Herbivores evolved powerful jaws to survive the Triassic

After the largest mass extinction in Earth’s history, herbivores rapidly evolved to eat different types of plants, according to a new study from the University of Bristol. The researchers discovered that herbivores with powerful jaws to chew tougher plants, including some of the earliest dinosaurs, were the most successful. 

The Permian-Triassic extinction event, also known as the Great Dying, took place about 250 million years ago and wiped out at least 90 percent of all species.  The Triassic Period that followed was a time of great change when many new plants and animals emerged. 

“I wanted to focus on the plant-eaters, including some of the first dinosaurs,” said Dr. Suresh Singh. “The main animals in any ecosystem tend to be the herbivores, and we found that they show remarkable evidence of specialization in the turbulent times after the great mass extinction.” 

“In fact, the environmental shocks that killed so many species, such as global warming and acid rain, were still returning from time to time, but the survivors were set on exploring new diets.”

“We were surprised to be able to identify definite specializations among the herbivores,” said Dr. Tom Stubbs. “We call these groups, the ingestion generalists, prehension specialists, durophagous specialists, shearing pulpers, and heavy oral processors; these names reflect the power of their jaws, their teeth, and the kinds of plants they would likely have been eating.”

Professor Emily Rayfield noted that it may seem hypothetical to identify the diets of animals that died over 200 million years ago. 

“But we measured hundreds of fossil jaws and compared their shapes with living animals, and we estimated key functional values from the fossil jaws such as mechanical advantage and bite force, measuring the leverage of the jaws and how fast or forcefully they could shut,” explained Professor Rayfield.

The investigation was focused on the 80 million years that followed the mass extinction event. 

“We were able to map the evolution of all the functional characters relating to feeding across the evolutionary tree and through time,” said Dr. Armin Elsler. 

“We found to our surprise that, during this time, several new groups diversified as herbivores, and they seemed to take over their roles by pushing potential competitors aside, not necessarily by wiping them out.”

Plants were also diversifying throughout the Triassic, and certain aspects of the evolution of herbivores can be linked to the availability of new kinds of plant food. 

“One key thing was the expansion of groups that were able to handle tough plant material, with powerful jaws for chopping and chomping,” explained Dr.  Singh. 

“This reflects the drying conditions especially in the Late Triassic, when many softer plant groups became less common, and dry-adapted conifers for example, spread worldwide. These changes combined with strictly enforced niche separation drove patterns of extinction as the generalist, hardy herbivores thrived, as other herbivores died out.”

“This kind of macroecological analysis represents a huge amount of work,” said Professor Mike Benton. “But it sheds real light on key processes at an ecological level and helps explain why some groups died out and were replaced by others such as the first dinosaurs.” 

“We can be reasonably certain of the results because we have excellent data on the geological ages of the specimens, their relationships to each other, and the key features of their jaws and teeth that correspond to their ancient diets. What was once quite a speculative kind of project has now become much more analytical and testable.”

The study is published in the journal Nature Communications.

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By Chrissy Sexton, Earth.com Staff Writer