Data from: Patterns of ecological diversification in thelodonts

Here we explore the spatial, temporal and phylogenetic patterns of ecological diversification for the entire clade of thelodonts, one of the earliest groups of vertebrates and longest lasting of the Palaeozoic agnathans in the fossil record. Parsimony and maximum-likelihood methods are used to reconstruct ancestral states of their geographical distributions, habitats and lifestyles. Our results support the concept that thelodonts originated during the Middle?–Late Ordovician probably in marine open waters of Laurasia, with a demersal lifestyle on hard substrates being the ancestral condition for the whole clade. Later, thelodonts underwent a complex ecological diversification and palaeobiogeographical history, comparable in many aspects to those of some major groups of living fishes. Different modes of life evolved repeatedly and a wide range of habitats were colonized by distinct groups, including deep waters and brackish marine and/or freshwater environments. Diadromous strategies presumably appeared on nine different occasions. The palaeobiogeographical history of thelodonts reveals significant differences in the dispersal potential of some major groups. Dispersal of thelodontiforms entailed displacements over long distances and the crossing of deep-water biogeographical barriers, whereas those of furcacaudiforms were always limited to areas interconnected by shallow platforms. We propose that the evolution of pelagic larval stages in thelodontiforms might explain this biogeographical pattern and could satisfactorily account for the greater evolutionary success of this group.

本研究针对鳍甲类（thelodonts）这一脊椎动物演化支的生态分异，从空间、时间与系统发育三个维度展开系统探究。鳍甲类既是已知最早的脊椎动物类群之一，同时也是化石记录中存续时间最长的古生代无颌类（Palaeozoic agnathans）。本研究采用简约法（parsimony）与最大似然法（maximum-likelihood），重建了该类群地理分布、栖息环境与生活方式的祖先状态。研究结果支持如下观点：鳍甲类起源于中奥陶世？–晚奥陶世，其起源地大概率为劳伦大陆（Laurasia）的开阔海洋，该演化支的祖先生活模式为栖息于硬质底质的底栖生活。后续演化过程中，鳍甲类经历了复杂的生态辐射与古生物地理历程，其诸多特征可与部分现生大型鱼类类群相提并论。不同的生活模式多次独立演化，多个类群相继拓殖了多样的栖息环境，涵盖深水水域、咸淡水海洋及淡水环境。推测洄游策略至少独立出现了九次。鳍甲类的古生物地理历史显示，其主要类群间的扩散潜力存在显著差异：鳍甲形类（thelodontiforms）的扩散可跨越长距离并穿越深水生物地理屏障，而叉鳞形类（furcacaudiforms）的扩散则始终局限于浅海台地相互连通的区域。我们提出，鳍甲形类演化出浮游幼体阶段，或许可以解释这一古生物地理格局，同时也能合理解释该类群更高的演化成功度。