The Earliest Evolutionary Arms Race Found in Ancient Shells

More than half a billion years ago, mystery predators bored into shelled animals’ defensive casings. Some of the holey shells became fossilized, and now tell us the story of the earliest known battle between predator and prey that influenced both species’ evolution.

“This critically important evolutionary record demonstrates, for the first time, that predation played a pivotal role in the proliferation of early animal ecosystems,” explains American Museum of Natural History paleontologist Russell Bicknell.

These fossilized shells belong to an early lamp shell relative, Lapworthella fasciculata, found at what’s now Flinders Ranges, South Australia. They lived and died amidst one of the Earth’s earliest explosions of life’s diversity, the Cambrian Explosion.

How such a rapid diversification of life occurred has long fascinated researchers. One mechanism well known to drive evolution is the battle between predators and prey, which often turns into a kind of “arms race.”

“Predator-prey interactions are often touted as a major driver of the Cambrian explosion, especially with regard to the rapid increase in diversity and abundance of biomineralizing organisms at this time,” says Bicknell. “Yet, there has been a paucity of empirical evidence showing that prey directly responded to predation, and vice versa.”

L. fasciculata’s shells provide that example, of ancient interacting species shaping each other’s evolution.

The positioning of the punctures and that they occurred in about the same point in all the examined shells, as well as shells from neighboring species, suggests they were the result of predatory action, the researchers explain.

L. fasciculata’s shells range from the size of a grain of sand to a sunflower seed, and the researchers were able to recover more than 200 of them with the telltale holes of a perforating predator.

By mapping them out according to their biological ages, Bicknell and team could see that the shells got thicker after a spate of hole punching occurred, reducing the frequency of shells with holes.

But over time, the predatory worm or mollusk clearly beefed up whatever its puncturing weapon was, resulting in perforated L. fasciculata shells peaking at rates of almost four percent. Shell thickness increased again, and perforated shell numbers dropped back down to around two percent.

This cycle of prey boosting its defenses and predator boosting its weapons seems to illustrate an evolutionary arms race – and at 517 million years old, it’s now the earliest known example.

It also “shows the rapid speed at which such phenotypic modifications arose during the Cambrian Explosion event,” Bicknell says.

Such strong selection pressures when coupled with separation can lead to the creation of new species.

This research was published in Current Biology.