Department of Organismal Biology and Anatomy, University of Chicago, Chicago, IL 60637;
Department of Biological Sciences, University of Alberta, Edmonton T6G 2E9, Canada.
Proc Natl Acad Sci U S A. 2019 Feb 5;116(6):2146-2151. doi: 10.1073/pnas.1814794116. Epub 2019 Jan 22.
Hagfish depart so much from other fishes anatomically that they were sometimes considered not fully vertebrate. They may represent: () an anatomically primitive outgroup of vertebrates (the morphology-based craniate hypothesis); or () an anatomically degenerate vertebrate lineage sister to lampreys (the molecular-based cyclostome hypothesis). This systematic conundrum has become a prominent case of conflict between morphology- and molecular-based phylogenies. To date, the fossil record has offered few insights to this long-branch problem or the evolutionary history of hagfish in general, because unequivocal fossil members of the group are unknown. Here, we report an unequivocal fossil hagfish from the early Late Cretaceous of Lebanon. The soft tissue anatomy includes key attributes of living hagfish: cartilages of barbels, postcranial position of branchial apparatus, and chemical traces of slime glands. This indicates that the suite of characters unique to living hagfish appeared well before Cretaceous times. This new hagfish prompted a reevaluation of morphological characters for interrelationships among jawless vertebrates. By addressing nonindependence of characters, our phylogenetic analyses recovered hagfish and lampreys in a clade of cyclostomes (congruent with the cyclostome hypothesis) using only morphological data. This new phylogeny places the fossil taxon within the hagfish crown group, and resolved other putative fossil cyclostomes to the stem of either hagfish or lamprey crown groups. These results potentially resolve the morphological-molecular conflict at the base of the Vertebrata. Thus, assessment of character nonindependence may help reconcile morphological and molecular inferences for other major discords in animal phylogeny.
八目鳗在解剖学上与其他鱼类有很大的不同，以至于它们有时被认为不完全是脊椎动物。它们可能代表：（1）基于形态的有头类假说中的一个解剖学上原始的祖群；或者（2）基于分子的圆口类假说中的一个解剖学退化的脊椎动物谱系姐妹群。这种系统上的难题已经成为形态学和分子系统发育之间冲突的一个突出案例。迄今为止，化石记录对这个长枝问题或八目鳗的进化历史提供的见解很少，因为该类群的明确化石成员尚不清楚。在这里，我们报告了一个来自黎巴嫩晚白垩世的明确的八目鳗化石。软组织解剖结构包括活八目鳗的关键属性：触须软骨、鳃器的后体位置和粘腺的化学痕迹。这表明，生活在八目鳗中的独特特征组合在白垩纪之前就已经出现了。这个新的八目鳗促使我们重新评估无颌脊椎动物之间的形态学特征。通过解决特征的非独立性，我们的系统发育分析仅使用形态学数据，在一个无颌类动物（与圆口类假说一致）的分支中恢复了八目鳗和七鳃鳗。这个新的系统发育将化石分类单元置于八目鳗冠群内，并将其他假定的化石圆口类分类单元置于八目鳗或七鳃鳗冠群的基部。这些结果可能解决了脊椎动物基部的形态学-分子冲突。因此，评估特征的非独立性可能有助于调和动物系统发育中其他主要分歧的形态学和分子推断。
