Illustrating ontogenetic change in the dentition of theNile monitor lizard,Varanus niloticus: a case study in the application of geometric morphometric methods for the quantification of shape–size heterodonty

Abstract

Many recent attempts have been made to quantify heterodonty in non-mammalian vertebrates, but the majority of these are limited to Euclidian measurements. One taxon frequently investigated is Varanus niloticus, the Nile monitor. Juveniles possess elongate, pointed teeth (caniniform) along the entirety of the dental arcade, whereas adults develop large, bulbous distal teeth (molariform). The purpose of this study was to present a geometric morphometric method to quantify V. niloticus heterodonty through ontogeny that may be applied to other non-mammalian taxa. Data were collected from the entire tooth row of 19 dry skull specimens. A semilandmark analysis was conducted on the outline of the photographed teeth, and size and shape were derived. Width was also measured with calipers. From these measures, sample ranges and allometric functions were created using multivariate statistical analyses for each tooth position separately, as well as overall measures of heterodonty for each specimen based on morphological disparity. The results confirm and expand upon previous studies, showing measurable shape-size heterodonty in the species with significant differences at each tooth position. Tooth size increases with body size at most positions, and the allometric coefficient increases at more distal positions. Width shows a dramatic increase at the distal positions with ontogeny, often displaying pronounced positive allometry. Dental shape varied in two noticeable ways, with the first composing the vast majority of shape variance: (i) caniniformy vs. molariformy and (ii) mesially leaning, 'rounded' apices vs. distally leaning, 'pointed' apices. The latter was twice as influential in the mandible, a consequence of host bone shape. Mesial teeth show no significant shape change with growth, whereas distal teeth change significantly due primarily to an increase in molariformy. Overall, heterodonty increases with body size concerning both tooth size and shape, but shape heterodonty changes in the mandible are much less pronounced. Although it is unclear to what degree V. niloticus specializes in hard prey items (durophagy), previous studies of varanid feeding behavior, along with research on analogous durophagous vertebrates, indicate a division of labor along the tooth row in adults, due to a possible transition to at least a partial durophagous niche. The geometric morphometric method proposed here, although not without its own limitations, may be ideal for use with a number of dental morphotypes in the future.

Repeated evolution of durophagy during ichthyosaur radiation after mass extinction indicated by hidden dentition

“…(2020) 10:7798 | https://doi.org/10.1038/s41598-020-64854-z www.nature.com/scientificreports www.nature.com/scientificreports/ Molariform teeth are usually an ontogenetic feature. Juveniles of relevant species have conical or tricuspid teeth that become larger and more rounded with growth, sometimes becoming almost flattened in extreme cases [32][33][34][35][36] . Such ontogenetic morphological changes are usually associated with dietary shifts [37][38][39] .…”

Claw morphometrics in monitor lizards: Variable substrate and habitat use correlate to shape diversity within a predator guild

“…Guilds are often defined by shared trophic resources (Simberloff & Dayan, 1991), and varanids typically have intersecting diets due to their opportunistic feeding strategies (example in Sutherland, 2011). Feeding structures such as skulls and teeth would be potentially poor ecomorphological characters for differentiating niche, as such traits have linked particularly well with diet in monitor lizards (Rieppel & Labhardt, 1979; D'Amore, 2015, unpublished data). We suggest that morphological traits associated with locomotion, such as claws, may be more reliable candidates for niche partitioning in these situations, as they link to the occupation of certain habitats.…”

Patterns of postnatal ontogeny of the skull and lower jaw of snakes as revealed by micro‐CT scan data and three‐dimensional geometric morphometrics

“…The skull and lower jaw of Varanus gilleni display limited shape change during ontogeny. This is not likely true of all Varanus species, some of which show distinctive changes in diet and tooth morphology as they mature (Mertens, 1942;Estes & Williams, 1984;D'Amore, 2015). A possible explanation is the small size of V. gilleni (pygmy mulga monitor), with adults reaching a maximum total length of about 35 cm (Cogger, 2014).…”