Transformations in morphology, physiology and behaviour along the mammalian stem lineage were accompanied by profound modifications to reproduction and growth, including the emergence of a reproductive strategy characterized by high maternal investment in a small number of offspring1,2 and heterochronic changes in early cranial development associated with the enlargement of the brain3. Because direct fossil evidence of these transitions is lacking, the timing and sequence of these modifications are unknown. Here we present what is, to our knowledge, the first fossil record of pre- or near-hatching young of any non-mammalian synapsid. A large clutch of well-preserved perinates of the tritylodontid Kayentatherium wellesi (Cynodontia, Mammaliamorpha) was found with a presumed maternal skeleton in Early Jurassic sediments of the Kayenta Formation. The single clutch comprises at least 38 individuals, well outside the range of litter sizes documented in extant mammals. This discovery confirms that production of high numbers of offspring represents the ancestral condition for amniotes, and also constrains the timing of a reduction in clutch size along the mammalian stem. Although tiny, the perinates have an overall skull shape that is similar to that of adults, with no allometric lengthening of the face during ontogeny. The only positive allometries are associated with the bones that support the masticatory musculature. Kayentatherium diverged just before a hypothesized pulse of brain expansion that reorganized cranial architecture at the base of Mammaliaformes4,5,6. The association of a high number of offspring and largely isometric cranial growth in Kayentatherium is consistent with a scenario in which encephalization—and attendant shifts in metabolism and development7,8—drove later changes to mammalian reproduction.
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We thank A. Zaman and B. Niessemeyer of the Navajo Nation Minerals Division for issuing the permit (dated 27 March 2000) under which this specimen was collected, and K. Calsoyas and T. Anderson for promoting our collaboration with the Navajo EcoScouts Program. Funding was provided by the National Science Foundation (EAR 1561622, IIS-9874781), by the Geology Foundation of The University of Texas and by the Jackson School of Geosciences. We thank B. Andres, S. Egberts, J. Franzosa, R. Gary, E. Gordon, T. Macrini, P. Owen, C. Sagebiel and R. S. Wallace for field, laboratory and curatorial assistance; M. Colbert, J. Maisano, J. Berlin and G. Rogers for computed tomography scanning the specimens described here; and S. Regnault, J. Hutchinson, C. Bell and digimorph.org for Sphenodon computed tomography scans.
Nature thanks H. Sues, L. Wilson and the other anonymous reviewer(s) for their contribution to the peer review of this work.