JERNVALL, J.*; KER�NEN, S.V.E.; THESLEFF, I.: Quantifying evolutionary modification of development in mammalian molar topography

Mammalian dentition is characterized by complex and diverse molar cusp patterns and subtle changes in molars are usually the earliest morphological evidence of modern orders in the fossil record. However, the exact ways that embryonic patterning of tooth shapes could change to result in the documented evolution have been difficult to characterize quantitatively. We have developed topographic methods using Geographic Information Systems (GIS) to analyze changes in developing teeth. We performed GIS analyses on cusp formation and gene expression in the first lower molar of two species of muroid rodents, the house mouse and the sibling vole. The evolutionary lineages of muroids are well represented in the fossil record, thus providing a good basis for discerning the types of developmental processes in evolutionary transformations. We identify two hierarchical developmental processes, prepatterning and propagation, that can be implicated in mouse and vole molar evolution. Firstly, digital elevation models and cross-correlation analysis show that evolution in lateral cusp topography, affecting location of cusps and crests, results from coordinated changes in molecular prepatterns preceding the first cusps. Secondly, evolutionary increase in longitudinal cusp number results from accelerated longitudinal growth and faster propagation of the established lateral topography. The diversity of mammalian molar cusp patterns may largely result from these two developmental processes, limiting the independence of individual morphological features used in evolutionary studies. GIS analyses on organ topography and gene expression enable new ways to quantify development and evolution.