Tooth counts through growth in diapsid reptiles: implications for interpreting individual and size-related variation in the fossil record

Abstract

Tooth counts are commonly recorded in fossil diapsid reptiles and have been used for taxonomic and phylogenetic purposes under the assumption that differences in the number of teeth are largely explained by interspecific variation. Although phylogeny is almost certainly one of the greatest factors influencing tooth count, the relative role of intraspecific variation is difficult, and often impossible, to test in the fossil record given the sample sizes available to palaeontologists and, as such, is best investigated using extant models. Intraspecific variation (largely manifested as size-related or ontogenetic variation) in tooth counts has been examined in extant squamates (lizards and snakes) but is poorly understood in archosaurs (crocodylians and dinosaurs). Here, we document tooth count variation in two species of extant crocodylians (Alligator mississippiensis and Crocodylus porosus) as well as a large varanid lizard (Varanus komodoensis). We test the hypothesis that variation in tooth count is driven primarily by growth and thus predict significant correlations between tooth count and size, as well as differences in the frequency of deviation from the modal tooth count in the premaxilla, maxilla, and dentary. In addition to tooth counts, we also document tooth allometry in each species and compare these results with tooth count change through growth. Results reveal no correlation of tooth count with size in any element of any species examined here, with the exception of the premaxilla of C. porosus, which shows the loss of one tooth position. Based on the taxa examined here, we reject the hypothesis, as it is evident that variation in tooth count is not always significantly correlated with growth. However, growth trajectories of smaller reptilian taxa show increases in tooth counts and, although current samples are small, suggest potential correlates between tooth count trajectories and adult size. Nevertheless, interspecific variation in growth patterns underscores the importance of considering and understanding growth when constructing taxonomic and phylogenetic characters, in particular for fossil taxa where ontogenetic patterns are difficult to reconstruct.

Keywords: Alligator; Crocodylus; Diapsida; Dinosauria; Reptilia; Varanus; allometry; dentition.

Figure 1

Size-related tooth count change for the premaxilla (n = 116), maxilla (n = 120), and dentary (n = 112) of Alligator mississippiensis. (A) Plot of tooth counts as a function of basal skull length for the premaxilla (white), maxilla (grey), and dentary (black). Dots indicate both left and right, ‘L’ indicates only left, and ‘R’ indicates only right. Lines represent the slope of the OLS regression, and grey areas indicate 95% confidence intervals. (B) Histogram of tooth counts for both left and right sides.

Figure 2

Size-related tooth count change for the premaxilla (n = 46), maxilla (n = 46), and dentary (n = 46) of Crocodylus porosus. (A) Plot of tooth counts as a function of basal skull length for the premaxilla (white), maxilla (grey), and dentary (black). Dots indicate both left and right, ‘L’ indicates only left, and ‘R’ indicates only right. Lines represent the slope of the OLS regression. (B) Histogram of tooth counts for both left and right sides.

Figure 3

Size-related tooth count change for the premaxilla (n = 36), maxilla (n = 42), and dentary (n = 40) of Varanus komodoensis. (A) Plot of tooth counts as a function of basal skull length for the premaxilla (white), maxilla (grey), and dentary (black). Dots indicate both left and right, ‘L’ indicates only left, and ‘R’ indicates only right. Lines represent the slope of the OLS regression. (B) Histogram of tooth counts for both left and right sides.

Figure 4

Tooth allometry for the fourth (Alligator mississippiensis), fifth (Crocodylus porosus), and seventh (Varanus komodoensis) tooth against tooth row length of both A. mississippiensis (A) C. porosus (B), and V. komodoensis (C), respectively. Left (circles) and rights (triangles) are plotted separately for both the maxilla (grey) and dentary (black). Dashed line indicates a slope of 1.

Figure 5

Ventral (palatal) view of the premaxilla of Crocodylus porosus specimen FMNH 10899. The left side shows a large foramen for receipt of the first dentary tooth (indicated by arrow), and as a result the second premaxillary tooth is lost. On the right side, this foramen is not developed and the second premaxillary tooth is retained. Scale bar: 5 cm.

Figure 6

Overview figure illustrating patterns of tooth count change across size in many species of extant and extinct diapsids. Each line indicates the best fit line for a species. Colours and pattern (solid, dotted, dashed) indicate different taxa, and thickness indicates element (thick = maxilla, thin = dentary). Taxon silhouettes for Figure 6 obtained from the PhyloPic website (phylopic.org) under attribution (creativecommons.org/licenses/by/3.0/) or public domain. Artist credit: Craig Dylke, Andrew Farke, Ghedo, Rebecca Groom, Scott Hartman, Michael Keesey, Michael Scroggie, Smokeybjb, Steve Traver, Sarah Werning, Emily Willoughby.