Osteostraci and Galeaspida are stem-gnathostomes, occupying a key phylogenetic position for resolving the nature of the jawless ancestor from which jawed vertebrates evolved more than 400 million years ago. Both groups are characterized by the presence of rigid headshields that share a number of common morphological traits, in some cases hindering the resolution of their interrelationships and the exact nature of their affinities with jawed vertebrates. Here, we explore the morphological and functional diversity of osteostracan and galeaspid headshields using geometric morphometrics and computational fluid dynamics to constrain the factors that promoted the evolution of their similar morphologies and informing on the ecological scenario under which jawed vertebrates emerged. Phylomorphospace, Mantel analysis and Stayton metrics demonstrate a high degree of homoplasy. Computational fluid dynamics reveals similar hydrodynamic performance among morphologically convergent species, indicating the independent acquisition of the same morphofunctional traits and, potentially, equivalent lifestyles. These results confirm that a number of the characters typically used to infer the evolutionary relationships among galeaspids, osteostracans and jawed vertebrates are convergent in nature, potentially obscuring understanding of the assembly of the gnathostome bodyplan. Ultimately, our results reveal that while the jawless relatives of the earliest jawed vertebrates were ecologically diverse, widespread convergence on the same hydrodynamic adaptations suggests they had reached the limits of their potential ecological diversity—overcome by jawed vertebrates and their later innovations.

中文翻译：

---

干颚动物形态同源性的功能评估

Osteostraci 和 Galeaspida 是干颚类动物，在解决 4 亿多年前有颚脊椎动物进化而来的无颚祖先的性质方面占据了关键的系统发育位置。这两个群体的特点是存在具有许多共同形态特征的刚性头罩，在某些情况下阻碍了它们相互关系的解决以及它们与有颚脊椎动物亲缘关系的确切性质。在这里，我们使用几何形态测量学和计算流体动力学来探索骨质和盖帽头罩的形态和功能多样性，以限制促进它们相似形态进化的因素，并告知有颌脊椎动物出现的生态情景。形态空间，Mantel 分析和Stayton 指标显示了高度的同质性。计算流体动力学揭示了形态趋同物种之间相似的流体动力学性能，表明独立获得相同的形态功能特征和潜在的等效生活方式。这些结果证实，一些通常用于推断 galeaspids、osteostracans 和有颚脊椎动物之间进化关系的特征在本质上是趋同的，这可能会掩盖对 gnathostome bodyplan 组装的理解。最终，我们的研究结果表明，虽然最早的有颌脊椎动物的无颌亲属在生态上是多样化的，