Why were there no tiny dinosaurs? You might blame the mammals

WASHINGTON, August 6, A new study has shed light on a longstanding mystery in paleontology: why there were no tiny dinosaurs. While some of the largest land animals in history were dinosaurs, such as the gargantuan Argentinosaurus, which could weigh up to 90 tons and stretch over 100 feet, the smallest known dinosaurs were significantly larger than modern-day creatures like the bee hummingbird or the Etruscan shrew. Researchers suggest that the presence of early mammals may have played a key role in preventing dinosaurs from evolving into truly miniature forms. According to the findings published in the journal Evolution, scientists used mathematical models to analyze how body size evolved across different vertebrate groups. These models accounted for factors such as energy requirements, physiological constraints, and reproductive strategies. Birds, mammals, and turtles fit neatly within these patterns, but dinosaurs deviated significantly. The researchers argue that this discrepancy points to a unique set of ecological pressures affecting dinosaurs. Early mammals, many of which were no bigger than shrews, appear to have occupied the same ecological niches that would have allowed dinosaurs to evolve into small body sizes. Paleontologist Stephanie Lechki, a co-author of the study, explained that small animals generally require less food and can reproduce more rapidly than their larger counterparts. “This gives them a competitive advantage,” she said. “If small animals are already filling those niches, it becomes harder for other species to follow.” The study highlights the diversity of small dinosaurs that did exist. Species such as Microraptor, which weighed around 15 ounces, and Anchiornis, at just 19 ounces, were relatively diminutive compared to their larger relatives. However, even these small dinosaurs were substantially larger than today’s smallest birds, mammals, and reptiles. For example, the bee hummingbird weighs just 1.75 grams, while the Etruscan shrew tips the scales at 1.8 grams. The smallest known lizard, the dwarf gecko, is even tinier, measuring around 0.15 grams. Roger Benson, another lead researcher, noted that the majority of current species are small. Approximately 90 percent of bird species and 75 percent of mammal species are small in stature. “This suggests that there are many more niches available to small-bodied species,” he said. “The absence of tiny dinosaurs means that all those abundant niches were closed to them.” Dinosaurs first emerged around 230 million years ago and dominated terrestrial ecosystems until an asteroid impact 66 million years ago triggered a mass extinction. During the time of the dinosaurs, mammals were small and largely nocturnal, occupying niches such as insectivory and seed eating. According to the researchers, these early mammals may have outcompeted dinosaurs for access to small body sizes. Conversely, large dinosaurs likely suppressed the growth of early mammals. It wasn’t until after the asteroid strike that mammals began to diversify and occupy a broader range of ecological roles. With the extinction of non-avian dinosaurs, mammals were free to evolve into larger forms and fill the vacant niches left behind. While the study offers compelling explanations, some questions remain unanswered. One possibility is that tiny dinosaurs simply did not exist. The researchers acknowledge that it is possible that such species were present but left few traces in the fossil record. However, they point out that fossil sites rich in dinosaur remains often contain evidence of small animals, making this scenario unlikely.