Megaconus

Megaconus is an extinct genus of mammaliaform from the Middle Jurassic Tiaojishan Formation of Inner Mongolia, China. The type and only species, Megaconus mammaliaformis (nicknamed Jurassic Squirrel), was described in the journal Nature in 2013. Megaconus is thought to have been a herbivore that lived on the ground, having a similar posture to modern-day armadillos and rock hyraxes. Megaconus is a member of a group called Haramiyida. A phylogenetic analysis published along with its first description showed that haramiyidans originated before the appearance of true mammals, but an accompanying description of the haramiyidan Arboroharamiya in the same issue of Nature indicated that haramyidans were true mammals. If haramiyidans are not mammals, Megaconus would be one of the most basal ("primitive") mammaliaforms to possess fur, and an indicator that fur evolved in the ancestors of mammals.

Megaconus is one of the few early mammaliaforms known from a complete skeleton. The skeleton includes both the jaw bones and the teeth, which are the most informative features because they allow for comparisons with other mammaliaforms known only from dental features. Megaconus has a dentition similar to those of rodents, with large incisors at the front of the jaws and broad molars in the back. One distinguishing feature of Megaconus is a pair of enlarged premolar teeth in the lower jaw. The teeth of Megaconus have many cusps, allowing them to interlock tightly when the jaws are closed. If Megaconus is a non-mammalian mammaliaform, it is one of the most basal mammaliaforms to possess such complex teeth.