Jurassic stem-mammal perinates and the origin of mammalian reproduction and growth

Eva A. Hoffman () and Timothy B. Rowe
Additional contact information
Eva A. Hoffman: The University of Texas at Austin
Timothy B. Rowe: The University of Texas at Austin

Nature, 2018, vol. 561, issue 7721, 104-108

Abstract: Abstract 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 Mammaliaformes4–6. 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.

Keywords: Tritylodontids; Positive Allometry; Kayenta Formation; Clutch Size; Early Jurassic Sediments (search for similar items in EconPapers)
Date: 2018
References: Add references at CitEc
Citations:

Downloads: (external link)
https://www.nature.com/articles/s41586-018-0441-3 Abstract (text/html)
Access to the full text of the articles in this series is restricted.

Related works:
This item may be available elsewhere in EconPapers: Search for items with the same title.

Export reference: BibTeX RIS (EndNote, ProCite, RefMan) HTML/Text

Persistent link: https://EconPapers.repec.org/RePEc:nat:nature:v:561:y:2018:i:7721:d:10.1038_s41586-018-0441-3

Ordering information: This journal article can be ordered from
https://www.nature.com/

DOI: 10.1038/s41586-018-0441-3

Access Statistics for this article

Nature is currently edited by Magdalena Skipper

More articles in Nature from Nature
Bibliographic data for series maintained by Sonal Shukla () and Springer Nature Abstracting and Indexing ().