A new predatory actinopterygian from the Tournaisian of Nova Scotia provides insight into the evolution of actinopterygian feeding

The Devonian–Carboniferous transition represents a fundamental shift in vertebrate faunal composition and ocean ecology. Tournaisian-aged outcrops of the Horton Bluff Formation from Blue Beach, Nova Scotia capture this moment and yield a diverse fauna of actinopterygians and other vertebrates. Here, we report an actinopterygian mandible preserved in 3D, representing a new genus and species, <i>Sphyragnathus tyche</i>. This mandible is elongate, deeply curved, and bears a primary dentition of heterodont fangs. Actinopterygian identity is established by the characteristic ornamentation, dentition, and overall mandible construction observed in the specimen. Analysis of the relationship between mandible and body length in Paleozoic actinopterygians establishes <i>S. tyche</i> as a relatively large actinopterygian. Mandible length, curvature, and fang morphology combine to produce a functionally differentiated dentition with distinct regions for prey capture and prey processing. Comparison with modern actinopterygians places <i>S. tyche</i> as a back-fanged macrodont, distinguishing it from front-fanged macrodont actinopterygians of the Late Devonian. This earliest known instance of back-fanged macrodonty in the actinopterygian fossil record provides further evidence of actinopterygian morphological differentiation post-Devonian and implies experimentation in feeding mode. Apparent changes in feeding mode are underscored by analysis of stress distribution across the dentition of Devonian front-fanged macrodont actinopterygians and <i>S. tyche</i>. Although this specimen is compatible with a previous ‘head-first’ model of morphological diversification in early Carboniferous actinopterygians, we argue that a ‘feeding-first’ model is a better fit.

泥盆纪-石炭纪过渡期（Devonian–Carboniferous transition）代表了脊椎动物群组成与海洋生态系统的根本性变革。产自加拿大新斯科舍省蓝海滩的杜内阶（Tournaisian）霍顿布拉夫地层（Horton Bluff Formation）露头记录了这一过渡期，并产出了包含辐鳍鱼类（Actinopterygii）与其他脊椎动物在内的多样化石群。本研究报道一件三维保存的辐鳍鱼类下颌骨（mandible），其代表一新属新种——*Sphyragnathus tyche*。该下颌骨修长且呈强烈弯曲状，其原生齿列分布有异型尖牙（heterodont fangs）。通过该标本的特征性骨表纹饰、齿列与整体下颌骨结构，可确定其辐鳍鱼类的分类地位。通过对古生代（Paleozoic）辐鳍鱼类下颌骨长度与体长之间关系的分析，确定*S. tyche*为体型相对较大的辐鳍鱼类。下颌骨的长度、弯曲度与尖牙形态共同造就了功能分化的齿列，该齿列拥有独立的猎物捕获与猎物处理区域。通过与现生辐鳍鱼类的对比，可将*S. tyche*归为后尖牙型大齿类（back-fanged macrodont），这使其区别于晚泥盆世（Late Devonian）的前尖牙型大齿类（front-fanged macrodont）辐鳍鱼类。辐鳍鱼类化石记录中目前已知最早的后尖牙型大齿类实例，为泥盆纪后辐鳍鱼类的形态分化提供了新证据，并暗示其取食模式存在演化尝试。通过对泥盆纪前尖牙型大齿类辐鳍鱼类与*S. tyche*的齿列应力分布进行分析，进一步凸显了其取食模式的显著变化。尽管该标本符合早期石炭世辐鳍鱼类形态分化的"头部优先"模型，但本研究认为"取食优先"模型更为契合该类群的演化特征。