He and his colleagues drew up a family tree for Megaconus on the basis of comparisons with animals from more than 100 other groups. The results suggest that the common ancestor of all extant mammals lived about 180 million years ago — and that the haramiyids, including Megaconus, branched away from the family tree some 40 million years before true mammals evolved.
Unfortunately, neither family tree is entirely consistent with all the data. Cifelli says that the confusion can be cleared up only with more fossils — preferably ones that include all or a significant part of a skull, whose anatomical features are particularly instructive in working out evolutionary relationships. “To break this tie, we really need more information,” he says.
But there’s another possibility, says Guillermo Rougier, a vertebrate paleontologist at the University of Louisville in Kentucky. Megaconus and Arboroharamiya “have been assigned to the same group, but they’re very different creatures,” he says. Indeed, he adds, Arboroharamiya, the more advanced of the two species in terms of its jawbone and other features, may actually belong in a long-successful but now-extinct group of mammals called multituberculates — a realignment that would explain the disparity between family trees drawn up by these individual studies.
One thing to do now, he suggests, is conduct a single analysis that includes both of the new species, and then see what the mammalian family tree looks like.