Organ reconstruction and systematic relationships of Late Cretaceous palm stems and roots

Permineralized palm stems and adventitious roots are classified under specific organ-type genera (<i>Palmoxylon</i> or <i>Rhizopalmoxylon</i>) without affinities below the family level. Fragments of vegetative basal axes provide an incomplete picture of histological changes due to axial and radial differences in cell growth. However, stem and root anatomical descriptors were previously reported based on general and consistent internal structural features. Two root specimens from the Tarahumara Formation and six stem specimens from the Olmos Formation are included as terminals in a previously reported family-wide morphological and molecular character matrix at the genus level enriched with stem and root anatomical descriptors to elucidate their systematic relationships. Late Cretaceous vegetative basal axes from northern Mexico have systematic affinities to subfamily Coryphoideae. The root system of <i>Palmoxylon polymorphum</i> is included in its original stem description and this fossil species is inferred to have a sister relationship to Cryosophileae. Circumscription of <i>Rhizopalmoxylon huepaciense</i> is emended to include former <i>R. teguachiense</i>, and a systematic affinity to the syncarpous clade is inferred. Circumscription of <i>Palmoxylon longum</i> is also emended, to include former <i>P. commune</i>, <i>P. enochii</i>, <i>P. fibrosum</i> and <i>P. kikaapoa</i>, and a systematic affinity to tribe Borasseae is inferred. These findings highlight the utility of cladistic analyses for reconstructing fossil fragments of palm vegetative basal axes and for inferring their systematic relationships. They also support a Late Cretaceous divergence of Coryphoideae and raise questions about its biogeographic history.

硅化棕榈茎与不定根可归入特定的器官型属：掌木化石属（Palmoxylon）或根掌木化石属（Rhizopalmoxylon），其亲缘关系仅能界定至科级，无法确定科下等级的亲缘关联。由于细胞生长存在轴向与径向差异，营养基轴的化石碎片无法完整反映其组织学变化过程。不过此前已有研究基于普遍且一致的内部结构特征，报道了茎与根的解剖学描述。本次研究将2件采自塔拉乌马拉组的根化石标本、6件采自奥尔莫斯组的茎化石标本作为终端分类单元，加入此前已发表的科级覆盖范围、属级水平的形态-分子特征矩阵，并补充茎与根的解剖学特征数据，以阐明这些化石的系统发育关系。产自墨西哥北部的晚白垩世营养基轴化石，其系统亲缘关系可归入扇叶棕榈亚科（Coryphoideae）。多形掌木化石（Palmoxylon polymorphum）的根系特征被纳入其原始茎部描述中，该化石物种被推断与菜棕族（Cryosophileae）构成姊妹群。本次研究对乌埃帕克根掌木化石（Rhizopalmoxylon huepaciense）的分类界定进行修订，将原特瓜奇根掌木化石（R. teguachiense）归入该种，并推断其与合心皮类群具有系统亲缘关系。同时修订长掌木化石（Palmoxylon longum）的分类界定，将原普通掌木化石（P. commune）、伊诺克掌木化石（P. enochii）、纤维掌木化石（P. fibrosum）与基卡波掌木化石（P. kikaapoa）纳入该种，并推断其与扇叶棕族（Borasseae）具有系统亲缘关系。本研究结果凸显了分支系统学分析在重建棕榈营养基轴化石碎片、阐明其系统发育关系中的应用价值，同时支持扇叶棕榈亚科在晚白垩世发生分化的观点，并为其生物地理历史研究提出了新的科学问题。