Articulated specimens provide new insights into the iconic Mesozoic shark genus Sphenodus

The fossil record of sharks and rays (Elasmobranchii) consists predominantly of isolated teeth, complicating their taxonomic and phylogenetic inferences. Consequently, the study of holomorphic specimens provides crucial information to reassess existing phylogenetic and taxonomic interpretations. The extinct Mesozoic shark genus †Sphenodus exemplifies these challenges. Initially classified based on dental traits, the genus has undergone multiple revisions, reflecting its taxonomic instability. However, the presence of skeletal remains assigned to †'Sphenodus' macer and †'S.' nitidus provides an opportunity to re-evaluate the phylogenetic and taxonomic relations of this genus. In this study, we critically reviewed the systematics and taxonomic relationships of †Sphenodus, through an exhaustive literature review and comparison with previous phylogenetic hypotheses. Our analysis of skeletal characters suggests the synonymy of †S. macer and †S. nitidus and supports previous Hexanchiformes classifications for the genus, placing it in a close relation to †Notidanoides forming a clade in sister relation to Hexanchidae. However, the teeth originally used to define †Sphenodus lack diagnostic traits for taxonomic identification, rendering†'Sphenodus' a nomen dubium. Consequently, we introduce a new name, †Archaeogracilidens, to include the species based on skeletal material and that present teeth with a well-preserved root and a crown, establishing a more stable systematic framework for this group.