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Stem mammals Morganucodon and Kuehneotherium dietary specializations

Ben Creisler

A new paper:

Pamela G. Gill, Mark A. Purnell, Nick Crumpton, Kate Robson Brown,
Neil J. Gostling, M. Stampanoni & Emily J. Rayfield (2014)
Dietary specializations and diversity in feeding ecology of the
earliest stem mammals.
Nature 512: 303–305

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 period. The earliest mammals, representing the first 50
million years of their evolution and including the most basal taxa,
are widely considered to be generalized insectivores. 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
joint—was decoupled from ecomorphological diversification. 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
gliding, but such well-preserved fossils of earlier mammals do not
exist, 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 view, this pattern suggests that lineage splitting
during the earliest stages of mammalian evolution was associated with
ecomorphological specialization and niche partitioning.

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