First report of the biomineral whitlockite from the fossil record of holocephalans

Abstract

Extant holocephalan dentitions consist of three pairs of dental plates that grow through life and are composed of a trabecular dentine of hydroxyapatite that surrounds elements of hypermineralized whitlockite. While this unique mineral composition has sparked a series of recent studies, the presence of whitlockite has not been investigated in fossil holocephalans. Using EDS mapping and XRD analyses, we have confirmed for the first time the presence of whitlockite in the tritoral pads and ovoids of the extinct holocephalans †Edaphodon minor, †Edaphodon cf. bucklandi, †Elasmodus hunteri and †Ischyodus dolloi from the Paleogene of the UK. These specimens were collected at sites well known for high-fidelity preservation and have clearly preserved tritoral tissue, suggesting that whitlockite has some preservation potential. Our study confirms the presence of whitlockite in the tritoral pads and ovoids of both extant and extinct crown Holocephali dental plates, extending its evolutionary record back to the Palaeocene. To understand when, phylogenetically, whitlockite was incorporated in their dentition, and to test the role of whitlockite in the adaptation towards durophagy in holocephalans, further studies of stem-group holocephalans, especially Mesozoic and Palaeozoic species with crushing dentitions and separated teeth are needed.