Atypical dentition of a Porbeagle Shark ( <i>Lamna nasus</i> ) highlights odontological considerations in the taxonomy and phylogenetics of sharks

Documenting naturally occurring variations in the dental morphology of sharks is important to better contextualize and assess the putative informativeness and diagnostic value of dental characters in selachian taxonomy and phylogenetic systematics. Here we report the occurrence of atypical tooth morphology, specifically the number and arrangement of lateral cusplets, and the presence of a supernumerary symphyseal tooth file in a mature female Porbeagle Shark, Lamna nasus (Lamniformes: Lamnidae). This is the first time such an occurrence has been documented in the Porbeagle using high-resolution computed tomography (CT). The occurrence of symphyseal teeth in the genus Lamna is regarded as an expression of a plesiomorphic condition. This condition is deeply rooted in the lamniform phylogeny, extending as far back as the Mesozoic, and it reflects that the secondary loss of tooth files is likely a key process in the evolution of modified patterns of tooth replacement in galeomorph sharks.