Sphenacanthus , a Palaeozoic freshwater shark

Recently discovered material from the Visén Oil Shale Groups of the Scottish Midland Valley allows a description of the anatomy of the head and dermal skeleton of Sphenacanthus serrulatus to be made for the first time. This suggests that it was a typical phalacanthous shark, rather larger, but otherwise quite similar to the contemporaneous Tristychius arcuatus . Like the latter, it appears to have had a long otic region on the neurocranium, a heterodont dentition and two dorsal finspines, but it differed in having longer, deeper jaws and a complete covering of scales (aggregated lepidornoria). Sphenacanthus serrulatus is absent from the British Coal Measures (Westphalian), but the very similar Sphenacanthus hybodoides is found here. Both these forms appear to be normal components of the freshwater ‘lagoonal’ stage of the cyclothems and provide further evidence that Palaeozoic sharks were more varied in their choice of habitat than those of today.