diverse than modern mammal dominated ones. Instead, diversity in the past may have been structured differently than scientists have assumed. He plans to continue examining how changes across dinosaur life stages shaped ancient ecosystems and how those systems ultimately transitioned into the modern world.

"We shouldn't just think dinosaurs are mammals cloaked in scales and feathers," Holtz said. "They're distinctive creatures that we're still looking to capture the full picture of."

Holtz's study, "Bringing up baby: preliminary exploration of the effect of ontogenetic niche partitioning in dinosaurs versus long-term maternal care in mammals in their respective ecosystems," was published in the Italian Journal of Geosciences.