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''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),〔http://news.msn.com/science-technology/jurassic-squirrel-fossil-hair-came-before-mammals〕 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. ==Description== ''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.〔 The middle ear of ''Megaconus'' is more primitive than that of modern mammals. The three bones that make up the middle ear in modern mammals — the malleus, incus, and stapes — originated from the lower jaw in the ancestors of mammals. In ''Megaconus'', these bones are still associated with the lower jaw, placed within a groove behind the dentary bone of the lower jaw.〔 ''Megaconus'' is estimated to have weighed about . It probably had an outwardly similar appearance to multituberculates, a major group of Mesozoic mammals. However, its body is longer than those of multituberculates and most other Mesozoic mammaliaforms. It has more back vertebrae (24) than other early mammals. The transition between the thoracic and lumbar vertebrae is more gradual in ''Megaconus'' than it is in multituberculates, and the boundary between the anterior and posterior epaxial muscles (the muscles that cover the front part and the back part of the back, respectively) is positioned farther back.〔 ''Megaconus'' is inferred to have been ambulatory, meaning that it walked on the ground. Its claws are short, meaning that they were not suitable for digging, and only slightly curved, meaning that they were not suitable for climbing. The lack of an elongated calcaneus or "heel" on the ankle means that it could not have run fast. However, ''Megaconus'' differs from most ambulatory mammals in having a fused tibia and fibula in its lower leg, a feature that is normally seen in jumping or bipedal mammals. Among the only living ambulatory mammals to have a fused tibia and fibula are the armadillo and the aardvark.〔 Remains of fur are preserved on parts of the skeleton. The fur consists of dark guard hairs and a lighter layer of underfur. Fur seems to be absent on the underside of the abdomen. A keratinous spur on the rear leg may have been used to deliver poison, like the spur of the living platypus.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Megaconus」の詳細全文を読む スポンサード リンク
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