Dietary specializations and diversity in feeding ecology of the earliest stem mammals

Abstract

Differences in function and dietary ecology between Morganucodon and Kuehneotherium show that lineage splitting during the earliest stages of mammalian evolution was associated with ecomorphological specialization and niche partitioning. The very earliest mammals, living in the Late Triassic and Early Jurassic around 200 million years ago, were small and are often presumed to have been generalized insectivores. Now a close study of two iconic Early Jurassic basal mammals, Morganucodon and Kuehneotherium, shows that niche partitioning and dietary specialization were well under way even at that early date. Analysis of tooth wear and jaw biomechanics shows that whereas Morganucodon had powerful jaws, capable of crushing hard prey such as beetles, Kuehneotherium was adapted for snapping at softer prey. The origin and radiation of mammals are key events in the history of life, with fossils placing the origin at 220 million years ago, in the Late Triassic period1. The earliest mammals, representing the first 50 million years of their evolution and including the most basal taxa, are widely considered to be generalized insectivores1,2. This implies that the first phase of the mammalian radiation—associated with the appearance in the fossil record of important innovations such as heterodont dentition, diphyodonty and the dentary–squamosal jaw joint1,3—was decoupled from ecomorphological diversification2,4. Finds of exceptionally complete specimens of later Mesozoic mammals have revealed greater ecomorphological diversity than previously suspected, including adaptations for swimming, burrowing, digging and even gliding2,5,6, but such well-preserved fossils of earlier mammals do not exist1, and robust analysis of their ecomorphological diversity has previously been lacking. Here we present the results of an integrated analysis, using synchrotron X-ray tomography and analyses of biomechanics, finite element models and tooth microwear textures. We find significant differences in function and dietary ecology between two of the earliest mammaliaform taxa, Morganucodon and Kuehneotherium—taxa that are central to the debate on mammalian evolution. Morganucodon possessed comparatively more forceful and robust jaws and consumed ‘harder’ prey, comparable to extant small-bodied mammals that eat considerable amounts of coleopterans. Kuehneotherium ingested a diet comparable to extant mixed feeders and specialists on ‘soft’ prey such as lepidopterans. Our results reveal previously hidden trophic specialization at the base of the mammalian radiation; hence even the earliest mammaliaforms were beginning to diversify—morphologically, functionally and ecologically. In contrast to the prevailing view2,4, this pattern suggests that lineage splitting during the earliest stages of mammalian evolution was associated with ecomorphological specialization and niche partitioning.