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（Bolca）地区始新世（Eocene）伊普雷阶（Ypresian）的动物群产出了著名的海洋热带真骨鱼类（teleosts）组合。博尔卡产区最具解剖学辨识度的真骨鱼类之一是神秘的†优雅巴哈鱼（†Bajaichthys elegans），该物种曾被普遍认为是月鱼目（Lampridiformes，皇带鱼及其近缘类群）的成员。本研究对该物种唯一的正模标本进行重新检视后，推翻了这一归类主张，并提出其最初被归为海鲂目（Zeiformes，海鲂类）的描述实则正确。†优雅巴哈鱼具备诸多海鲂目特有的衍征，而月鱼目并无此类特征，包括：腹鳍棘（pelvic spine）、与脑颅（neurocranium）紧密关联的第一椎骨、不与方骨（quadrate）接触的缩小的中翼骨（metapterygoid）。此前已知的月鱼目类特征——如极度延长的前上颌骨（premaxilla）升突——实则是该类群与海鲂目趋同演化的结果。本研究结合形态学与分子数据，证实了这一修正后的归类结论，并明确了†优雅巴哈鱼在海鲂目系统发育（phylogeny）中的演化位置。就整体形态而言，极度修长的†优雅巴哈鱼与高体型海鲂目类群形成鲜明反差，其生态习性大概率独具一格。我们推断的†优雅巴哈鱼及其他具有延长尾柄（caudal peduncle）的化石类群（如†Archaeozeus）的演化位置表明，修长形态型是整个海鲂目的祖先特征，而现生类群（extant taxa）典型的高体体型可能曾多次独立演化出现。不过，由于海鲂目内部亲缘关系的支持度较低，上述推断仍需被视作初步结论。对†优雅巴哈鱼的系统学重新归类，既拓展了博尔卡动物群的分类多样性，也丰富了海鲂类群的形态与生态多样性。