Homology of Lateral Cusplets in the Teeth of Lamnid Sharks (Lamniformes: Lamnidae)

Teeth are a potentially rich source of character data for phylogenetic studies of living and fossil sharks, but there are many uncertainties about the homologies of tooth loci as well as structural details of teeth in different taxa. Here, we provide new developmental data on the so-called lateral cusplets of the teeth in Lamniformes. We studied the five living species of Lamnidae (Lamna nasus, L. ditropis, Isurus oxyrinchus, I. paucus, and Carcharodon carcharias) and a more basal lamniform, the Sand Tiger Carcharias taurus, using light microscopy and micro-CT scanning. We also studied a single skeletal specimen of the smalltooth Sand Tiger, Odontaspis ferox. Structures in the teeth of juvenile White Sharks, C. carcharias, were interpreted as lateral cusplets in some previous studies, but we show that they develop differently from lateral cusplets in the teeth of Lamna and Carcharias. We conclude that the structures in Carcharodon are not homologous to the lateral cusplets of Lamna and Carcharias taurus and propose the new name, serrational cusplet, for them. The distinction is important because the presence of lateral cusplets has been used as a character in previous phylogenetic analyses of Lamniformes. According to our new interpretation, among the species we studied, C. taurus, O. ferox, L. nasus, and L. ditropis have lateral cusplets but I. oxyrinchus, I. paucus, and C. carcharias do not. We interpret that the loss of lateral cusplets is a synapomorphy of Carcharodon and Isurus. This new information and interpretation sets the stage for a more comprehensive reevaluation of dental characters and lamniform phylogeny.