Amelotin evolution in tetrapods. Exons are shown as grey squares (not at scale). Species names used in this study are given in Additional file 1. AMTN was present in the last common ancestor of sarcopterygians and was either composed of 9 or 10 exons depending on the presence of exon 5 before or after the actinistian lineage (coelacanthiformes) separated from the other sarcopterygians. Distinct loss and gain of exons occurred in the various lineages explaining the current gene structure. AMTN was invalidated (φ AMTN) independently in chelonian (turtles) and aves (modern birds) after the capability to develop teeth was lost in both lineages. Two important events have been identified in the mammalian lineage: loss of exon 2b that encodes a phosphorylation site, and recruitment of exon 9 after exon 8 was shortened through intraexonic splicing, leading to the loss of the integrin-binding RGD motif. We can establish a parallel between these structural changes and the differences observed in AMTN expression during amelogenesis in mice and non-mammalian tetrapods (see text), suggesting important changes in the protein function. Estimation dates for lineage divergence and phylogenetic relationships are from [13,44-47].