Data from: Mosaicism in a new Eocene pufferfish highlights rapid morphological innovation near the origin of crown tetraodontiforms

Tetraodontiformes (pufferfishes and kin) is a taxonomically and structurally diverse, widely-distributed clade of acanthomorphs, whose members often serve as models for genomics and, increasingly, macroevolutionary studies. Morphologically disparate Palaeogene fossils suggest considerable early experimentation, but these flattened specimens often preserve limited information. We present a three-dimensionally preserved beaked tetraodontiform from the early Eocene (c. 53 Ma) London Clay Formation, UK. Approximately coeval with the oldest crown tetraodontiforms, †Ctenoplectus williamsi gen. et sp. nov. presents an unprecedented combination of characters, pairing a fused beak-like dentition with prominent dorsal-fin spines that insert atop transversely-expanded pterygiophores roofing the skull. Bayesian total-evidence tip-dating analysis indicates that †Ctenoplectus represents the sister lineage of Triodontidae and highlights considerable levels of homoplasy in early tetraodontiform evolution. According to our dataset, rates of morphological character evolution were elevated at the origin of crown Tetraodontiformes, especially within gymnodonts, but declined after the principal body plans were established. Such ‘early burst’ patterns are regarded as a hallmark of adaptive radiations, but are typically associated with diversification at smaller spatiotemporal scales. However, denser sampling of Neogene and Recent taxa is needed to confirm this pattern.

鲀形目（Tetraodontiformes，涵盖鲀类及其近缘类群）是一类分类与结构多样性丰富、广泛分布的棘鳍鱼类（acanthomorphs）支系，其成员常被用作基因组学及日益增多的宏观进化研究的模式类群。古近纪（Palaeogene）形态差异悬殊的化石暗示该类群早期存在大量演化尝试，但这类压扁的标本通常仅能保存有限的信息。本文报道一件产自英国伦敦黏土组（London Clay Formation）始新世早期（约53 Ma）的三维保存的具喙鲀形目类群。该新属新种†栉齿鲀（Ctenoplectus williamsi gen. et sp. nov.）与最古老的冠群鲀形目（crown tetraodontiforms）类群时代大致同期，其兼具前所未有的性状组合：融合的喙状齿系与位于覆盖头骨的横向扩展鳍担骨（pterygiophores）上方的显著背鳍棘（dorsal-fin spines）。贝叶斯全证据末端定年分析（Bayesian total-evidence tip-dating analysis）显示，†栉齿鲀是三齿鲀科（Triodontidae）的姊妹支系（sister lineage），并揭示了早期鲀形目演化中存在大量同塑性（homoplasy）现象。基于本数据集的分析结果表明，冠群鲀形目起源时的形态性状演化速率（morphological character evolution）较高，尤其是在裸颌亚目（gymnodonts）类群中，但在主要身体构型确立后演化速率有所下降。这类“早爆发”模式被视为适应性辐射（adaptive radiations）的标志性特征，但这类模式通常仅与较小时空尺度下的类群分化相关。不过，仍需要对新近纪（Neogene）及现生类群进行更密集的采样，才能验证这一演化模式。