Abstract
BackgroundThe facial musculature is a remarkable anatomical complex involved in vital activities of fishes, such as food capture and gill ventilation. The evolution of the facial muscles is largely unknown in most major fish lineages, such as the Actinopterygii. This megadiverse group includes all ray-finned fishes and comprises approximately half of the living vertebrate species. The Polypteriformes, Acipenseriformes, Lepisosteiformes, Amiiformes, Elopiformes, and Hiodontiformes occupy basal positions in the actinopterygian phylogeny and a comparative study of their facial musculature is crucial for understanding the cranial evolution of bony fishes (Osteichthyes) as a whole.ResultsThe facial musculature of basal actinopterygians is revised, redescribed, and analyzed under an evolutionary perspective. We identified twenty main muscle components ontogenetically and evolutionarily derived from three primordial muscles. Homologies of these components are clarified and serve as basis for the proposition of a standardized and unifying myological terminology for all ray-finned fishes. The evolutionary changes in the facial musculature are optimized on the osteichthyan tree and several new synapomorphies are identified for its largest clades, including the Actinopterygii, Neopterygii, and Teleostei. Myological data alone ambiguously support the monophyly of the Holostei. A newly identified specialization constitutes the first unequivocal morphological synapomorphy for the Elopiformes. The myological survey additionally allowed a reinterpretation of the homologies of ossifications in the upper jaw of acipenseriforms.ConclusionsThe facial musculature proved to be extremely informative for the higher-level phylogeny of bony fishes. These muscles have undergone remarkable changes during the early radiation of ray-finned fishes, with significant implications for the knowledge of the musculoskeletal evolution of both derived actinopterygians and lobe-finned fishes (Sarcopterygii).
Highlights
The facial musculature is a remarkable anatomical complex involved in vital activities of fishes, such as food capture and gill ventilation
Justifications for the inferences of homologies and the consequent muscle terminology applied are discussed in the section “Homologies of the facial muscles in the Actinopterygii”
Clarification of the muscle homologies resulted in the proposition of a unifying terminology for the facial musculature of all actinopterygians
Summary
The facial musculature is a remarkable anatomical complex involved in vital activities of fishes, such as food capture and gill ventilation. The skeletal musculature exhibits an extraordinary diversification across multiple fish lineages and constitutes a rich source of phylogenetic information [1,2,3,4,5]. This anatomical system has been traditionally neglected in most evolutionary studies [6]. With only 49 extant species, occupy the basalmost region of the actinopterygian tree: Polypteriformes (bichirs and reedfish; 12 spp.), Acipenseriformes (sturgeons and paddlefishes; 29 spp.), Lepisosteiformes (gars; 7 spp.), and Amiiformes (bowfin; 1 sp.) [7, 10].
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