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.

本文记述了产自美国俄勒冈州中部约翰戴盆地克劳诺组始新世中期遗址克劳诺坚果层（Clarno Nut Beds）的一种雷兽科（Brontotheriidae）新物种。尽管克劳诺坚果层以产出丰富的植物化石而闻名，但其中的脊椎动物化石十分稀少，该新物种是该遗址中已知产出量最高的哺乳类化石。经放射性同位素测年限定，该坚果层动物群的时代为北美陆生哺乳动物Uintan期，距今约43.76 Ma。该新分类单元以大量颅骨、下颌骨及牙齿标本为代表，相较于其他雷兽体型相对小巧，其最具辨识度的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——终末后巨兽（Metatelmatherium ultimum）、短鼻威克兽（Wickia brevirhinus）与切齿强肢兽（Sthenodectes incisivum）——共享。系统发育分析显示，该克劳诺坚果层雷兽的姊妹群可能为以下两个物种之一：产自科罗拉多州沙洗盆地与怀俄明州瓦沙基组的短鼻威克兽（Wickia brevirhinus），或是分布于犹他州Uintan组（及其他Uintan年代沉积）与中国内蒙古伊尔丁曼哈组（Irdin Manha Formation）的泛白令陆区物种终末后巨兽（Metatelmatherium ultimum）。系统发育分析结果同时表明，该克劳诺坚果层雷兽属于侏儒型分类单元。尽管雷兽类以演化出巨型体型而著称，但近数十年来发现的该新雷兽与其他数种同类化石均表明，体型的演化缩小在雷兽科（Brontotheriidae）中或许相对常见。