From PBS Eons.

In 2003, a fossil belonging to a mammaliaform was discovered in an ancient lakebed in what’s now China. It was an almost complete skeleton the size of a platypus, a find that complicated the history of mammaliaforms. It painted a picture of their explosive diversification, their mysterious disappearance, and how our own ancestors might have survived thanks to a leg up from some leafy allies…

*They weren’t /exactly/ mammals…

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References:
https://docs.google.com/document/d/1VeRmfSTKqgnd1rKOzjkL8nK4yckEPGYFTtiAqzyKAcg/edit?usp=sharing