Data from: Ancient collagen reveals evolutionary history of the endemic South American ‘ungulates’

Since the late eighteenth century, fossils of bizarre extinct creatures have been described from the Americas, revealing a previously unimagined chapter in the history of mammals. The most bizarre of these are the ‘native’ South American ungulates thought to represent a group of mammals that evolved in relative isolation on South America, but with an uncertain affinity to any particular placental lineage. Many authors have considered them descended from Laurasian ‘condylarths’, which also includes the probable ancestors of perissodactyls and artiodactyls, whereas others have placed them either closer to the uniquely South American xenarthrans (anteaters, armadillos and sloths) or the basal afrotherians (e.g. elephants and hyraxes). These hypotheses have been debated owing to conflicting morphological characteristics and the hitherto inability to retrieve molecular information. Of the ‘native’ South American mammals, only the toxodonts and litopterns persisted until the Late Pleistocene–Early Holocene. Owing to known difficulties in retrieving ancient DNA (aDNA) from specimens from warm climates, this research presents a molecular phylogeny for both Macrauchenia patachonica (Litopterna) and Toxodon platensis (Notoungulata) recovered using proteomics-based (liquid chromatography–tandem mass spectrometry) sequencing analyses of bone collagen. The results place both taxa in a clade that is monophyletic with the perissodactyls, which today are represented by horses, rhinoceroses and tapirs.

自18世纪晚期以来，美洲地区已有诸多奇异已灭绝生物的化石被记述，揭示了哺乳动物演化史上一段此前从未被认知的篇章。其中最为奇特的当属被认为是在南美洲相对孤立环境中演化而来的“本土”南美有蹄类动物，但它们与任何特定胎盘类演化支的亲缘关系均尚不明确。诸多学者认为它们起源于劳亚大陆的踝节目（condylarths）动物，该类群同时可能也是奇蹄目（perissodactyls）和偶蹄目（artiodactyls）的祖先；而另有学者则提出，它们要么与仅分布于南美洲的异关节类（xenarthrans，食蚁兽、犰狳和树懒）亲缘关系更近，要么与基部非洲兽总目（basal afrotherians，如大象、蹄兔）关系更近。由于形态学特征存在冲突，且此前始终无法获取其分子信息，这些假说一直饱受争议。在“本土”南美哺乳动物类群中，仅有箭齿兽类（toxodonts）和滑距骨兽类（litopterns）延续至晚更新世-早全新世时期。鉴于从温暖气候地区的标本中获取古DNA（aDNA）存在公认的难度，本研究通过基于蛋白质组学的液相色谱-串联质谱（liquid chromatography–tandem mass spectrometry）技术对骨胶原进行测序分析，成功获取了巴塔哥尼亚马驼兽（Macrauchenia patachonica，滑距骨目Litopterna）与扁齿兽（Toxodon platensis，南方有蹄目Notoungulata）的分子系统发育数据。研究结果表明，这两个类群均归入一个与奇蹄目构成单系群的演化支，现生奇蹄目类群涵盖马、犀牛和貘。