Feeding mechanisms and ecology of pycnodont fishes (Neopterygii, Pycnodontiformes)

Abstract

The functional morphology of the jaw apparatus and the skull and the feeding habits of the extinct pycnodont fishes are reconstructed in comparison with some extant halecostomes. For this a short review of the functional units of the pycnodont head is given. The feeding mechanisms of pycnodonts exhibit a transition from simple stereotypic feeding kinematics, which are characteristic for primitive actinoptergians, to the modulating feeding kinematics of advanced teleosts and is called limited modulating feeding kinematics herein. Two structural specialisations which are found in halecostomes (operculum with distinct m. levator operculare and the interopercular bone with the interopercular ligament) are supposed to be absent in pycnodonts, whereas they maintain the two primitive couplings for direct mandibular depression (epaxial muscles ‐ neurocranium, hypaxial muscles ‐cleithrum‐m. sternohyoideus ‐ hyoid apparatus). Advanced pycnodonts developed a new structure (upper jaw protrusion, resulting in an enlargment of the buccopharyngeal cavity), that is absent in halecomorphs (e.g., Amia calva) and basal pycnodonts (e.g., Anduafrons, Mesturus). The premaxillae and maxillae are firmly fixed in basal pycnodonts, whereas the premaxillae and maxillae are free and movable in advanced pycnodonts. Pycnodonts were benthic foragers with a combination of biting or nipping and suction feeding based on the „truncated cone morphology”︁ of the buccopharyngeal cavity. It is concluded, that pycnodonts certainly were omnivorous feeders with a general broad range of prey. But they were also a highly specialised group on generic level in respect to their prey. This is indicated by gut contents, as far as they are known, which comprise only monospecific remains of shelled invertebrates (e.g., spines of echinoderms, shells of bivalves). The ecological demands of pycnodonts are discussed.