Dental histology of Cretaceous mosasaurs (Reptilia, Squamata) : incremental growth lines in dentine and implications for tooth replacement

Abstract: The dentine of teeth from five genera of mosasaurs [i.e., Clidastes, Halisaurus, Dollosaurus (Prognathodon),cf. Platecarpus, and Tylosaurus] and a Mesozoic crocodylian (Aigialosuchus) from the Campanian of the Kristianstad Basin, southernmost Sweden, displays two types of incremental growth lines; i.e., von Ebner’s and, for the first time in extinct animals, Andresen’s lines. These lines are homologous to incremental growth lines found in the dentine of extant mammals and reptiles, as well as in the teeth of non-avian dinosaurs and extinct mammal-like reptiles, and are probably homologous for the entire Amniota. The incremental lines document different accumulation rates, where the lines of von Ebner are deposited daily and the Andresen’s lines are deposited roughly every 7–8 day. Incremental lines were measured and counted to obtain replacement rate values for the teeth being analysed. For mosasaurs, the tooth replacement rates varied between 260 (Platecarpus) and 593 days (Tylosaurus), whereas the corresponding rate for the contemporaneous crocodylian Aigialosuchus was 240 days. The dentine accretion rates were similar to one another independent on phylogenetic relationship and tooth-size, to suggest that the tempo was genetically controlled rather than an effect of diet or habits. A replacement tooth was observed growing inside the functional Aigialosuchus tooth, thus providing evidence that tooth replacement was a continuous process where a germ tooth was always ready to replace an older one.