The skeletal adaptations of Amblyrhynchus cristatus (Marine Iguanas) are of particular interest based on their amphibious lifestyle, which is unique among living lacertilian squamates. The well-known ecological data are applied to new bone histological findings, which revealed expected and unexpected congruencies. The cortical bone matrix consists of avascular lamellar-zonal bone tissue type. The geometrical disposition of the growth marks (i.e., their spacing) shows an unusual pattern for lizards: the growth cycles maintain a constant thickness until the growth is terminated, which is marked by the development of the external fundamental system (efs). Minor resorption processes within the inner periosteal cortical region and the occurrence of these thick growth cycles in A. cristatus result in high mean bone compactness values. The reported life-history data from ecological studies and the hypothesized annuality of the growth cycles indicate that this first decline in annual bone deposition rate is not congruent with the attainment of sexual maturity. In contrast, this event might be indicated by other histological changes in the growth record of A. cristatus, which they share exclusively with their sister group, Conolophus subcristatus (Land Iguana). The bone matrix of the growth zones and annuli differ in their thickness, their color in polarized light, and vary slightly in the amount and shape of osteocyte lacunae in both A. cristatus and C. subcristatus. These well-recognizable growth zones and annuli of the growth cycles change their thickness abruptly within the reported time frame of the attainment of sexual maturity in A. cristatus.