School of Earth Sciences, University of Bristol, Life Sciences Building, Tyndall Avenue, Bristol BS8 1TQ, UK.
Instituto Cavanilles de Biodiversidad i Biología Evolutiva, Universitat de València, C/ Catedrático José Beltrán Martínez, 2, 46980 Paterna, Valencia, Spain.
Proc Biol Sci. 2021 Jan 27;288(1943):20202719. doi: 10.1098/rspb.2020.2719. Epub 2021 Jan 20.
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.
盔甲鱼目和盔甲鱼亚纲是有颌脊椎动物的祖先，它们在解决 4 亿多年前从无颌祖先进化而来的有颌脊椎动物的本质方面占据着关键的进化位置。这两个群体的特点是都有坚硬的头甲，具有许多共同的形态特征，在某些情况下，这使得它们的相互关系以及与有颌脊椎动物的确切亲缘关系难以确定。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盔甲鱼目和盔甲鱼亚纲头甲的形态和功能多样性，以限制促进其相似形态进化的因素，并了解有颌脊椎动物出现的生态背景。分支形态空间、Mantel 分析和 Stayton 度量法表明了高度的同形性。计算流体动力学揭示了形态上趋同的物种具有相似的流体动力学性能，表明了相同形态功能特征的独立获得，并且可能具有相同的生活方式。这些结果证实，有颌脊椎动物、盔甲鱼目和盔甲鱼亚纲之间的进化关系通常是趋同的，这可能会掩盖对有颌脊椎动物体节模式组装的理解。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近亲在生态上是多样化的，但它们对相同的流体适应的广泛趋同表明，它们已经达到了其潜在生态多样性的极限——这是有颌脊椎动物及其后来的创新所克服的。
