The study of the so-called vertebral ‘pachyostosis’ of Carentonosaurus mineaui, a plesiopedal mosasauroid sensu Bell & Polcyn from the Cenomanian (Late Cretaceous) of Charente-Maritime (Western France), has revealed that it actually corresponds to pachyosteosclerosis resulting from the combination of cortical hyperplasy with bone compaction due to an inhibition of chondroclastic and osteoclastic activities. This characteristic also occurs in other Cretaceous squamates such as Pachyvaranus crassispondylus and Simoliophis rochebrunei but it is absent in extant squamates. On the contrary, vertebrae of the latter display a very strong porosity due to intense bone remodelling during growth. The phylogenetic significance of pachyosteosclerosis in squamates is thus discussed. The peculiar structure of the vertebrae of Carentonosaurus may be regarded as the result of a heterochronic process, more specifically neoteny. Its association with an adaptation to shallow marine environment is consistent with the inferred ecology of C. mineaui. Moreover, the histological features of the periosteal bone of Carentonosaurus vertebrae provide information about its growth pattern (asymmetry, rate, cyclicity) which may be compared to the ones of Pachyvaranus and Simoliophis.