The tangled world of the early mammals

Independent reports of two newly discovered fossils from the Jurassic of China (around 160–165 million years old) produce conflicting reconstructions of the origins of mammals. Haramiyids were Mesozoic mammals with strange, highly derived rodent-like teeth. Because of this they have been allied with multituberculates, a larger and highly successful group of rodent-like mammals that lived until the Eocene. The problem with haramiyids is that they were until recently known only from teeth. A report by Jin Meng and colleagues reveals a much more complete creature whose features ally it with multituberculates, confirming earlier views — but also implying that the roots of extant mammals lie well back in the Triassic. By contrast, the haramiyid described by Zhe-Xi Luo and colleagues is startlingly primitive in many features of the jaw and ankle, implying that the haramiyids go way back in the mammalian scheme of things and are not related to multituberculates at all. This dichotomy is a reminder of just how little we know about fossils whose interpretation is crucial to the early evolution of mammals.