Plant-eating sauropod dinosaurs were ecosystems engineers, profoundly changing their environments by knocking down trees and eating high volumes of vegetation. Following the end-Cretaceous mass extinction about 66 million years ago, the forests grew back thicker, blocking the Sun from reaching the ground layer, which, many generations later, led to the growth of large seeds and fruit; in time, these fruits became a primary food source for many animal species, including our primate ancestors. New research led by Northern Arizona University scientists provides mechanistic evidence in favor of this theory.
A herd of the Jurassic sauropod dinosaur Camarasaurus walks through a mostly coniferous floodplain forest in what will one day be Utah. Such large dinosaurs transform the landscape with their massive footfalls and bodies, damaging trees and increasing light levels for the saplings on the forest floor. Image credit: Victor O. Leshyk / Northern Arizona University.
“At first glance, the darker forest understory caused by dinosaur extinctions may seem unimportant, but it could have directly led to the evolution of our fruit-eating primate ancestors,” said Northern Arizona University’s Professor Christopher Doughty.
More than 66 million years ago, when the world still had big lumbering dinosaurs, the average seed size of plants was small, and fruits were rare. After their extinction, seeds and fruits increased exponentially in size.
Researchers have hypothesized this happened because in denser forests, competition for light incentivized trees to grow taller and faster than their neighbors, and trees grown from larger seeds had a head start in that competition.
As a bonus, investing in lush, tasty fruit made them more likely to be ingested and dispersed by animals, helping the plants to thrive. However, there hasn’t been much evidence to support this theory.
To address this, Professor Doughty and his colleagues created a model where seed and fruit size increased as a response to the darker forest understory that followed the dinosaur extinction, matching actual seed size trends in the past 65 million years.
They incorporated recently gained understanding of how large animals affect forest structure, how seeds grow into seedlings and saplings and how animal size has changed over time.
The model closely replicated the observed trends in seed and animal size over time.
But what came next, when they continued running the model, was a surprise.
The data pointed to a mysterious phenomenon in the fossil record: about 35 million years ago, seeds reversed course and started getting smaller, because land animals had once again become sufficiently large to have a similar effect on the forests as the dinosaurs did, though proportionately smaller.
“Our model predicted these animals would open the forest enough that sufficient light began to enter the understory, and larger seeds were no longer successful over smaller seeds,” Professor Doughty said.
“The evolutionary pressure for seed size to increase began to diminish.”
“Thus, we were able to explain the trends in seed size over time without resorting to external influences such as climate change.”
“These results provide a striking example of how large dinosaurs — and their eventual extinction — not only shaped their contemporary environment but also triggered cascading effects on ecosystems for millions of years,” said Benjamin Wiebe, a Ph.D student at Northern Arizona University.
“The next time you’re eating fruit or pondering, ‘why am I here,’ consider the impact of the dinosaur extinctions!”
“Another change occurred about 50,000 years ago, when another large extinction event wiped out prehistoric mammals such as mammoths.”
“Without these ecosystem engineers, forest understories again darkened, and the model predicted a long-term increase in seed size in response to the absence of these animals.”
The team’s paper was published today in the journal Palaeontology.
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Christopher E. Doughty et al. 2025. Ecosystem engineers alter the evolution of seed size by impacting fertility and the understory light environment. Palaeontology 68 (1): e70002; doi: 10.1111/pala.70002