由于我们密切联系我们赖以生存的土地，，，，as well as the daily interactions we have with domesticates and wildlife around us, we may often forget that collectively, amphibians, reptiles, birds and mammals represent less than half of the ‘leaves’ of the vertebrate tree of life.
Indeed, over 50% of living species of vertebrates are ray-finned bony fishes (Actinopterygii), a clade that has been around for almost half a billion years. Despite being masked by an overall fish-like appearance, actinopterygians comprise fossil and living evolutionary lineages that are, in loose evolutionary terms, as distinct as those of frogs and humans.
†Saurichthyiformes were an iconic group of early actinopterygians, which reigned aquatic ecosystems for almost 80 million years and even survived the biggest mass extinction in the history of life.
然而，主要的雷鳍谱系的起源仍然晦涩难懂。将早期的actinopterygian化石分配到生命之树的正确分支仍然是获取更完整的脊椎动物进化图像及其通过重大灭绝事件的过程的障碍。
Typically, ray-finned fossils are found crushed in two dimensions by overlying sediments. This does not allow the observation of tremendously important internal skeletal features (e.g. the braincase, sensory structures and gill bones). Luckily, on rare occasions, carcasses were encrusted quickly in pebble-shaped nodules that secured their three-dimensional preservation, protecting the integrity of internal structures.
经典调查，从20岁开始Th世纪，在3D早期actinopterygians的内部解剖结构中，仍然构成了我们有关该主题的知识的核心。早期作品采用了破坏性技术，例如串行研磨，以描述内部特征。可以想象，这些结果的精度有限，并且不可再生，因为原始标本在此过程中被破坏。
最新的古生代 - 梅生代†Saurichthyiformes是一群标志性的Actinopterygians，统治了水生生态系统已有近8000万年的历史，甚至幸免于生命历史上最大的批评。他们是第一个进化典型的“伏击捕食者”（鱼雷般的）身体形状的人，如今，它是在GARS或针鱼中遇到的。
Destructive investigations of the internal skeleton of †Saurichthyiformes, conducted in the 1920s, pointed towards a close relationship with modern paddlefishes and sturgeons (which are best known for their delicious eggs ‒ caviar). Although widely accepted, this hypothesized close relationship rested on weak anatomical arguments.
在这项工作中，我们通过对两种†的3D化石进行了计算的X射线显微光检查来重新审视†Saurichthyiformes的内部解剖结构。Saurichthysfrom the Early Triassic of Nepal and East Greenland. This method is non-destructive, and results in high-resolution digital models that can be easily studied from different angles, enabling accurate comparisons with other animals.
我们通过数字化重建的内部骨骼结构，例如脑壳，下巴和g。我们还可以观察到生活中被大脑，颅神经和血管以及内耳所占据的空间。这使我们能够推断实际软组织结构的形状和/或过程。我们的重建导致了新的解剖学发现，但也揭示了古典作品中的关键缺陷。
在我们的新发现中，有鼻子中存在大型空空间。此外，我们发现骨骼（Dermohyal）以前被认为是†中缺失的Saurichthys，实际上被融合到另一个骨头（卫生）。这两个功能都是原始的。有趣的是，我们发现†的g覆盖骨结构Saurichthysis very similar to that of modern sturgeons and paddlefishes.
To put this new anatomical knowledge in its evolutionary context, we compared †Saurichthys带有各种化石和活射线的鱼类。在专门的算法的帮助下，我们分析了数十种骨鱼类的大量形态特征数据集，以根据解剖学相似性追踪进化亲和力。
Contrasting historical phylogenetic hypotheses, the evolutionary trees produced by our analyses do not recover †Saurichthyiformes as closely related to sturgeons and paddlefishes, implying that many of their similarities have likely evolved independently (convergently). However, the lack of internal skeletal data for many other early actinopterygians precludes a clear picture of the early ray-fin tree of life.
There will always be too many fossil fishes out there and not enough time (and resources) to study them. It might even seem that this work only scratches the surface of early actinopterygian evolution. Yet, through works like this, we are able to better locate specific morphological characters and also fossil species that are responsible for ‘reshuffling’ the branches of the largest partition of the vertebrate tree of life.
The unprecedented anatomical knowledge of the internal skeleton of †Saurichthyiformes will doubtlessly form a solid basis for further comparisons with new fossils. Moreover, our research highlights the fact that established classical interpretations of actinopterygian anatomy need to be critically reappraised in light of new material and modern methods of anatomical imaging.
Comments