How and when did mammals transition from laying eggs to live birth? The fossil record doesn’t typically preserve soft tissues like embryos and placentas making this a tough problem. This week we take a look at two new fossil discoveries that begin to answer the question of mammal evolution, and challenge orthodoxy. A cute baby dicynodont fossil from the Triassic of South Africa is the first mammal to be captured ‘in ovo’ i.e. inside its egg. Secondly, we discuss a new fossil cynodont from the Triassic of Argentina that might have recorded its growth and birth in its bone histology. Using this we begin to piece together our own evolutionary history and estimate when placental mammals ‘got cracking’. Along we way we ask if viviparity evolved twice and whether teenagers can be trusted to go to the shops to get eggs.

The papers discussed this week are “The first non-mammalian synapsid embryo from the Triassic of South Africa” by Julien Benoit and colleagues published in PLoS One in April 2026 https://doi.org/10.1371/journal.pone.0345016 and “Early origin of viviparity in the mammalian lineage” by Leandro Carlos Gaetano and colleagues published in Frontiers in Mammal Science https://doi.org/10.3389/fmamm.2026.1845319

Palaeoart by Sophie Vrard.