Denticle-embedded ampullary organs in a Cretaceous shark provide unique insight into the evolution of elasmobranch electroreceptors
Abstract
Here, we report a novel type of dermal denticle (or
placoid scale), unknown among both living and fossil
chondrichthyan fishes, in a Cretaceous lamniform shark. By
their morphology and location, these dermal denticles,
grouped into clusters in the cephalic region, appear to have
been directly associated with the electrosensory ampullary
system. These denticles have a relatively enlarged (∼350 μm
in diameter), ornamented crown with a small (∼100 μm)
asterisk- or cross-shaped central perforation connected to a
multi-alveolate internal cavity. The formation of such a complex
structure can be explained by the annular coalescence and
fusion, around an ampullary vesicle, of several developmental
units still at papillary stage (i.e. before mineralization), leading
to a single denticle embedding an alveolar ampulla devoid of
canal. This differs from larger typical ampullae of Lorenzini
with a well-developed canal opening in a pore of the skin and
may represent another adaptive response to low skin resistance.
Since it has been recently demonstrated that ampullary
organs arise from lateral line placodes in chondrichthyans, this
highly specialized type of dermal denticle (most likely nondeciduous)
may be derived from the modified placoid scales
covering the superficial neuromasts (pit organs) of the
mechanosensory lateral line system of many modern sharks.