The debate regarding the evolutionary relationships of the extinct South American native ungulates (SANUs) to the major placental clades Afrotheria and Boreoeutheria is exciting and has profound implications for our understanding of their early diversification and paleobiogeography. Although this controversy has not yet proven resolvable using morphological evidence, paleoproteomic and ancient DNA analyses support that at least some SANUs (i.e., Litopterna and Notoungulata) are members of Boreoeutheria, closely related to Perissodactyla (the Panperissodactyla hypothesis). Here we present a critical assessment of a recently published morphology-based study that claims that: (1) some SANUs (i.e., Notoungulata, Astrapotheria, Pyrotheria, and Xenungulata) represent a monophyletic supraordinal group, the Sudamericungulata, closely related to the Afrotherian hyracoids; and (2) the remaining SANUs (i.e., Litopterna and Didolodontidae, placed in a separate taxon, Panameridiungulata) are boreoeutherian in origin. Because this proposal (hereafter, the Sudamericungulata - Panameridiungulata or S-P hypothesis) is based on an incongruously reduced sample of boreoeutherians (including only a single perissodactyl) and inadequate character sampling restricted to dental and mandibular traits, it cannot be regarded as a satisfactory test of SANU relationships. Moreover, the S-P hypothesis fails to recover monophyletic Boreoeutheria and/or Afrotheria, making it incompatible with all well-established hypotheses of placental diversification. We find that the introduction of molecular constraints forcing the monophyly of Boreoeutheria and Afrotheria produces new trees, all recovering Sudamericungulata and Panameridiungulata nested within Boreoeutheria. These results are consistent with our analyses using a corrected version of the S-P matrix. Although we acknowledge that boreoeutherian affinities have still not been conclusively demonstrated for all nominal SANUs, it is beyond argument that any further credible testing must be based on much more exhaustive surveys than are currently available.

关于已灭绝的南美本土有蹄类动物 (SANUs) 与主要胎盘分支 Afrotheria 和 Boreoeutheria 的进化关系的辩论令人兴奋，并且对我们了解它们的早期多样化和古生物地理学具有深远的影响。尽管这一争议尚未证明可以使用形态学证据解决，但古蛋白质组学和古代 DNA 分析支持至少一些SANU（即Litopterna 和Notoungulata）是与Perissodactyla 密切相关的Boreeutheria 的成员（Panperissodactyla 假说）。在这里，我们对最近发表的一项基于形态学的研究进行了批判性评估，该研究声称：（1）一些SANU（即Notoungulata、Astrapotheria、Pyrotheria和Xenungulata）代表一个单系的超序群，Sudamericungulata，与 Afrotherian hyracoids 密切相关；(2) 其余的SANUs（即Litopterna 和Didolodontidae，置于单独的分类群Panameridiungulata 中）起源于boreeutherian。因为这个提议（以下称为 Sudamericungulata - Panameridiungulata 或 SP 假设）是基于一个不协调的减少的北兽类样本（仅包括单个 perissodactyl）和仅限于牙齿和下颌特征的不充分的字符抽样，它不能被视为令人满意的测试SANU 关系。此外，SP 假设未能恢复单系 Boreeutheria 和/或 Afrotheria，使其与所有已确立的胎盘多样化假设不相容。我们发现引入分子约束迫使 Boreeutheria 和 Afrotheria 单系产生新树，所有正在恢复的 Sudamericungulata 和 Panameridiungulata 都嵌套在 Boreeutheria 内。这些结果与我们使用 SP 矩阵的校正版本的分析一致。尽管我们承认所有名义上的 SANU 仍然没有最终证明 boreeutherian 的亲和力，但毫无疑问，任何进一步的可信测试必须基于比目前可用的更详尽的调查。