Lepanus parapisoniae, a species that was sequenced for this study. Cate Lemann/Australian National Insect Collection/CSIRO
Janet Fang 16 May 2016, 09:14

Researchers studying scarab beetle DNA have revealed that dung eaters were around in the Lower Cretaceous 115 million years ago, or 30 million years earlier than we thought. Dung beetles may have evolved and diversified with dinosaurs and the rise of flowering plants. The work, published in PLOS One this week, is the first to demonstrate that the speciation of a group was tied to the use of dinosaur dung as an important ecological resource. 

There are about 30,000 species of scarab beetles, and while the majority of them feed on leaves, fruit, flowers, pollen, and wood, there are several thousands of species that feed on dead or decaying matter: leaf litter, fallen logs, rotting fruit, carcasses, and of course, feces. In fact, there are 5,000 species of dung specialists. Researchers have long debated whether the key event in the evolution of dung beetles (subfamily Scarabaeinae) was an adaptation to feeding on dinosaur or mammal dung. 

To investigate, an international team led by Nicole Gunter of Palacky University generated DNA sequence data on 125 scarab beetles collected from Australia. Together with previously published work downloaded from GenBank, their entire dataset spanned 450 types of scarab beetles. The team then created a molecular family tree that allowed them to compare evolutionary relationships among scarab beetles that fed on plants and those that fed on dead or decaying matter. 

Not surprisingly, they found that the evolution of plant-eating scarab beetles tracked the ecological dominance of flowering plants (or angiosperms). However, they also found that non-plant-eating dung beetles underwent a similar diversification pattern as the herbivorous ones.

These findings place the evolution of dung beetles at about 115 to 130 million years ago in the Lower Cretaceous – some 30 million years earlier than previous studies have suggested. The only mammals around at the time were rodent-sized, and they produced dry pellets of poop. "We hypothesize that the incorporation of flowering plants in the diet of dinosaurs resulted in the first palatable dung source for feeding – providing a new niche for evolution," Gunter said in a statement. Dung beetles may have first evolved with this less fibrous, more nutritious dung. Afterall, fossilized dinosaur feces have previously been found with evidence of tunneling attributed to dung beetle feeding. 

So what happened to dung beetles when non-avian dinosaurs went extinct some 65 million years ago? We know they survived the Cretaceous-Paleogene extinction event, but some dung beetle groups did go extinct around the same time as the dinosaurs. Modern dung beetles may have descended from species that were already adapted to feeding on Cretaceous mammal dung.

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