The study of mammalian evolution depends greatly on understanding the evolution of teeth and the relationship of tooth shape to diet. Links between gross tooth shape, function and diet have been proposed since antiquity, stretching from Aristotle to Cuvier, Owen and Osborn. So far, however, the possibilities for exhaustive, quantitative comparisons between greatly different tooth shapes have been limited. Cat teeth and mouse teeth, for example, are fundamentally distinct in shape and structure as a result of independent evolutionary change over tens of millions of years. There is difficulty in establishing homology between their tooth components or in summarizing their tooth shapes, yet both carnivorans and rodents possess a comparable spectrum of dietary specializations from animals to plants. Here we introduce homology-free techniques to measure the phenotypic complexity of the three-dimensional shape of tooth crowns. In our geographic information systems (GIS) analysis of 441 teeth from 81 species of carnivorans and rodents, we show that the surface complexity of tooth crowns directly reflects the foods they consume. Moreover, the absolute values of dental complexity for individual dietary classes correspond between carnivorans and rodents, illustrating a high-level similarity between overall tooth shapes despite a lack of low-level similarity of specific tooth components. These results suggest that scale-independent forces have determined the high-level dental shape in lineages that are widely divergent in size, ecology and life history. This link between diet and phenotype will be useful for inferring the ecology of extinct species and illustrates the potential of fast-throughput, high-level analysis of the phenotype.

Fossils discovered in Lower Cretaceous (Aptian) rocks in the Ténéré Desert of central Niger provide new information about spinosaurids, a peculiar group of piscivorous theropod dinosaurs. The remains, which represent a new genus and species, reveal the extreme elongation and transverse compression of the spinosaurid snout. The postcranial bones include blade-shaped vertebral spines that form a low sail over the hips. Phylogenetic analysis suggests that the enlarged thumb claw and robust forelimb evolved during the Jurassic, before the elongated snout and other fish-eating adaptations in the skull. The close phylogenetic relationship between the new African spinosaurid and Baryonyx from Europe provides evidence of dispersal across the Tethys seaway during the Early Cretaceous.

The Cretaceous-Paleogene mass extinction approximately 66 million years ago is conventionally thought to have been a turning point in mammalian evolution. Prior to that event and for the first two-thirds of their evolutionary history, mammals were mostly confined to roles as generalized, small-bodied, nocturnal insectivores, presumably under selection pressures from dinosaurs. Release from these pressures, by extinction of non-avian dinosaurs at the Cretaceous-Paleogene boundary, triggered ecological diversification of mammals. Although recent individual fossil discoveries have shown that some mammalian lineages diversified ecologically during the Mesozoic era, comprehensive ecological analyses of mammalian groups crossing the Cretaceous-Paleogene boundary are lacking. Such analyses are needed because diversification analyses of living taxa allow only indirect inferences of past ecosystems. Here we show that in arguably the most evolutionarily successful clade of Mesozoic mammals, the Multituberculata, an adaptive radiation began at least 20 million years before the extinction of non-avian dinosaurs and continued across the Cretaceous-Paleogene boundary. Disparity in dental complexity, which relates to the range of diets, rose sharply in step with generic richness and disparity in body size. Moreover, maximum dental complexity and body size demonstrate an adaptive shift towards increased herbivory. This dietary expansion tracked the ecological rise of angiosperms and suggests that the resources that were available to multituberculates were relatively unaffected by the Cretaceous-Paleogene mass extinction. Taken together, our results indicate that mammals were able to take advantage of new ecological opportunities in the Mesozoic and that at least some of these opportunities persisted through the Cretaceous-Paleogene mass extinction. Similar broad-scale ecomorphological inventories of other radiations may help to constrain the possible causes of mass extinctions.

Lower Cretaceous fossils from central Niger document the succession of sauropod dinosaurs on Africa as it drifted into geographic isolation. A new broad-toothed genus of Neocomian age ( approximately 135 million years ago) shows few of the specializations of other Cretaceous sauropods. A new small-bodied sauropod of Aptian-Albian age ( approximately 110 million years ago), in contrast, reveals the highly modified cranial form of rebbachisaurid diplodocoids. Rates of skeletal change in sauropods and other major groups of dinosaurs are estimated quantitatively and shown to be highly variable.