Are flatfishes (Pleuronectiformes) monophyletic?

Area of Science:

• Evolutionary Biology
• Molecular Phylogenetics
• Ichthyology

Background:

• Flatfish (Pleuronectiformes) are traditionally considered monophyletic due to shared traits like bilateral asymmetry and ocular migration.
• Previous molecular phylogenetic studies have yielded inconclusive results regarding flatfish monophyly, with some placing the genus Psettodes as an outlier.

Purpose of the Study:

• To rigorously investigate the monophyly of Pleuronectiformes using diverse molecular markers and advanced phylogenetic inference methods.
• To resolve conflicting phylogenetic signals and potential artifacts in previous flatfish evolutionary studies.

Main Methods:

• Analysis of six phylogenetically informative nuclear loci across an extensive taxonomic sample of flatfish and related taxa.
• Application of multiple phylogenetic inference methods designed to mitigate analytical artifacts.
• Fossil-calibrated Bayesian relaxed clock analysis to estimate divergence times.

Main Results:

• Phylogenetic analyses consistently indicate that Pleuronectiformes are non-monophyletic, with Psettodes being systematically excluded from the flatfish clade.
• Estimated divergence times suggest the Pleuronectoidei originated approximately 73 million years ago (Ma), with a common ancestor for related taxa dating to 77 Ma.
• These molecularly inferred ages significantly predate the oldest known fossil records of pleuronectiforms.

Conclusions:

• The molecular evidence strongly supports a polyphyletic origin for Pleuronectiformes, challenging the long-held hypothesis of a single common ancestor.
• The findings necessitate a re-evaluation of flatfish evolutionary history and classification, integrating both molecular and morphological data.
• The study highlights the importance of robust molecular markers and extensive taxonomic sampling in resolving deep evolutionary relationships.