Data from: A small-bodied species of Brontotheriidae from the Middle Eocene Nut Beds of the Clarno Formation, John Day Basin, Oregon

We diagnose a new species of Brontotheriidae from a middle Eocene locality, the Clarno Nut Beds, from the Clarno Formation, John Day Basin, Central Oregon. Though renowned for its richness in fossil flora, fossil vertebrates are rare in the Clarno Nut Beds and this new species is the most abundantly represented mammal. Radiometric dating constrains the age of the Nut Beds fauna to about 43.76 Ma within the Uintan North American Land Mammal Age. This new taxon, represented by numerous cranial, mandibular and dental specimens, is comparatively small for a brontothere and notable for its cranio-caudally shorted nasal incision, a trait shared with three larger-bodied middle Eocene species, Metatelmatherium ultimum, Wickia brevirhinus, and Sthenodectes incisivum. Phylogenetic analysis suggests the sister taxon of the Nut Beds brontothere could be one of two species, Wickia brevirhinus from the Sand Wash Basin of Colorado and Washakie Formation of Wyoming, or Metatelmatherium ultimum, a pan-Beringian species known from the Uintan Formation of Utah (and other Uintan age deposits) and the “Irdin Manha” Formation of Inner Mongolia, China. Phylogenetic results also indicate that the Nut Beds brontothere is a dwarf taxon. Though brontotheres are renowned for having evolved very large body sizes, this new brontothere is one of several discovered in recent decades that suggest evolutionary reductions in body size may have been relatively common in Brontotheriidae.我们对产自俄勒冈州中部约翰·戴盆地（John Day Basin）克拉诺组（Clarno Formation）的始新世中期化石点克拉诺坚果层（Clarno Nut Beds）的一种雷兽科（Brontotheriidae）新物种开展了分类鉴定研究。尽管克拉诺坚果层以产出丰富的植物化石闻名，但其脊椎动物化石十分稀少，该新物种是其中最具代表性的哺乳类类群。通过放射性同位素测年，将该坚果层动物群的年代限定在北美陆地哺乳动物分期的Uintan期内，约为43.76 Ma（即4376万年前）。该新分类群以大量头骨、下颌骨及牙齿标本为代表，相较于其他雷兽体型相对小巧，其显著特征为颅尾向缩短的鼻切迹，这一性状与另外三种体型更大的始新世中期物种——终极后槽雷兽（Metatelmatherium ultimum）、短鼻威克雷兽（Wickia brevirhinus）以及切齿强雷兽（Sthenodectes incisivum）共有。系统发育分析结果显示，该克拉诺坚果层雷兽的姊妹群可能为以下两个物种之一：产自科罗拉多州桑德沃什盆地与怀俄明州沃沙基组的短鼻威克雷兽（Wickia brevirhinus），或是分布于犹他州乌因塔组（Uintan Formation，及其他Uintan期地层）与中国内蒙古的伊尔丁曼哈组（"Irdin Manha" Formation）的泛白令区物种终极后槽雷兽（Metatelmatherium ultimum）。系统发育分析结果同时表明，该坚果层雷兽属于矮小型分类群。尽管雷兽类以演化出极为庞大的体型而闻名，但该新雷兽是近数十年发现的多个雷兽类物种之一，这些物种表明体型的演化缩减在雷兽科中可能相对普遍。