Larger animals are assumed to ingest larger seeds and consume larger fruits, but empirical studies reveal inconsistent trends between body mass and the average size of fruits and seeds ingested. Furthermore, no studies have explored seed size relationships with morphological traits, such as skull dimensions. Such characteristics might provide more reliable estimates of ingestion ability and allow for accurate predictions of seed dispersal capacity in species for which we lack empirical data, especially extinct species. To determine whether (i) mammalian skull dimensions are better predictors of the maximum size of ingested seeds and fruits, compared to body mass and (ii) body mass are the better predictors of mean fruit and seed sizes, we studied these relationships across three mammalian orders: Chiroptera, Primates and Carnivora. We collected novel data on skull dimensions and collated available data on body mass and maximum and mean sizes of ingested fruits and seeds for mammals (N = 100) across the Neotropics, Asia, Africa and Madagascar. We explored the relationships between anatomical traits and fruit and seed sizes of extant species and made predictions for five extinct species. Our results revealed that body mass and skull dimensions are essential determinants of ingested fruit and seed size in mammals. The latter traits can generate predictions for extinct species, especially coronoid height and maximum jaw gape. Nevertheless, body mass predicted larger ingested fruits and seeds than skull dimensions and explained a greater part of the variance for both maximum and mean sizes in our dataset. Our results show how body mass and cranial anatomy constrain seed size and reinforce the importance of maintaining functional diversity in seed dispersers to maintain tropical forest structure. We also show that scientists can use morphological characteristics to predict the seed dispersal potential of extinct mammals allowing better inferences on past and future consequences of frugivore extinctions within tropical forests.