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，包含鲀类及其近缘类群）是一类分类学与形态结构兼具多样性、广布的棘鳍类（acanthomorphs）支系，其成员常被用作基因组学乃至日益增多的宏观演化研究的模式类群。形态差异显著的古近纪化石暗示早期鲀形目存在过大量演化尝试，但这类压扁保存的标本通常所能提供的信息较为有限。本文报道一件产自英国早始新世（约5300万年前）伦敦黏土组的三维保存具喙状结构的鲀形目新类群。该新属新种†栉鳍鲀（Ctenoplectus williamsi gen. et sp. nov.）与最古老的冠群鲀形目（crown tetraodontiforms）物种大致同时代，呈现出前所未有的性状组合：兼具愈合的喙状齿系与着生于覆盖颅骨的横向扩展鳍担骨顶端的显著背鳍棘。贝叶斯全证据末端定年分析（Bayesian total-evidence tip-dating analysis）显示，†栉鳍鲀是三齿鲀科（Triodontidae）的姊妹群，同时揭示出早期鲀形目演化过程中存在大量同塑性（homoplasy）现象。基于本数据集的分析结果显示，冠群鲀形目起源时的形态性状演化速率显著升高，尤其在裸颌类（gymnodonts）类群中；但在主要体型构型确立后，演化速率逐渐下降。这类“早期爆发”模式被视为适应性辐射（adaptive radiations）的标志性特征，但通常仅与较小时空尺度（spatiotemporal scales）下的类群分化相关。不过，要验证这一演化模式，仍需对新近纪与现生类群开展更密集的采样工作。