Osteology of †Heteronectes chaneti (Acanthomorpha, Pleuronectiformes), an Eocene stem flatfish, with a discussion of flatfish sister-group relationships

ABSTRACT

The first detailed description of the fossil acanthomorph teleost †Heteronectes chaneti is presented based on a single acid-transfer prepared specimen from the lower Eocene (Ypresian, NP 14) of Bolca, Italy. This genus shows striking cranial asymmetry, with the left orbit displaced toward the top of the skull but failing to reach the dorsal midline. Outside of the orbital region, most paired cranial bones show no clear asymmetry. The postcranial skeleton of †Heteronectes is comparable to that of many ‘generalized’ percomorphs, with a vertebral column comprising 10 abdominal and 14 caudal vertebrae, and a caudal fin with a procurrent spur and 17 principal rays in a I-8/7-I pattern, a caudal-fin endoskeleton consisting of a parphyural, five hypurals, three epurals, and two uroneurals, and pelvic fins with one spine followed by five rays. Evidence for the phylogenetic position of †Heteronectes is reviewed, and a previous interpretation of this genus as a stem member of the percomorph clade Pleuronectiformes is corroborated. However, †Heteronectes retains many primitive percomorph features transformed in other flatfishes, indicating that orbital migration was among the earliest of the distinctive pleuronectiform features to arise. Comparisons between †Heteronectes and candidates for the living sister group of flatfishes reveal the greatest number of similarities with latids (Nile perches and allies). These characters, most of which are of ambiguous polarity, should be the target of future analyses to determine whether they represent symplesiomorphies, homoplasies, or derived features uniting flatfishes and latids.

ACKNOWLEDGMENTS

O. Schultz (NHMW) permitted preparation of the specimen of †Heteronectes, which was completed while the author was a graduate student in the Committee on Evolutionary Biology, University of Chicago, and the Department of Geology, Field Museum (FMNH), and supported by a grant from the Lerner-Grey Fund for Marine Research, a Hinds Fund Grant, an Evolving Earth Grant, a National Science Foundation Graduate Research Fellowship (award number DGE-0228235), and an Environmental Protection Agency STAR Fellowship (award number FP916730). A. Shinya, D. Wagner, and particularly J. Holstein (FMNH) provided invaluable assistance with acid-transfer techniques. K. Swagel (FMNH) took radiographs of the specimen. Morphological comparisons with Recent materials were made in the Department of Vertebrate Zoology, AMNH, while the author worked as a research associate in the Department of Paleontology at the same institution. B. Brown and R. Arrindell kindly allowed me free access to the dry skeletal collection in their care and, along with A. Murray (University of Alberta) and M. Lowe (University Museum of Zoology, Cambridge), provided images of Lates material. Reviews of an earlier version of this contribution were improved by O. Otero (Université de Poitiers) and E. Hilton (Virginia Institute of Marine Science).

Handling editor: Charlie Underwood