Extrinsic labyrinth infillings imply open endolymphatic ducts in Lower Devonian osteostracans, acanthodians, and putative chondrichthyans

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Abstract
Acanthodians, osteostracans, and putative chondrichthyans from the Lower Devonian (Lochkovian) ‘MOTH’ locality are found to have granular labyrinth infillings located immediately posterior to the orbits and composed of fine, sand-sized particles. They were examined under light and scanning electron microscopes and an elemental analysis (EDX) was performed. The infillings contain large proportions of silica, calcium carbonate, and dolomite, as well as minor proportions of elements such as aluminum, iron, and potassium. These results were similar to elemental analysis of the rock matrix. The inability of the fish to manufacture some of the minerals confirms that the labyrinth infilling consists at least partly of extrinsic grains. Endolymphatic pores, the openings through which this material probably entered the labyrinth, are seen in osteostracans, but have seldom been reported in acanthodians and never in basal (putative) chondrichthyans. No undoubted otoliths were seen in any of the specimens. The presence of similar statoconia, including extrinsic grains, in all three groups indicates a common inner ear physiology and presence of open endolymphatic ducts. The phylogenetic distribution of this feature suggests that it is primitive for osteostracans + gnathostomes, and that placoderm ancestors also had open endolymphatic ducts that functioned to allow entry of extrinsic grains into the labyrinth.