Abstract
Here, we report a novel type of dermal denticle (orplacoid scale), unknown among both living and fossilchondrichthyan fishes, in a Cretaceous lamniform shark. Bytheir morphology and location, these dermal denticles,grouped into clusters in the cephalic region, appear to havebeen directly associated with the electrosensory ampullarysystem. These denticles have a relatively enlarged (∼350 μmin diameter), ornamented crown with a small (∼100 μm)asterisk- or cross-shaped central perforation connected to amulti-alveolate internal cavity. The formation of such a complexstructure can be explained by the annular coalescence andfusion, around an ampullary vesicle, of several developmentalunits still at papillary stage (i.e. before mineralization), leadingto a single denticle embedding an alveolar ampulla devoid ofcanal. This differs from larger typical ampullae of Lorenziniwith a well-developed canal opening in a pore of the skin andmay represent another adaptive response to low skin resistance.Since it has been recently demonstrated that ampullaryorgans arise from lateral line placodes in chondrichthyans, thishighly specialized type of dermal denticle (most likely nondeciduous)may be derived from the modified placoid scalescovering the superficial neuromasts (pit organs) of themechanosensory lateral line system of many modern sharks.