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.

A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system

“…The pattern of an initial increase to a later decrease is also seen in the recently published data for T. rex of Brown et al 51,. where an increase in tooth count of the dentary (from 16 to 17) is seen in the two least mature specimens (CMNH 7541, BMRP 2002.4.1).…”

“…Work in progress by one of us (TDC) finds that the smaller CMNH 7541 is the less mature specimen of the two. An increase is potentially also seen in the maxilla, where the tooth count for one specimen (BMRP 2002.4.1) is different between sides (left: 16, right: 15), but the authors only included the lower count in their data set51. Including the higher count would result in an increase from 15 to 16 teeth, followed by a decrease in number with increasing size, as seen in their data.…”

Assessment of trophic ecomorphology in non‐alligatoroid crocodylians and its adaptive and taxonomic implications

“…A previous study agrees with this observation by showing positive allometry in the diameter of ‘caniniform’ tooth relative to tooth row length (maxillary tooth #5 in C. porosus and #4 in Alligator mississippiensis ), which is positioned at the apex of the ‘first wave’ of maxillary tooth row (Brown et al. ).…”

“…These tooth size, tooth spacing and tooth count patterns in the upper jaw can almost certainly be generalized for the lower jaw (Brown et al. ). The size of supratemporal fenestra (STF) was measured as an area enclosed by landmarks and semi‐landmarks (Fig.…”

The Permian reptile Opisthodontosaurus carrolli: a model for acrodont tooth replacement and dental ontogeny

“…Most reptiles exhibit minimal change in tooth position and tooth count, with a few documented exceptions (Brown et al. ). Reptilian dentitions tend to be homodont (Montanucci, ), readily masking any external anatomical evidence of a change in tooth position that would be associated with tooth replacement, as the new tooth would look very similar in size and shape to its predecessor.…”