Antiarch placoderm fishes were an abundant component of the Middle Paleozoic vertebrate assemblages. Despite a large number of known taxa and specimens, the morphology and function of the skeletal elements of their jaws is inadequately known. Because of this, questions regarding their feeding modes and their roles in the trophic webs remains open. We present a skeleto-muscular model of the antiarch jaw apparatus with an attempt to reconstruct its potential biomechanical function. The position of the upper jaw suborbital bones within the plane of the ventral side of the fish armor is suggested to represent the natural ‘mouth closed’ position. During mouth opening the suborbitals rotated rostrally with simultaneous depression and inward rotation of the infragnathals. The ball-and-socket jaw articulation might ensure this combined movement. Recently described lower jaw elements of Livnolepis zadonica (Obrucheva, 1983) and Bothriolepis sp. from the Upper Devonian (Lower Famennian) of Central Russia demonstrating very deep and porous blades of the oral division of the infragnathals attracted attention as to the structure of these bones in other antiarchs. Observed porosity reflects intense vascularization to supply blood to a connective tissue underlying a supposed keratinous sheath, which protected and strengthened the jaws, as well as made possible scraping tough food objects, such as thallus algae, from the substrate.

Having evolved during the Silurian in the Pan-Cathaysian zoogeographical province, antiarchs migrated to Gondwana during the Emsian and later to Euramerica during the Eifelian. Supposedly, antiarchs became the first macrophytophagous vertebrates occupying the trophic level of primary consumers during the late Silurian – early Devonian. This event diversified the only previously existing predator-prey interrelationships between filter-feeding agnathans and predatory gnathostomes.

The specimens are infragnathal (lower jaw) bones assigned by Moloshnikov (2004, 2008) to Livnolepis zadonica (H. Obrucheva, 1983) (specimen PIN 3725/1118) and to Bothriolepis sp. indet. (specimen PIN 3725/1119). Both specimens come from the same locality, a limestone factory quarry by the Gornostayevka village, Livny District, Orel Region, Central Russia, 52°23'30.2"N, 37°33'16.8"E (Upper Devonian, Lower Famennian, Zadonskian Regional Stage). The Zadonskian Regional Stage correlates to Upper triangularis - crepida interval of the SCZ (Sobolev and Evdokimova, 2008).

These infragnathals were CT-scanned at the All-Russian Research Geological Oil Institute (VNIGNI) using the ProCon X-Ray CT-compact system. The following scan parameters were used for PIN 3725/1118: AxialCT = 3840 angles, voxel size = 19.771 μm, exposure time = 0.65 sec, averages = 9, voltage = 90 kV, current = 210 μa, focal spot mode = high resolution, bit depth=16, detector resolution = 1888:1504, detector pixel size=127 μm, binning mode = 1x1 12.5 fps high sensitivity; for PIN 3725/1119: AxialCT = 3840 angles, voxel size = 9.224 μm, exposure time=1.43 sec, averages = 7, voltage = 90 kV, current = 97 μa, focal spot mode = high resolution, bit depth = 16, detector resolution = 1888:1504, detector pixel size = 127 μm, binning mode = 1x1 12.5 fps high sensitivity. Scans were reconstructed by means of CERA and segmented using VolumeGraphics software, with movies made using VolumeGraphics and Movavi Video Suite software. Before macrophotography, specimens were dusted with ammonium chloride.

Antiarch jaw kinematics were reconstructed based on the best-preserved Bothriolepis sp. specimen CPC25205 illustrated by Young (1984). Apart from the dermal jaw bones (suborbital and infragnathal) this specimen also demonstrates the endoskeletal palatoquadrate and Meckelian cartilages. An enlarged jaw model (approximately 5 times) was created of acrylic plaster, aluminum plates and plasticine, mainly based on the graphic reconstructions presented by Young (1984) but including some corrections regarding the lower jaw and jaw articulation made according to Young’s photos of the original specimen CPC25205.