Convergent dental adaptations in the serrations of hypercarnivorous synapsids and dinosaurs

Abstract

Theropod dinosaurs are well known for having a ziphodont dentition: serrated, blade-shaped teeth that they used for cutting through prey. Serrations along the carinae of theropod teeth are composed of true denticles, a complex arrangement of dentine, enamel, and interdental folds. This structure would have supported individual denticles and dissipated the stresses associated with feeding. These particular serrations were previously thought to be unique to theropod dinosaurs and some other archosaurs. Here, we identify the same denticles and interdental folds forming the cutting edges in the teeth of a Permian gorgonopsian synapsid, extending the temporal and phylogenetic distribution of this dental morphology. This remarkable instance of convergence not only represents the earliest record of this adaptation to hypercarnivory but also demonstrates that the first iteration of this feature appeared in non-mammalian synapsids. Comparisons of tooth serrations in gorgonopsians with those of earlier synapsids and hypercarnivorous mammals reveal some gorgonopsians acquired a complex tissue arrangement that differed from other synapsids.

Keywords: dentition; histology; hypercarnivory; synapsid; theropod.

Figure 1.

Gorgonopsian serration morphology. (a) Skull of the gorgonopsian Lycaenops ornatus (BP/1/2470). (b) Distal serrations on the canine of NHCC LB248. (c) Serrations on the premaxillary incisors of gorgonopsian NHCC LB396. (d) Thin section through distal serrations of NHCC LB334 displaying both interdental folds and denticles. Dashed box indicates location of high magnification images in both plain (e) and cross polarized light (f). Both (e) and (f) highlight features of the denticles and interdental folds including the enamel spindles that cross the enamel–dentine junction as well as the globular dentine and dentine tubules found at the furthest extent of the interdental folds. d, dentine; de, denticle; dt, dentine tubules; e, enamel; edj, enamel–dentine junction; es, enamel spindles; idf, interdental fold; gd, globular dentine; rtd, reduced tubular dentine.

Figure 2.

Comparative serration histology in a phylogenetic context. Histological images of serrations of (a) theropod dinosaurs, (b) Dimetrodon, (c) gorgonopsians and (d) Smilodon fatalis at lower magnification in plain light (top) and higher magnification under polarized light (bottom). All scale bars, 100 µm.