Dental plate histology of †Ischyodus dolloi (Chondrichthyes, Holocephali), from Antarctica

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Abstract
In contrast to other cartilaginous fishes (Chondrichthyes), the dentition of the Holocephali consists of six dental plates, four in the upper jaw (two vomerines and two palatines) and two in the lower jaw (mandibulars). While the microstructure of holocephalan dentition has been extensively studied in extant species, similar analyses in fossil taxa are less common. Here, we provide a detailed description and interpretation of the microanatomy and histology of dental plates of the extinct taxon †Ischyodus dolloi Leriche (Chimaeriformes: ‘Edaphodontidae’). The study is based on two palatine and two mandibular plates from the Eocene of the Antarctic Peninsula. Our main objectives were to (1) identify the presence and distribution of different tissues; (2) analyze morphological diversity in hypermineralized structures (i.e., tritors) within the dental plates; (3) compare the microanatomy and histology of †I. dolloi with other living and fossil holocephalans; and (4) infer the sequence and mode of tissue formation within the plates. The dental plates are composed of an outer shell enclosing a framework of trabecular dentine, in which hypermineralized dentine (i.e., whitlockin) is deposited to form the tritors. Trabecular dentine becomes compacted by centripetal deposition of circumvascular dentine around vascular canals. The tritors exhibit two morphological types: tritoral rods (first report for ‘Edaphodontidae’) and tritoral pads. Additionally, a distinct tissue, which we term ‘parallelitubular dentine’, is formed in the aboral and symphyseal regions. Regarding the growth of dental plates, we identified a developmental pattern in which the full formation of tritors and complete compaction of trabecular dentine follows a mesial‐to‐lingual, oral‐to‐aboral, and labial‐to‐symphyseal sequence. This growth pattern aligns with the spatial order in which the dentinal tissues are progressively removed due to feeding‐related wear.

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