Department of Biological Sciences, Humboldt State University, Arcata, CA, 95521, USA.
Am J Bot. 2018 Aug;105(8):1264-1285. doi: 10.1002/ajb2.1102. Epub 2018 Jun 12.
The deep origin and early evolution of seed plants (spermatophytes) are poorly understood. Starting in the Early Devonian, euphyllophytes diversified rapidly into several groups. Two of these groups, progymnospems and Stenokoleales, along with satellite taxa, have been involved in discussions of seed plant origins. Because these early lineages are extinct, the key to the origin and early evolution of seed plants lies in the fossil record. Decades-long paleobotanical work has produced data on the diversity and anatomy of fossil species, which now provide a foundation for exploring seed plant origins in a phylogenetic context.
We address phylogenetic relationships between early seed plants, aneurophytalean progymnosperms, Stenokoleales, and several Devonian species of uncertain affinities using parsimony analyses that include 28 anatomically preserved species (the most comprehensive taxon sampling to date) and 49 morphoanatomical characters (including nine continuous characters).
Our analyses recover monophyletic seed plants, Stenokoleales, and aneurophytes, with the latter placed as sister to a clade (termed the bilateral clade) that includes the former two. When added in the analysis, continuous characters based on anatomy improve phylogenetic resolution.
Our results support the groups defined by traditional taxonomy, resolve Stenokoleales nested among the lignophytes, and indicate that seed plants may share a closer ancestor with Stenokoleales than with aneurophytes. Additionally, our trees suggest a Givetian minimum age for the seed plant ancestor, a late Emsian minimum age for the Stenokoleales, and early Emsian minimum ages for lignophytes, the bilateral clade, and the aneurophyte ancestor.
种子植物（有胚植物）的深层起源和早期进化仍不清楚。早泥盆世开始，真蕨类植物迅速多样化为几个类群。其中两个类群，原叶植物和石松目，以及一些卫星类群，一直是探讨有胚植物起源的焦点。由于这些早期谱系已灭绝，因此种子植物起源和早期进化的关键在于化石记录。几十年来的古植物学工作已经产生了关于化石物种多样性和解剖结构的数据，这些数据现在为在系统发育背景下探索种子植物的起源提供了基础。
我们使用包含 28 个解剖保存物种（迄今为止最全面的分类群采样）和 49 个形态解剖特征（包括 9 个连续特征）的简约分析，探讨早期种子植物、无叶植物原叶植物、石松目和几个泥盆纪亲缘关系不确定的物种之间的系统发育关系。
我们的分析结果恢复了单系的种子植物、石松目和无叶植物，后者与包括前两者的一个分支（称为双侧分支）为姐妹群。当加入分析时，基于解剖结构的连续特征提高了系统发育分辨率。
我们的结果支持传统分类学定义的类群，解决了石松目在木质植物中的嵌套问题，并表明种子植物与石松目可能比与无叶植物有更近的共同祖先。此外，我们的树状图表明种子植物的祖先是在吉维特期（Givetian），石松目的祖先是在爱姆期晚期（Emsian），木质植物、双侧分支和无叶植物祖先的最早出现时间则在爱姆期早期。
