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转变。产自加拿大新斯科舍省蓝滩的霍顿布拉夫地层（Horton Bluff Formation）杜内期（Tournaisian-aged）露头记录了这一关键地质时段，并产出包含辐鳍鱼类（actinopterygians）在内的多样脊椎动物群。本文报道一件以三维立体状态保存的辐鳍鱼类下颌骨（mandible），其隶属于一新属新种——*Sphyragnathus tyche*（暂译棘颌鱼）。该下颌骨修长且强烈弯曲，具由异型齿尖牙（heterodont fangs）组成的主齿列。通过标本的特征性骨表面纹饰（ornamentation）、齿列结构与整体下颌骨构造，可确定其归属辐鳍鱼类。通过分析古生代（Paleozoic）辐鳍鱼类下颌骨长度与体长度的相关性，确认*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塑造了功能分化的齿列，具备分别用于猎物捕获（prey capture）与猎物处理（prey processing）的专属区域。通过与现生辐鳍鱼类对比，可将*S. tyche*归为后位大齿型类群（back-fanged macrodont），区别于晚泥盆世（Late Devonian）的前位大齿型辐鳍鱼类（front-fanged macrodont actinopterygians）。这是目前已知辐鳍鱼类化石记录（fossil record）中最早的后位大齿型类群实例，为泥盆纪之后辐鳍鱼类的形态分异（morphological differentiation）提供了新证，并暗示其摄食模式（feeding mode）存在演化尝试。通过对晚泥盆世前位大齿型辐鳍鱼类与*S. tyche*齿列应力分布（stress distribution）的分析，摄食模式的显著变化得到进一步佐证。尽管该标本契合此前提出的石炭纪早期辐鳍鱼类形态分异“头部优先”演化模型（head-first model of morphological diversification），但本文认为“摄食优先”演化模型（feeding-first model）更为贴合实际。