Megazostrodon

Megazostrodon is a Mammaliaform that is widely accepted as being one of the first mammals and which appeared in the fossil record approximately 200 million years ago.[1] It did have some non-mammalian characteristics: the first two vertebrae (atlas and axis) were still unfused as in cynodonts, and it only had three sacral vertebrae instead of the usual mammalian five.[2] An interclavicle is also present, another primitive trait still visible in monotremes. However, these were deemed sufficiently minor to warrant the analysis that this animal probably represents the final stage of the transition between cynodonts and true mammals.[3]