An adult, mammal-like fossil found preserved with its young is described in a paper published online this week in Nature. Unearthed in Arizona, the new specimens date back to 184 million years ago, and shine light on how the reproduction and growth strategies of modern mammals evolved.

Mammals are essentially defined by reproduction, with almost all bearing live young (rather than laying eggs) that they nurse with milk. As the mammal class developed, it grew to favour both a high investment in relatively few offspring, and changes in early skull development to accommodate bigger brains. However, with the young of both mammals and their forebears rarely preserved as fossils - let alone newborns or embryos - the exact timing of these transitions is unclear.

Eva Hoffman and Timothy Rowe report the discovery of a clutch of young Kayentatherium wellesi individuals buried alongside an adult - presumably their mother - from the early Jurassic of what is now northeastern Arizona. K. wellesi is not a true mammal, but one of a group of mammal-like animals called tritylodonts.

The clutch contains 38 young - twice that which would be expected for any mammal, but comparable with that of reptiles. The young have skulls similar in shape, if not size, to those of the adult. This suggests that K. wellesi grew like modern-day reptiles, without undergoing the cranial lengthening seen in modern mammals as they mature. Together, the association of large litter sizes with uniform cranial growth supports the notion that the evolution of bigger brains drove the later changes in mammalian reproduction and development.