Interpretation of the toothplates of chimaeroid fishes

Abstract

It has been argued that the toothplates of chimaeroid fishes exhibit a mode of growth that is fundamentally different from that of other chondrichthyans. Chimaeroid toothplates are supposed to be statodont, growing from the basal surface, whereas other chondrichthyan dentitions are lyodont, growing from the lingual towards the labial surface of the jaw. That idea is shown to be mistaken, because chimaeroid toothplates grow from the lingual surface, like other chondrichthyan dentitions. The mistake resulted from confusion about the nomenclature of toothplate surfaces, and on the choice of Chimaera as a Recent model. Callorhynchus is a more appropriate model, since it is shown to exhibit a primitive toothplate conformation, with the labial and symphysial margins of the occlusal surface bounded by a descending lamina which is applied to the margin of the jaw cartilage and grows basally throughout life. The descending lamina is well developed in toothplates of the extinct chimaeroid genera Ischyodus, Pachymylus and Brachymylus, but is much reduced in all Recent genera other than Callorhynchus. A basally-growing descending lamina also bounds the labial and symphysial margins of the principal toothplates in the Mesozoic myriacanthoids and Squaloraja. The toothplates of the Palaeozoic ‘cochliodonts' are reviewed; amongst them, the chondrenchelyids are the only forms with a basally growing descending lamina. So far as the dentition and its mode of growth arc concerned, the closest Palaeozoic relatives of chimaeroids seem to be the chondrenchelyids. The only statodont (basally growing) toothplates found in the course of this work are those of ptyctodont placoderms, which are therefore unlikely to be related to any chondrichthyans. Statodonty in its original sense (failure to shed teeth) is shown to be widespread and possibly primitive in chondrichthyans. Cochliodont and chimaeroid toothplates grow in a logarithmic spiral. Toothplates of primitive chimaeroid type, with basally growing marginal descending laminae, can develop only when the constant angle of the spiral is small (less than about 35°), and when the oral surface of the jaw grows to the same logarithmic spiral.