Apaper published August 29 in Nature describes the discovery of a mammal-like adult creature, named Kayentatherium wellesi, and 38 of its probable young, fossilized in the early Jurassic, 184 million years ago in present-day Arizona. These offspring are the youngest mammal-like creatures discovered in fossil form to date.
E.A. Hoffman, T.B. Rowe, “Jurassic stem-mammal perinates and the origin of mammalian reproduction and growth,” Nature, doi:10.1038/s41586-018-0441-3, 2018.