The size of marine invertebrate predators has increased over the past 500 million years, while the size of their prey has not, a new study reveals. The results support a popular hypothesis that has been difficult to confirm. The so-called "escalation hypothesis" suggests that top-down pressure from increasingly powerful and metabolically active predators has driven evolutionary trends in prey; for example, by increasing their motility, burrowing, or defensive arsenal. Yet, reconstructing the ancient, complex interactions among predator and prey species, to gather insights into the likelihood of the escalation hypothesis, has been challenging. Here, Adiël A. Klompmaker and colleagues reconstructed predator-prey interactions of marine organisms in the Phanerozoic period, which spans the past 540 million years, by analyzing attack marks, or "drill holes," in the fossilized shells of various types of marine prey. First, they compared the sizes of hundreds of drill holes to the sizes of predators, confirming drill hole size increased as predator size did. Then they examined Phanerozoic trends in drill-hole size, prey size, and predator-prey size ratios across nearly 7,000 specimens, representing more than 360 taxa. While the median shell size of prey remained more or less constant over the course of a vast portion of the Phanerozoic period, the median drill-hole diameter increased from 0.35 to 3.25 millimeters, indicating a significant increase in the size of predators relative to their prey. The authors propose that the increase in the predator-prey size ratio is driven by dramatic changes in the energetic structure of marine ecosystems, where marine predators had to consume more prey in order to satisfy their ever-growing body size and appetites. Coinciding with this increase in predator size is a shift towards prey that are more mobile and better at burrowing, an effect that is consistent with predator-driven evolution, the authors say.