Fig. 6.

Thelodonti. (A–D) Schematic illustrations of four of the five recognized structural forms of thelodont odontode-like scales (Janvier, 1996). (A) Kawalepis (achanolepid type, early Silurian). (B) Thelodus (thelodontid type, Silurian) (note that this form of thelodont scale is very similar to the odontodes of modern chondrichthyans). (C) Loganellia (loganiid type, late Silurian). (D) Phlebolepis (katoporid type, late Silurian). All thelodont scales are composed of a non-growing dentine-rich crown with a variably defined pulp cavity, and an attachment process of acellular bone which may be the only vestige of an osteogenic skeletal derivative. (E) SEM section of a Thelodus scale demonstrating the presence of a thin superficial layer of enameloid (arrowhead) covering the dentine. Scale bars: B = 100 µm; E = 250 µm.