New specimens of the arthrodire Bullerichthys fascidens Dennis and Miles 1980 show incipient site-specific osteichthyan-like tooth addition and resorption

Abstract The arthrodiran placoderm Bullerichthys fascidens, from the Late Devonian Gogo Formation, Western Australia, was originally described from an incomplete headshield with only the spinal and interolateral plates known from the trunk shield. Two new specimens, both incomplete, preserve additional headshield plates enabling the reconstruction of the head. In addition, both new specimens represent different ontogenetic stages relative to the holotype, which is best illustrated in the dental plates, preserved in all three known specimens. B. fascidens presents an unusual dentition for placoderms with multiple tooth rows obliquely crossing the central region of the posterior supragnathal, one of the upper tooth plate elements. New tomographic data of the dentition support the presence of resorption in arthrodiran dentitions, associated with remodelling rather than tooth-specific loss and replacement. In the lower jaw resorbed teeth are not replaced but the biting division is modified through expansion of a new bony shearing blade presumed to have been derived from the original tooth row. Nevertheless, resorption in arthrodires such as Bullerichthys mirrors the resorption and remodelling characteristics of the osteichthyan dentition, with tooth site-specific features, differing in all respects from chondrichthyans.