DOI: 10.5061/dryad.9zw3r22gw
title: Feeding in the Devonian antiarch placoderm fishes: a study based upon morpho-functional analysis of jaws

description: 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.
description: The specimens are infragnathal
 (lower jaw) bones assigned by Moloshnikov (2004, 2008)
 to <em>Livnolepis zadonica</em> (H. Obrucheva, 1983) (specimen
 PIN 3725/1118) and to <em>Bothriolepis</em> 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 <em>Bothriolepis</em> 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.