Data from: Bajaichthys elegans from the Eocene of Bolca (Italy) and the overlooked morphological diversity of Zeiformes (Teleostei, Acanthomorpha)

The Eocene (Ypresian) fauna of Bolca, Italy yields a famous assemblage of marine tropical teleosts. One of the most anatomically distinctive teleosts from Bolca is the enigmatic †Bajaichthys elegans, generally interpreted as a member of Lampridiformes (oarfishes and their allies). Re-examination of the type and only specimen of †Bajaichthys contradicts this attribution, and we propose that its original description as a member of Zeiformes (dories) was in fact correct. †Bajaichthys bears numerous derived features of zeiforms not found in lampridiforms, including: a pelvic spine, a first vertebra closely associated with the neurocranium, and a reduced metapterygoid not contacting the quadrate. Lampridiform-like attributes, including a greatly elongated ascending process of the premaxilla, are known to be convergent between this group and Zeiformes. Using a combination of morphological and molecular data, we confirm this revised interpretation and resolve the position of †Bajaichthys within zeiform phylogeny. In terms of overall shape, the very elongate †Bajaichthys contrasts with deep-bodied Zeiformes and probably had distinctive ecological habits. Our inferred placement of †Bajaichthys and other fossil taxa with extended caudal peduncles (e.g. †Archaeozeus) suggests that the elongate morphotype is ancestral for Zeiformes as a whole, and that the deep-bodied geometry typical of extant taxa probably appeared several times independently. However, these inferences must be considered preliminary due to low support for patterns of relationships within Zeiformes. The systematic reattribution of †Bajaichthys expands the taxonomic diversity of the Bolca fauna as well as the morphological and ecological diversity of the zeiform clade.意大利博尔卡化石点的始新世（Eocene）伊普雷阶（Ypresian）动物群产出了闻名遐迩的海洋热带真骨鱼类组合。博尔卡产出的真骨鱼类中，解剖学特征最为独特的类群之一便是神秘的†优雅巴亚鱼（†Bajaichthys elegans），此前学界普遍将其归入月鱼目（Lampridiformes，即桨鱼及其近缘类群）。对†优雅巴亚鱼的正模且唯一标本的重新检视驳斥了这一分类归属，我们提出其最初被描述为海鲂目（Zeiformes，即海鲂类）成员的观点才是正确的。†优雅巴亚鱼具备诸多海鲂目类群的衍生特征，而这些特征在月鱼目类群中并未发现，包括：腹鳍棘、与脑颅紧密关联的第一椎骨，以及不与方骨相接的退化后翼骨。学界已知，类似月鱼目的特征（如前上颌骨极度延长的上升突）在月鱼目与海鲂目之间属于趋同演化的结果。结合形态学与分子数据，我们证实了这一修订后的分类解读，并明确了†优雅巴亚鱼在海鲂目系统发育中的位置。从整体形态来看，极度修长的†优雅巴亚鱼与体高较高的海鲂目类群形成鲜明对比，其生态习性大概率也颇具特殊性。我们推断的†优雅巴亚鱼以及其他具有延长尾柄的化石类群（如†古海鲂属（Archaeozeus））的系统位置表明，修长形态型是整个海鲂目的祖征，而现生类群典型的高体形态大概率独立演化出现了多次。不过由于海鲂目内部类群间的系统发育关系支持度较低，这些推断仍需被视为初步结论。对†优雅巴亚鱼的分类学重新归属，不仅拓展了博尔卡动物群的分类多样性，也丰富了海鲂目演化支的形态与生态多样性。