The development of complex tooth shape in reptiles

Abstract

Reptiles have a diverse array of tooth shapes, from simple unicuspid to complex multicuspid teeth, reflecting functional adaptation to a variety of diets and eating styles. In addition to cusps, often complex longitudinal labial and lingual enamel crests are widespread and contribute to the final shape of reptile teeth. The simplest shaped unicuspid teeth have been found in piscivorous or carnivorous ancestors of recent diapsid reptiles and they are also present in some extant carnivores such as crocodiles and snakes. However, the ancestral tooth shape for squamate reptiles is thought to be bicuspid, indicating an insectivorous diet. The development of bicuspid teeth in lizards has recently been published, indicating that the mechanisms used to create cusps and crests are very distinct from those that shape cusps in mammals. Here, we introduce the large variety of tooth shapes found in lizards and compare the morphology and development of bicuspid, tricuspid, and pentacuspid teeth, with the aim of understanding how such tooth shapes are generated. Next, we discuss whether the processes used to form such morphologies are conserved between divergent lizards and whether the underlying mechanisms share similarities with those of mammals. In particular, we will focus on the complex teeth of the chameleon, gecko, varanus, and anole lizards using SEM and histology to compare the tooth crown morphology and embryonic development.

Developmental mechanisms driving complex tooth shape in reptiles

“…To analyze the development of tooth complexity in reptiles, we selected the veiled chameleon ( C. calyptratus ) as a species from Acrodonta lineage of squamates and ocelot gecko ( P. picta ) as a representative of the basal squamates of Gekkota lineage, which both exhibit distinct structures (ridges/crests) on their tooth surface. These ridges in the gecko have been shown to arise due to asymmetrical enamel deposition leading to thick enamel production at the crests and reduced enamel in the grooves . Tooth morphology during development was compared to that in the Siamese crocodile ( C. siamensis ), member of Archosauria lineage of reptiles, which exhibits a simpler conical tooth shape.…”

“…However, co‐expression of Shh and Fgf8 orthologs is present in this area . In reptiles, structures resembling primary EKs have been indicated in the alligator, which has single cuspid teeth, and also in several squamate species with both single and multicuspid teeth . On the other hand, an evaluation of Shh and Fgf4 expression in crocodilians did not confirm the presence of an EK .…”

“…Toothed vertebrates have evolved many types of dentition with different tooth complexity level as an adaptation to distinct food requirements. The shape of teeth in vertebrates varies from simple conical to multicuspid, and their surface morphology can include tooth cusps, enamel ridges, or tubercles with serrated edges . Such heterogeneity brought us to the question of whether there is any conserved mechanism driving the process of tooth shaping.…”

Mosasaurs and snakes have a periodontal ligament: timing and extent of calcification, not tissue complexity, determines tooth attachment mode in reptiles

“…In general, the teeth of most lizards fuse to the jaws extremely rapidly and the probability of capturing a tooth at an intermediate stage of attachment tissue mineralization is extremely low (Zahradnicek et al. ). Most snakes, with the exclusion of hinged teeth, appear to be no exception (Figs and ).…”

“…), and developmental work by Zahradnicek et al. () has shown the presence of cementoblast‐like cells forming a portion of the mineralized attachment tissue complex in a species of extant gecko. These findings challenge the conventional, single‐tissue model of reptilian tooth attachment, where ‘bone of attachment’ fuses the tooth to the jaws.…”