Revisiting Hybodus and Mesozoic hybodontiform evolution

Understanding the evolution of hybodontiforms, a diverse group of shark-like chondrichthyans ranging from the Late Devonian to the Late Cretaceous, is challenging, as most species are only known from their teeth. The genus Hybodus, established in the 19th century based on both dental and skeletal material from the Early Jurassic Lias Group (UK), has traditionally been regarded as one of the most speciose hybodontiforms, encompassing numerous nominal species from across the globe and spanning a stratigraphic range from the Early Permian to the Late Cretaceous. However, recent research has revealed that Hybodus, as historically defined, represents an unnatural assemblage of unrelated species characterized by unspecialized multicuspid teeth. Here, we present preliminary results from an ongoing study of abundant, largely undescribed fossil material referred to Hybodus, including articulated and whole-bodied specimens from some of the world's most productive Jurassic vertebrate sites. Our findings provide evidence for restricting Hybodus to its type species, H. reticulatus from the Early Jurassic Lias Group, and further bolster the hypothesis that Jurassic hybodontids were ecological generalists exhibiting an arrested evolutionary trajectory.