Diet and body mass are inextricably linked in vertebrates, but a 66-million-year-old relationship linked to animal evolution and survival is being interrupted by humans, according to new research.

Animals that are exclusively herbivores or carnivores are generally much larger in size than omnivores or invertivores -- animals that only eat invertebrates -- according to the study, published Thursday in the journal Nature Ecology and Evolution.

Described as a "roughly U-shaped relationship," University of Nebraska researchers found the pattern has existed for at least 66 million years.

The shape represents carnivores and herbivores on either raised end, with the smaller invertivores and omnivores in the lower middle area of the letter.

Scientists believe that the fundamental feature of past and present ecosystems is being interrupted by humans, who are systematically eliminating the largest carnivores and herbivores from the face of the Earth through extinction.

The consequences of doing so are unpredictable, researchers say.

"We're not sure what's going to happen, because this hasn't happened before," study co-author Will Gearty said in a press release.

"But because the systems have been in what seems to be a very steady state for a very long time, it's concerning what might happen when they leave that state," said Gearty, a postdoctoral researcher at Nebraska.

The plant-based diet of herbivores is relatively poor in nutrition, meaning they often grow very large for the sake of covering more ground to forage more food.

On the other hand, carnivores generally grow large enough to both keep up with and take down those herbivores, keeping their stomachs continually full.

"You can be as big as your food will allow you to be. At the same time, you're often as big as you need to be to catch and process your food. So there's an evolutionary interplay there," Gearty said.

The result of this interplay is the U-shaped distribution of both average and maximum body sizes in mammals, the researchers said.

This U curve stretches back at least 66 million years, to a time when non-avian dinosaurs had just been wiped out, and mammals had yet to diversify and dominate the planet's surface.

"To my knowledge, this is the most extensive investigation of the evolution of body size and especially diet in mammals over time," Gearty said.

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