Although the initial formulation of modern concepts of adaptive radiation arose from consideration of the fossil data, rigorous attempts to identify this phenomenon in the fossil record are largely uncommon. Here I focus on direct evidence of the diet (through tooth-wear patterns) and ecologically-relevant traits of one of the most renowned fossil vertebrates-the Miocene ruminant Hoplitomeryx from the island of Gargano-to deepen our understanding of the most likely causal forces under which adaptive radiations emerge on islands. Results show how accelerated accumulation of species and early-bursts of ecological diversification occur after invading an island, and provide insights on the interplay between diet and demographic (population-density), ecological (competition/food requirements) and abiotic (climate-instability) factors, identified as drivers of adaptive diversification. A pronounced event of overpopulation and a phase of aridity determined most of the rate and magnitude of radiation, and pushed species to expand diets from soft-leafy foods to tougher-harder items. Unexpectedly, results show that herbivorous mammals are restricted to browsing habits on small-islands, even if bursts of ecological diversification and dietary divergence occur. This study deepens our understanding of the mechanisms promoting adaptive radiations, and forces us to reevaluate the role of diet in the origins and evolution of islands mammals.