Alternative interpretations of early mammalian history
Reproduced with permission by Macmillan Publisher Ltd.
from Cifelli and Davis (here) copyright 2013
Crown Mammalia are the extant monotremes, marsupials and placentals together with all extinct species with which they share a common ancestor. It is widely accepted that they include the multituberculates that became extinct in the Oligocene.
Two well preserved Jurassic fossils from another group of mammaliforms, the haramiyids, are described in the current issue of Nature. In one paper (here) the fossil form (Arboroharamiya jenkinsi) is found to group with the multituberculates and thus within Crown Mammalia. Since the haramyids have a deep fossil record this pushes the common ancestor of crown mammals back into the Late Triassic.
The second analysis (here) of a different fossil (Megaconus mammaliaformis) places haramiyids outside crown placentals, which thus can retain a Jurassic origin.
An accompanying commentary (here) notes that a Triassic origin is in accordance with the "long fuse" hypothesis of mammalian evolution whereas a Jurassic origin better corresponds to the "explosive" hypothesis. These authors tend to favour the conclusion based on Megaconus as it is a more complete fossil and the counterparts of the middle ear bones remain attached to the mandible, differing in this respect from extant mammals and multituberculates.