Study: Early mammals blocked dinosaurs from evolving tiny body sizes
Researchers find competition with small mammals, not physiology, explains why dinosaurs never became mouse-sized.
Conversation activity · last 41 hours peak 8/hr
Summary, timeline and people extracted by Claude from 10 items across 2 sources · 5h ago. Quotes are verbatim.
A new study by researchers from the American Museum of Natural History and Princeton University suggests that early small mammals, not biology alone, prevented non-avian dinosaurs from evolving into tiny forms. Using mathematical models of body size evolution, the team found that while physiology can explain size ranges in modern mammals and birds, it cannot account for the surprisingly large minimum size of dinosaurs—roughly 450 grams, compared to animals today that weigh as little as 0.15 grams. The researchers propose that ecological competition for small-body-size niches forced dinosaurs to remain relatively large.
- Mathematical models of body size evolution show physiology alone cannot explain why dinosaurs never became tiny—the smallest non-avian dinosaurs weighed ~450g vs. modern animals weighing as little as 0.15 grams.
- New research proposes ecological competition with early mammals, not biological constraints, prevented dinosaurs from shrinking to mouse or sparrow sizes during the Mesozoic era.
- Fossil evidence supports this theory: tiny creatures are preserved at dinosaur sites, yet no tiny dinosaur fossils appear alongside them, suggesting they were extremely rare or absent rather than undiscovered.
- The findings reverse the known dynamic where dinosaurs prevented mammals from evolving to large sizes—suggesting both groups constrained each other's size evolution.
How it unfolded
-
Event Research becomes public
Study results widely reported across major news outlets including Science Daily, Times of India, CTV News, and Phys.org.
-
Analysis Key finding on mammalian competition
Study suggests that small mammals occupying ecological niches prevented dinosaurs from shrinking to mouse or sparrow sizes, reversing the known dynamic where dinosaurs prevented mammals from growing large.
“We already knew that dinosaurs prevented mammals from evolving to large sizes before the end-Cretaceous mass extinction. Here we suggest that mammals in turn prevented dinosaurs from evolving to small sizes.”
Roger Benson · Press ↗ -
Analysis Scale of size disparity
Smallest non-avian dinosaurs weighed ~450 grams; smallest modern birds (bee hummingbird) weigh 1.75 grams, and smallest mammals (Etruscan shrew) weigh 1.8 grams—a difference of over 200 times.
-
Analysis Fossil record rules out missing specimens
Researchers argue that if mouse-sized dinosaurs had been common, fossils would appear at sites where smaller creatures like mammals, lizards, and amphibians are frequently preserved. Their absence suggests tiny non-avian dinosaurs were extremely rare or non-existent.
“If dinosaurs as small as mice had been widespread, researchers would expect at least some of their fossils to have turned up alongside these other tiny animals.”
Study authors · Press ↗ -
Report Study published in Evolution journal
Researchers from American Museum of Natural History and Princeton University publish findings examining dinosaur body size evolution using mathematical models and energetic fitness analysis.
What people are saying verbatim
“Everyone loves a giant dinosaur. But we decided to look at the other end of the scale. The absence of tiny dinosaurs may be just as interesting as the existence of giant ones.”
Roger Benson, Study co-author, Museum paleobiologist · Science Daily ↗ · Aug 8
“We already knew that dinosaurs prevented mammals from evolving to large sizes before the end-Cretaceous mass extinction. Here we suggest that mammals in turn prevented dinosaurs from evolving to small sizes.”
Roger Benson, Study co-author, Museum paleobiologist · Science Daily ↗ · Aug 8
“Small animals dominate modern ecosystems. If we want to understand how today's biodiversity evolved, we need to understand why tiny dinosaurs appear to have been missing.”
Stephanie Lechki, Lead author, Princeton postdoctoral fellow · Science Daily ↗ · Aug 8
“The smallest adult non-avian dinosaurs weighed around 400g, hundreds of times heavier than the tiniest living birds, mammals or reptiles.”
Study authors, Researchers · Times of India ↗ · Aug 8