The Bunkers Graben, Flinders Ranges, in South Australia, where the fossil shells were discovered.© John Paterson
Throughout the natural world, there are many examples of what scientists call an evolutionary “arms race.” Think of snakes evolving potent venom while the rodents they eat develop venom resistance.
This type of ongoing cycle of adaptation between competing species that leads to increasing specialization can be seen in the relationships between predators and prey, parasites and hosts, and even in the development of drug-resistant pathogens.
Now, researchers have discovered the oldest known example of an evolutionary arms race, dating back 517 million years. These ancient marine predator-prey interactions took place between a microscopic shelled animal distantly related to brachiopods and an unknown hole-punching predator. It is the first record of an evolutionary arms race in the Cambrian, a transformative time in Earth’s history between about 541-485 million years ago that saw a burst of evolutionary activity and set the stage for much of today’s biodiversity.
“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. Yet there has been a paucity of empirical evidence showing that prey directly responded to predation, and vice versa,” said Russell Bicknell, a postdoctoral researcher in the Museum’s Division of Paleontology and lead author of the study, which is published today in the journal Current Biology.
Bicknell worked with colleagues from two universities in Australia, the University of New England and Macquarie University, to study a large sample of fossilized shells of an early Cambrian tommotiid species, Lapworthella fasciculata. Hundreds of these micrometer-scale specimens, ranging in size from slightly larger than a grain of sand to just smaller than an apple seed, were preserved in a fossil bed in South Australia.
Using scanning electron microscopy, the researchers identified more than 200 shells with holes that were likely made by a single predator, probably a soft-bodied mollusk or worm. When the research team analyzed these fossils in relation to their geologic ages, they found an increase in shell wall thickness that coincides with an increase in the number of punctured shells.
Their study suggests L. fasciculata and its mystery predator were in a microevolutionary arms race: the shelled animal continuously evolved to strengthen its shell against predation while the predator evolved the ability to pierce its prey despite the thicker armor.
“This critically important evolutionary record demonstrates, for the first time, that predation played a pivotal role in the proliferation of early animal ecosystems and shows the rapid speed at which such phenotypic modifications arose during the Cambrian explosion event,” Bicknell says.