Australia: The Land Where Time Began

A biography of the Australian continent 

Parareptilia - Colobomycter pholeter, a parareptile from the Early Permian- Plicidentine

Previously plicidentine, an infolding pattern of dentine, was believed to be a characteristic found only in amniotic clades, which includes Parareptilia. It is usual for most parareptiles to be assumed to lack plicidentine as the external indicators, such as tooth bases with plications, are not present, when a detailed analysis of their dentine has not been carried out. The detailed study within the dentition of this unusual parareptile, as well as coeval members of the same clade, presented here resulted from the finding by McDougall et al., (2014) that in Colobomycter pholeter the largest premaxillary and maxillary teeth clearly lacked this dentinal feature. There is a large range of variability within the dentition of C. pholeter of the degree of folding in the dentine, which is also the case in the dentition of closely related parareptiles. The variability found ranges from no plications on the teeth to amniote-like plicidentine that is very complex. The authors¹ also demonstrated, by the use of CAT scans in conjunction with histological sections, the utility of CAT scans to conduct sampling that is not destructive for the identification of plicidentine. They hypothesise that a function of plicidentine is to increase the surface area for attachment tissues, based on the variability of the plicidentine found in this sample of parareptiles, and they suggest it may be misleading to use the plicidentine as a character for phylogenetic analyses of parareptiles.

It has been revealed by comparisons of several taxa of coeval parareptilians that the presence of plicidentine in Parareptilia is much more common than previously believed. A wide array of dentine folding is displayed by different members of the clade, the range extending from a complete lack of folding, as in the case of Bolosaurus, to complex folding to the extent that it is reminiscent of the plicidentine found in temnospondyl amphibians. Variation of plicidentine within a single taxon has also been described for the first time by the athors¹ in Colobomycter pholeter, in which it exhibits 3 different patterns of folding, which is dependent on the size and position of the tooth. The likelihood of heterodonty in C. pholeter is also reinforced by this variation, being associated with different functions for different teeth. The hypothesis that a function of plicidentine is increased surface area for that attachment of tissue between the dentine and the periodontium is supported by the presence of plicidentine in parareptiles. This function can, however, be attributed to teeth that are implanted shallowly and display the most extensive patterns of infolding towards the tooth base, at the point where it interacts with the attachment tissues. Plicidentine is not present in the deeply implanted teeth of bolosaurids. The authors¹ suggest that in bolosaurids the relationship between the dentine and periodontium may be related to the evolution of dental occlusion (Reiscz, R.R., 2006). It is now apparent, based on the data gained from this study, that in the parareptiles the presence of plicidentine has a complicated evolutionary history. Within the clade the large variability of plicidentine, the variability of its content in a single taxon, as well as its apparent relation to the size of a tooth, reduces the phylogenetic utility of the plicidentine. The authors¹ suggest that plicidentine should not be used as a character for phylogenetic analyses of Parareptilia, given the problematic nature of the dentitions of the Parareptilia; though at least a better understanding is gained of the distribution and evolution history of this feature.

MacDougall MJ, LeBlanc ARH, Reisz RR (2014) Plicidentine in the Early Permian Parareptile Colobomycter pholeter, and Its Phylogenetic and Functional

Significance among Coeval Members of the Clade. PLoS ONE 9(5): e96559. doi:10.1371/journal.pone.0096559