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.

意大利博尔卡地区的始新世（伊普雷斯期，Ypresian）动物群产出了著名的海洋热带真骨鱼类（teleosts）组合。其中，博尔卡动物群中解剖特征最为独特的真骨鱼类之一，便是神秘的†优雅博尔卡鱼（†Bajaichthys elegans），长期以来该物种被认为是月鱼目（Lampridiformes，皇带鱼及其近缘类群）的成员。我们对其正模且唯一标本的重新检视推翻了这一归类，并提出：该物种最初被归入海鲂目（Zeiformes，海鲂类）的描述实则正确。†优雅博尔卡鱼具备诸多海鲂目特有的衍征，而这些特征并未见于月鱼目，包括：腹鳍棘、与神经颅紧密相连的第一椎骨，以及不与方骨（quadrate）相接的缩小后翼骨。前颌骨极度延长的上升突等类似月鱼目的特征，已被证实是该类群与海鲂目之间的趋同演化特征。我们结合形态学与分子数据，证实了这一修订后的分类观点，并明确了†优雅博尔卡鱼在海鲂目系统发育中的系统位置。就整体形态而言，极度延长的†优雅博尔卡鱼与体高较高的现生海鲂目类群差异显著，其生态习性大概率独具特色。我们对†优雅博尔卡鱼及其他具延长尾柄的化石类群（如†古海鲂（Archaeozeus））的推断系统位置表明，延长形态型是整个海鲂目的祖先特征，而现生类群典型的高体体型可能独立演化了多次。不过，由于海鲂目内部类群间的亲缘关系支持度较低，这些推断仍需视作初步结论。对†优雅博尔卡鱼的重新系统归类，不仅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。