A skeleton of a Uintan machaeroidine ‘creodont’ and the phylogeny of carnivorous eutherian mammals

An edentulous partial skeleton of a carnivorous mammal from the Uinta Formation (middle Eocene) of Utah is referred to the rare and enigmatic sabre-tooth clade Machaeroidinae primarily on the basis of alveolar patterns and cranial morphology. The newly recognized skeleton includes portions of both girdles, all long bones, and the first known tarsal and phalangeal material of a machaeroidine. The specimen permits a preliminary reconstruction of the locomotor habits of machaeroidines, which appear to have been adapted to scansorial or arboreal rather than terrestrial locomotion. The new material also prompts a review of machaeroidine affinities, which have been unresolved for over a century, with consensus opinion favouring a link to either oxyaenid or limnocyonine hyaenodontid ‘creodonts’. Postcranial evidence favours a link to oxyaenids, as machaeroidines share numerous features with oxyaenids that are lacking in hyaenodontids. To test this relationship machaeroidines were included in a phylogenetic analysis broadly sampling early carnivorous eutherians, including members of both ‘Creodonta’ and Carnivoramorpha. Results place Machaeroidinae within Oxyaenidae but fail to support either ‘creodont’ or carnivoramorphan monophyly. Instead, Oxyaenidae is linked with Carnivoraformes, while Viverravidae is basally positioned among carnivorous eutherians. Reconsideration of the character evidence cited in support of Carnivoramorpha indicates that many features are ambiguous in the context of a broad sample of ‘creodonts’ and early carnivoramorphans. Hyaenodontid monophyly is also not recovered but this likely reflects the influence of one morphologically divergent genus, Arfia.

来自犹他州 Uinta 形成层（中新世中期）的一具无齿肉食哺乳动物部分骨骼，主要基于牙槽模式及颅骨形态，被归入罕见的神秘剑齿类群 Machaeroidinae。这具新发现的骨骼包括两具骨盆的部分、所有长骨以及已知的第一批 Machaeroidinae 的跗骨和指骨材料。该标本允许对 machaeroidines 的运动习性进行初步重建，它们似乎适应了攀爬或树栖而非地面运动。新材料亦促使对 machaeroidine 的亲缘关系进行审视，这一关系自一个多世纪以来尚未解决，共识观点倾向于与氧齿科或淡湖科鬣狗科‘肉食目’存在联系。脊椎骨的证据倾向于与氧齿科相关，因为 machaeroidines 与氧齿科共享许多在鬣狗科中缺失的特征。为了测试这种关系，将 machaeroidines 包含在广泛的早期食肉真兽目的系统发育分析中，包括‘肉食目’和肉食形目的成员。结果将 Machaeroidinae 放在 Oxyaenidae 内部，但未能支持‘肉食目’或肉食形目的单系群。相反，Oxyaenidae 与肉食形目相联系，而 Viverravidae 则在食肉真兽目中处于基位。对支持肉食形目的特征证据的重新审视表明，在广泛的‘肉食目’和早期肉食形目样本中，许多特征都具有模糊性。鬣狗科的单系群也未恢复，这很可能反映了形态上差异较大的一个属——Arfia的影响。